Cambridge O Level Biology · Syllabus 5090 · Excretion
Urea
What is Urea?
A soluble, nitrogen-containing waste product formed in the liver from the nitrogen-containing part of excess amino acids. Urea is toxic if it accumulates to a high concentration, so it is not stored: it dissolves in the blood plasma, is carried to the kidneys, is removed from the blood there, and leaves the body dissolved in urine. Urea is produced by the liver and removed by the kidneys, and the two organs must not be interchanged; urea is also distinct from urine, which is the fluid containing urea together with excess water and excess ions.
This definition is part of the Excretion chapter in Cambridge O Level Biology.
Urea in context
Excretion is the removal from organisms of toxic materials, the waste products of metabolism, and substances in excess of requirements. The two metabolic wastes you must know are carbon dioxide, made by respiration in every cell and excreted by the lungs in expired air, and urea, made in the liver from excess amino acids and removed from the blood by the kidneys in urine. The kidney does this in three steps: filtration at the glomerulus forces small molecules out of the blood, selective reabsorption returns all the glucose, some ions and most of the water to the blood, and what is left — urea, excess water and excess ions — is urine.
Glomerular filtrate has the same composition as blood plasma except that the large plasma proteins have been left behind. Everything else that was dissolved in the plasma — water, glucose, ions and urea — passes into the capsule.
Common mistakes with Urea
- “The kidneys make urea.” Why wrong It puts manufacture and removal in the same organ, which destroys the whole liver–kidney structure of the topic. Say instead The liver makes urea by deamination; the kidneys remove it from the blood.
- “The liver stores excess amino acids until they are needed.” Why wrong There is no store for amino acids anywhere in the body. If there were, no urea would need to be made. Say instead Excess amino acids cannot be stored, so the liver deaminates them and urea is formed.
- “All the filtered water is reabsorbed.” Why wrong If it were, there would be no urine, because urea and excess ions leave the body dissolved in water. Say instead Most of the filtered water is reabsorbed; the excess remains in the tubule and leaves in the urine.
- “The collecting duct carries blood”, or “carbon dioxide is excreted by the kidneys.” Why wrong The collecting duct carries urine, not blood; blood travels in the capillaries alongside it. And carbon dioxide is excreted by the lungs, in expired air. Say instead The collecting duct carries urine toward the ureter. Carbon dioxide is excreted by the lungs; the kidneys excrete urea, excess water and excess ions.
Examiner tips on Urea
- Examiner's eye. When a question says explain, it is asking for a mechanism, not a list. Compare these two answers to “Explain how filtrate is produced at the glomerulus (3 marks)”: Answer A: “The blood is filtered at the glomerulus and small molecules go into the Bowman's capsule.” — one mark. It says what happens twice and never says why. Answer B: “Blood in the glomerulus is under high pressure, so small molecules such as water, glucose, urea and ions are forced out through the capillary wall into the Bowman's capsule; blood cells and large plasma proteins are too large to pass through, so they stay in the blood.” — three marks.
- A one-mark question worth rehearsing. “Name two waste products of metabolism and, for each, name the organ that excretes it.” The full answer is: carbon dioxide, excreted by the lungs; urea, excreted by the kidneys. Do not write “urea, excreted by the liver” — the liver produces it. Do not offer faeces as a third example.
Questions students ask about Urea
What is excretion in biology?
Excretion is the removal from organisms of toxic materials, the waste products of metabolism, and substances in excess of requirements. In humans the two required examples are carbon dioxide, made by respiration in every cell and excreted by the lungs, and urea, made in the liver from excess amino acids and excreted by the kidneys in urine.
Where is urea made, and where is it removed?
Urea is made in the liver, by deamination of excess amino acids. It is removed from the blood by the kidneys and leaves the body dissolved in urine. The two organs must not be interchanged: the liver never excretes urea and the kidney never manufactures it.
What is deamination?
Deamination is the removal of the nitrogen-containing part of amino acids, which takes place in the liver and results in the formation of urea. It is necessary because excess amino acids cannot be stored as amino acids. The remaining carbon-containing part is used in other reactions of the body.
How does filtration work in the kidney?
Blood in the glomerulus is under high pressure, and that pressure forces small molecules through the capillary wall and the wall of the Bowman's capsule. Water, glucose, urea and ions pass through; blood cells and large plasma proteins are too large and remain in the blood. Filtration separates by size alone, so it is not selective.
What is selective reabsorption?
Selective reabsorption is the return of useful substances from the nephron tubule to the blood in the surrounding capillaries. All of the glucose, some of the ions and most of the water are reabsorbed. Urea is not reabsorbed and stays in the tubule.
What is the difference between filtrate and urine?
Filtrate is the fluid in the Bowman's capsule immediately after filtration; it contains water, glucose, urea and ions. Urine is what remains after selective reabsorption has returned all the glucose, some ions and most water to the blood, so urine contains urea, excess water and excess ions but no glucose.
Why must urea be excreted?
Urea is toxic at high concentration and is produced continuously by the liver, so if it were not removed its concentration in the blood would keep rising and would disrupt the normal functioning of the body.
Does the kidney excrete carbon dioxide?
No. Carbon dioxide is excreted by the lungs, in expired air. The kidneys excrete urea, excess water and excess ions in urine.