Transcript Bez nadpisu - Masaryk University

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
• 2. Special pathophysiology – disturbances of
intravascular volume and tonicity
Etiopathogenesis of individual disorders
Edematous conditions
Disturbing factors in the relationship
PNa - osmolality – tonicity
1. Physiology and general pathophysiology
of water and electrolytes
Compartements of body fluids (Fig. 1)
ECS 1/3
ICS 2/3
cytopl.
membrane
1
ISS
3/4
TRANSCELLULAR SPACE
IVS
1/4
endothelium
epithelium
A quantity of (micromolecular) osmolytes in a compartment  a volume of the liquid in it (Fig. 2)
27 L
IC
10 L
IS
3L
IV
2
0.9
3
9
Osm
Note: Normal plasma Na concentrations 
roughly normal plasma osmolality  normal
electrolyte content in the cells (when they should
be isoosmolal with plasma)  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 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.
4
Sodium: (Fig. 5)
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
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.
Combinations of volume and osmolality disorders in the extracellular space
9 combinations are possible theoretically (Fig. 6)
6
Tonicity disorders  disorders of water:
states 1, 4, 6, 9
Volume disorders  sodium disorders:
states 2, 3, 8, 7
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 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