This chapter vaguely recalls Section I1(i) of the 2023 CICM Primary Syllabus, which expects the exam candidates to "explain the composition, distribution and movement of body fluids". It is also probably related to the old SectionG5(iv), "explain the humoral regulation of blood volume and flow". The author, bound by his unexplainable attachment to the CICM syllabus document, found it rather difficult to navigate the narrow channel between "humoral regulation of blood volume" and "distribution and movement of body fluids", as both will inevitably involve some of the same physiological systems.
The main problem of trying to write about this was that the volume of the extracellular fluid is controlled by two parallel and interdependent regulatory pathways: the regulation of body water (by vasopressin and thirst) and the regulation of osmotically active solute in the extracellular fluid (which is mainly sodium, and mainly mediated by the humoral factors like renin angiotensin aldosterone and the natriuretic peptides). Both are necessary for the maintenance of normal extracellular fluid volume and tonicity, and both respond to changes in both volume and tonicity, and also each can activate the other. The problem is that most textbooks divide these into "the defence of volume" and "the defence of tonicity", as if these are somehow completely separate. In defense of this strategy, neurohormonally mediated changes in volume do not necessarily produce changes in tonicity, as the retained fluid should generally be isotonic, whereas changes in tonicity usually do produce changes in volume because they involve the manipulation of water. On this shaky premise, what follows will focus mainly on the defence of volume, leaving discussions of tonicity to the ridiculously apocryphal chapter dealing with the osmoregulatory role of vasopressin.
So as not to reproduce content already available elsewhere in this site, the short summary offered here is linked to relevant pages.
- The volume of the extracellular fluid is mainly dependent on two factors:
- Total body water content, which is carefully regulated mainly by vasopressin, by adjusting thirst and the renal handling of water
- Osmotic forces, i.e. how much of the total body water is osmotically drawn into the extracellular fluid, which is:
- the function of the osmotically active content of the extracellular fluid (mainly sodium and chloride)
- mainly regulated by neurohumoral mechanisms that control the renal handling of sodium in response to changes in the function of the circulatory system:
- The shape, size and pressure of the actual extracellular compartment is largely irrelevant, as the fluid it in is incompressible, and animal cells cannot sustain pressure gradients because they lack a cell wall.
- Both systems affect each other's function, and are complimentary:
- Vasopressin-mediated retention of water to restore normal tonicity in a hypertonic state also restores volume, reducing RAAS activity.
- RAAS-mediated changes in sodium handing by the kidney produce a reabsorption of sodium, increasing volume and inhibiting vasopressin secretion.
- Conflict between these systems is resolved in favour of volume:
- Where the tonicity of the extracellular fluid is low, but the volume is also low, vasopressin secretion is sustained even though it may lead to a decrease in tonicity by the retention of water.
Alternatively, in a sensor-controller-effector matrix beloved by college examiners:
Why only this abbreviated point form, and not a 10,000-word monograph? Gauer et al (1970) said it best when they started their paper with the words,
"A complete review of the above topic would be formidable indeed since it would comprise a good part of circulatory physiology, plus the physiology of salt and water balance and, last but not least, the physiology of fluid exchange through cell membranes."
They went on to complain that they were "encouraged by the editors" to keep their word count to some manageable minimum, and still ended up publishing something that spanned across forty-eight pages of the Annual Reviews of Physiology. Looking at the paper, it seems the authors' error was to reach across fluid physiology into circulatory control (i.e. they expanded on the role of the neurohormonal mechanisms that control blood volume distribution and blood flow), which did critical hubris damage to their word count.