Locations of secretion and reabsorption in the nephron
In
renal physiology, reabsorption or tubular reabsorption is the process by which the
nephron removes water and solutes from the
tubular fluid (pre-urine) and returns them to the
circulating blood.[1] It is called reabsorption (and not absorption) because these substances have already been absorbed once (particularly in the
intestines) and the body is reclaiming them from a post
glomerular fluid stream that is on its way to becoming
urine (that is, they will soon be lost to the urine unless they are reabsorbed from the tubule into the
peritubular capillaries. This happens as a result of
sodium transport from the lumen into the
blood by the
Na+/K+ATPase in the basolateral membrane of the
epithelial cells. Thus, the glomerular filtrate becomes more
concentrated, which is one of the steps in forming
urine. Nephrons are divided into five segments, with different segments responsible for reabsorbing different substances.[2] Reabsorption allows many useful
solutes (primarily
glucose and
amino acids),
salts and
water that have passed through
Bowman's capsule, to return to the circulation. These solutes are reabsorbed
isotonically, in that the
osmotic potential of the fluid leaving the proximal convoluted tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids,
inorganic phosphate, and some other solutes are reabsorbed via secondary
active transport through
cotransport channels driven by the sodium gradient.