💧 Too much water
The fluid around the cells is too dilute. Water enters the cells by osmosis and the cells swell.
Hormonal Coordination in Humans » Water Balance and the Kidneys
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.5.3.3
- Why water balance matters for the cells of your body
- How water, ions and urea leave the body
- How the kidneys make urine by filtration and selective reabsorption
- How to read tables and bar charts of substances before and after filtration
- Higher tier: how urea is made and how ADH controls water
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Unlock This CourseWhy water balance matters
Every cell in your body is surrounded by fluid. The concentration of that fluid must stay in the right range. If it changes, water moves into or out of your cells by osmosis. You met osmosis in the earlier lesson called Osmosis, so here we only use the idea.
If the fluid around the cells becomes too dilute, water moves into the cells. If it becomes too concentrated, water moves out of the cells. Either way, body cells that lose or gain too much water by osmosis do not function efficiently. This is why the body must keep its water level under control.
🌵 Too little water
The fluid around the cells is too concentrated. Water leaves the cells by osmosis and the cells shrink.
Key terms:
- Water balance: keeping the amount of water in the body steady, so water gained matches water lost.
- Permeable: lets substances such as water pass through easily. Higher tier only.
- Excrete: to remove waste substances from the body.
How the body loses water, ions and urea
You lose water and other substances all the time, in three main ways.
🌬 Lungs
Water leaves the body as water vapour when you breathe out. You can see this when your breath mists a cold window.
💦 Skin
Sweat contains water, ions and urea. These are lost from the skin when you sweat.
⚗ Kidneys
Excess water, ions and urea are removed by the kidneys in the urine.
There is no control over how much water, ions or urea you lose from the lungs or the skin. You cannot decide to breathe out less water vapour. So the body has to do its controlling at the kidneys. By changing how much water leaves in the urine, the kidneys balance whatever has been lost or gained elsewhere.
Think about a hot day. You lose a lot of water in sweat, so your kidneys make a small amount of urine. After you drink a big bottle of water, they make a large amount.
How the kidneys make urine
Blood flows through the kidneys all the time. The kidneys make urine in two steps.
- Filtration. Blood is filtered. Small molecules such as water, glucose, ions and urea are squeezed out of the blood into the kidney tubules. Large molecules such as proteins stay in the blood.
- Selective reabsorption. Useful substances are taken back into the blood. This includes all the glucose, some of the ions and some of the water.
What is not reabsorbed carries on and leaves as urine. Urine is a mixture of the excess water, excess ions and urea. The kidneys are very good at keeping what you need and getting rid of what you do not.
Key terms:
- Filtration: forcing small molecules out of the blood into the kidney tubules.
- Selective reabsorption: taking useful substances, such as glucose, some ions and water, back into the blood.
- Urine: the liquid made by the kidneys, containing excess water, ions and urea.
You do not need to know
The spec does not ask for the structure of the kidney, the structure of a nephron or other parts of the urinary system. Just learn the two steps and what they do.
Reading tables and bar charts
Exam questions often give you data on glucose, ions and urea in the blood, in the fluid after filtration and in the urine. You need to translate what the numbers tell you.
Worked example
The table shows example data in arbitrary units.
Glucose: blood before filtration 10, fluid after filtration 10, urine 0.
Urea: blood before filtration 3, fluid after filtration 3, urine 60.
Ions: blood before filtration 20, fluid after filtration 20, urine 15.
Reading it: the values before and after filtration are the same, because small molecules pass through the filter freely. Glucose is 0 in the urine, so all of it was reabsorbed. Urea is much higher in the urine, because it is not reabsorbed and a lot of water is. Ions are slightly lower, so some were reabsorbed.
The same method works for a bar chart. Compare the heights of the bars for each substance, and say what happened between each stage.
Higher tier: making urea and controlling water
Higher tier only
Where urea comes from. Digesting the protein in your diet can give you excess amino acids, which need to be excreted safely. In the liver these amino acids are deaminated (the amino acids are broken down) to form ammonia. Ammonia is toxic, so it is immediately converted to urea. The urea goes into the blood and is removed by the kidneys.
ADH. The water level in the body is controlled by the hormone ADH, which acts on the kidney tubules. When the blood is too concentrated, the pituitary gland releases ADH. ADH makes the kidney tubules more permeable to water, so more water is reabsorbed back into the blood. Less water is lost, and the urine is small in volume and concentrated.
When the blood is too dilute, less ADH is released. The tubules are less permeable, less water is reabsorbed, and you make a large volume of dilute urine.
This is negative feedback. As the blood water level returns to normal, the change that started the response is reversed, just as with blood glucose.
Common mistakes
Saying that glucose is lost in urine in a healthy person. It is all reabsorbed. Another mistake is saying that sweat only contains water. It also contains ions and urea. Higher tier: do not say that ADH makes the kidneys produce water. It changes how much water is reabsorbed.
Exam-style question
(a) Name two substances, other than water, that are lost from the skin in sweat. [2 marks]
(b) Higher tier only: a person's blood becomes too concentrated. Explain how the body responds to bring the water level back to normal. [3 marks]
(c) Glucose is found in the blood entering the kidney. None is found in the urine of a healthy person. Explain why. [2 marks]
Model answer
(a) Ions (1) and urea (1).
(b) Higher tier only: The pituitary gland releases more ADH (1). ADH makes the kidney tubules more permeable to water (1). More water is reabsorbed into the blood, so less water is lost in the urine (1).
(c) The glucose is filtered out of the blood into the kidney tubules (1). It is all selectively reabsorbed back into the blood (1).
Exam tip
In part (b), which is Higher tier only, name the hormone, the organ it acts on and the effect on water. Each one is a separate mark. In part (a), give two different substances, and do not write water.