Nephrons
Kidney produces urine by its microscopic functional units called nephrons. A nephron is essentially a long coiled duct in which the coiling takes a definite course. A nephron along with the collecting tubule is also called as a uriniferous tubule. There are about 1 to 1.2 million nephrons in each kidney.
Structure of nephron :
A nephron is a thin walled, coiled, duct lined by a single layer of epithelial cells. Its proximal end is blind i.e. has no opening anywhere while the distal end opens into a collecting tubule. The proximal and distal ends of nephron both lie in the cortex of the kidney while middle region lies in the medulla. Total length of each nephron is about 40 to 60 nm in mammals.
A nephron can be divided into two regions- the Bowman's capsule and Renal tubule.
The Bowman's capsule is the proximal blind end of the nephron. Its widest part is double walled and cup-shaped. Outer layer is called parietal later and inner layer tis visceral layer which continues along the rim of the cup.
Each layer is formed of squamus epithelium. The space enclosed by the two layer is called urinary space. It is continuous with the space of the renal tubule which originates from the base of Bowman's capsule.
Within the cup shaped space formed by the visceral layer of the Bowman's capsule lies a globular network of capillaries called glomerulus.
A thin branch of the renal artery is afferent arteriole which supplies blood to the glomerulus while slightly thinner efferent arteriole carries blood away from the glomerulus. There is an intimate connection between glomerulus and Bowman's capsule. Two together are referred as Renal corpuscles or Malpighian body or Pygmalion corpuscle.
The renal tubule arises from the base of Broman's capsule. The connection between this is the neck. Just near the neck, the tubule is highly coiled and this region is called proximal convoluted tubule (PCT). This leads to a 'U' shaped region of the tubule which enters the medulla region of the kidney. It takes a hairpin turn and returns to the cortex. This middle region of tubule is called loop of Henle. It has two limbs - a) thin walled descending limb and b) thick walled ascending limb. It is not permeable to water. Henle's loop is mainly meant for concentration of urine. After reentering the cortex the tubule again gets coiled to give rise to the distal convoluted tubule (DCT). It opens into a collecting tubule i.e. Duct of Bellini which opens into the calyx at the apex of the medullary pyramid.
PCT shows the cells with brush border formed of microvilli. Microvilli are absent in DCT. The entire tubule is embedded within the network of capillaries. The efferent arteriole which leaves the glomerulus breaks up into capillaries which form a network all over called peri tubular capillaries. These capillaries join to form a venule. Venules join to form a renal vein.
DCT lies close to glomerulus of the same nephron and forms a complex structure called juxta glomerular apparatus. Cells of JG apparatus secrete enzyme Renin that controls blood pressure.