Glomerulus

A tuft of small blood veins (capillaries) found at the beginning of a nephron in the kidney is known as a glomerulus (plural glomeruli). There are around one million nephrons in each of the two kidneys. The mesangium (the space between the blood vessels), which is made up of intraglomerular mesangial cells, supports the tuft structurally. The glomerular filtration barrier filters blood across the capillary walls of this tuft, yielding a filtrate of water and soluble substances to a cup-like sac known as Bowman's capsule. The filtrate then enters the nephron's renal tubule. Thus, the job of our kidneys is to filter our blood and remove wastes. The kidney also controls the fluid balance in our bodies and maintains the level of electrolytes. 

Blood in human bodies passes through the kidney approximately 40 times a day. Kidney primarily does the following work:

• Blood comes inside the kidney, kidney removes wastes in the blood.

• The kidney adjusts the level of water, salt, and other minerals in our bodies.

• The extra water and waste are passed out from our bodies in the form of urine.

• Two thin tubes of muscles (called ureters that exist on either side of the bladder) carry this urine from the kidney to the bladder.

• The bladder stores this urine. The bladder along with the kidney and ureters form the urinary tract in our bodies.

Each of the kidneys is made up of millions of nephrons and each nephron has a tubule and a filtering unit called the glomerulus. In this article, we will look closely at the glomerulus histology where we will see the glomerulus diagram, understand glomerulus structure, learn in detail about glomerulus function, and other important information about this body part.

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Glomerulus Structure

The glomerulus is the filtering unit of the kidney which is made up of a distinct bundle of capillaries. Let us take a closer look at the glomerulus structure. 

• The glomerular capillary membranes are contained within Bowman’s capsule. They are the only capillary membranes in human bodies which are not surrounded by intestinal tissues. The glomerular capillary membranes are made up of:

• Delicate Perforated Endothelial - Endothelial is a complex mesh of proteins that act as the basement membrane for the glomerulus. It has large pores whose diameter ranges between 70 to 100 nanometers. These pores allow plasma proteins, solutes, and fluid to pass through it, but not blood. 

• Specialized Epithelial Cells - Also called podocytes, these visceral epithelial cells envelop the glomerular capillaries and are attached to the basement membrane by pedicels (foot processes). They have slits between them called filtration slits. There is a thin diaphragm between these filtration slits that provides a final filtration barrier to the fluid entering the glomerular space.

• The glomerulus together with the glomerular filtration membrane is termed the renal capsule. The glomerulus diagram below shows the various parts that make up the glomerulus anatomy.

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Filtering Process of Glomerulus

Let us take a look at the glomerular filtration diagram presented below:

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In the above diagram:

• Podo is podocytes.

• GBM is glomerular basement membrane

• Endo is fenestrated endothelial cells

• ESL is endothelial cell surface layer which is often referred to as glycocalyx

• GFR is glomelur filtration rate

• Qp is the plasma flow rate

Now we will go into the details of glomerular filtration:

1. The process of making urine begins with the separation of plasma (the liquid part of the blood which has all the dissolved solutes) from the blood cells.

2. The blood which is going to be filtered enters the glomerulus which is sandwiched between two arteries:

• Afferent Arterioles - These deliver blood to the glomerulus.

• Efferent Arterioles - These carry away blood from the glomerulus. 

The constriction of these arterioles when blood exits the glomerulus provides resistance to the flow of blood. This prevents pressure drop which would not have been possible if blood flowed into venules (these are small vessels that deliver blood to the glomerular capillary bed and also carry blood from it).

3. The arteries change the size to decrease or increase blood pressure inside the glomerulus.

4. The glomerular filtration rate in humans is 125 ml per minute and the rate of plasma flow is near about 700 ml per minute. The filtration factor is 20% which is the fraction of RPF (renal plasma flow) filtered across glomerulus and we get this by the formula GFR/RPF.