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Metabolism 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

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2. Special pathophysiology – disturbances of intravascular volume and tonicity Etiopathogenesis of individual disorders Edematous conditions Disturbing factors in the relationship P Na - osmolality – tonicity

1 1. Physiology and general pathophysiology of water and electrolytes Compartements of body fluids (Fig. 1) ECS 1/3 ICS 2/3 cytopl.

membrane ISS 3/4 IVS 1/4 endothe lium TRANSCEL LULAR SPACE epithelium

A quantity of (micromolecular) osmolytes in a compar tment

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a volume of the liquid in it (Fig. 2) 27 L IC 2 10 L IS 3 L IV 0.9 3 9 Osm

Note: Normal plasma Na concentrations

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roughly normal plasma osmolality

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normal electrolyte content in the cells (when they should be isoosmolal with plasma)

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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

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a small percentual derangement has large consequences for the whole-body water and electrolyte balance

Regulation of volume and tonicity (osmolality) Water

: about l,5 L is exchanged compulsatorily 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 ultimately regulated by water, the circulating volume by sodium

(Fig. 3)

3

Regarding adiuretine and thirst regulation: osmoreception (feedback No. 3) is functioning more sensitively, volumoreception (feedback No. 1) more sluggish, later 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.

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5 Sodium: (Fig. 5)

The sodium appetite is rudimentary in Man, Na regulation is mediated by Na urinary excretion exclusively (feedback No. 2). Three ways:

- GFR

  Na filtration - aldosterone: the renin secretion is a consequence both of sympathetic activity and of declined

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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 solute´s reabsorption 

pressure diuresis

The three feedback circuits regulating tonicity and volume work in concert of course 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. 3, of course.

6 Combinations of volume and osmolality disorders in the extra cellular space

9 combinations are possible theoretically (Fig. 6)

Tonicity disorders

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disorders of water: states 1, 4, 6, 9

Volume disorders

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sodium disorders: states 2, 3, 8, 7

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Fi g. 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 and 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.

8a 8b