CoachThe osmolarity of a solution is its total concentration of dissolved particles (in osmoles per litre); it sets the osmotic pressure, for dilute solutions, which drives water across any membrane that passes water but not solute. A mammal’s body fluids sit at about — of body mass inside the cells, outside, the extracellular fluid of which a quarter is plasma — and cells swell or shrink within seconds if the outside changes, since their membranes pass water freely. An osmoconformer (most marine invertebrates, sharks) keeps its fluids at the osmolarity of the sea and regulates only their composition; an osmoregulator (vertebrates, insects, freshwater animals) holds its osmolarity constant against the environment, which costs energy in proportion to the gradient. The second task, coupled to the first, is to dispose of the nitrogen of protein breakdown: as ammonia into abundant water (fish), as urea, soluble and non-toxic but needing water to carry it (mammals), or as uric acid, a paste excreted almost dry (birds, reptiles, insects).
Examples
Example 20.9 (Two days)
A day without water: the plasma osmolarity rises from toward , vasopressin rises within minutes, the collecting ducts open to water, the urine concentrates to and falls to half a litre; the person is thirsty; the volume lost is mostly from the cells, whose water follows the extracellular salt. A litre of water drunk at once: osmolarity falls , vasopressin falls to nothing within twenty minutes, the ducts close, and over the next two hours the kidney passes a litre of urine at . A haemorrhage of a litre: pressure falls, renin and vasopressin both rise (volume overrides osmolarity), the arterioles constrict, sodium and water are kept, and over a day the plasma volume is refilled — with fluid drawn from the interstitium and then drunk. Diabetes insipidus, the absence of vasopressin or of its receptor, produces of dilute urine a day and a thirst to match; kidney failure, the absence of filtration, leaves a body that must be filtered by a machine three times a week, through a membrane whose clearance is a fraction of two kidneys’.