Distribution of non-ionic solutes such as glucose and urea

This chapter is barely relevant to Section I1(iii) of the 2023 CICM Primary Syllabus, which expects the exam candidates to "describe the distribution, regulation and physiological importance of sodium, chloride, potassium, magnesium, calcium, and phosphate ions", which of course does not include glucose.

Total body glucose content

• Given the below concentrations in mmol/L, and knowing the total body fluid volume of a random 70kg guy, you can estimate that he will have a total of about 95mmol of glucose in the extracellular fluid.  That comes to about 17 grams, rounded up to 20 according to G.F. Cahill.
• Glucose is neither acidic nor alkaline. It does not dissociate in solution.

Total body urea content

Like bicarbonate and lactate, its hard to say exactly how much urea one has at any given moment.

• Given the below concentrations in mmol/L, and knowing the total body fluid volume of a random 70kg guy, you can estimate that he will have a total of about 168mmol of urea in his body, evenly distributed among all compartments.

Urea is neither acidic nor alkaline. It is merely two amine groups strapped to a carbonyl group.
It does not dissociate in solution.

When the body loses water, urea reabsorption is increased, and the urea concentration increases.

The reason for this seems to be an increase in the renal medulalry urea concentration, which is then reflected in the total serum urea. This effect is mediated by vasopressin-sensitive transport proteins on the endothelium of the collecting duct- by reabsorbing more urea, you set up a larger concentration gradient in the medulla, and therefore you can reabsorb more water (and make more concentrated urine).