Maintaining water balance in the body


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

The uninary system

Blue and red blood vessels.  The blue show blood coming from the kidneys going to the heart. The red show blood going to the kidneys from the heart. Bladder, urethra and kidney are labelled.

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.

The nephron

The role of the kidney

Diagram showing how the kidney functions

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:

Stage 1 - Filtration

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.

Stage 2 - Selective reabsorption

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:

  • 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.

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.