Maintaining water balance in the body

Kidneys

The kidneys are organs of the - which remove excess water, salts and urea from the body.

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 the blood has been purified, it returns to the circulatory system through the .

The kidneys produce . The amount of water in the urine is controlled by the kidney, which helps to maintain water balance. The urine is taken from the kidneys to the by the . The bladder stores the urine until it is convenient to expel it from the body.

Note that 'ureter' differs from the word ''. 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, and ions. Urea is produced in the liver when excess 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 kidney tubule and is responsible for 'cleaning' the blood by removing urea, excess water and mineral ions. The nephrons reabsorb as much water as is needed by the body to maintain the water balance. This keeps the blood plasma at the correct concentration to avoid water entering or leaving cells and causing damage by osmosis .

Diagram showing how the kidney functions

A number of different stages take place in the nephrons:

Stage 1 - Filtration

Blood passes through capillaries and enters the nephron at high pressure. This aids of the blood where small molecules are filtered out and pass into the nephron tubule. These small molecules include urea, water, ions, and glucose. Large molecules, such as blood proteins and cells, are too big to fit through the capillary wall and remain in the blood.

Stage 2 - Selective reabsorption

Having filtered out small molecules from the blood - the kidneys must reabsorb the molecules which are needed, but still leave molecules that are not needed to pass out in the urine. 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

Stage 3 - The formation of urine

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 via the ureter.

These steps allow the kidney to fulfil its functions of regulating the water and ion balance of the blood plasma, as well as keeping the level of urea low.

Why do we need to maintain a constant internal environment?Maintaining water balance in the body