KIN 268: Fluid, Electrolyte, and Acid-Base Balance

KIN 268

Body fluid

body substance that consists of water and dissolved solutes; 55-65% of body mass; 2/3 is inside cells, 1/3 is outside cells

Intracellular fluid

cytosol within cells

Extracellular fluid

80% interstitial fluid (lymph, cerebrospinal fluid, synovial fluid, aqueous and vitreous humours, and fluids between serous membranes [pleural, pericardial and peritoneal]) and 20% blood plasma

Plasma membrane

cell barrier; separates intracellular fluid from interstitial fluid

Blood vessel and capillary walls

divide interstitial fluid from blood plasma

Fluid balance

required amounts of water and solutes are present and proportioned among compartments

Osmosis

water movement; direction is determined by solute concentration

Starling forces

hydrostatic and osmotic forces at capillaries determine how much fluid leaves the arterial end then is reabsorbed at the venous end as blood flow to tissues

Blood hydrostatic pressure

promotes filtration; generated by heart pumping; decreases from arterial to venous end

Interstitial fluid osmotic pressure

weakly promotes filtration

Blood colloid osmotic pressure

promotes reabsorption; due to presence of plasma proteins too large to cross out of capillary

Interstitial fluid hydrostatic pressure

promotes reabsorption; close to 0 mmHg unless in a state of edema

Thirst centre

area of the hypothalamus that regulates water intake; stimulated by decreased blood volume and pressure, increased blood osmolarity, dry mouth (dehydration)

Aldosterone

responds to decreased blood pressure/Na+ deficiency in plasma

ANP

respond to increased blood volume; increases excretion of Na+ and slows renin release

Water intoxication

excess body water swells cells; person consumes water faster then kidneys can secrete; symptoms: mental confusion, seizures, coma, death

Electrolytes

control osmosis of water between fluid compartments; maintain acid-base balance; carry electrical current; serve as cofactors; concentration of ions in mEq/litre; inorganic compounds that dissociate into ions

Sodium

most abundant extracellular cation; used for impulse transmission in neurons, AP causing muscle contractions, fluid and electrolyte balance; controlled by aldosterone, ADH, and ANP; too low = hyponatremic, too high = hypernatremia

Chlorine

most abundant extracellular anion; helps regulate osmotic pressure; shifts between RBCs and blood plasma because of CO2; for HCl in stomach; controlled by aldosterone and ADH

Potassium

most abundant intracellular cation; involved in fluid volume, impulse conductions, repolarization during muscle contraction, regulating pH; controlled by aldosterone; too high = hyperkalemia

Bicarbonate

second most abundant extracellular anion; acid-base buffer system; reabsorbed/secreted by kidneys for acid-base balance

Calcium

most abundant mineral; structural component of bones and teeth; used for blood coagulation, neurotransmitter release, muscle tone, excitability of nerves and muscles; controlled by parathyroid hormone and calcitonin

Phosphate

appears as calcium salt; buffer system; controlled by parathyroid hormone and calcitonin

Magnesium

second most abundant intracellular cation; 54% found as salts in bone extracellular matrix; coenzyme involved in carb/protein metabolism; needed for neuromuscular function, used in myocardial function, CNS transmission, Na+ pump operation, PTH secretion; controlled by how much is excreted by kidneys, Ca+, ECF volume, PTH, pH

Acid-base balance: Buffer system

temporarily binds to H+

Protein buffer system

most abundant in intracellular fluid and blood plasms; carboxyl group dissociates to act like an acid when pH rises → dissociated H+ can now form water with OH-; amino group combines with H+ to act like a base when pH falls

Hb buffer system

CO2 enter capillaries then RBCs → forms H2CO3 → dissociate into HCO3- and H+ ions → Hb picks up free H+ ions

Carbonic acid-bicarbonate buffer system

pH falls → HCO3- picks up excess H+ → forms H2CO3→ dissociates into CO2 and H20; pH rises → H2CO3 dissociates into H+ and HCO3 ions; CO2 is a prerequisite

Phosphate buffer system

H2PO4- acts as a weak acid to form a weak base → buffers strong bases; HPO42- acts as weak base, picks up H+ ions to form a weak acid → buffer strong acids

Acid-base balance: Exhalation of CO2

exhaling CO2 → less acid production → pH rises; retaining CO2 → more acid production → pH lower in minutes; changes in ventilation and rate/depth of breathing stimulates the DRG which contracts respiratory muscles more forcefully/frequently

Acid-base balance: kidney H+ excretion

removes nonvolatile acids; some secreted H+ is buffered by HPO4-2 and NH3 which are secreted along with the H+

Acidosis

blood pH below 7.35; results in severe CNS depression by depressed synaptic transmission, disorientation, comatose, death

Alklalosis

blood pH above 7.45; results in overexcitability of CNS and PNS, impulses are conducted when not stimulated resulting in muscle spams, nervousness, death

Compensation

respiratory vs. renal; physiological response to imbalances to normalize blood pH

Renal compensation

respiratory acidosis: increases excretion of H+ and reabsorption of HCO3- → pH returns normal but P_CO2 will be high; respiratory alkalosis: decreases excretion of H+ and reabsorption of HCO3- → pH returns normal but P_CO2 will be low

Respiratory compensation

metabolic acidosis: hyperventilation increases loss of CO2 → pH returns to normal but HCO3- will be low; metabolic alkalosis: hypoventilation slows loss of CO2 → pH returns to normal but HCO3- will be high

Respiratory acidosis

blood pH drops due to excessive retention of CO2 leading to excess H2CO3

Respiratory alkalosis

blood pH rises due to excessive loss of CO2 during hyperventilation

Metabolic acidosis

arterial blood levels of HCO3- fall

Metabolic alkalosis

arterial blood levels of HCO3- rise