ADH changes collecting duct permeability
Definition
Antidiuretic hormone (ADH)
A hormone released by the pituitary gland that increases the permeability of the collecting duct to water, so more water is reabsorbed into the blood.
- Antidiuretic hormone is released from the pituitary gland into the blood.
- Its target is the kidney where it increases the permeability of the collecting duct to water.
- A more permeable collecting duct allows more water to move by osmosis from the filtrate into the blood.
- More water reabsorption produces a smaller volume of more concentrated urine.
- Less water reabsorption produces a larger volume of more dilute urine.
The brain detects low water content
Definition
Osmoregulation
The control of the water and ion content of the body within narrow limits.
- Osmoregulation keeps the water and ion content of the body within narrow limits.
- The hypothalamus detects when the blood contains too little water and is therefore too concentrated.
- The pituitary gland responds by releasing more ADH into the blood.
- More ADH makes the collecting duct more permeable to water.
- More water returns to the blood by osmosis, so less water remains in the urine.
- The result is a small volume of concentrated urine and restoration of the blood water content.
High water content lowers ADH
Definition
Collecting duct
A kidney tubule whose permeability to water is controlled by ADH as urine passes towards the renal pelvis.
- When the blood contains too much water the hypothalamus detects that it is too dilute.
- The pituitary releases less ADH so the collecting duct becomes less permeable to water.
- Less water returns to the blood and more water remains in the filtrate.
- The result is a large volume of dilute urine and a fall in the water content of the blood towards normal.
Example
Predicting urine after exercise
- Situation: sweating during a long run removes water from the body, so the blood becomes more concentrated.
- Detection: the hypothalamus detects the low water content and the pituitary releases more ADH.
- Kidney response: the collecting duct becomes more permeable and more water is reabsorbed by osmosis.
- Prediction: the runner produces a smaller volume of more concentrated urine.
Negative feedback reverses each change
Definition
Negative feedback
A control mechanism in which a change away from the normal level causes responses that reverse the change and restore the normal level.
- Too little water causes more ADH and more water reabsorption, which reverses the original decrease.
- Too much water causes less ADH and less water reabsorption, which reverses the original increase.
- As normal water content returns the corrective change in ADH release becomes smaller.
Common Mistake
More ADH means more permeability, more water reabsorption and less water in the urine.
Urine volume and concentration move oppositely
- A small urine volume is usually more concentrated because more water has returned to the blood.
- A large urine volume is usually more dilute because less water has returned to the blood.
- Concentrated urine contains less water per quantity of dissolved waste, not more urea produced by the liver.
Exam technique
Writing the complete ADH chain
- State the water condition: identify whether the blood contains too little or too much water.
- Name the control organs: the hypothalamus detects the change and the pituitary alters ADH release.
- Link ADH to permeability: more ADH makes the collecting duct more permeable, while less ADH makes it less permeable.
- Link permeability to osmosis: a more permeable duct allows more water to return to the blood.
- Finish with both urine properties: state the correct volume and concentration.
Every direction must stay consistent
- For low blood water write more ADH, greater permeability, more reabsorption, smaller urine volume and more concentrated urine.
- For high blood water write less ADH, lower permeability, less reabsorption, larger urine volume and more dilute urine.
Self review
- Which gland releases ADH?
- How does ADH change the collecting duct?
- What happens when the blood contains too little water?
- What urine forms when ADH concentration is low?
- Why is ADH control an example of negative feedback?
