Edexcel Biology Gcse - Homeostasis in Humans Part 3

When excessive amounts of protein are eaten, the excess amino acids produced from digesting proteins are transported to the liver from the small intestine. The liver controls the amino acid concentration in the body, as excess amino acids which need to be excreted safely. The body is unable to store proteins or amino acids.

In the liver, ammonia is formed by the deamination of amino acids. It is highly toxic and cannot be allowed to accumulate in the body. Excess ammonia is converted to urea. Urea and water are released from the liver cells in to the bloodstream and transported to the kidneys where the blood is filtered and the urea is passed out of the body in the urine.

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 are organs of the urinary system - which remove excess water, mineral ions and urea.

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.

The ureters are tubes that carry urine from the kidneys to the bladder.

The urethra is the tube that carries urine out of the body.

Renal artery carries oxygenated blood to the kidneys

Renal veins carries deoxygenated blood away from the kidneys

Kidneys regulates water content and filters blood

Ureter carries urine from the kidneys to the bladder

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.

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.

Stage 1 - Filtration

Stage 2 - Selective reabsorption

Stage 3 - The formation of urine

Blood passes into the many capillaries inside the kidney. The blood is under high pressure at the start of the nephron, which helps the ultrafiltration of the blood. Small molecules are squeezed out and pass into the nephron tubule. These small molecules include urea, 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 to form urine. Therefore, the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream.

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

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.

The hypothalamus detects changes in the blood plasma.

anti-diuretic hormone

pituitary gland

Different amounts of ADH are released into the bloodstream according to the concentration of water in the blood plasma. ADH is released by the pituitary gland when the blood is too concentrated and it causes the kidney tubules to become more permeable. This allows more water to be reabsorbed back into the blood during selective reabsorption.

Answer: If a person has consumed a large volume of water and has not lost much as sweat, too much water might be detected in the blood plasma. If this occurs, less ADH will be released, which results in less water being reabsorbed and a dilute and larger volume of urine will be produced.

1) Hypothalamus detects water level

2) Pituitary gland releases less ADH

3) Less water reabsorbed by kidneys

4) More water lost in urine

Blood water level returns to normal

Answer: If a person becomes too hot and sweats a lot, but doesn't drink enough water to replace what was lost, too little water might be detected in the blood plasma. More ADH will be released, which results in water being reabsorbed and a more concentrated but smaller volume of urine will be produced.

1) Hypothalamus detects water level

2) Pituitary gland releases ADH

3) More water reabsorbed by kidneys

4) Less water lost in urine

Blood water level returns to normal

Too little water

Too much water

ADH release increases