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Jai Ganesh

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Bowman's Capsule

Bowman's Capsule

Gist

The glomerular capsule, also known as Bowman's capsule, is the blind expanded end of a renal tubule. It is a double layered epithelial capsule surrounding the glomerulus. The glomerular capsule together with the glomerulus are termed the renal corpuscle, the site of blood filtration within the kidneys.

The fluid entering Bowman's capsule is called glomerular filtrate, which consists of plasma from blood minus large proteins and cells that cannot pass through the filtration barrier.

Bowman's capsule looks like a pouch, sac or cup. You can only see it under a microscope. Bowman's capsule contains fluid like blood plasma, but with no red or white blood cells or platelets.

Bowman's capsule surrounds the glomerular capillary loops and participates in the filtration of blood from the glomerular capillaries. Bowman's capsule also has a structural function and creates a urinary space through which filtrate can enter the nephron and pass to the proximal convoluted tubule.

Summary

Bowman's capsule (or the Bowman capsule, capsula glomeruli, or glomerular capsule) is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac. Fluids from blood in the glomerulus are collected in the Bowman's capsule.

Structure

Outside the capsule, there are two poles:

* The vascular pole is the side with the afferent arteriole and efferent arteriole.
* The tubular pole, is the side with the proximal convoluted tubule.

Inside the capsule, the layers are as follows, from outside to inside:

* Parietal layer—A single layer of simple squamous epithelium. Does not function in filtration.
* Bowman's space (or "urinary space", or "capsular space")—Between the visceral and parietal layers, into which the filtrate enters after passing through the filtration slits.
* Visceral layer—Lies just above the thickened glomerular basement membrane and is made of podocytes. Beneath the visceral layer lie the glomerular capillaries.
* Filtration barrier—The filtration barrier is composed of the fenestrated endothelium of the glomerular capillaries, the fused basal lamina of the endothelial cells and podocytes, and the filtration slits of the podocytes. The barrier permits the passage of water, ions, and small molecules from the bloodstream into the Bowman's space. The barrier prevents the passage of large and/or negatively charged proteins (such as albumin). The basal lamina of the filtration barrier is composed of three layers. The first layer is the lamina rara externa, adjacent to the podocyte processes. The second layer is the lamina rara interna, adjacent to the endothelial cells. The final layer is the lamina densa which is a darker central zone of the basal lamina. It consists of the meshwork of type IV collagen and laminin which act as a selective macromolecular filter.

Function

The process of filtration of the blood in the Bowman's capsule is ultrafiltration, and the normal rate of filtration is 125 ml/min, equivalent to 80 times the daily blood volume. It is a major site for blood filtration (including glomerulus).

Any proteins under roughly 30 kilodaltons can pass freely through the membrane, although there is some extra hindrance for negatively charged molecules due to the negative charge of the basement membrane and the podocytes.

Any small molecules such as water, glucose, salt (NaCl), amino acids, and urea pass freely into Bowman's space, but cells, platelets and large proteins do not.

As a result, the filtrate leaving the Bowman's capsule is very similar to blood plasma (filtrate or glomerular filtrate is composed of blood plasma minus plasma protein i.e. it contains all the components of blood plasma except the proteins) in composition as it passes into the proximal convoluted tubule.

Details

Bowman’s capsule is part of a nephron, a filter in your kidney. One million nephrons in each kidney clean your blood. A Bowman’s capsule in each nephron plays a part in the filtering process. After filtering, nutrients stay in your blood, and waste goes out through urine.

Overview

Bowman’s capsule surrounds blood vessels in each nephron that filters blood in your kidneys.

What is Bowman’s capsule?

Bowman’s capsule is a part of each filtering unit (nephron) in your kidney. You have about 1 million nephrons that filter blood in each of your two kidneys. Kidneys clean blood and return it to your body.

Every nephron has a glomerulus. A two-walled pouch, Bowman’s capsule covers the glomerulus. This group of tiny blood vessels is the starting point for filtering waste products out of your blood. Bowman’s capsule and the glomerulus make up the renal corpuscle.

You may hear other names for Bowman’s capsule, like:

* Glomerular capsule
* Malpighian capsule
* Renal corpuscular capsule

The space in between the walls (layers) of Bowman’s capsule is called Bowman’s space. You may hear healthcare providers refer to Bowman’s space as:

* Glomerular capsule space
* Filtration space
* Urinary space

Function:

What is the function of the Bowman’s capsule in the kidney?

The function of Bowman’s capsule is to help the glomerulus filter blood. Small molecules from your blood pass freely into Bowman’s space. Cells and large proteins stay in your blood.

The glomerular capsule also protects cells called podocytes by keeping white blood cells from getting in. White blood cells can’t pass through Bowman’s capsule. Podocytes in a kidney capsule have pedicels that manage what stays and what goes. Finger-like pedicels link together like they’re holding hands. The way they join creates slits that only let certain things go through. When you’re healthy, protein and cell content can’t get through.

The small molecules then pass through tubes in your kidney. Your kidney regulates which molecules your blood absorbs and which leave your body in pee (urine).

Waste materials go out of your body as urine through tubules (tiny tubes). The blood pressure in the glomerulus helps move the blood along. As the fluids leave, water and nutrients go back into your blood.

Two arterioles go into the Bowman’s capsule. One brings blood into the glomerulus. The other lets blood out.

Anatomy:

Where is Bowman’s capsule located?

Bowman’s capsule is in the renal cortex, part of your kidney. Your kidneys are in your back, below your rib cage. Usually, you have one kidney on either side of your spine. Your kidneys are between your intestines and diaphragm. Each kidney connects to your bladder by a tube called a ureter.

What does it look like?

Bowman’s capsule looks like a pouch, sac or cup. You can only see it under a microscope. Bowman’s capsule contains fluid like blood plasma, but with no red or white blood cells or platelets.

What are the parts of Bowman’s capsule?

The glomerular capsule has two layers. A type of body tissue, simple squamous epithelium, makes up the outer (parietal) layer. These parietal cells give structure to Bowman’s capsule. Cells called podocytes form the inner (visceral) layer.

Additional Information:

Introduction

Bowman’s capsule is a part of the nephron that forms a cup-like sack surrounding the glomerulus. Bowman’s capsule encloses a space called “Bowman’s space,” which represents the beginning of the urinary space and is contiguous with the proximal convoluted tubule of the nephron. Bowman’s capsule, Bowman’s space, and the glomerular capillary network and its supporting architecture can collectively be thought of as composing the glomerulus. There are an estimated 900000 glomeruli within the cortex of a mature human kidney.

Structure and Function

In the kidney, the glomerulus represents the initial location of the renal filtration of blood. Blood enters the glomerulus through the afferent arteriole at the vascular pole, undergoes filtration in the glomerular capillaries, and exits the glomerulus through the efferent arteriole at the vascular pole.

Bowman’s capsule surrounds the glomerular capillary loops and participates in the filtration of blood from the glomerular capillaries. Bowman’s capsule also has a structural function and creates a urinary space through which filtrate can enter the nephron and pass to the proximal convoluted tubule. Liquid and solutes of the blood must pass through multiple layers to move from the glomerular capillaries into Bowman’s space to ultimately become filtrate within the nephron’s lumen.

The first step of filtration occurs through the endothelial layer of the capillaries, which is composed of fenestrated endothelial cells. These fenestrations, or slits between endothelial cells, are approximately 60 to 80 nm wide and restrict the movement of matter above this size. In addition to filtering based on size, the fenestrated endothelium carries negative charges that preferentially restrict the movement of negatively charged substances into Bowman’s space.

Filtrate next moves through the glomerular basement membrane (GBM). From the direction of the capillaries and moving towards Bowman’s capsule, three layers compose the GBM – the lamina rara interna, the lamina densa, and the lamina rara externa. Mesangial cells within the glomerulus play a role in creating and maintaining the GBM, as well as holding capillary loops together.

Following the GBM, filtrate must pass through the epithelial layer of Bowman’s capsule, which is composed of podocytes. The podocytes feature finger-like projections of cytoplasm referred to as “foot processes” or “pedicels.” These foot processes interdigitate with one another and create a further barrier through which filtrate must pass. Structures called “slit diaphragms” bridge nearby foot processes and provide structural support. The podocytes are the primary cells of the epithelium adjacent to the capillaries (the visceral epithelium) and play a role in filtration. The parietal epithelium of Bowman’s capsule is the outer layer and is composed of simple squamous epithelial cells called “parietal cells.” The parietal layer is not directly involved with filtration from the capillaries. Parietal cells play a structural role in maintaining Bowman’s capsule and are also speculated to have the ability to differentiate into podocytes to replace damaged or old podocytes. Bowman’s space is the area between the visceral and parietal epithelium of Bowman’s capsule.

In summary, filtrate entering Bowman’s space traverses through glomerular capillaries, the GBM, and the interdigitated foot processes of the podocytes and is filtered based on size and electric charge. The filtrate entering Bowman’s space has a very similar composition to that of the blood in the glomerular capillaries except for the protein, and cell content as these are the components largely prevented from entering Bowman’s space when glomerular filtration is functioning properly.

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