Nephrons can be defined as basic functional units in the kidney that are complex and tubular in structure. There are two parts for each nephron namely Glomerulus and Renal Tubule. The word nephron is derived from the Greek word; nephron, which is kidney. There are millions of nephrons in each human kidney.
Structure of a Nephron
The structure of the nephron is rather complex. The figure below illustrates the different parts of the nephron.
The structure of the nephron consists of two major portions:
- Renal Tubule
- Renal Corpuscle
The renal tubule of which the kidney is for the most part made up, is a long and convoluted structure that emerges from the glomerulus. It can be divided into three parts based on function.
- The first part is the proximal convoluted tubule (PCT) due to its proximity to the glomerulus and it stays in the renal cortex.
- The second part is the Henle’s Loop or nephritic loop as it forms a loop (with descending and ascending limbs) that goes through the renal medulla.
- The third part of the renal tubule is the distal convoluted tubule (DCT). This part is restricted to the renal cortex.
Proximal Convolutes Tubule (PCT)
The other component of the nephron is the Renal Tubule. The Renal Tubule starts from the Bowman’s capsule and forms a highly coiled network of nerves known as the Proximal Convoluted Tubule.
The tubule further forms a hairpin-like structure known as Henle’s loop. The Henle’s loop is a curved structure along with two sides known as the ascending limb and descending limb.
Distal Convoluted Tubule (DCT)
The ascending limb of the Henle’s loop again forms a coiled tubular structure called Distal Convoluted Tubule (DCT). It connects to a straight tube called the collecting duct which in turn is connected to the renal pelvis from where urine is emptied into the urinary bladder.
One of the two components of the renal corpuscle is the Glomerulus which is a collection of capillaries branching out from a fine branch of an efferent arteriole (a renal artery). Another renal arteriole carries blood away from the glomerulus. The glomerulus is enclosed in a cup-like structure called the Bowman’s Capsule. The part of the nephron consisting of the glomerulus along with the Bowman’s capsule is known as the renal corpuscle.
The Bowman’s capsule is again classified into three layers:
- Outer Parietal layer: With minute pores of diameter 12nm it is made up of epithelial cells.
- Middle Basement membrane: This layer is selectively permeable.
- Inner Visceral Layer: It comprises large nucleated cells called podocytes which bear finger-like projections named podocel.
Functions of a Nephron
The main functions of the nephrons include filtration of blood and urine formation. These functions are carried out by different parts of the nephron. Urine formation is carried out by three main processes namely Glomerular Filtration, Reabsorption, and Secretion.
- Glomerular Filtration
Glomerular Filtration is a process that involves the filtration of blood by the glomerulus. The filtration of blood takes place through 3 layers:
- Endothelium of glomerular blood vessels
- Epithelium of Bowman’s capsule
- Thin membranes between the layers
The epithelial cells on the Bowman’s capsule are arranged in such a way the blood is filtered very finely through the membranes. Almost all the constituents of the blood except proteins are filtered. As the filtration process here is very fine, it is known as the Ultrafiltration process.
About 99% of the filtrate which is obtained after the Ultrafiltration process, is absorbed back into the blood by the renal tubules through active or passive mechanisms. This process is called reabsorption. Most of the reabsorption of electrolytes and water takes place at the Proximal Convoluted Tubule (PCT) and partial or conditional reabsorption takes place at Henle’s Loop, Distal Convoluted Tubule (DCT), and Collecting Duct.
Secretion of hydrogen ions, potassium ions and ammonia takes place during urine formation. These components are secreted by the tubular cells. This tubular secretion is a very important process to maintain the pH level of the urine and balance of body fluids.
The nephrons also play a vital role in the regulation of kidney function through the Renin-Angiotensin and ANF (Atrial Natriuretic Factor) mechanisms.
Types of Nephron
Cortical Nephrons: These are short nephrons present in the Cortex and are high in number.
Juxtamedullary Nephrons: These are long loops of nephrons that extend to the medulla. These nephrons are small in number.
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By Team Learning Mantras