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Metabolismus of water and electrolytes
1. Physiology and general pathophysiology
Compartments of body fluids
Regulation of volume and tonicity (osmolality)
Combinations of volume and osmolality disorders in the extracellular space
2. Special pathophysiology – disturbances of intravascular volume and tonicity
Etiopathogenesis of individual disorders
Edematous conditions
Disturbing factor in the relationship PNa - osmolality – tonicity
1. Physiology and general pathophysiology of water and electrolytes
Compartments of body fluids (Fig. 1)
Fig. 2: A quantity of (micromolecular) osmolytes in a compartment  a volume of the
liquid in it
Note: Normal plasma Na concentrations  roughly normal plasma osmolality  normal
osmolality of the cells. The elektrolyte content in the cells is roughly fixed  normal volume
of liquid in the cells (IC space)
A large quantity of water is exchanged between an organismus and the environment via
kidneys and a gut  a small percentual derangement has large consequences for the wholebody water and electrolyte balance
Regulation of volume and tonicity (osmolality)
Water: about l,5 L is exchanged compulsorily per day (of that, urine volume must be 0,5 L
as a minimum), the rest of the exchange is facultative. The water balance is regulated by
intake (thirst) and by urine excretion (adiuretin)
Tonicity is regulated by water, the circulating volume by sodium ultimately (Fig. 3)
Regarding adiuretine and thirst regulation: osmoreception (feedback No. 3) is functioning
more sensitively, volumoreception (feedback No. 1) more sluggish, afterwards more
forcefully, however  „volume overrides tonicity“ when the large deviations of volume and
tonicity from a norm take place. It is a consequence of the type of dependency of the ADH
production on both these factors (Fig. 4). A circulatory failure is apparently evaluated to be
more dangerous acutely than the CNS disturbances.
Sodium: (Fig. 5)
The sodium appetite is rudimentary in Man, Na regulation is mediated by Na urinary
excretion exclusively (feedback No. 3). Three ways:
- GFR  Na filtration
- aldosterone: the renin secretion is a consequence both of sympathetic activity and of
declined pressure in the afferent arteriole
- „third factor“: a.o., changes in the hydrostatic and oncotic pressures in the peritubular
capillaries in hypervolemia and arterial hypertension  Na, water and solutes
reabsorption  pressure diuresis
The three feedback circuits regulating tonicity and volume work of course in concert and
represent a certain redundancy of the regulation. They differ, a.o., by their dynamic lag. The
definitive correction of volume is conditioned by proper functioning of the circuit No. 2, of
course.
Combinations of volume and osmolality disorders in the extracellular space
9 combinations are possible theoretically (Fig. 6)
Tonicity disorders  disorders of water: states 1, 4, 6, 9
Volume disorders  sodium disorders: states 2, 3, 8, 7
Fig. 7: Volume disorders correspond roughly to the hydratation derangements (with the
exception of generalized edemas), as water penetrates easily all boundaries between the
compartments. Tonicity is roughly the same in IC end EC space.
Paralelly with establishing these pathological conditions, transports of water (and
electrolytes) take place between the IC a EC space: the differences of osmolality equalize
quickly and buffering follows by means of electrolyte transports between IC and EC space –
volume regulation effected by the cells (Fig. 8a, b). It follows from these Fig. a.o. that the Na
concentration in ECS (and in plasma) does not reflect the total Na content in the body.