Question 1

College Answer

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.

Discussion

By "quantification of responses", do they mean "we want vasopressin levels in picomoles per litre"? Very well. 

Extracellular tonicity is the sensed variable.

• The normal value is about 285 mOsm/kg
• Tonicity is the main determinant ( ineffective osmoles are less stimulating)

Hypothalamic osmoreceptors are the sensors

• Located in the organum vasculosum lamina terminalis (OVLT) and the subfornical organ
• These are circumventricular organs and lack a blood-brain barrier
• These are mechanoreceptors which change their firing rate in response to osmotic swelling or shrinking

Afferent signal is via OVLT and subfornical organ nerves fibres to the hypothalamus

Another mechanism of stimulating the same system is via hypotension

• The sensed variable (blood pressure) is measured by the carotid baroreceptors
• Afferents are the carotid sinus nerve (branch of the glossopharyngeal)

Central controller/integrator is the hypothalamus

• Specifically, the hypothalamic supraoptic nucleus and periventricular nucleus
• There, the vasopressin-producing magnocellular neuron cell bodies reside

Efferent signal is vasopressin

• Vasopressin is released from nerve terminals in the posterior pituitary
• Vasopressin secretion is minimal below an osmolality of 285 mOsm/kg,
• At normal euvolemia, usually the plasma level is 3 pmol/L
• It increases markedly (twentyfold) when extracellular tonicity increases
• It increases massively (a thousand fold) in the presence of hypotension

Effector organ is the renal collecting duct

• V2 receptor binding leads to expression of apical aquaporins
• This increases the water permeability of the collecting duct cells

Effect on urine concentration

• Range is from 1200-1400 mOsm/kg to 40-50 mOsm/kg
• Maximal response occurs at a vasopressin concentration of about 5 pmol/L, corresponding to an extracellular fluid osmolality of around 290 mOsm/kg

Another effector mechanism is thirst

• Hypothalamic neurons project to the anterior cingulate gyrus and mediate thirst sensation as well as behavioural changes (drinking, water-seeking)

Indirectly, total body water is also affected through sodium regulation by:

• Renin-angiotensin system
• Aldosterone
• Natriuretic peptides

Baylis, PETER H. "Osmoregulation and control of vasopressin secretion in healthy humans." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 253.5 (1987): R671-R678.