Cambridge O Level Biology · Syllabus 5090 · Excretion
Urine
What is Urine?
The fluid produced by the kidneys, consisting of urea together with excess water and excess ions dissolved in it. Urine is what remains in the nephron tubule after filtration has removed small molecules from the blood and selective reabsorption has returned all of the glucose, some of the ions and most of the water to the blood. Normal urine contains no glucose, no blood cells and no large plasma proteins. Urine passes from the collecting ducts out of the kidney, along a ureter to the bladder, where it is stored, and leaves the body through the urethra.
This definition is part of the Excretion chapter in Cambridge O Level Biology.
Urine 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 Urine
- “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.
- “Filtrate and urine are the same thing.” Why wrong Filtrate contains all the glucose that was filtered, plus far more water and ions. Urine contains none of that glucose. Say instead Filtrate becomes urine only after selective reabsorption has returned the useful substances to the blood.
- “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.
- “Glucose is normally present in urine.” Why wrong All of the filtered glucose is reabsorbed, so none reaches the collecting duct. Say instead Glucose is filtered but all of it is reabsorbed, so normal urine contains no glucose.
- “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 Urine
- Four words that lose marks here. “The blood is filtered into the capsule” — whole blood does not enter the nephron, only part of the plasma does. “The waste is filtered out” — glucose is filtered too, and glucose is not waste. “The proteins are filtered and then reabsorbed” — large plasma proteins never leave the blood in the first place. “Urine collects in the capsule” — it is filtrate until reabsorption has happened.
- Two absences, two different reasons. Glucose is absent from urine because it was reabsorbed. Protein is absent because it was never filtered. A question that asks you to explain both is testing whether you know that the two absences arise at completely different stages, and a single blanket answer will only score half of it.
- 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 Urine
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.
Why does normal urine contain no glucose?
Glucose is small enough to be filtered, so it does enter the nephron and is present in the filtrate. All of it is then reabsorbed into the blood by active transport as the filtrate passes along the tubule, so none of it reaches the collecting duct.
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.
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.