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#676 2020-05-19 00:32:50

ganesh
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Registered: 2005-06-28
Posts: 30,280

Re: Miscellany

556) Meteorite crater

Meteorite crater, depression that results from the impact of a natural object from interplanetary space with Earth or with other comparatively large solid bodies such as the Moon, other planets and their satellites, or larger asteroids and comets. For this discussion, the term meteorite crater is considered to be synonymous with impact crater. As such, the colliding objects are not restricted by size to meteorites as they are found on Earth, where the largest known meteorite is a nickel-iron object less than 3 metres (10 feet) across. Rather, they include chunks of solid material of the same nature as comets or asteroids and in a wide range of sizes—from small meteoroids up to comets and asteroids themselves.

Meteorite crater formation is arguably the most important geologic process in the solar system, as meteorite craters cover most solid-surface bodies, Earth being a notable exception. Meteorite craters can be found not only on rocky surfaces like that of the Moon but also on the surfaces of comets and ice-covered moons of the outer planets. Formation of the solar system left countless pieces of debris in the form of asteroids and comets and their fragments. Gravitational interactions with other objects routinely send this debris on a collision course with planets and their moons. The resulting impact from a piece of debris produces a surface depression many times larger than the original object. Although all meteorite craters are grossly similar, their appearance varies substantially with both size and the body on which they occur. If no other geologic processes have occurred on a planet or moon, its entire surface is covered with craters as a result of the impacts sustained over the past 4.6 billion years since the major bodies of the solar system formed. On the other hand, the absence or sparseness of craters on a body’s surface, as is the case for Earth’s surface, is an indicator of some other geologic process (e.g., erosion or surface melting) occurring during the body’s history that is eliminating the craters.

The Impact-Cratering Process

When an asteroidal or cometary object strikes a planetary surface, it is traveling typically at several tens of kilometres per second—many times the speed of sound. A collision at such extreme speeds is called a hypervelocity impact. Although the resulting depression may bear some resemblance to the hole that results from throwing a pebble into a sandbox, the physical process that occurs is actually much closer to that of an atomic bomb explosion. A large meteorite impact releases an enormous amount of kinetic energy in a small area over a short time. Planetary scientists’ knowledge of the crater-formation process is derived from field studies of nuclear and chemical explosions and of rocket missile impacts, from laboratory simulations of impacts using gun-impelled high-velocity projectiles, from computer models of the sequence of crater formation, and from observations of meteorite craters themselves.

Immediately after a meteorite strikes the surface of the planet, shock waves are imparted both to the surface material and to the meteorite itself. As the shock waves expand into the planet and the meteorite, they dissipate energy and form zones of vaporized, melted, and crushed material outward from a point below the planet’s surface that is roughly as deep as the meteorite’s diameter. The meteorite is usually vaporized completely by the released energy. Within the planet, the expanding shock wave is closely followed by a second wave, called a rarefaction, or release, wave, generated by the reflection of the original wave from the free surface of the planet. The dissipation of these two waves sets up large pressure gradients within the planet. The pressure gradients generate a subsurface flow that projects material upward and outward from the point of impact. The material being excavated resembles an outward-slanted curtain moving away from the point of impact.

The depression that is produced has the form of an upward-facing parabolic bowl about four times as wide as it is deep. The diameter of the crater relative to that of the meteorite depends on several factors, but it is thought for most craters to be about 10 to 1. Excavated material surrounds the crater, causing its rim to be elevated above the surrounding terrain. The height of the rim accounts for about 5 percent of the total crater depth. The excavated material outside the crater is called the ejecta blanket. The elevation of the ejecta blanket is highest at the rim and falls off rapidly with distance.

When the crater is relatively small, its formation ends when excavation stops. The resulting landform is called a simple crater. The smallest craters require no more than a few seconds to form completely, whereas craters that are tens of kilometres wide probably form in a few minutes.

As meteorite craters become larger, however, the formation process does not cease with excavation. For such craters the parabolic hole is apparently too large to support itself, and it collapses in a process that generates a variety of features. This collapse process is called the modification stage, and the final depression is known as a complex crater. The modification stage of complex crater formation is poorly understood because the process is mostly beyond current technological capability to model or simulate and because explosion craters on Earth are too small to produce true complex crater landforms. Although conceptually the modification stage is considered to occur after excavation, it may be that collapse begins before excavation is complete. The current state of knowledge of complex crater formation relies primarily on inferences drawn from field observations of Earth’s impact structures and spacecraft imagery of impact craters on other solid bodies in the solar system.

Features associated with complex craters are generally attributed to material moving back toward the point of impact. Smaller complex craters have a flat floor caused by a rebound of material below the crater after excavation. This same rebound causes large complex craters to have a central peak; even-larger craters have a raised circular ring within the crater. Analogues to the central peak and ring are the back splash and outward ripple that are seen briefly when a pebble is dropped into water. Also associated with the modification stage is downward faulting, which forms terraces of large blocks of material along the inner rim of the initial cavity. In the case of very large craters, discrete, inward-facing, widely spaced faults called megaterraces form well outside the initial excavation cavity. Craters with megaterraces are called impact basins.

Variations In Craters Across The Solar System

Although impact craters on all the solid bodies of the solar system are grossly similar, their appearances from body to body can vary dramatically. The most-notable differences are a result of variations among the bodies in surface gravity and crustal properties. A higher surface gravitational acceleration creates a greater pressure difference between the floor of the crater and the surface surrounding the crater. That pressure difference is thought to play a large role in driving the collapse process that forms complex craters, the effect being that the smallest complex craters seen on higher-gravity bodies are smaller than those on lower-gravity bodies. For example, the diameters of the smallest craters with central peaks on the Moon, Mercury, and Venus decrease in inverse proportion to the bodies’ surface gravities; Mercury’s surface gravity is more than twice that of the Moon, whereas Venus’s gravity is more than five times that of the Moon.

The inherent strength of the impacted surface has an effect similar to that of surface gravity in that it is easier for craters to collapse on bodies with weaker near-surface materials. For example, the presence of water in the near-surface materials of Mars, a condition thought to be likely, would help explain why the smallest complex craters there are smaller than on Mercury, which has a similar surface gravity. Layering in a body’s near-surface material in which weak material overlies stronger strata is thought to modify the excavation process and contribute to the presence of craters with flat floors that contain a central pit. Such craters are particularly prominent on Ganymede, the largest moon of Jupiter.

Observations of the solid planets show clearly that the presence of an atmosphere changes the appearance of impact craters, but details of how the cratering process is altered are poorly understood. Comparison of craters on planets with and without an atmosphere shows no obvious evidence that an atmosphere does more than minimally affect the excavation of the cavity and any subsequent collapse. It does show, however, that an atmosphere strongly affects emplacement of the ejecta blanket. On an airless body the particles of excavated material follow ballistic trajectories.

In the presence of an atmosphere most of this material mixes with the atmosphere and creates a surface-hugging fluid flow away from the crater that is analogous to volcanic pyroclastic flow on Earth. On an airless body an ejecta blanket shows a steady decrease in thickness away from the crater, but on a planet with an atmosphere the fluid flow of excavated material lays down a blanket that is relatively constant in thickness away from the crater and that ends abruptly at the outer edge of the flow. The well-preserved ejecta blankets around Venusian craters show this flow emplacement, and field observations of Earth’s impact structures indicate that much of their ejecta were emplaced as flows. On Mars most of the ejecta blankets also appear to have been emplaced as flows, but many of these are probably mudflows caused by abundant water near the Martian surface.

Meteorite Craters As Measures Of Geologic Activity

A common misconception is that Earth has very few impact craters on its surface because its atmosphere is an effective shield against meteoroids. Earth’s atmosphere certainly slows and prevents typical asteroidal fragments up to a few tens of metres across from reaching the surface and forming a true hypervelocity impact crater, but kilometre-scale objects of the kind that created the smallest telescopically visible craters on the Moon are not significantly slowed by Earth’s atmosphere (see meteor and meteoroid: Meteorites—meteoroids that survive atmospheric entry). The Moon and Earth certainly experienced similar numbers of these larger impact events, but on Earth subsequent geologic processes (e.g., volcanism and plate tectonic processes) completely eliminated or severely degraded the craters. The dominant role of erosion as a geologic process that destroys craters is unique to Earth among the solid bodies that have been well-studied. Erosional processes may be important in eliminating craters on Titan, Saturn’s largest moon, if methane proves to play the role there that water does in Earth’s hydrologic cycle. Elsewhere only volcanic and tectonic processes are capable of eliminating large meteorite craters.

An absence or sparseness of craters in a given region of a large solid body indicates that relatively recent geologic activity has resurfaced it or otherwise greatly altered its surface appearance. For example, on the Moon the dark mare regions are much less heavily cratered than the light highland areas because the mare were flooded by basaltic volcanic flows about one billion years after formation of the highland areas. From simple counts of the craters larger than a given size per unit area for different regions of a body, it is possible to determine relative surface ages of different regions in order to gain insight into a body’s geologic history. For the Moon absolute ages can be assigned to regions with different numbers of craters per unit area because surface samples from several regions were collected during Apollo lunar landing missions and dated in laboratories on Earth. For other large bodies, assigning absolute ages to given regions based on the number of craters is based on estimates of asteroidal and cometary impact rates, the size range of those objects, and the size of the crater that forms from a given impacting object. Very little data exist as a basis for these estimates, particularly for impact rates. Absolute ages determined for planetary surfaces other than the Moon consequently have large uncertainties relative to the age of the solar system.

MeteorCrater.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#677 2020-05-20 06:09:03

ganesh
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Registered: 2005-06-28
Posts: 30,280

Re: Miscellany

557) Melanoma

Overview

Melanoma, the most serious type of skin cancer, develops in the cells (melanocytes) that produce melanin — the pigment that gives your skin its color. Melanoma can also form in your eyes and, rarely, inside your body, such as in your nose or throat.

The exact cause of all melanomas isn't clear, but exposure to ultraviolet (UV) radiation from sunlight or tanning lamps and beds increases your risk of developing melanoma. Limiting your exposure to UV radiation can help reduce your risk of melanoma.

The risk of melanoma seems to be increasing in people under 40, especially women. Knowing the warning signs of skin cancer can help ensure that cancerous changes are detected and treated before the cancer has spread. Melanoma can be treated successfully if it is detected early.

Symptoms

Melanomas can develop anywhere on your body. They most often develop in areas that have had exposure to the sun, such as your back, legs, arms and face.
Melanomas can also occur in areas that don't receive much sun exposure, such as the soles of your feet, palms of your hands and fingernail beds. These hidden melanomas are more common in people with darker skin.
The first melanoma signs and symptoms often are:
•    A change in an existing mole
•    The development of a new pigmented or unusual-looking growth on your skin

Melanoma doesn't always begin as a mole. It can also occur on otherwise normal-appearing skin.

Normal moles

Normal moles are generally a uniform color — such as tan, brown or black — with a distinct border separating the mole from your surrounding skin. They're oval or round and usually smaller than 1/4 inch (about 6 millimeters) in diameter — the size of a pencil eraser.

Most moles begin appearing in childhood and new moles may form until about age 40. By the time they are adults, most people have between 10 and 40 moles. Moles may change in appearance over time and some may even disappear with age.

Unusual moles that may indicate melanoma

To help you identify characteristics of unusual moles that may indicate melanomas or other skin cancers, think of the letters ABCDE:
•    A is for asymmetrical shape. Look for moles with irregular shapes, such as two very different-looking halves.
•    B is for irregular border. Look for moles with irregular, notched or scalloped borders — characteristics of melanomas.
•    C is for changes in color. Look for growths that have many colors or an uneven distribution of color.
•    D is for diameter. Look for new growth in a mole larger than 1/4 inch (about 6 millimeters).
•    E is for evolving. Look for changes over time, such as a mole that grows in size or that changes color or shape. Moles may also evolve to develop new signs and symptoms, such as new itchiness or bleeding.

Cancerous (malignant) moles vary greatly in appearance. Some may show all of the changes listed above, while others may have only one or two unusual characteristics.
Hidden melanomas

Melanomas can also develop in areas of your body that have little or no exposure to the sun, such as the spaces between your toes and on your palms, soles, scalp or genitals. These are sometimes referred to as hidden melanomas because they occur in places most people wouldn't think to check. When melanoma occurs in people with darker skin, it's more likely to occur in a hidden area.

Hidden melanomas include:
•    Melanoma under a nail. Acral-lentiginous melanoma is a rare form of melanoma that can occur under a fingernail or toenail. It can also be found on the palms of the hands or the soles of the feet. It's more common in people of Asian descent, black people and in others with dark skin pigment.
•    Melanoma in the mouth, digestive tract, urinary tract or math. Mucosal melanoma develops in the mucous membrane that lines the nose, mouth, esophagus, math, urinary tract and math. Mucosal melanomas are especially difficult to detect because they can easily be mistaken for other far more common conditions.
•    Melanoma in the eye. Eye melanoma, also called ocular melanoma, most often occurs in the uvea — the layer beneath the white of the eye (sclera). An eye melanoma may cause vision changes and may be diagnosed during an eye exam.

When to see a doctor

Make an appointment with your doctor if you notice any skin changes that seem unusual.

Causes

Melanoma occurs when something goes wrong in the melanin-producing cells (melanocytes) that give color to your skin.

Normally, skin cells develop in a controlled and orderly way — healthy new cells push older cells toward your skin's surface, where they die and eventually fall off. But when some cells develop DNA damage, new cells may begin to grow out of control and can eventually form a mass of cancerous cells.

Just what damages DNA in skin cells and how this leads to melanoma isn't clear. It's likely that a combination of factors, including environmental and genetic factors, causes melanoma. Still, doctors believe exposure to ultraviolet (UV) radiation from the sun and from tanning lamps and beds is the leading cause of melanoma.
UV light doesn't cause all melanomas, especially those that occur in places on your body that don't receive exposure to sunlight. This indicates that other factors may contribute to your risk of melanoma.

Risk factors

Factors that may increase your risk of melanoma include:
•    Fair skin. Having less pigment (melanin) in your skin means you have less protection from damaging UV radiation. If you have blond or red hair, light-colored eyes, and freckle or sunburn easily, you're more likely to develop melanoma than is someone with a darker complexion. But melanoma can develop in people with darker complexions, including Hispanic people and black people.
•    A history of sunburn. One or more severe, blistering sunburns can increase your risk of melanoma.
•    Excessive ultraviolet (UV) light exposure. Exposure to UV radiation, which comes from the sun and from tanning lights and beds, can increase the risk of skin cancer, including melanoma.
•    Living closer to the equator or at a higher elevation. People living closer to the earth's equator, where the sun's rays are more direct, experience higher amounts of UV radiation than do those living farther north or south. In addition, if you live at a high elevation, you're exposed to more UV radiation.
•    Having many moles or unusual moles. Having more than 50 ordinary moles on your body indicates an increased risk of melanoma. Also, having an unusual type of mole increases the risk of melanoma. Known medically as dysplastic nevi, these tend to be larger than normal moles and have irregular borders and a mixture of colors.
•    A family history of melanoma. If a close relative — such as a parent, child or sibling — has had melanoma, you have a greater chance of developing a melanoma, too.
•    Weakened immune system. People with weakened immune systems have an increased risk of melanoma and other skin cancers. Your immune system may be impaired if you take medicine to suppress the immune system, such as after an organ transplant, or if you have a disease that impairs the immune system, such as AIDS.

Prevention

You can reduce your risk of melanoma and other types of skin cancer if you:

•    Avoid the sun during the middle of the day. For many people in North America, the sun's rays are strongest between about 10 a.m. and 4 p.m. Schedule outdoor activities for other times of the day, even in winter or when the sky is cloudy.

You absorb UV radiation year-round, and clouds offer little protection from damaging rays. Avoiding the sun at its strongest helps you avoid the sunburns and suntans that cause skin damage and increase your risk of developing skin cancer. Sun exposure accumulated over time also may cause skin cancer.
•    Wear sunscreen year-round. Use a broad-spectrum sunscreen with an SPF of at least 30, even on cloudy days. Apply sunscreen generously, and reapply every two hours — or more often if you're swimming or perspiring.
•    Wear protective clothing. Cover your skin with dark, tightly woven clothing that covers your arms and legs, and a broad-brimmed hat, which provides more protection than does a baseball cap or visor.

Some companies also sell protective clothing. A dermatologist can recommend an appropriate brand. Don't forget sunglasses. Look for those that block both types of UV radiation — UVA and UVB rays.
•    Avoid tanning lamps and beds. Tanning lamps and beds emit UV rays and can increase your risk of skin cancer.
•    Become familiar with your skin so that you'll notice changes. Examine your skin often for new skin growths or changes in existing moles, freckles, bumps and birthmarks. With the help of mirrors, check your face, neck, ears and scalp.
Examine your chest and trunk and the tops and undersides of your arms and hands. Examine both the front and back of your legs and your feet, including the soles and the spaces between your toes.

melanoma-1016082.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#678 2020-05-21 14:26:59

ganesh
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Registered: 2005-06-28
Posts: 30,280

Re: Miscellany

558) Kingfisher

Kingfisher, any of about 90 species of birds in three families (Alcedinidae, Halcyonidae, and Cerylidae), noted for their spectacular dives into water. They are worldwide in distribution but are chiefly tropical. Kingfishers, ranging in length from 10 to 42 cm (4 to 16.5 inches), have a large head, a long and massive bill, and a compact body. Their feet are small, and, with a few exceptions, the tail is short or medium-length. Most species have vivid plumage in bold patterns, and many are crested.

These vocal, colourful birds are renowned for their dramatic hunting techniques. Typically, the bird sits still, watching for movement from a favourite perch. Having sighted its quarry, it plunges into the water and catches the fish usually no deeper than 25 cm (10 inches) below the surface in its dagger-shaped bill. With a swift downstroke of the wings, it bobs to the surface. It then takes the prey back to the perch and stuns the fish by beating it against the perch before swallowing it. Many species also eat crustaceans, amphibians, and reptiles.

The typical kingfishers (subfamily Alcedininae) are river dwellers, like the belted kingfisher (Megaceryle alcyon), the only widespread North American species. This handsome crested bird flies off over the water when disturbed, uttering a loud rattling call. It is about 30 cm (12 inches) long and is bluish gray above and across the breast and white below. Only the females sport the brownish red band or “belt” across the lower breast. The male in its courtship ritual offers fish to the female as she perches. After copulation the pair circle high overhead and chase each other while crying shrilly.

Stretching 43 cm (17 inches) long and weighing 465 grams (16 ounces), the largest of all kingfishers is the kookaburra, known throughout Australia for its laughing call. The kookaburra’s white head has a brown eye stripe, the back and wings are dark brown, and the underparts are white. Often found in urban and suburban areas, it can become quite tame and may be fed by hand. A member of the subfamily Daceloninae, the forest kingfishers, it captures insects, snails, frogs, reptiles, and small birds on the ground. It lives in family groups that roost together at night.

The IUCN Red List of Threatened Species classifies most kingfishers as species of least concern. Many species, such as the common kingfisher (Alcedo atthis), have large populations and vast geographic ranges. However, ecologists have observed that the populations of some species endemic to specialized habitats in Southeast Asia and the islands of the tropical Pacific Ocean are in decline. Aggressive logging activities resulting in the deforestation of large areas of Indonesia, Malaysia, and the Philippines have been associated with dramatic population decreases in several species, including the blue-banded kingfisher (A. euryzona), the Sulawesi kingfisher (Ceyx fallax), the brown-winged kingfisher (Pelargopsis amauropterus), and some of the paradise kingfishers (Tanysiptera) of New Guinea.

The Marquesan kingfisher (Todiramphus godeffroyi), one of the most endangered kingfishers, faces a different suite of threats. Once found on a handful of islands in the Marquesas chain, the species is now limited to only one, Tahuata. The bird’s decline has been attributed to habitat degradation caused by feral livestock coupled with predation by introduced species such as the great horned owl (Bubo virginianus), common mynah (Acridotheres tristis), and house rat (Rattus rattus).

common-kingfisher-alcedo-atthis-perched-branch-bird-animals_132416-596.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#679 2020-05-23 00:03:21

ganesh
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Registered: 2005-06-28
Posts: 30,280

Re: Miscellany

559) Sparrow

Sparrow, any of a number of small, chiefly seed-eating birds having conical bills. The name sparrow is most firmly attached to birds of the Old World family Passeridae (order Passeriformes), particularly to the house sparrow (Passer domesticus) that is so common in temperate North America and Europe, but also to many New World members of the Emberizidae.

Most members of the New World family Emberizidae are called sparrows. Examples breeding in North America are the chipping sparrow (Spizella passerina) and the tree sparrow (S. arborea), trim-looking little birds with reddish-brown caps; the savanna sparrow (Passerculus sandwichensis) and the vesper sparrow (Pooecetes gramineus), finely streaked birds of grassy fields; the song sparrow (Melospiza melodia) and the fox sparrow (Passerella iliaca), heavily streaked skulkers in woodlands; and the white-crowned sparrow (Zonotrichia leucophrys) and the white-throated sparrow (Z. albicollis), larger species with black-and-white crown stripes. The rufous-collared sparrow (Z. capensis) has an exceptionally wide breeding distribution: from Mexico and Caribbean islands to Tierra del Fuego. A great many emberizid sparrows are native to Central and South America.

swsp-8.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#680 2020-05-25 00:08:51

ganesh
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Registered: 2005-06-28
Posts: 30,280

Re: Miscellany

560) Heron

Heron, any of about 60 species of long-legged wading birds, classified in the family Ardeidae (order Ciconiiformes) and generally including several species usually called egrets. The Ardeidae also include the bitterns (subfamily Botaurinae). Herons are widely distributed over the world but are most common in the tropics. They usually feed while wading quietly in the shallow waters of pools, marshes, and swamps, catching frogs, fishes, and other aquatic animals. They nest in rough platforms of sticks constructed in bushes or trees near water; the nests usually are grouped in colonies called heronries.

Herons commonly stand with the neck bent in an S shape. They fly with the legs trailing loosely and the head held back against the body, instead of stretching the neck out in front as most birds do. They have broad wings, long straight sharp-pointed bills, and powder downs; the latter are areas of feathers that continually disintegrate to a fine powder which is used for preening (absorbing and removing fish oil, scum, and slime from the plumage).

Herons are subdivided into typical herons, night herons, and tiger herons. Typical herons feed during the day. In breeding season some develop showy plumes on the back and participate in elaborate mutual-courtship posturing. Best known of the typical herons are the very large, long-legged and long-necked, plain-hued, crested members of the genus Ardea—especially the 130-cm (50-inch) great blue heron (A. herodias) of North America, with a wingspan of 1.8 metres (6 feet) or more, and the similar but slightly smaller gray, or common, heron (A. cinerea), widespread in the Old World. Largest of all is the goliath heron (A. goliath) of Africa, a 150-cm (59-inch) bird with a reddish head and neck. The purple heron (A. purpurea) is a darker and smaller Old World form.

The typical herons also include the black heron, Hydranassa (or Melanophoyx) ardesiaca, of Africa, and several species of the genus Egretta (egrets), such as the tricoloured heron (E. tricolor), of the southeastern United States and Central and South America, and the little blue heron (E. caerulea). The green heron (Butorides virescens), a small green and brown bird widespread in North America, is notable for its habit of dropping bait on the surface of the water in order to attract fish.

Night herons have thicker bills and shorter legs and are more active in the twilight hours and at night. The black-crowned night heron (Nycticorax nycticorax) ranges over the Americas, Europe, Africa, and Asia; the Nankeen night heron (N. caledonicus) in Australia, New Caledonia, and the Philippines; and the yellow-crowned night heron (Nyctanassa violacea) from the eastern and central United States to southern Brazil. Another night heron is the boat-billed heron, or boatbill (Cochlearius cochlearius), of Central and South America, placed by some authorities in its own family (Cochleariidae).

The most primitive herons are the six species of tiger herons (formerly called tiger bitterns), shy, solitary birds with cryptic, often barred, plumage. The lined, or banded, tiger heron (Tigrisoma lineatum), 75 cm (30 inches) long, of central and northern South America, is a well-known example. Another is the Mexican, or bare-throated, tiger heron (T. mexicanum) of Mexico and Central America.

heronblack-crowned_night.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#681 2020-05-27 00:05:12

ganesh
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Posts: 30,280

Re: Miscellany

561) Cactus

Cactus, (family Cactaceae), plural cacti or cactuses, flowering plant family (order Caryophyllales with more than 2,000 species and about 175 genera. Cacti are native through most of the length of North and South America, from British Columbia and Alberta southward; the southernmost limit of their range extends far into Chile and Argentina. Mexico has the greatest number and variety of species. The only cacti possibly native to the Old World are members of the genus Rhipsalis, occurring in East Africa, Madagascar, and Sri Lanka. Although a few cactus species inhabit tropical or subtropical areas, most live in and are well adapted to dry regions.

Physical Characteristics

Cacti are succulent perennial plants. Cacti generally have thick herbaceous or woody chlorophyll-containing stems. Cacti can be distinguished from other succulent plants by the presence of areoles, small cushionlike structures with trichomes (plant hairs) and, in almost all species, spines or barbed bristles (glochids). Areoles are modified branches, from which flowers, more branches, and leaves (when present) may grow.

In most species, leaves are absent, greatly reduced, or modified as spines, minimizing the amount of surface area from which water can be lost, and the stem has taken over the photosynthetic functions of the plant. Only the tropical genera Pereskia and Pereskopsis, both vines, have conventional-looking functional leaves, while the leaves of the Andean Maihuenia are rounded, not flattened. The root systems are generally thin, fibrous, and shallow, ranging widely to absorb superficial moisture.

Cacti vary greatly in size and general appearance, from buttonlike peyote (Lophophora) and low clumps of prickly pear (Opuntia) and hedgehog cactus (Echinocereus) to the upright columns of barrel cacti (Ferocactus and Echinocactus) and the imposing saguaro (Carnegiea gigantea). Most cacti grow in the ground, but several tropical species—including leaf cactus (Epiphyllum), Rhipsalis, and Schlumbergera—are epiphytes, growing on other plants; others live on hard substrates such as rocks, while yet others climb far up trees. Epiphytic species tend to have thin, almost leaflike flattened stems. The appearance of the plant varies also according to whether the stem surface is smooth or ornamented with protruding tubercles, ridges, or grooves.

The primary method of reproduction is by seeds. Flowers, often large and colourful, are usually solitary. All genera have a floral tube, often with many petal-like structures, and other less colourful and almost leaflike structures; the tube grows above a one-chambered ovary. A style topped by many pollen-receptive stigmas also arises from the top of the ovary. The fruit is usually a berry and contains many seeds. Soon after pollination, which may be effected by wind, birds, insects, or bats, the entire floral tube detaches from the top of the ovary to leave a prominent scar.

Several cacti develop plantlets at ground level that, as offsets, reproduce the species vegetatively. Many others can reproduce by fragmentation, whereby segments broken from the main plant will readily root to form clonal individuals. Tissues of cacti are broadly compatible so that terminal portions of one species may be grafted on top of another.

The internal structure of cacti stems conforms to the pattern of broad-leaved angiosperms; a cambium layer of dividing cells, located between the woody inner tissues and those near the outside of the stem, is present. The bulk of the stem, however, consists of thin-walled storage cells that contain mucilaginous substances that prevent the loss of moisture. The stem of cacti is the main food-manufacturing and food-storage organ.

Uses

Cacti are widely cultivated as ornamentals. In addition, various species, notably prickly pears and chollas (Opuntia and Cylindopuntia, respectively), are cultivated as food. In South America, species of Opuntia, Cereus, and others are used as living fences, and wood from columnar cacti is used as fuel in some desert regions. In times of drought, the spines are removed from cacti such as mandacaru (Cereus jamacaru) to use as fodder for livestock. Peyote, from Lophophora williamsii, has been used ceremonially since pre-Columbian times for its hallucinogenic properties, and many cactus species are of local importance in traditional medicine.

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#682 2020-05-28 00:05:20

ganesh
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Registered: 2005-06-28
Posts: 30,280

Re: Miscellany

562) Pituitary gland

Pituitary gland, also called hypophysis, ductless gland of the endocrine system that secretes hormones directly into the bloodstream. The term hypophysis (from the Greek for “lying under”)—another name for the pituitary—refers to the gland’s position on the underside of the brain. The pituitary gland is called the “master gland” because its hormones regulate other important endocrine glands—including the adrenal, thyroid, and reproductive glands — and in some cases have direct regulatory effects in major tissues, such as those of the musculoskeletal system.

Anatomy Of The Pituitary Gland

The pituitary gland lies at the middle of the base of the skull and is housed within a bony structure called the sella turcica, which is behind the nose and immediately beneath the hypothalamus. The pituitary gland is attached to the hypothalamus by a stalk composed of neuronal axons and the so-called hypophyseal-portal veins. Its weight in normal adult humans ranges from about 500 to 900 mg (0.02 to 0.03 ounce).

In most species the pituitary gland is divided into three lobes: the anterior lobe, the intermediate lobe, and the posterior lobe (also called the neurohypophysis or pars nervosa). In humans the intermediate lobe does not exist as a distinct anatomic structure but rather remains only as cells dispersed within the anterior lobe. Nonetheless, the anterior and posterior lobes of the pituitary are functionally, anatomically, and embryologically distinct. Whereas the anterior pituitary contains abundant hormone-secreting epithelial cells, the posterior pituitary is composed largely of unmyelinated (lacking a sheath of fatty insulation) secretory neurons.

The Anterior Pituitary

The cells of the anterior pituitary are embryologically derived from an outpouching of the roof of the pharynx, known as Rathke’s pouch. Although the cells appear to be relatively homogeneous under a light microscope, there are in fact at least five different types of cells, each of which secretes a different hormone or hormones. The thyrotrophs synthesize and secrete thyrotropin (thyroid-stimulating hormone; TSH); the gonadotrophs, both luteinizing hormone (LH) and follicle-stimulating hormone (FSH); the corticotrophs, adrenocorticotropic hormone (ACTH; corticotropin); the somatotrophs, growth hormone (GH; somatotropin); and the lactotrophs, prolactin.

Somatotrophs are plentiful in the anterior pituitary gland, constituting about 40 percent of the tissue. They are located predominantly in the anterior and the lateral regions of the gland and secrete between one and two milligrams of GH each day.

Structure and function of anterior pituitary hormones

The hormones of the anterior pituitary are proteins that consist of one or two long polypeptide chains. TSH, LH, and FSH are called glycoproteins because they contain complex carbohydrates known as glycosides. Each of those hormones is composed of two glycopeptide chains, one of which, the alpha chain, is identical in all three hormones. The other chain, the beta chain, differs in structure for each hormone, thereby explaining the different actions of TSH, LH, and FSH. As is the case for all protein hormones, the hormones of the anterior pituitary are synthesized in the cytoplasm of the cells as large inactive molecules called prohormones. Those prohormones are stored in granules, within which they are cleaved into active hormones and are secreted into the circulation.

Each pituitary hormone plays a vital role in endocrine function. Thyrotropin stimulates the production of thyroid hormone. ACTH stimulates the production of cortisol and androgenic hormones by the adrenal cortex. FSH stimulates the production of estrogens and the growth of egg cells (oocytes) in the women and male gamete cells in men. LH stimulates the production of estrogens and progesterone by the ovaries in women and the production of testosterone by the testes in men. GH stimulates linear growth in children and helps to maintain bone and other tissues in adults. Prolactin stimulates milk production.

Regulation of anterior pituitary hormones

The production and secretion of each of the major anterior pituitary hormones are regulated by peptides that are released from the median eminence neurons of the hypothalamus into the hypophyseal-portal veins, which traverse a short distance to the pituitary microvasculature. Among those peptides are thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone, gonadotropin-releasing hormone, and growth-hormone-releasing hormone. The hypothalamus also produces dopamine and somatostatin, which are potent inhibitors of prolactin and GH, respectively.

Feedback loops involving the pituitary hormones and their target glands play an important role in pituitary-hormone signaling. TRH secretion, for example, is inhibited by thyroid hormone, which also inhibits the effect of TRH on thyrotrophs. Such negative feedback loops help to maintain a stable balance between the secretion of pituitary hormones and the secretion of hormones produced by pituitary target glands. Physiological perturbations, such as the effects of stress on the pituitary-adrenal axis and neuroendocrine rhythms, can override that balance.

Posterior Pituitary Hormones

The posterior lobe of the pituitary gland consists largely of extensions of processes (axons) from two pairs of large clusters of nerve cell bodies (nuclei) in the hypothalamus. One of those nuclei, known as the supraoptic nuclei, lies immediately above the optic tract, while the other nuclei, known as the paraventricular nuclei, lies on each side of the third ventricle of the brain. Those nuclei, the axons of the cell bodies of nerves that form the nuclei, and the nerve endings in the posterior pituitary gland form the neurohypophyseal system. There are neural connections that run from those nuclei to other regions of the brain, including to regions that sense osmolality (solute concentrations) and regulate thirst.

The major neurohypophyseal hormones are vasopressin (antidiuretic hormone) and oxytocin, which are synthesized and incorporated into neurosecretory granules in the cell bodies of the nuclei. Those hormones are synthesized as part of a precursor protein that includes one of the hormones and a protein called neurophysin. After synthesis and incorporation into neurosecretory granules, the precursor protein is cleaved, forming separate hormone and neurophysin molecules, which remain loosely attached to one another. Those granules are carried through the axons and are stored in the posterior lobe of the pituitary gland. Upon stimulation of the nerve cells by internal or external events (e.g., milk suckling in the case of oxytocin-secreting neurons), the granules fuse with the cell wall of the nerve endings, the hormone and neurophysin dissociate from one another, and both the hormone and the neurophysin are released into the bloodstream. The hormones are released into the circulation in response to nerve signals that originate in the hypothalamus and are transmitted to the posterior pituitary lobe.

Oxytocin stimulates contraction of the uterus, an important aspect of labour and parturition and of milk ejection during breast-feeding. Vasopressin regulates blood pressure and increases reabsorption of water from the kidneys, thus conserving body water and defending against dehydration. Vasopressin secretion is stimulated by increased serum osmolality, which is an indication of dehydration.

Diseases Of The Anterior And Posterior Pituitary

Decreased secretion of anterior and posterior pituitary hormones is known as panhypopituitarism, a serious and sometimes fatal disorder. The term panhypopituitarism is also commonly used when only anterior pituitary hormones are deficient. Patients with panhypopituitarism usually have features of adrenal insufficiency, hypothyroidism, and gonadal failure, along with poor responses to stress. Pituitary vascular insufficiency, autoimmunity, infections, and neoplasms can cause panhypopituitarism. If central diabetes insipidus is present, the lesion generally involves the posterior as well as the anterior pituitary. Isolated deficiencies of one or two pituitary hormones also may occur, often on a heritable basis. Those conditions are rare. Some patients may present with infertility due to LH and FSH deficiency. Proportionate congenital growth failure due to GH deficiency is a predominant type of isolated deficiency.

Tumours that secrete individual anterior pituitary hormones are recognized. Acromegaly due to GH-secreting tumours and Cushing syndrome due to ACTH-producing tumours are among the most-common disorders produced by functional pituitary tumours, though even those conditions are rare. Autonomous hypersecretion of prolactin is a common feature of pituitary tumours, since such growths tend to interfere (via tissue compression) with prolactin-suppressing signals from the hypothalamus. Excess prolactin typically is associated with varying degrees of gonadal failure and in some cases with spontaneous breast-milk secretion (galactorrhea) in men and women. Posterior pituitary tumours that secrete excess vasopressin or oxytocin do not occur; however, functional states of excess vasopressin (inappropriate vasopressin secretion) and transient vasopressin deficiency have been described.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#683 Yesterday 00:03:58

ganesh
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Posts: 30,280

Re: Miscellany

563) Burn

Burn, damage caused to the body by contact with flames, hot substances, certain chemicals, radiation (sunlight, X rays, or ionizing radiation from radioactive materials), or electricity. The chief effects of contact with flame, hot water, steam, caustic chemicals, or electricity are apparent promptly. There is a delay of several hours before the full effects of sun or ultraviolet burns are apparent and a delay of 10 to 30 days before the full effects of ionizing radiation burns are apparent.

The severity of a burn depends largely on the depth of tissue destruction and the amount of body surface affected. Other factors—including the patient’s age and prior state of health, the location of the burn wound, and the seriousness of any associated injuries—can also influence recovery from a burn.

For an appreciation of how depth and size of a burn affect the severity of the injury, some understanding of the anatomy and physiology of the skin is necessary. Human skin is composed of two layers: an upper layer called the epidermis, and a lower layer known as the dermis (or corium). The largest of the body’s organs, skin performs a number of vital functions. Its foremost job is to separate the external environment from the body’s interior. The epidermis, the outer surface of which consists of dead, cornified cells, prevents infectious microorganisms and other harmful environmental agents from gaining entrance to the body. The dermis, by contrast, is made up of fibrous connective tissues that prevent the evaporation of body fluids. Embedded within the dermis and opening to the skin surface are the sweat glands. These secrete perspiration, the evaporation of which helps regulate body temperature. Perspiration also contains small amounts of sodium chloride, cholesterol, aluminum, and urea; it thus plays a role in regulating the composition of body fluids. The dermis also contains all of the skin’s blood vessels and nerves, including sensory nerve endings that respond to touch, pressure, heat, cold, and pain. The skin therefore also serves as a sense organ that enables a person to adjust to changing environmental conditions. One final function of the skin is the synthesis of vitamin D, a compound essential to growth and maintenance, particularly of bone. Vitamin D is formed by the action of sunlight on certain cholesterol compounds in the dermis. Destruction of the skin by deep or extensive burns can disrupt all of these functions, subjecting the victim to serious complications.

Physicians have traditionally categorized burns as first-, second-, or third-degree injuries, according to the depth of skin damage. In a first-degree burn, only the epidermis is affected. These injuries are characterized by redness and pain; there are no blisters, and edema (swelling due to the accumulation of fluids) in the wounded tissue is minimal. A classic example of a first-degree burn is moderate sunburn.

The damage in a second-degree burn extends through the entire epidermis and part of the dermis. These injuries are characterized by redness and blisters. The deeper the burn the more prevalent the blisters, which increase in size during the hours immediately following the injury. Like first-degree burns, second-degree injuries may be extremely painful. The development of complications and the course of healing in a second-degree burn depend on the extent of damage to the dermis. Unless they become infected, most superficial second-degree burns heal without complications and with little scarring in 10 to 14 days.

Third degree, or full-thickness, burns destroy the entire thickness of the skin. The surface of the wound is leathery and may be brown, tan, black, white, or red. There is no pain, because the pain receptors have been obliterated along with the rest of the dermis. Blood vessels, sweat glands, sebaceous glands, and hair follicles are all destroyed in skin that suffers a full-thickness burn. Fluid losses and metabolic disturbances associated with these injuries are grave.

Occasionally burns deeper than a full thickness of the skin are incurred, as when part of the body is entrapped in a flame and not immediately extricated. Electrical burns are usually deep burns. These deep burns frequently go into the subcutaneous tissue and, at times, beyond and into the muscle, fascia, and bone. Such burns are of the fourth degree, also called black (because of the typical colour of the burn), or char, burns. Fourth-degree burns are of grave prognosis, particularly if they involve more than a small portion of the body. In these deep burns toxic materials may be released into the bloodstream. If the char burn involves only a small part of the body, it should be excised down to healthy tissue. If an extremity is involved, amputation may be necessary.

Surgeons measure the area of a burn as a percentage of the body’s total skin area. The skin area on each arm is roughly 9 percent of the body total, as is the skin covering the head and neck. The percentage on each leg is 18, and the percentage on the trunk is 18 on the front and 18 on the back. The percentage of damaged skin affects the chances of survival. Most people can survive a second-degree burn affecting 70 percent of their body area, but few can survive a third-degree burn affecting 50 percent. If the area is down to 20 percent, most people can be saved, though elderly people and infants may fail to survive a 15 percent skin loss.

Severe burns cause immediate nervous shock. The victim grows pale and is confused, anxious, and frightened by the pain and may faint. Much more dangerous is the secondary shock that comes a few hours later. Its chief features are a dramatic fall in blood pressure that leads to pallor, cold extremities, and eventual collapse. This secondary shock is precipitated by loss of fluid from the circulation, not just the fluid lost in the destroyed tissue but fluid that leaks from the damaged area that has lost its protective covering of skin.

Burns kill not just by damaging tissue but by allowing this leakage of fluid and salts. If more than a fifth of the blood volume is lost to the circulation, insufficient blood returns to the heart for it to maintain blood pressure. And the loss of salts, particularly sodium and potassium salts, not only disturbs their balance in the body but changes the osmotic balance of the blood and body fluids. The significance of these physiological changes was understood in 1905, but not until the 1930s were doctors able to correct them with transfusions of blood or plasma.

The treatment of a burn is, of course, dependent upon the severity of the injury. In general, first-degree burns can be adequately treated with proper first-aid measures. Second-degree burns that cover more than 15 percent of an adult’s body or 10 percent of a child’s, or that affect the face, hands, or feet, should receive prompt medical attention, as should all third-degree burns, regardless of size.

First Aid.

Following a first-degree or a small second-degree burn, the best first aid is to quickly immerse the wound under cool tap water. This action will stop the burning process and dissipate the heat energy from the wound. The wound should then be cleansed with mild soap and water and gently blotted dry. After cleansing, the burn can be left exposed, provided it is small and will be frequently washed. If the wound is larger, a dry, bulky, sterile dressing can be placed over it to minimize pain and exposure to the environment. Home remedies, such as butter or petroleum jelly, should not be applied to the wound, as these trap heat within the injury and can cause further damage. The application of antiseptics and other irritating substances should also be avoided; a good rule of thumb is to refrain from applying any substance that one would be afraid to put into one’s eye.

Third-degree burns are true medical emergencies, and the victim should receive professional medical attention as quickly as possible. These wounds should not be immersed, as cool water can intensify the circulatory shock that accompanies third-degree burns. The injuries can be covered with bulky, sterile dressings or with freshly laundered bed linens. Clothing stuck to the wound should not be removed, nor should any ointments, salves, sprays, etc. be applied. Burned feet and legs should be elevated, and burned hands should be raised above the level of the heart. The victim’s breathing must be closely watched; artificial respiration should be given if breathing stops.

Outpatient Treatment.

The majority of burn victims that are brought to hospital emergency rooms are released for outpatient burn care. As in first-aid treatment, small wounds can be left open if frequently washed; larger wounds are covered with a dry, bulky dressing. The pain involved in removing the dressing can be reduced by soaking it with tepid water prior to removal or by using a nonadhering dressing such as gauze impregnated with a bland emulsion.

Hospital Treatment.

All patients with severe burns should be hospitalized. The first priority in treating the burn victim is to ensure that the airway (breathing passages) remains open. Associated smoke inhalation injury is very common, particularly if the patient has been burned in a closed space, such as a room or building. Even patients burned in an open area may sustain smoke inhalation. Risk for smoke inhalation is greatest in victims who have injuries to the upper torso or burns of the face and in victims who cough up carbonaceous material or soot. If inhalation injury seems likely, an anesthesiologist or surgeon passes a tube through the patient’s nose or mouth into the trachea. This endotracheal tube allows the administration of high concentrations of oxygen and the use of a mechanical ventilator.

The next priority is to treat the associated burn shock. This requires the placement of intravenous lines through which resuscitating fluid can be administered; special lines are also placed into the circulation to monitor the resuscitation. A catheter is passed into the bladder to monitor urine output, another index of fluid resuscitation. Most burn centres treat the burn victim during the first 24 hours with intravenous administrations of a balanced salt solution (Ringer’s lactate); this solution replaces the fluids lost into the burn wound and from the burn wound into the environment. The administration of blood is not usually necessary, because in most burns blood loss is minimal, and less than 10 percent of the blood suffers hemolysis (i.e., the destruction of red blood cells). This hemolysis of blood, however, can cause serious secondary injuries, particularly to the kidneys; if severe enough, it may even cause the kidneys to fail. This danger can be minimized by rapidly establishing fluid resuscitation and by stimulating urine output with diuretics such as mannitol. A careful medical history is taken, and tetanus toxoid is administered.

After this initial treatment of the airway and resuscitation of the burn shock, a decision must be made as to the disposition of the patient. If the patient is admitted to a burn centre, he is usually placed into a special tub, where the wound is cleansed with mild soap solutions. The wound is then dressed. Derivatives of sulfa—particularly mafenide—and other antibiotics are now used with great success in preventing the infection of burn wounds and the subsequent spread of bacteria and toxins through the bloodstream and tissues (sepsis).

Almost immediately there are other problems that the burn surgeon must address. The patient’s ongoing fluid balance must be monitored and regulated, his nutritional needs must be met, pain must be controlled, and the burn wound itself must be repaired. Pain is most problematic in patients with partial or deep second-degree burns and is aggravated by the necessity of frequent dressing changes and physical therapy. In addition, pain leads to increased catecholamine release, which aggravates the patient’s nutritional needs and energy expenditure. Burn centres have employed innovative measures to control pain, including the use of morphine intravenously, the administration of incomplete anesthetic drugs at the time of dressing changes, and even the use of general anesthesia during major debridements.

Nutrition can be a particularly vexing problem because the caloric needs are often greater than the patient can consume in a normal fashion. Thus, supplementary feedings administered intravenously or through a feeding tube placed into the stomach are commonplace in treating severe burns. One of the major advances in the treatment of the critically burned has been the use of hyperalimentation, a procedure in which total nutritional support can be provided through a catheter placed into a large central vein.

The goals in managing the burn lesion are to prevent infection, to avoid further injury to the damaged tissues, and to close the wound as soon as possible. There are three major methods of therapy for the burn wound: exposure, occlusive dressings, and primary excision.
Exposure therapy is indicated for surfaces that are easily left exposed, such as the face. The burn is initially cleansed and then allowed to dry. A second-degree burn forms a crust, which falls off after two or three weeks, revealing minimally scarred skin beneath. Full-thickness burns will not form a crust because of the overlying dead skin, or eschar. The goal of exposure therapy is to soften the eschar and remove it. Exposure allows the eschar to dry. After it dries, saline-soaked gauzes are applied to the eschar to soften it and hasten its spontaneous separation from the underlying tissues. The advantage of exposure therapy is that the patient is not immobilized in bulky dressings. It is particularly useful in burns that cover less than 20 percent of the body area. The chief disadvantage is that the protection against infection afforded by sterile dressings is absent. In addition, pain and heat loss are greater in exposed wounds. Exposure therapy is usually combined with the use of antibacterial creams.

Occlusive dressings, usually combined with topical antibacterial agents, are more commonly used in the treatment of extensive burns. The antibacterial ointment or cream may be applied to the patient or to the gauze. The use of occlusive dressings provides a sterile barrier against airborne infection; the dressings also help minimize heat loss and pain. On the other hand, the bandages must be absorptive as well as occlusive and thus are usually bulky and restrictive. Furthermore, the dressings must be changed as often as every eight hours to prevent the growth of bacteria in the warm, moist environment of the covered wound. As pointed out previously, these frequent dressing changes may increase the amount of pain and need for anesthetics.

In both of the above methods of wound treatment, the patient is usually immersed daily in a special tank, where remaining dressings and creams are washed off and loose tissue is debrided. The patient is encouraged to move about to reduce scar formation and subsequent disabling contractures (permanent contractions of scar, muscles, and tendons) over the joints.

Primary excision—that is, the surgical removal of necrotic tissues within 24 to 48 hours of the injury—is used to prepare full-thickness burns for grafting at the earliest possible time. After the dead skin has been removed, the surgeon’s primary goal is to cover the burned area as rapidly as possible with autografts—that is, grafts of the patient’s own skin harvested from uninjured areas of the body. Often, there is a discrepancy between the amount of harvestable skin and the extent of the potential recipient sites. This discrepancy can be addressed by covering the debrided or excised areas with allografts of skin obtained from cadavers, or by treating the burn with porcine xenografts (pigskin), antibiotic solutions, or special plastic dressings. These measures are only temporary, however, and skin autografting is the final method of coverage for most full-thickness injuries. Most autografts use split-thickness skin (i.e., thin slices of skin including the epidermis and part of the dermis), which the surgeon obtains from unburned areas using an instrument called a dermatome. The face, neck, and surfaces around joints receive first priority for grafting. Grafts are usually dressed and inspected frequently to be sure they are taking.

Complications.

The use of topical antibacterial agents has reduced the incidence of post-burn infection, but infection remains one of the most serious complications of burns. Burn surgeons often obtain cultures of the burn wound and of sputum and other body secretions; these are examined for signs of infection. Early detection and prompt treatment of infection with antibiotics and surgical debridement can minimize its consequences. Acute gastrointestinal ulcers are another frequent complication of burns; they appear as small, circumscribed lesions within the lining of the stomach or duodenum. These ulcers can be detected by endoscopy and are treated with antacids and drugs that reduce the amount of acid secretion.

The occurrences of post-burn seizures is a complication unique to children. These seizures may result from electrolyte imbalances, abnormally low levels of oxygen in the blood, infection, or drugs. The cause is unknown in about a third of the cases. Post-burn hypertension is also somewhat unique to children and is probably related to the release of catecholamines and other stress hormones.

A common complication of deep dermal burns and skin grafts is the formation of fibrous masses of scar tissue called hypertrophic scars and keloids. This complication is especially common in brown-skinned races. Reddened, inflamed tissue is biologically active; it has a rich vascular supply, and it rapidly forms collagen, the primary wound protein and major component of scars. Direct pressure on inflamed tissue reduces its blood supply and collagen content, thereby minimizing the formation of hypertrophic scars and keloids. Such pressure can be provided by tailored splints, sleeves, stockings, and body jackets. Skeletal traction may be necessary in special instances.

Respiratory complications rank as the major cause of death in burn patients. Potentially fatal respiratory complications include inhalation injuries, aspiration of fluids by unconscious patients, bacterial pneumonia, pulmonary edema, obstruction of pulmonary arteries, and postinjury respiratory failure. Direct-inhalation injuries, which can lead to other respiratory complications, are especially common. The three basic categories of direct-inhalation injuries are inhalation of dry heat and soot, carbon monoxide poisoning, and smoke inhalation.

Any patient likely to have suffered inhalation injuries should receive a bronchoscopic examination of the airway. This examination can reveal the degree of respiratory injury and help in planning the appropriate treatment. Constant one-on-one nursing care is often necessary to provide the required pulmonary treatment. In most instances, an endotracheal tube is passed into the lungs, and the patient is placed on a mechanical ventilator. By delivering air under constant pressure, the ventilator helps keep the lungs inflated; this aids in the control and prevention of atelectasis (collapse of the air sacs). The ventilator can also be used to reexpand collapsed lungs. In addition, the machine can deliver varying concentrations of oxygen and mists in the inspired air. Patients who have suffered smoke inhalation are given high concentrations of humidified oxygen. Those with carbon monoxide poisoning receive 100 percent oxygen until their blood level of carboxyhemoglobin falls below 20 percent.

Rehabilitation.

Physically and cosmetically debilitating scars are the most common aftereffects of extensive burns. Such scars often require additional plastic surgery—sometimes years after the initial skin grafting—to release contractures over joints and to achieve acceptable cosmetic results. Realistically, the results are almost never as good as the patient’s preinjury condition. Most burn scars are unsightly, and, though the patient may realistically hope for improvement, complete restoration is usually not possible.

Burn scars require special care. The patient should avoid exposing the scars to sunlight. Scars in areas that are frequently exposed to the sun, such as the face and hands, should be protected by an ultraviolet screening agent (a sunblock). Because full-thickness burns can destroy sweat glands, sebaceous glands, and hair follicles, it may be necessary to apply lanolin and other emollient creams and lotions to the scarred skin in order to prevent drying and cracking and to reduce itching.

Many victims of severe burns face years of often painful physical therapy as they work to regain or maintain mobility in damaged joints. The psychological adjustment to disfigurement may be traumatic, and many patients require extended counseling to come to grips with their altered appearance and physical disabilities. Yet, with the help of understanding family, friends, and professionals, even severely injured burn victims can make successful adjustments and lead productive lives.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#684 Today 00:06:33

ganesh
Administrator
Registered: 2005-06-28
Posts: 30,280

Re: Miscellany

564) Catalyst

Catalyst, in chemistry, any substance that increases the rate of a reaction without itself being consumed. Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions.

Most solid catalysts are metals or the oxides, sulfides, and halides of metallic elements and of the semimetallic elements boron, aluminum, and silicon. Gaseous and liquid catalysts are commonly used in their pure form or in combination with suitable carriers or solvents; solid catalysts are commonly dispersed in other substances known as catalyst supports.

In general, catalytic action is a chemical reaction between the catalyst and a reactant, forming chemical intermediates that are able to react more readily with each other or with another reactant, to form the desired end product. During the reaction between the chemical intermediates and the reactants, the catalyst is regenerated. The modes of reactions between the catalysts and the reactants vary widely and in solid catalysts are often complex. Typical of these reactions are acid–base reactions, oxidation–reduction reactions, formation of coordination complexes, and formation of free radicals. With solid catalysts the reaction mechanism is strongly influenced by surface properties and electronic or crystal structures. Certain solid catalysts, called polyfunctional catalysts, are capable of more than one mode of interaction with the reactants; bifunctional catalysts are used extensively for reforming reactions in the petroleum industry.

Catalyzed reactions form the basis of many industrial chemical processes. Catalyst manufacture is itself a rapidly growing industrial process.

Catalytic processes and their catalysts:

process    : catalyst
ammonia synthesis :    iron
sulfuric acid manufacture :    nitrogen(II) oxide, platinum
cracking of petroleum    : zeolites
hydrogenation of unsaturated hydrocarbons    : nickel, platinum, or palladium
oxidation of hydrocarbons in automobile exhausts    : copper(II) oxide, vanadium(V) oxide, platinum, palladium
isomerization of n-butane to isobutane :     aluminum chloride, hydrogen chloride

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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