🚫 Role 1: excretion
The kidneys remove urea, a waste product, from the blood. Urea would be harmful if it built up, so it leaves the body dissolved in urine.
Excretion » The Urinary System and Kidney Roles
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.72, 2.73
- The two roles of the kidney: excretion and osmoregulation
- The parts of the urinary system and the job of each part
- How the renal artery and renal vein fit in
- How to label a diagram of the urinary system
Note: this lesson is Paper 2 only. It will not be asked on Paper 1.
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Unlock This CourseWhat the kidneys do
You have two kidneys, one on each side of your body, towards the back. Blood flows through them all the time. The kidneys clean the blood and decide what to keep and what to get rid of. They have two main roles.
Key terms:
- Kidney: an organ that cleans the blood and makes urine.
- Osmoregulation: controlling the water content and the ion content of the blood.
💧 Role 2: osmoregulation
The kidneys control the amount of water and the amount of ions in the blood. Extra water and extra ions leave in urine. If the body needs them, they are kept.
Remember from the lesson Excretion in Plants and Humans that water and ions only count as excretory products when there is too much of them. Osmoregulation is how the kidneys make sure the amounts in your blood stay about right.
Two roles, one liquid
Both roles happen at the same time and both end up in the same place: urine. Urine carries the urea, the extra water and the extra ions out of the body.
The urinary system
The kidneys do not work alone. They are part of the urinary system, a set of organs and tubes that make urine, store it and remove it from the body.
Key terms:
- Ureter: a tube that carries urine from a kidney to the bladder. You have two.
- Bladder: a muscular bag that stores urine.
- Urethra: the tube that carries urine from the bladder out of the body.
🩸 Kidneys
Take urea, extra water and extra ions out of the blood and make urine.
🔄 Ureters
Carry urine from each kidney down to the bladder.
📦 Bladder
Stores urine until it is released.
The urine is then released through the urethra, which leads out of the body.
Follow the urine
Kidney, then ureter, then bladder, then urethra, then out of the body. Learn this order and you can label any diagram of the system.
Blood vessels to and from the kidney
Each kidney needs a blood supply, because the blood is what it cleans. You met these two vessels in the lesson The General Plan of the Circulation.
- The renal artery carries blood to the kidney. This blood contains urea.
- The renal vein carries blood away from the kidney. This blood has had the urea removed, and its water and ion content has been adjusted.
Be careful not to mix up the tubes. The renal artery and renal vein carry blood. The ureter carries urine. Blood goes in and out of the kidney by the renal vessels, while urine leaves by the ureter.
Labelling the urinary system
In the exam you may be given a diagram and asked to label it. Here is what you should know about each part.
| Part | What it carries or does |
|---|---|
| Renal artery | Blood with urea into the kidney |
| Renal vein | Cleaned blood out of the kidney |
| Kidney | Removes urea, controls water and ions, makes urine |
| Ureter | Urine from kidney to bladder |
| Bladder | Stores urine |
| Urethra | Urine from bladder to outside |
Worked example
Question: Urine leaves a kidney. Name the parts it passes through to leave the body.
Answer: It passes along the ureter to the bladder, where it is stored. It then leaves through the urethra.
Common mistakes
Saying the ureter carries urine out of the body. It carries urine to the bladder. The urethra carries it out. Also, do not say the kidney "makes" urea. The kidney removes urea from the blood. Finally, do not call the renal vein a tube for urine. It carries blood.
Exam-style question
(a) Name the tube that carries urine from the bladder out of the body. [1 mark]
(b) Describe the two roles of the kidney. [2 marks]
(c) The renal artery and the ureter are both connected to the kidney. Explain how the liquids they carry are different. [2 marks]
Model answer
(a) The urethra. (1)
(b) Excretion: removing urea (waste) from the blood. (1) Osmoregulation: controlling the water and ion content of the blood. (1)
(c) The renal artery carries blood (to the kidney), which contains urea. (1) The ureter carries urine (away from the kidney to the bladder). (1)
Exam tip
For "two roles" questions, use both key words, excretion and osmoregulation, and say what each removes or controls.