Download Physiology د. نصير جواد المختار Lecture VIII: Regulation of ECF

‫ نصير جواد المختار‬.‫د‬
Physiology
Lecture VIII: Regulation of ECF Volume and Osmolarity
Na+ and Cl- are most abundant ions in ECF consequently. Increase
NaCl content of ECF causes changes in ECF volume.
Two control system operate in association to regulate ECF, Na +
concentration and osmolarity.
1. Osmosodium Receptor (ADH feedback system):
Osmoreceptor located in supraoptic nuclei of hypothalamus are
sensitive to slight increase in osmolarity of interstitium. Thus when there
is low osmolarity (low solute content). There will be osmosis of water
into these receptor causing their swelling, decrease rate of ??????
discharge causing posterior pituitary to stop releasing ADH eventually
causing decrease permeability of distal tubule, collecting tubule,
collecting duct → decrease water reabsorption, increase secretion of
diluted urine and reverse is true.
2. Thirst:
Any factor cause increase dehydration causes thirst sensation, most
common cause increase osmolar concentration of ECF which causes
osmosis of fluid from neural cells of thirst center.
3. Atrial Natriuretic Factor (Peptide):
Hormone secreted by right atrium whenever blood pressure in right
atrium increase. It exert hormonal influence on kidney. Its release causes:
1. Inhibition of rennin and aldosteron secretion.
2. Dilate arteries and veins, reducing peripheral vascular resistance,
lower blood pressure.
3. Inhibition of ADH secretion, lead to production of large volume of
diluted urine.
Regulation of K+:
Extracellular K+ concentration normally regulated precisely at (4.2
meq/L), the difficulty in regulating EC K+ concentration because of more
than 98% in intracellular, maintains of K+ balance depends on excretion
by kidney as amount of K+ excreted (5-10%).
K+ excretion by kidney is determined by sum of 3 renal process:
1. Rate of K+ filtration.
2. Rate of K+ reabsorption.
3. Rate of K+ secretion by distal tubule.
Most of daily variation of K+ excretion occur by increase of K+
secretion in distal tubules.
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‫ نصير جواد المختار‬.‫د‬
Physiology
Cells secrete K+ (Distal CT) called (Principal cells) and secretion
of K+ is 3 step process:
1. Passive transport of K+ from inter to principal cells by Na+/K+
ATPase.
2. Passive diffusion of K+ from cells to tubular lumen.
Factors control K+ secretion:
1. Increased EC K+ secretion.
2. Increase aldosterone concentration.
3. Increase tubular flow rate.
These increase secretion of K+.
4. Increase H+ concentration lead to decrease K+ secretion
(Competition).
K+ secretion to tubular fluid of distal tubules is coupled with active
transport of Na+ by basolateral membrane of tubular epithelial cells.
Transport of Na+ from interior of cell to interstitium increase (-ve) of cell,
cause additional diffusion of (+ve) charge K+ in opposite direction.
Consequently K+ will diffuse down concentration gradient to
tubular lumen because renal interstitium is part of ECF, increase ECF K +
concentration rate of K+ secretion.
Effect of Aldosterone:
Aldosterone increase Na+ reabsorption by distal tubule and at same
time increase K+ secretion to tubule and latter effect is secondary to ?????
effect.
That is greater Na+ reabsorption, more rapid is transport of K+ to
tubules.
Second effect of aldosterone is increase permeability of luminal
membrane to K+.
Aldosterone K+ feedback control system:
Rate of aldosterone secretion by adrenal cortex is controlled to
high extent by: ECF K+ concentration.
Increase plasma K+ concentration causes increase aldosteron
secretion and concentration in blood. It causes mark increase in K +
excretion by kidneys and eventually ECF K+ concentration decreases
back towards normal.
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