How the Glomerulus Filters Blood and Forms Urine
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:
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.
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).
The arteries change the size to decrease or increase blood pressure inside the glomerulus.
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.
FAQs on Glomerulus: Structure and Function in the Kidney
1. What is a glomerulus and what is its primary function in the kidney?
A glomerulus is a tiny, tangled cluster of capillaries located at the beginning of a nephron in the kidney. Its primary function is ultrafiltration, the first step in making urine. During this process, high pressure forces water, salts, glucose, and waste products like urea from the blood into the Bowman's capsule, while larger molecules like proteins and blood cells are retained in the blood.
2. What is the difference between a glomerulus and a nephron?
The key difference lies in their scale and role. A nephron is the entire microscopic structural and functional unit of the kidney, responsible for filtering blood and producing urine. In contrast, the glomerulus is just one component of the nephron—specifically, the capillary network where filtration begins. The glomerulus and its surrounding Bowman's capsule together form the renal corpuscle, the first part of the nephron.
3. Where is the glomerulus located and what is its relationship with the Bowman's capsule?
The glomerulus is located inside the Bowman's capsule, a cup-shaped structure at the start of the nephron within the kidney's cortex. They have a close functional relationship:
- The glomerulus acts as the high-pressure filter.
- The Bowman's capsule collects the fluid, known as glomerular filtrate, that passes through this filter.
4. Why is the blood pressure in the glomerulus so high?
The high blood pressure in the glomerulus is crucial for filtration and is created by a unique structural feature. The afferent arteriole, which brings blood into the glomerulus, has a wider diameter than the efferent arteriole, which carries blood away. This difference in diameter creates resistance to blood outflow, causing blood to back up in the glomerulus and significantly increasing the glomerular hydrostatic pressure, which drives the filtration process.
5. How is the Glomerular Filtration Rate (GFR) regulated to remain stable?
The Glomerular Filtration Rate (GFR) is kept remarkably constant through two main types of mechanisms:
- Intrinsic Regulation (Autoregulation): The kidney can manage its own GFR. This includes the myogenic mechanism, where the afferent arteriole adjusts its diameter in response to blood pressure changes, and tubuloglomerular feedback, where the juxtaglomerular apparatus (JGA) senses filtrate composition and adjusts filtration accordingly.
- Extrinsic Regulation: The nervous and endocrine systems can override autoregulation. Hormonal control via the Renin-Angiotensin-Aldosterone System (RAAS) can constrict arterioles to conserve fluid during low blood pressure, while the sympathetic nervous system can also reduce GFR during extreme stress.
6. What are the key components of the glomerular filtration membrane?
The glomerular filtration membrane is a highly selective barrier made of three distinct layers. For a substance to pass from the blood into the Bowman's capsule, it must cross:
- The endothelium of the glomerular capillaries, which contains pores (fenestrations).
- The basement membrane, a non-cellular layer that prevents the filtration of large proteins.
- The visceral layer of the Bowman's capsule, which is composed of specialised cells called podocytes. These cells have foot-like processes that form narrow filtration slits.
7. What happens if large proteins or blood cells pass through the glomerulus?
Under normal conditions, the glomerular filtration membrane prevents large proteins and blood cells from passing into the filtrate. If these components are found in urine, it signals potential glomerular damage or disease. The presence of significant protein in urine is called proteinuria, and the presence of red blood cells is called hematuria. These conditions often indicate underlying kidney problems, such as glomerulonephritis or kidney damage from other diseases, and require medical evaluation.
8. What is the difference between glomerular filtrate and urine?
Glomerular filtrate and urine are two different stages of the same process. Glomerular filtrate is the initial fluid filtered from the blood into the Bowman's capsule. It is essentially blood plasma without the large proteins and cells, containing water, glucose, salts, amino acids, and urea. Urine, on the other hand, is the final product after this filtrate has travelled through the rest of the nephron. During this journey, about 99% of the water and essential substances are reabsorbed back into the blood, leaving behind a concentrated solution of waste products and excess water.
