Your kidneys filter over 150 litres a day and hand back 99%
Each day an adult pushes more than 150 litres of fluid through the kidneys' microscopic filters, then reclaims 99% of the water before it can leave. The work is done by one to one and a half million tiny units in each kidney, called nephrons, each a filter bolted to a long, busy pipe.
A nephron starts with the renal corpuscle: a knot of capillaries, the glomerulus, cupped inside Bowman's capsule. Blood pressure alone drives the filtering, helped by a neat trick of plumbing: the vessel leaving the glomerulus is narrower than the one entering, which raises the pressure inside. Fluid is squeezed through three layers, the capillary lining, a basement membrane and the foot-like processes of cells called podocytes. Only about a fifth of the plasma gets through; proteins and blood cells normally stay behind.
The filtrate then travels a tube with distinct sections. The proximal convoluted tubule, lined with brush-bordered cells that multiply the absorbing surface, recovers all the filtered glucose and amino acids and more than half the salt and water. Neighbouring capillaries wrap the tubule and carry the recovered material straight back into the bloodstream. Hormones fine-tune the rest: antidiuretic hormone adjusts water, aldosterone sodium and potassium, parathyroid hormone calcium and phosphate.
The loop of Henle is the clever part. Its descending limb lets water out but holds salt; its thick ascending limb is watertight but pumps sodium out. That arrangement salts the surrounding tissue of the medulla, so water can later be drawn out of the urine and returned to the blood, a countercurrent system. Long-looped juxtamedullary nephrons, found only in birds and mammals, make the most concentrated urine. They are only about 15% of human nephrons, yet they are the kind usually drawn in textbooks.
Diseases tend to strike either the filters, as in diabetic nephropathy and IgA nephropathy, or the tubules, as in polycystic kidney disease. A mouse study suggests preterm birth stresses the cells that build nephrons, leaving a permanent shortfall that shows up as kidney damage in adulthood. Another line of work finds the anti-ageing protein Klotho plays different roles in the nephron's early and late segments.
Source: Nephron
