Meaning, Number, and Size:
A nephron is the structural and functional unit of the kidney. A kidney contains about a million nephrons, each about 3 cm long. There is thus a great surface area for the exchange of materials in a kidney.
Structure of Nephron:
The body of nephron is differentiated into four regions-
Bowman’s Capsule or Glomerular Capsule:
It is a large double-walled cup lies in the renal cortex. Its outer wall is composed of squamous cells while the inner wall is composed of special types of cells called podocytes. These cells bear distinct finger-like processes which entwine around the capillaries of the Glomerulus. The glomerulus is a network of about 50 parallel capillaries present at the concavity of Bowman’s capsule. The Glomerulus capillaries have high permeability as compared to blood capillaries. Bowman’s capsule and Glomerulus collectively form the renal corpuscle and helps in the ultrafiltration of urine.
Proximal Convoluted Tubule (PCT):
After Bowman’s capsule, the nephron becomes much convoluted to form a proximal convoluted tubule. It is lined by brush bordered cuboidal epithelium with numerous microvilli. These epithelial cells are specialized in the transport of salts and other substances from lumen to interstitial fluid. Its cells have numerous mitochondria for reabsorption of salts by active transport.
Loop of Henle:
After proximal convoluted tubule, the nephron forms a ‘U’ shaped loop known as Henle’s Loop. It is present in the renal medulla. This loop has a descending limb and an ascending limb. Like the proximal convoluted tubule, it is also lined with cuboidal epithelium but the cells have fewer mitochondria and microvilli as compared to the proximal convoluted tubule. Both the descending limb and the ascending limb have thin and thick segments also lined by cuboidal epithelium. Loop of Henle is much longer in mammals and birds which pass out hypertonic urine.
Distal Convoluted Tubule (DCT):
Like a proximal convoluted tubule, it is greatly twisted and lies in the renal cortex. A short terminal straight part of DCT is known as a collecting duct. It is lined by cuboidal epithelial cells with few microvilli. Many collecting tubules open into a large collecting duct known as ducts of Bellini which opens into the renal pelvis.