Cambridge O Level Biology · Syllabus 5090 · Excretion
Kidney
What is Kidney?
One of a pair of organs that remove urea, excess water and excess ions from the blood and form urine. Blood reaches each kidney through the renal artery and leaves through the renal vein. Inside the kidney, about a million microscopic nephrons filter the blood under pressure and then selectively reabsorb the substances the body needs, so that the fluid leaving the kidney contains urea together with excess water and excess ions. The kidney removes urea from the blood but does not produce it: urea is formed in the liver.
This definition is part of the Excretion chapter in Cambridge O Level Biology.
Kidney 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.
Common mistakes with Kidney
- “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.
- “Deamination happens in the kidneys.” Why wrong Deamination is a chemical process carried out by the liver. The kidney filters and reabsorbs; it does not break down amino acids. Say instead Deamination is the removal of the nitrogen-containing part of amino acids, in the liver.
- “The ureters carry urine out of the body.” Why wrong The ureters end at the bladder. Nothing leaves the body through them. Say instead The two ureters carry urine from the kidneys to the bladder; the single urethra carries it from the bladder to the outside.
- “Urine travels kidney → urethra → bladder.” Why wrong It puts the exit tube before the storage bag, which is physically impossible. Say instead Collecting duct → ureter → bladder → urethra → outside the body.
- “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 Kidney
- 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.
- Never diagnose. Data questions in this topic sometimes give the composition of a urine sample and ask what it suggests. The safe answer describes kidney function — “glucose is present, so not all of the filtered glucose has been reabsorbed” — and stops there. Naming a disease is not what is being asked and is not creditworthy at this level.
Questions students ask about Kidney
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 the difference between a ureter and the urethra?
There are two ureters, one from each kidney, and each carries urine from a kidney down to the bladder. There is one urethra, and it carries urine from the bladder to the outside of the body. The complete route is collecting duct, ureter, bladder, urethra, outside.
What are the parts of a nephron?
The Bowman's capsule, the glomerulus inside it, the nephron tubule, the loop of Henle, the collecting duct, and the blood vessels associated with them. Each kidney contains about a million nephrons, and each nephron is made of many cells rather than being a single cell.
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.
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.