GCSE Biology: Excretion

Justina: Excretion is the removal of metabolic waste substances, such as carbon dioxide and water, which are waste products creating during aerobic respiration, toxins and other excess substances from the body.

Georgina: If waste products are allowed to build up, they can have a range of negative effects on the body.Let’s take a closer look at how we get rid of them.

Justina: Our bodies have organs that are specialised in the removal of certain excretory products.The lungs excrete carbon dioxide and water.The skin excretes excess water and mineral ions, such as sodium, through sweating.And the kidneys excrete water, mineral ions and a substance called urea.Most people have two kidneys, and their main function is to regulate the water content of the blood and filter waste substances, including urea, from the blood.

Georgina: The digestion of proteins results in an excess of amino acids being taken to the liver, where they are converted into ammonia, which is highly toxic.Ammonia reacts with carbon dioxide and forms urea, which is less toxic, enabling it to be safely removed by the kidneys.

Justina: Blood enters the kidneys via the renal artery.Each kidney contains over one million nephrons, which are microscopic filtering units that are responsible for filtering the blood.

Georgina: The blood enters these nephrons at very high pressure.Two main processes occur in the kidneys.The first process is filtration.Due to high pressure, small molecules such as urea, water and minerals are filtered out of the blood and pass into the nephron tubules in the cortex of the kidney.However, large molecules such as blood proteins are too big to be filtered and remain in the blood.The second process is selective reabsorption.Some vital molecules such as water and glucose, are small enough to be filtered out of the blood by the nephrons.But our bodies still need them.

Justina: So, some water, minerals and/or glucose are reabsorbed back into the blood in the medulla, in a controlled way.

Georgina: This ensures the correct concentrations of these substances are in the blood.

Justina: the excess water, excess minerals and urea are then passed into the ureter and stored in the bladder, ready for a trip to the toilet.

Georgina: Let’s look a little more closely at the water balance in the body, also known as osmoregulation; an example of homeostasis.If your body has too much water in the blood, more will move into the cells via osmosis.The cells will expand and swell, which can be deadly.If the opposite happens and you have too little water in the blood, water leaves the cells via osmosis and they may not have enough water to function properly.

Justina: When the hypothalamus in the brain detects lower than normal water levels in the blood, the pituitary gland releases more of a hormone called ADH.ADH travels in the blood to the kidneys, where it causes more water to be reabsorbed by the nephrons,by making the connecting ducts with the nephron tubules more permeable to water.Due to more water being reabsorbed back into the body, the urine in the bladder is more concentrated and of a lower volume.

Georgina: When water levels return to an ideal level, ADH levels also return to normal.And the reverse happens if water levels are high in the blood.Less ADH is produced, meaning less water is reabsorbed back into the body by the nephrons.There is then a higher volume of more dilute urine that passes to the bladder, ready to be excreted.

Justina: Kidneys are pretty important things.

Georgina: They are.

Justina: But you can actually live with one kidney.

Georgina: But if both are damaged, medical care is needed.Kidney dialysis uses a machine that recreates the job of the kidneys by filtering your blood outside of your body.

Justina: So there we go.Excretion is a pretty complex and amazing process.