The nephron is the unit of the kidney responsible for ultrafiltration of the blood and reabsorption or excretion of products in the subsequent filtrate. Each nephron is made up of:
- A filtering unit- the glomerulus. 125ml/min of filtrate is formed by the kidneys as blood is filtered through this sieve-like structure. This filtration is uncontrolled.
- The proximal convoluted tubule. Controlled absorption of glucose, sodium, and other solutes goes on in this region.
- The loop of Henle. This region is responsible for concentration and dilution of urine by utilising a counter-current multiplying mechanism- basically, it is water-impermeable but can pump sodium out, which in turn affects the osmolarity of the surrounding tissues and will affect the subsequent movement of water in or out of the water-permeable collecting duct.
- The distal convoluted tubule. This region is responsible, along with the collecting duct that it joins, for absorbing water back into the body- simple maths will tell you that the kidney doesn't produce 125ml of urine every minute. 99% of the water is normally reabsorbed, leaving highly concentrated urine to flow into the collecting duct and then into the renal pelvis.