The kidney is the most important organ of
excretion in
vertebrates. It is here that the body is able to selectively release toxins and other substances that exist in overabundance. At the
cellular level, the functional component of the kidney is the
nephron. These cells use a complex set of physical and chemical processes to selectively releases substances from the body.
The first stage of kidney function occurs at the
Bowman’s capsule. Here,
blood in the
glomerulus, a
capillary bed, is forced by
pressure filtration into said capsule. The blood in the
glomerulus comes from the
afferent arteriole, which in turn connects to the
renal artery. The kidneys process 180 litres of
blood per day, making them both vital and busy. This blood leaves the
glomerulus through the
efferent arteriole. For the remainder of the length of the
nephron, it is surrounded by the
peritubular capillaries. Material is exchanged between them and the
nephron through the processes of
active transport,
diffusion, and
osmosis – which is a particular kind of diffusion
involving only water.
Pressure filtration basically draws from the
blood everything except the
blood cells, red and white, and the
blood proteins, like
albumin. Thus,
water,
amino acids,
glucose,
vitamins,
nitrogenous wastes,
proteins, and other small molecules are introduced into the
nephron.
The majority of these substances are subsequently returned to the blood at the
proximal convoluted tubule. Here, the process of
selective re-absorption occurs and all molecules desired by the body are pumped into the
peritubular capillaries by
active transport. Examples of molecules that the body re-absorbs are
glucose and
vitamins.
From here, the
nephon contents enter the
Loop of Henle. The most important function of this segment of the
nephron is the removal of excess water. This is accomplished by a mechanism called the
countercurrent effect. Because water cannot be moved by
active transport, the nephron moves salts out of itself and into the area around it. This decreases the concentration of water outside the nephron and therefore draws water out, into the
renal cortex, through
osmosis.
Having had much of its water content removed, the contents of the
nephron enter the
distal convoluted tubule. Here,
active transport works to move unwanted substances out of the
peritubular capillaries and into the
nephron. Excess
salts and
nitrogenous wastes, among a great many other things, enter the
nephron thus.
Finally, the contents of the
nephron enter the collecting duct. This conveys them to the
renal pelvis where, by way of the
ureter, they enter the
urinary bladder. When the
sphincter here relaxes, the
urine flows out the
urethra and exits the body.
It should be noted that the countercurrent effect also occurs in the
collecting duct. Here, the
hormone ADH (
antidiuretic hormone) controls the
active transport of
urea into the area surrounding the
collecting duct, as well as increasing its
permeability to water. This allows
osmosis to remove an additional quantity of water from the contents of the
nephron, which can now be called urine.