Describe the regulation of body water.
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.
By "quantification of responses", do they mean "we want vasopressin levels in picomoles per litre"? Very well.
Extracellular tonicity is the sensed variable.
Hypothalamic osmoreceptors are the sensors
Afferent signal is via OVLT and subfornical organ nerves fibres to the hypothalamus
Another mechanism of stimulating the same system is via hypotension
Central controller/integrator is the hypothalamus
Efferent signal is vasopressin
Effector organ is the renal collecting duct
Effect on urine concentration
Another effector mechanism is thirst
Indirectly, total body water is also affected through sodium regulation by:
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.