Organs of the Urinary System
The urinary system removes nitrogenous waste and helps control water and salt balance. It consists of two kidneys, two ureters, a urinary bladder and a urethra. Blood enters each kidney through the renal artery and leaves, cleaned, through the renal vein. The kidney is the organ of both excretion and osmoregulation.
The Nephron
Each kidney contains about a million tiny functional units called nephrons. A nephron has a cup-shaped Bowman's capsule holding a knot of capillaries (the glomerulus), followed by a long, coiled tubule. Three main processes take place:
- Ultrafiltration: high blood pressure in the glomerulus forces water, glucose, salts, amino acids and urea out of the blood into Bowman's capsule. Large molecules such as proteins and blood cells stay behind.
- Selective reabsorption: as the filtrate flows along the tubule, useful substances (all glucose and amino acids, most water and needed salts) are reabsorbed back into the blood.
- Osmoregulation: the amount of water reabsorbed is adjusted so that the correct water balance is kept. The remaining fluid is urine (water, urea, excess salts).
Key idea
Filtrate is NOT urine. Filtrate becomes urine only after useful substances have been reabsorbed along the tubule.
Control of Water Balance
Water balance is controlled by the hormone ADH (antidiuretic hormone). When the blood is too concentrated (little water), the pituitary gland releases more ADH; the tubules become more permeable and reabsorb more water, so a small volume of concentrated urine is made. When the blood is dilute, less ADH is released and more, dilute urine is produced.
Example
After heavy exercise with lots of sweating, the blood becomes concentrated. More ADH is secreted, more water is reabsorbed, and only a small amount of dark, concentrated urine is passed.
Remember
Urine leaves the kidney through the ureter, is stored in the bladder, and passes out through the urethra.