Maintaining internal environments - OCR GatewayMaintaining water balance in the body

Kidneys

The kidneys are organs of the urinary system - which remove excess water, mineral ions 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, mineral 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

Urine contains water, urea and mineral ions. 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.

The nephron

The role of 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, excess water and mineral ions.

A three stage process occurs in each nephron: filtration, selective reabsorption and finally excretion.

Stage 1 - Filtration

The glomerulus filters the blood and removes water, glucose, salts and waste urea from it. The blood is under high pressure at the start of the nephron, which aids the ultrafiltration of the blood. These waste substances all pass from the capillaries in the glomerulus into the Bowman’s capsule. This purifies the blood. These waste substances then move from the Bowman’s capsule towards the loop of Henle. Proteins are too large to pass through here and so remain in the blood. This process is called filtration.

Stage 2 - Selective reabsorption

The kidneys must now 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:

• all of the glucose which was originally filtered out
• as much water as the body needs to maintain a constant water level in the blood plasma
• as many ions as the body needs to maintain a constant balance of mineral ions in the blood plasma

This reabsorption comes from the solution passing along the loop of Henle from the Bowman’s capsule. Here water and salts needed by the body are reabsorbed into the blood.

Stage 3 - The formation of urine

The molecules which are not selectively reabsorbed (the urea, excess water and ions) continue along the nephron tubule to the collecting ducts 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.

Learn more about maintaining water balance with Dr Alex Lathbridge.

Listen to the full series on BBC Sounds.