structure of kidney

In humans, the excretory system consists

of a pair of kidneys, one pair of ureters, a

urinary bladder and a urethra . Kidneys are reddish brown, bean

shaped structures situated between the

levels of last thoracic and third lumbar

vertebra close to the dorsal inner wall of

the abdominal cavity. Each kidney of an

adult human measures 10-12 cm in

length, 5-7 cm in width, 2-3 cm in

thickness with an average weight of 120-

170 g. Towards the centre of the inner

concave surface of the kidney is a notch

called hilum through which ureter, blood

vessels and nerves enter. Inner to the hilum

is a broad funnel shaped space called the

renal pelvis with projections called calyces.

The outer layer of kidney is a tough

capsule. Inside the kidney, there are two

zones, an outer cortex and an inner

medulla. The medulla is divided into a few

conical masses (medullary pyramids)

projecting into the calyces (sing.: calyx).

The cortex extends in between the

medullary pyramids as renal columns called

Columns of Bertini.

Each kidney has nearly one million

complex tubular structures called nephrons which are the functional units.

Each nephron has two parts – the

glomerulus and the renal tubule.

Glomerulus is a tuft of capillaries formed by

the afferent arteriole – a fine branch of renal

artery. Blood from the glomerulus is carried

away by an efferent arteriole.

The renal tubule begins with a double

walled cup-like structure called Bowman’s

capsule, which encloses the glomerulus.

Glomerulus alongwith Bowman’s capsule, is

called the malpighian body or renal

corpuscle . The tubule

continues further to form a highly coiled

network – proximal convoluted tubule

not to be republished. A hairpin shaped Henle’s loop is the

next part of the tubule which has a

descending and an ascending limb. The

ascending limb continues as another highly

coiled tubular region called distal

convoluted tubule (DCT). The DCTs of many

nephrons open into a straight tube called

collecting duct, many of which converge and

open into the renal pelvis through medullary

pyramids in the calyces.

The Malpighian corpuscle, PCT and DCT

of the nephron are situated in the cortical

region of the kidney whereas the loop of Henle

dips into the medulla. In majority of

nephrons, the loop of Henle is too short and

extends only very little into the medulla. Such

nephrons are called cortical nephrons. In

some of the nephrons, the loop of Henle is

very long and runs deep into the medulla.

These nephrons are called juxta medullary

nephrons.

The efferent arteriole emerging from the glomerulus forms a fine

capillary network around the renal tubule called the peritubular

capillaries. A minute vessel of this network runs parallel to the Henle’s

loop forming a ‘U’ shaped vasa recta. Vasa recta is absent or highly

reduced in cortical nephrons.