(3.5) Hormone controlled concentration of water in the blood and urine - ADH negative feedback mechanism

Urine volume and concentration is regulated through the same processes that regulate blood volume.

The  antidiuretic hormone (ADH), produced by the pituitary gland, increases the amount of water reabsorbed in the kidney tubules.

The concentration of urine in your body fluids is controlled by the anti-diuretic hormone (ADH) - which makes the kidney tubules more permeable - so more water can be reabsorbed into the bloodstream.

In effect, the ADH controls the permeability of the urine collecting duct in the kidney.

ADH is released into the blood by the pituitary gland in the brain.

The brain's hypothalamus monitors the water content of the blood via osmotic pressure changes - osmoregulation.

If the water content is not right, the brain instructs the pituitary gland to release or not release ADH into the bloodstream depending on how much water is needed.

The hormone ADH makes the collecting duct membranes of the nephrons more permeable so that more water can be reabsorbed into the blood.

This process is controlled by a negative feedback system so that if the water content of your body gets too high or too low the ADH mechanism is initiated to bring the water levels to normal.

 

(i) If you drink a lot of water it may dilute the blood too much. If this happens, little, if any ADH is released into the bloodstream. This results in less water being reabsorbed by the nephrons in the kidney and more water passes through the tubules to produce urine.

In passing (if you will pardon the pun), drinking alcohol inhibits the production of ADH so you produce much more urine, so heavy drinking of alcoholic beverages can produce dehydration.

 

(ii) If the blood starts to become too concentrated it makes you feel thirsty and the brain instructs the pituitary gland to release antidiuretic hormone ADH into the bloodstream. ADH stimulates the nephrons in the kidneys to reabsorb water into the blood from the urine. Reducing water loss in the urine helps to conserve water and reduce the likelihood of dehydration.

(iii) Note that the more water your kidneys reabsorb, the smaller the volume of water you excrete as urine.

However, you will still excrete all the waste products produced by the kidney, but your urine will be more concentrated and probably a darker colour than the usual pale straw colour.

If your water intake is in excess of what is need for a balanced water content, the hypothalamus detects this increase 'above normal' and instructs the pituitary gland to secrete less ADH and this instructs the kidneys to reabsorb less water. This produces a greater volume of more dilute urine - more water lost to restore the water content balance in the body.

If you take in a lot of salt, the kidney responds by excreting more salt in concentrated urine.

 

A description of a body's homeostasis negative feedback system for controlling water-urine content via ADH
(a) The negative feedback in action for too high level of water content in the body e.g. after drinking a large volume of water	(b) The negative feedback in action for too low level of water content in the body e.g. hot and thirsty due to high outside temperature or physical exercise
1. A receptor in the brain's hypothalamus detects a stimulus that the water level is too high - blood solute concentrations too low - too dilute - detection is to do with osmotic pressure (osmoregulators).	1. A receptor in the brain's hypothalamus detects a stimulus that the water level is too low - blood solute concentrations too high - too concentrated - detection is to do with osmotic pressure (osmoregulators).
2. The hypothalamus coordination centre in the brain receives and processes the stimulus information and then organises a response by the effector - pituitary gland.	2. The hypothalamus coordination centre in the brain receives and processes the stimulus information and then organises a response by the effector - pituitary gland.
3. The effector, the pituitary gland, responds by releasing none or very little ADH - so less water is reabsorbed by the blood from the kidney tubules. This results in larger volumes of dilute urine - pale yellow colour.	3. The effector, the pituitary gland, responds by releasing more ADH into the blood stream - so more water is reabsorbed into the blood from the kidney tubules . This results in a smaller volume of concentrated urine - dark yellow colour.
4. The pituitary gland will carry on producing less of the ADH - the 'water reducing' response, as long as the coordination centre is stimulated by the receptors are saying ,too much water'. Eventually the correct water balance is reached.	4. The pituitary gland will carry on producing the increased ADH 'water increasing' response as long as the coordination centre is stimulated by the receptors are saying 'too little water'. Eventually the correct water balance is reached.
5. If the pituitary effector response is more than required, and the water content becomes too low, the receptors will detect this, and the negative feedback will stimulate the effectors to increase the water level (1. - 3. on the right).	5. The pituitary effector response is more than required, and the water content becomes too high, the receptors will detect this and the negative feedback will stimulate the effectors to decrease the water level (1. - 3. on the left).
This is all automatically done by the organism's complex control systems and enables the organism e.g. your body, to maintain as near as possible the 'ideal' water content of your body.

Schematic simplified diagram of the ADH negative feedback system for controlling the water content of the body.

Its all about getting the blood plasma concentration normalised.

A simple graphical representation of the body's 'water content' homeostasis mechanism.

(iv) Ion concentration and the feedback mechanism

If the concentration of sodium ions increases in the bloodstream, the concentration of water will fall.

Therefore, the brain detects that the blood needs more water.

More ADH is released so more water will be reabsorbed from the collecting ducts and the water content of the blood will rise.

This can happen when you eat 'salty' food (NaCl salt gives sodium ions), you may feel thirsty soon after if you haven't taken in enough water to be reabsorbed.

To much salt in your bloodstream is bad for you. It draws water out of cells by osmosis and a high salt diet is associated with high blood pressure.

Most experts think we have too much salt in our diet.

 

(v) Sweating and dehydration

Losing a significant amount of water can cause dehydration.

This most likely to happen in hot weather or after vigorous exercise - both result in sweating - loss of body fluid - mainly water containing salt (sodium chloride).

As we have described already, the reduction in water content is detected by receptors in the hypothalamus (osmoregulators).

The hypothalamus registers the change and instructs the pituitary gland to release more of the hormone ADH.

This instructs the kidneys to reabsorb more water so the volume of blood increases and a smaller volume of more concentrated urine is made.

At the same time, the brain triggers feelings of thirst when you are dehydrated.

Therefore your brain makes you want to drink more and hence restore the water content balance in your body.

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points about osmoregulation

Osmoregulation: Feedback Mechanism Explained

What Is Osmoregulation?

Osmoregulation is the control of the body’s water content and solute (ion) concentration to maintain a stable internal environment. It’s a vital part of homeostasis.

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Negative Feedback in Osmoregulation

The body uses a negative feedback loop to keep water levels within a narrow range. This loop involves:

1. Receptors – Detect changes in blood water concentration.

2. Coordination Centre – The hypothalamus in the brain processes the information.

3. Effector – The pituitary gland releases or inhibits ADH (Anti-Diuretic Hormone).

4. Response – The kidneys adjust how much water is reabsorbed into the blood.

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How It Works: Two Scenarios

When Water Levels Are Too Low (Dehydration)

• Stimulus: Blood becomes more concentrated (low water).

• Receptor: Hypothalamus detects this change.

• Response:

  • Pituitary gland releases more ADH.

  • ADH makes the kidney tubules more permeable to water.

  • More water is reabsorbed into the blood.

  • Urine becomes concentrated and in small volume.

• Outcome: Blood water concentration increases back to normal.

When Water Levels Are Too High (Overhydration)

• Stimulus: Blood becomes too dilute (high water).

• Receptor: Hypothalamus detects this change.

• Response:

  • Pituitary gland releases less ADH.

  • Kidney tubules become less permeable to water.

  • Less water is reabsorbed into the blood.

  • Urine becomes dilute and in large volume.

• Outcome: Blood water concentration decreases back to normal.

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Why This Is Important in Human Biology

• Demonstrates how the endocrine system (hormones) and excretory system (kidneys) work together.

• Helps explain how the body responds to exercise, hot weather, or illness.

• Shows how negative feedback maintains internal balance.

• Links to real-life conditions like dehydration, kidney failure, and diabetes insipidus (where ADH doesn’t function properly).

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Exam Tips

• Use key terms: ADH, hypothalamus, pituitary gland, negative feedback, permeability, urine concentration.

• Be able to label diagrams of the feedback loop.

• Understand how the body responds to changing water levels.

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Keywords, phrases and learning objectives for this part on ADH and water concentration control

In the context of homeostasis be able to describe and understand the hormone controlled concentration of water in the blood and urine - osmoregulation.

Be able to interpret diagrams or descriptions involving the antidiuretic hormone ADH (from the pituitary gland) and the negative feedback mechanism for water content balance and the regulation of urine and ion level concentrations in the blood and cell fluids.

Know the hypothalamus monitors the water concentration and responds to changes brought on the body from sweating and dehydration.

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