Osmoreceptors (specialized neurons) in the hypothalamus monitor the osmotic pressure of the circulating blood, i.e. detect changes in solute concentration (mainly Na conc.)
If dehydration (output> input) occurs:
Osmoreceptors → excite hypothalamus → ADH release from posterior hypothalamus→ ↑ water reabsorption in kidneys
Water loss → renin production (by kidneys)→ angiotensin II → adrenal cortex → aldosterone → Na & water retention (kidneys)
Angiotensin II → stimulates thirst centers in the hypothalamus →
↓ production of saliva → ↑ sense of thirst
End result, water is reabsorbed by kidneys or taken in by person, and the blood osmotic pressure decreases back to normal level
Electrolytes
Compounds that separate in solution into + ve ions and - ve ions
cations (+ve ions) and anions (-ve ions)
Conduct electric current in solution
Important constituents of body fluid
Exist in the blood as acids, bases, and salts
Only a small % of molecules in fluids are non-electrolytes:
glucose, urea and creatinine
Functions of Electrolytes
Certain ions control the osmosis of water between body compartments
Help maintain the acid-base balance necessary for cellular activity
Carry electric current, which allows for action potentials and secretion of neurotransmitters
Several ions are cofactors needed for the optimal activity of enzymes
Sodium (Na+ ): the major +ve ion in ECF
responsible for maintaining osmotic balance & fluid volume
necessary for generation of action potentials in nerve & muscle cells
important in keeping acid-base balance
↑ ADH → ↓ Na+ blood levels by increasing water reabsorption
Aldosterone increases Na+ reabsorption in the kidneys
Potassium (K+ ): the major +ve ion in ICF
necessary for membrane potential and repolarization
involved in cellular enzymes activities
Aldosterone causes the elimination of K+ by the kidneys
Sodium-Potassium Balance
Na+ and K+ concentration in body fluids are critical for water and electrolyte balance and cell functions
Hypernatremia = excess of Na+ in body fluid
↓ ADH (diabetes insipidus), renal failure, dehydration, severe vomiting
Hyponatremia = a deficiency of Na+ in body fluids
Addison’s disease (↓ aldosterone), water intoxication, heart or kidney failure, liver cirrhosis, ph imbalance or endocrine disorders
Hyperkalemia = excess of K+ in body fluid
Acidosis, Addison’s disease kidney failure, dehydration
Hypokalemia = a deficiency of K+ in body fluids
Alkalosis, diuretics (K+ is lost with water), ph imbalance or ↑ levels of aldosterone
