Every cell in your body is busy with chemical reactions, and some of their products are poisonous if they build up. Excretion removes them: carbon dioxide leaves through the lungs, and the kidneys remove urea, excess water and excess ions as urine. Where urea comes from, the liver and its amino acids, is Extended content.
Excretion is the removal of the waste products of metabolism and substances in excess of requirements. Carbon dioxide is excreted through the lungs, and the kidneys excrete urea and excess water and ions as urine, which passes down the ureters to the bladder and out through the urethra. Extended: the liver joins amino acids into proteins (assimilation); excess amino acids are deaminated, their nitrogen-containing part being removed to form urea, which is toxic and so must be excreted.
Fig. 1: the human excretory organs; urine flows from each kidney down a ureter to the bladder and leaves through the urethra
Section 01
What the body excretes.
Respiration in every cell makes carbon dioxide, and the breakdown of excess amino acids makes urea. Both are waste products of metabolism. The body also has to get rid of water and ions it has taken in beyond its needs. Click each organ in the model and follow the yellow drops of urine and the grey dots of carbon dioxide.
Lungs: excrete carbon dioxide, the waste product of respiration. It diffuses from the blood into the alveoli and is breathed out.
Kidneys: excrete urea and excess water and ions. These leave the blood in the kidneys and form urine.
Ureters: two tubes, one from each kidney, carrying urine to the bladder.
Bladder: a muscular bag that stores urine until it is released.
Urethra: a single tube carrying urine from the bladder out of the body.
Renal artery and vein: the renal artery brings blood containing urea to each kidney; the renal vein carries the cleaned blood away.
Examiner trap
Do not mix up the ureter and the urethra. There are two ureters, each running from a kidney to the bladder; there is one urethra, running from the bladder to the outside. Remember the order: kidney, ureter, bladder, urethra.
Section 02
Amino acids in the liver.
Extended. Amino acids absorbed from the small intestine travel in the blood to the liver. In the model, coloured spheres are amino acids and the small blue sphere on each one is its nitrogen-containing part. Switch between a normal diet and a high-protein diet and watch the two counters.
Assimilation: the liver uses amino acids to make proteins, joining them into chains. Plasma proteins such as fibrinogen are made this way, and other cells also build proteins from amino acids.
Excess amino acids: the body cannot store amino acids, so those not needed are broken down in the liver.
Deamination: the removal of the nitrogen-containing part of an amino acid, which is used to form urea. The rest of the molecule can be respired to release energy.
Urea in the blood: urea leaves the liver in the blood and travels to the kidneys, which excrete it in urine.
High-protein diet: more amino acids arrive than the body can use, so more are deaminated and more urea is made and excreted.
Examiner trap
Urea is made in the liver, not in the kidneys. The kidneys only remove urea from the blood and excrete it. Deamination removes the nitrogen-containing part of the amino acid; it does not destroy the whole molecule.
Stage 1 · Learn
Check what the sim just showed you
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In the excretory organs model, which organ is shown excreting carbon dioxide?
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In the model, the yellow drops of urine pass along which route?
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Extended: in the liver tab, what happens when you switch from a normal diet to a high-protein diet?
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Extended: in the liver tab, what is removed from each excess amino acid (the blue part) to form urea?
Section 03
Why urea must go.
Extended. Excretion matters because the waste products of metabolism are harmful if they are allowed to accumulate. For urea the reason is simple: it is toxic.
Toxicity: urea is toxic. If it were not excreted, its concentration in the blood and tissue fluid would rise and it would poison cells.
Continuous removal: the liver makes urea all the time, so the kidneys must filter the blood all the time to keep the urea concentration low.
Kidney failure: if the kidneys stop working, urea builds up in the blood; without treatment such as dialysis this is fatal.
Carbon dioxide too: carbon dioxide that is not breathed out makes the blood more acidic, another reason waste products of metabolism must be excreted.
Examiner trap
Egestion is not excretion. Faeces are mostly undigested food that never entered the body cells, so removing them is egestion. Excretion removes substances made by metabolism, such as carbon dioxide and urea.
Stage 2 · Exam
Exam-style questions
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Which substances are excreted by the kidneys?
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Extended: which term describes the liver converting amino acids into proteins?
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Extended: urea is formed in the:
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Extended: why must urea be excreted from the body?
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