The kidneys are organs of the urinary system - which remove excess water, salts and urea.
Blood is transported to the kidney through the renal artery. The blood is filtered at a high pressure and the kidney selectively reabsorbs any useful materials such as glucose, salt ions and water. After it has been purified, the blood returns to the circulatory system through the renal vein.
The kidneys produce urine and this helps maintain water balance. The urine is taken from the kidneys to the bladder by the ureters. The bladder stores the urine until it is convenient to expel it from the body.
Note that 'ureter' differs from the word 'urethra'. The ureters are tubes that carry urine from the kidneys to the bladder, whereas the urethra is the tube that carries urine out of the body.
Urine contains water, urea and salts. Urea is produced in the liver when excess amino acids are broken down. Urea is the main waste product removed in the urine, as it is not reabsorbed in the kidney.
Each kidney contains over one million microscopic filtering units called nephrons. Each nephron is made of a tubule and is responsible for 'cleaning' the blood by removing urea and excess water and mineral ions.
The kidney works in a number of different stages:
Blood passes through the nephron inside the kidneys, there are many capillaries inside the kidney, and the blood is under high pressure at the start of the nephron, which aids the ultrafiltration of the blood. Small molecules are filtered out and pass into the nephron tubule. These small molecules include ureas, water, ions, and glucose. However, large molecules, such as blood proteins, are too big to fit through the capillary wall and remain in the blood.
Having filtered out small essential molecules from the blood - the kidneys must reabsorb the molecules which are needed, while allowing those molecules which are not needed to pass out in the urine. Therefore, the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream.
The reabsorbed molecules include:
The molecules which are not selectively reabsorbed (the urea, excess water and ions) continue along the nephron tubule as urine. This eventually passes down to the bladder.
In carrying out these processes, the kidney is able to fulfil its functions of regulating the water and ion balance of the blood plasma, as well as keeping the level of urea low.