1.2.7Structure and Functions in Living Organisms: Part 2
ADH and Composition of Urine
5 min read 5 parts Lesson 16 of 29 Unit quiz
Section 1 of 5Composition of Urine
Urine is what remains after filtration and reabsorption, so it contains wastes and excess water rather than useful nutrients.
Water
- Most of urine is water, and the volume lost depends on how much water the body needs to keep.
Urea
- This waste product from the breakdown of excess amino acids leaves the body dissolved in the urine.
Ions
- Excess ions, such as sodium and chloride, are removed in the urine in amounts that vary with the diet.
Absent substances
- Glucose, proteins and blood cells are not present in the urine of a healthy person.
Section 2 of 5ADH Action
When the blood loses water, for example during sweating, the body saves water as follows.
1
The water content of the blood falls, so the blood becomes more concentrated.
2
Osmoreceptors in the hypothalamus detect the change and signal the pituitary gland.
3
The pituitary gland releases more antidiuretic hormone (ADH) into the blood.
4
ADH makes the walls of the collecting ducts more permeable to water, so more water is reabsorbed by osmosis.
5
A small volume of concentrated urine is produced, and the water content of the blood returns to normal.
Section 3 of 5Too Much Water
The same system works in reverse after a large drink.
Too Much Water
Detection
- The osmoreceptors detect blood that is too dilute because it contains too much water.
Less ADH
- The pituitary gland releases less ADH into the blood.
Less permeable
- The collecting ducts become less permeable, so less water is taken back into the blood.
Result
- A large volume of dilute urine is produced and the blood returns to its normal water content.
| Condition | ADH released | Collecting duct permeability | Urine |
|---|---|---|---|
| Blood too concentrated | More | Higher | Small volume, concentrated |
| Blood too dilute | Less | Lower | Large volume, dilute |
Section 4 of 5Negative Feedback
Negative Feedback
Stimulus
- A change in the water content of the blood away from its normal level starts the response.
Receptor
- Osmoreceptors in the hypothalamus are sensitive to the solute concentration of the blood.
Effector
- The collecting ducts of the kidney change how much water they reabsorb under the action of ADH.
Outcome
- The change is reversed, which keeps the water content of the blood within narrow limits.
Section 5 of 5Exam Checklist & Tips
1.2.7 – Key Points for the Exam
- ADH Antidiuretic hormone, released by the pituitary gland, which increases the permeability of the collecting ducts to water.
- Osmoregulation Control of the water content of the blood.
- Osmoreceptor A receptor in the hypothalamus that detects the concentration of the blood.
- Negative feedback A response that reverses a change and returns conditions to normal.
Common Mistakes
- Saying that the kidney releases ADH; it is released by the pituitary gland.
- Reversing the effect, since more ADH gives less urine that is more concentrated.
- Saying that ADH changes the volume of water filtered in the glomerulus; it acts on the collecting ducts.
- Forgetting that the hypothalamus detects the blood concentration and does not release ADH itself.