🏜 Blood too concentrated
You have lost water, for example by sweating on a hot day. The pituitary gland releases more ADH. The collecting duct becomes more permeable. More water is reabsorbed. You make a small volume of concentrated urine.
Excretion » Water Reabsorption, ADH and Urine
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.76, 2.78, 2.79
- How water is reabsorbed into the blood from the collecting duct
- What ADH is and how it controls the water content of the blood
- Why you make a little concentrated urine or a lot of dilute urine
- What urine contains
Note: this lesson is Paper 2 only. It will not be asked on Paper 1.
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseWater reabsorption in the collecting duct
In the last lesson, Ultrafiltration and Glucose Reabsorption, you saw that the filtrate holds a lot of water. Most of it must go back into the blood, or you would run out of water in minutes. The final part of the nephron, the collecting duct, has a very important job: it decides how much water is taken back.
Key terms:
- ADH: a hormone that controls how much water is reabsorbed from the collecting duct.
- Pituitary gland: a small gland at the base of the brain that releases ADH into the blood.
- Dilute: having a lot of water compared with the substances dissolved in it.
- Concentrated: having only a little water compared with the substances dissolved in it.
Water leaves the collecting duct and goes back into the blood by osmosis. Remember, osmosis is the net movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane. The tissue around the collecting duct is very salty, so water moves out of the duct and into the blood capillaries nearby. The wall of the duct can let a lot of water through, a little water through, or almost none. That is how the body adjusts the amount of water it keeps.
Think of a tap
Picture the collecting duct wall as a tap on a pipe. Open the tap wide and lots of water flows through to the blood. Turn the tap down and most of the water stays in the duct and leaves the body as urine. The hormone ADH is the hand that turns the tap.
ADH controls the water content of the blood
The body must keep the water content of the blood steady. This is part of homeostasis, which is keeping conditions inside the body steady. ADH is the hormone that does this job for water.
Here is what ADH does, step by step.
- ADH is made and released into the blood by the pituitary gland.
- ADH travels in the blood to the kidneys.
- ADH makes the wall of the collecting duct more permeable to water.
- More water can pass out of the duct by osmosis and back into the blood.
The pituitary gland releases more ADH when the blood is too concentrated, which means it has too little water. It releases less ADH when the blood has too much water.
💧 Blood too dilute
You have drunk a lot of water. The pituitary gland releases less ADH. The collecting duct becomes less permeable. Less water is reabsorbed. You make a large volume of dilute urine.
Notice that this is a loop. A change in the blood causes a response that puts the blood back to normal. When the water content returns to normal, the amount of ADH returns to normal too. This is why ADH is an example of homeostasis in action.
Memory trick
More ADH means more water saved and less urine. Less ADH means less water saved and more urine.
What does urine contain?
By the time the liquid leaves the collecting duct, it is urine. The spec says you must know three things that urine contains.
- Water: the amount left over after reabsorption. This changes from person to person and day to day.
- Urea: a waste product made in the liver from the breakdown of excess amino acids. It is filtered out of the blood and is not reabsorbed.
- Ions: such as sodium ions. Extra ions that the body does not need leave in the urine.
A healthy person's urine contains no glucose, because all of it was reabsorbed earlier. The urine is then carried on to the bladder, as you saw in The Urinary System and Kidney Roles.
Worked example
Question: A runner loses a lot of water as sweat during a race. Explain why the runner makes only a small volume of urine.
Answer: Sweating makes the blood more concentrated, because it has lost water. The pituitary gland releases more ADH. ADH makes the collecting duct more permeable to water. More water is reabsorbed into the blood by osmosis. So less water is left in the urine, and the volume is small and concentrated.
Common mistakes
Saying that ADH is made by the kidney. It is made by the pituitary gland, and it acts on the kidney. Another slip is forgetting to say that it is the collecting duct wall that becomes more permeable. Also, do not mix up the amounts: more ADH means more water reabsorbed, so less urine. Finally, do not say the urine is "more watery" when ADH is high. It is more concentrated.
Exam-style question
(a) Name the gland that releases ADH. [1 mark]
(b) A student drinks a large volume of water. Describe what happens to the amount of ADH released and explain the effect on the urine. [3 marks]
(c) State two substances, other than water, that are found in urine. [2 marks]
Model answer
(a) The pituitary gland. (1)
(b) Less ADH is released. (1) The collecting duct becomes less permeable to water, so less water is reabsorbed into the blood. (1) A large volume of dilute urine is made. (1)
(c) Urea (1) and ions (1).
Exam tip
For any ADH question, write the chain in order: the state of the blood, the amount of ADH, the permeability of the collecting duct, the water reabsorbed, then the urine. Each link is often a mark.