Figure 6-2. A section of a human kidney.
c. The Nephron. (See Figure 6-3 for an illustration of a nephron.) Nephrons
are the functional units of the human kidney. Their primary function is to remove the
wastes of protein usage from the blood. In addition, they serve to conserve water and
other materials for continued use by the body. The result of nephron function is more or
less concentrated fluid called urine. The kidneys contain great numbers of nephrons,
about a million for each kidney. The main subdivisions of a nephron are the renal
corpuscle and a tubular system.
(1) Renal corpuscle. The renal corpuscle has a hollow double walled sac
called the renal capsule ("Bowman's capsule"). Leading into the capsule is a very small
artery called the afferent arteriole. Within the capsule, this artery becomes a mass of
capillaries known as the glomerulus. An efferent arteriole drains the blood away from
the capsule. The capsule and the glomerulus together are known as the renal
(2) Tubules. Each renal capsule is drained by a renal tubule. The first part
of this tubule runs quite a distance in a coiled formation and is called the proximal
convoluted tubule. A long loop, the renal loop (of Henle) extends down into the medulla
with two straight parts and a sharp bend at the bottom. As the tube returns to the cortex
layer, it becomes coiled once more and here is known as the distal convoluted tubule.
(3) Filtration/reabsorption. Except for the blood cells and the larger proteins,
the fluid portion of the blood passes through the walls of the glomerulus into the cavity
between the two layers of the renal capsule. This fluid is called the glomerular filtrate.
By a process of taking back (resorption), the majority of the fluid is removed from the
tubules and the concentrated fluid is called urine.