22A08: Exam Report

Describe the regulation of body water

43% of candidates passed this question.

Better answers for this question used the “sensor, integrator/controller, effector” structure.

They also included appropriate detail relating to the site and mechanism of angiotensin II and the subsequent stimulation of ADH and aldosterone release.

A detailed description of ADH was necessary to score well. Lengthy descriptions of body water distribution or renal handling of water did not attract additional marks. Answers that scored less well were often disorganised, with limited structure and incorrect facts.

21B01: Exam Report

Describe the regulation of body water

28% of candidates passed this question.

This is a level 1 topic. An understanding as to how the body regulates water is crucial to the daily practice of critical care, this topic is well described in the major texts.

This type of question lends itself to the basic template of sensor mechanisms, central processing and integration with effector limbs and feedback loops.

However, high scoring answers require a quantification of responses and an introduction into how these processes are integrated and fine-tuned.

I1i / 21B01: Describe the regulation of body water


  • Total body water (TBW) makes up approximately 60% of total body weight e.g. 42 L in a 70 kg male.
  • TBW maintained fairly constant on day-to-day basis. TBW regulated by feedback loop:



  • Located in hypothalamus
  • Very sensitive. Can detect 1-2% changes in plasma osmolarity, from normal set point of 280 mOsmol/L.

Volume receptors (low pressure baroreceptors)

  • Stretch receptors located in walls of right atrium and great veins
  • Monitor effective intravascular volume by assessing central venous pressure
  • Less sensitive than osmoreceptors. Respond to 8-10% change in blood volume.
  • More potent response. Hypovolaemia more potent stimulus than hyperosmolarity. i.e. in hypovolaemia, release of ADH stimulated even when plasma osmolarity low.

Volume receptors (low pressure baroreceptors)

  • Located in carotid sinus and aortic arch
  • Respond to changes in mean arterial pressure
  • If pressure ↓, rate of neuronal firing from carotid sinus ↓. Results ­↑ sympathetic tone and stimulates ↑ ADH release

Central controller

Hypothalamus acts as central controller. Receives input from sensors (or contains sensors), integrates the input and initiates effector response


Hypothalamus responds to increase in plasma osmolarity or hypovolaemia by:

1) Stimulating Thirst

  • = Physiological urge to drink
  • Regulates input of water
  • Originates in thirst centre of hypothalamus
  • Stimulated by:
    • Hypertonicity
    • Hypovolaemia
    • Hypotension
    • Angiotensin II

2) Increasing ADH release

  • ↓ Water excretion by the kidney and regulates output of water
  • ADH synthesised by supraoptic and paraventricular nuclei in hypothalamus
  • Transported to posterior pituitary and stored
  • Hypothalamus stimulates posterior pituitary to secrete ADH into systemic circulation
  • Secretion stimulated by:
    1. ↑­ Plasma osmolarity
    2. Hypovolaemia
    3. Hypotension
    4. Angiotensin II
    5. Stress
    6. Certain drugs e.g. barbituates
  • Inhibited by: ↓ plasma osmolarity, ethanol, ANP, alpha adrenergic agonists
  • ADH binds to V2 receptors on collecting ducts in kidneys > leads to insertion of aquaporin water 2 channels > ↑ water reabsorption, which ↓ urine output > ↑ intravascular volume + ↓ plasma [Na+]
  • ADH short ½ life. Approx. 15 mins. Inactivated by liver + kidney.

Author: Madeline Coxwell Matthewman