Microscopic Structure

Microscopic Structure and

Function of the Kidney

Each of the million or so nephrons in each kidney is a microscopic

tubule consisting of two major parts: a glomerulus

and a renal tubule. The glomerulus is a tangled capillary knot

that filters fluid from the blood into the lumen of the renal

tubule. The function of the renal tubule is to process that

fluid, also called the filtrate. The beginning of the renal

tubule is an enlarged end called the glomerular capsule,

which surrounds the glomerulus and serves to funnel the filtrate

into the rest of the renal tubule. Collectively, the

glomerulus and the glomerular capsule are called the renal

corpuscle.

As the rest of the renal tubule extends from the glomerular

capsule, it becomes twisted and convoluted, then dips

sharply down to form a hairpin loop, and then coils again before

entering a collecting duct. Starting at the glomerular capsule,

the anatomical parts of the renal tubule are as follows:

the proximal convoluted tubule, the loop of Henle (nephron

loop), and the distal convoluted tubule.

Two arterioles supply each glomerulus: an afferent arteriole

feeds the glomerular capillary bed and an efferent arteriole

drains it. These arterioles are responsible for blood flow

through the glomerulus. Constricting the afferent arteriole

lowers the downstream pressure in the glomerulus, whereas

constricting the efferent arteriole will increase the pressure in

the glomerulus. In addition, the diameter of the efferent arteriole

is smaller than the diameter of the afferent arteriole,

restricting blood flow out of the glomerulus. Consequently,

the pressure in the glomerulus forces fluid through the endothelium

of the glomerulus into the lumen of the surrounding

glomerular capsule. In essence, everything in the blood except

the cells and proteins are filtered through the glomerular

wall. From the capsule, the filtrate moves into the rest of the

renal tubule for processing. The job of the tubule is to reabsorb

all the beneficial substances from its lumen while allowing

the wastes to travel down the tubule for elimination from

the body.

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