This chapter is loosely associated with Section I1(i) of the 2017 CICM Primary Syllabus, which asks the candidates to "explain the distribution and movement of body fluids and their measurement". The specific question of how the total body water content is controlled seems fundamental but does not appear in the syllabus, contrary to the examiners referring to it as a "Level 1 topic" in their comments to Question 1 from the second paper of 2021. It appeared again in Question 8 from the first paper of 2022. These were the only primary exam SAQs which mentioned asked the candidates to describe this essential feature of multicellular life.
To be sure, this question could be discussed from several different angles. One could talk about the movement of water between each different body fluid compartment (as this would fit into the distribution and movement part of the syllabus item), or one could discuss the mechanisms that maintain the tonicity and volume of each compartment (as they are all different), or one could focus on the actions of vasopressin (the dominant osmoregulatory hormone), or one could look pragmatically at the physiological responses to the loss or gain of water. Each would be a reasonable option for the discussion of body water behaviour, and being unable to choose between them, the author ultimately did all of the above:
However, from what the examiners wrote in response to Question 1 from the second paper of 2021, it would appear that the answer needed to be in a sensor-processor-effector format, similar to what was expected from the discussion of the humoral regulation of blood volume and flow. To satisfy this exam-centric requirement, this slightly expanded exam answer was fashioned from the rearranged bones of older chapters.
Extracellular tonicity is the sensed variable.
Hypothalamic osmoreceptors are the sensors
Afferent signal is via
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 is the reabsorption (or non-reabsorption) of urinary water
Another effector mechanism is thirst
Indirectly, total body water is also affected through sodium regulation by:
Sure, there's plenty of peer-reviewed papers out there to act as references, but literally none of them seem to set things out in this exact way, so it seemed pointless to reference them.