Water, Electrolyte, and pH Balance It’s all a balancing act

Where do we keep our body fluids? “Compartments” Intracellular fluid Inside the cells 2/3 of body fluid Extracellular fluid Interstitial fluid Plasma Intracellular fluid has an exacting composition Plasma has an exacting composition Interstitial fluid is the “slosh” compartment that lends to both as needed

Premises of Fluid and Electrolyte Regulation Changes in extracellular fluid are monitored not intracellular fluid Homeostatic response mechanisms are for the big picture not the individual cell Fluid and electrolyte balance is not directly monitored, just hydrostatic and osmotic pressures Water cannot be moved by active transport Water can follow ions, remember the kidney If gains exceed losses, you will gain water weight and visa versa

Regulatory Hormones Antidiuretic hormone Aldosterone Produced by hypothalamus when it detects increase in osmolarity Causes kidneys to concentrate urine Stimulates thirst and desire for salty food Aldosterone Released by adrenal cortex when juxtaglomerular apparatus releases renin or when decreased levels of sodium and potassium are detected Causes increased reabsorption of water

Regulatory Hormones cont. Atrial naturitic peptide Brain naturitic peptide Both released when increase in blood pressure (stretch of right atrium or vessels in brain) is noted Reduce thirst Block ADH and aldosterone production

Fluid and electrolyte balance are linked When water is up, electrolyte concentrations are down When water is down, electrolyte concentrations are up

Water Movement Water moves freely into and out of the ECF compartment Peritoneal cavity Ventricles of brain Gains Metabolic generation of water Absorption from digestive tract Losses Urine Respiration Feces Sweat

Remember Hypertonic and Hypotonic? When the ECF gaines water it becomes hypotonic to the ICF Which direction will the water flow? When the ECF loses water it becomes hypertonic to the ICF

Remember Hypertonic and Hypotonic? Cont. What can happen to ECF then ICF volumes during dehydration? What can happen to ECF then ICF volumes during over hydration? What does this mean for the burn victim? What does this mean for the dialysis patient?

Remember – Water follows sodium Increase in sodium Osmoreceptors stimulated Increases ADH Increase thirst Decrease urine Increase ECF Dilution of sodium Homeostasis restored Decrease in sodium Osmoreceptors inhibited Decrease ADH Decrease thirst Increase urine production Concentration of sodium Homeostasis restored

Combined Water/Sodium Regulation ECF increase Inc blood volume Atrial stretch Release of atrial naturitic peptide Decrease ADH, aldosterone, thirst Decrease water intake, increase urination, increase sodium loss Dec blood volume

Combined Water/Sodium Regulation cont. ECF decreased Dec blood volume/blood pressure Inc renin, angiontensin Inc aldosterone and ADH release Inc thirst Dec urine production Inc sodium retention Inc blood volume/blood pressure

Other Electrolytes Potassium Calcium Phosphate Kidneys regulate When pH falls so does potassium secretion When sodium retained potassium is secreted Calcium Calcitonin Calcitriol (Vitamin D) Parathyroid hormone Phosphate Calcitriol stimulates reabsorption along proximal convoluted tubule

Maintaining Proper pH What is an acid? What is a salt? H+ donor What is a salt? An acid that has given up its H+ and taken up a metal ion HCl  H+ + Cl- Cl- + Na+  NaCl What is a buffer? A weak acid and its salt that give up or take up H+ as needed and prevent wild swings in pH

Maintaining Proper pH cont. Types of acids Volatile Can come out of solution and enter atmosphere Carbonic – H2CO3 Fixed Stay in solution Sulfuric and phosphoric Organic Participate in or produced by glucose metabolism Acetyl-CoA Lactic acid

Maintaining Proper pH cont. Protein Buffer System C R OH O H N If pH drops, extra H+ can bind here If pH increases, H+ can leave here

Maintaining Proper pH cont. Bicarbonate Buffer System CO2 + H2O  H2CO3  H+ + HCO3- H+ + HCO3- + NaCl  HCl + NaHCO3

Maintaining Proper pH cont. Phosphate Buffer System H3PO4-  H+ + HPO42-

Maintaining Proper pH cont. Respiratory Compensation When pH drops respiration increases When pH rises respiration decreases Renal compensation Regulation of H+ and HCO3- secretion and reabsorption

Acid/Base Imbalance Disorders Acidosis Respiratory Alveolar respiration can’t keep up Metabolic Too much lactic acid Ketone bodies from alcoholism Too much aspirin Alkalosis Hyperventilation Rare – overuse of bicarbonates