Nephron is the structural and functional unit of kidney.
Structure of nephron:
A nephron (uriniferous tubule) is a thin walled, coiled duct, lined by a single layer of epithelial cells.
Each nephron is divided into two main parts:
i. Malpighian body
ii. Renal tubule
i. Malpighian body: Each Malpighian body is about 200pm in diameter and consists of a Bowman’s capsule and glomerulus.
a. Glomerulus:
Glomerulus is a bunch of fine blood capillaries located in the cavity of Bowman’s capsule. A small terminal branch of the renal artery, called as afferent arteriole enters the cup cavity (Bowman capsule) and undergoes extensive fine branching to form network of several capillaries. This bunch is called as glomerulus. The capillary wall is fenestrated (perforated). All capillaries reunite and form an efferent arteriole that leaves the cup cavity. The diameter of the afferent arteriole is greater than the efferent arteriole. This creates a high hydrostatic pressure essential for ultrafiltration, in the glomerulus.
b. Bowman’s capsule:
It is a cup-like structure having double walls composed of squamous epithelium. The outer wall is called as parietal wall and the inner wall is called as visceral wall. The parietal wall is thin consisting of simple squamous epithelium. There is a space called as capsular space / urinary space in between two walls. Visceral wall consists of special type of squamous cells called podocytes having a foot-like pedicel. These podocytes are in close contact with the walls of capillaries of glomerulus.
There are small slits called as filtration slits in between adjacent podocytes.
ii. Renal tubule:
a. Neck:
The Bowman’s capsule continues into the neck. The wall of neck is made up of ciliated epithelium. The lumen of the neck is called the urinary pole. The neck leads to proximal convoluted tubule.
b. Proximal Convoluted Tubule :
This is highly coiled part of nephron which is lined by cuboidal cells with brush border (microvilli) and surrounded by peritubular capillaries. Selective reabsorption occurs in PCT. Due to convolutions (coiling), filtrate flows slowly and remains in the PCT for longer duration, ensuring that maximum amount of useful molecules are reabsorbed.
c. Loop of Henle :
This is ‘U’ shaped tube consisting of descending and ascending limb. The descending limb is thin walled and permeable to water and lined with simple squamous epithelium. The ascending limb is thick walled and impermeable to water and is lined with simple cuboidal epithelium. The LoH is surrounded by capillaries called vasa recta.
Its function is to operate counter current system – a mechanism for osmoregulation. The ascending limb of Henle’s loop leads to DCT.
d. Distal convoluted tubule:
This is another coiled part of the nephron. Its wall consists of simple cuboidal epithelium. DCT performs tubular secretion / augmentation / active secretion in which, wastes are taken up from surrounding capillaries and secreted into passing urine. DCT helps in water reabsorption and regulation of pH of body fluids.
e. Collecting tubule:
This is a short, straight part of the DCT which reabsorbs water and secretes protons. The collecting tubule opens into the collecting duct.
