 2009 Cengage-Wadsworth Chapter 14 Body Fluid & Electrolyte Balance

 2009 Cengage-Wadsworth Water Distribution in the Body Intracellular compartment Extracellular compartment –Plasma (cell-free, intravascular) –Interstitial fluid (ISF) Changes during growth/aging –Total water decreases –Extracellular decreases, intracellular increases

 2009 Cengage-Wadsworth Maintenance of Fluid Balance Osmotic pressure –Theoretical osmotic pressure –Effective (actual) osmotic pressure –Osmolarity –Osmolality –Proteins confer higher osmotic pressure on plasma as compared to interstitial fluid

 2009 Cengage-Wadsworth Maintenance of Fluid Balance Filtration forces –Hydrostatic pressure (P pl ) - heart –ISF colloid osmotic pressure (II isf ) –Negative ISF hydrostatic pressure (P isf ) –Countered by reabsorption force Plasma osmotic pressure (II pl )

 2009 Cengage-Wadsworth The Kidney’s Role Anatomy of the kidney –Nephron = functional unit Bowman’s capsule –Glomerulus - capillary network Proximal convoluted tubule Loop of Henle Distal convoluted tubule Collecting duct

 2009 Cengage-Wadsworth The Kidney’s Role Glomerular filtrate Urine formation –Filtration - formation of glomerular filtrate –Reabsorption of selected filtrate substances into the blood –Secretion of materials into tubules from surrounding capillaries

 2009 Cengage-Wadsworth The Kidney’s Role Hormonal regulation –Antidiuretic hormone (vasopressin) Increases water permeability of distal convoluted tubule & collecting duct Facilitates reabsorption of water –Aldosterone Renin-angiotensin-aldosterone system Stimulates Na reabsorption  increases extracellular fluid osmolality  promotes fluid retention

 2009 Cengage-Wadsworth The Kidney’s Role –Water output area of hypothalamus Effects of food intake on fluid & electrolyte balance –Early fasting –Prolonged fasting –Refeeding

 2009 Cengage-Wadsworth Maintenance of Electrolyte Balance Sodium –99.5% reabsorbed –Active reabsorption of Na ions results in passive reabsorption of Cl ions, bicarbonate ions, & water –Na + /K + -ATPase pump –Water follows Na

 2009 Cengage-Wadsworth Maintenance of Electrolyte Balance Chloride –Concentration parallel to Na –Generally accompanies Na

 2009 Cengage-Wadsworth Maintenance of Electrolyte Balance Potassium –Enough excreted to maintain level –Mechanisms of K control: K intake causes increase of K in distal renal tubular cells - concentration gradient favors secretion of K Aldosterone - enhances K secretion Active reabsorption + secretion of protons in collecting duct - catalyzed by H + /K + -ATPase

 2009 Cengage-Wadsworth Maintenance of Electrolyte Balance Calcium & magnesium –Tubular reabsorption of Ca associated with that of Na & phosphate –Ca excreted mostly from GI tract Balance achieved via control of absorption –Mg reabsorption parallel to Ca

 2009 Cengage-Wadsworth Maintenance of Electrolyte Balance Imbalances impair body functions Anion gap –Cations (Na + ) - anions (CL - + HCO 3 - ) Sodium & blood pressure –Salt sensitivity: responders vs. nonresponders –Other cations involved (e.g. Ca, Mg, K)

 2009 Cengage-Wadsworth Acid-Base Balance: The Control of Hydrogen Ion Concentration Acidity = concentration of H + ions –pH = -log [H + ] Blood pH must remain within a narrow range to sustain life –Normal ECF pH = 7.4 Acids release H + ions (protons) –E.g. lactic acid, pyruvic acid, fatty acids, ketone bodies, carbonic acid

 2009 Cengage-Wadsworth Acid-Base Balance: The Control of Hydrogen Ion Concentration Acidosis vs. alkalosis Acid-base regulatory systems: –Buffer systems - neutralize acidic or basic compounds –Respiratory center - CO 2 –Renal regulation - acidic or alkaline urine formation

 2009 Cengage-Wadsworth Acid-Base Buffers Buffer = anything that can reversibly bind proteins –Usually weak acid & conjugate base Henderson-Hasselbalch equation –pH = pK a + log ([A - ] / [HA]) –pK a - constant for a particular acid that reflects its strength (proton- releasing tendency)

 2009 Cengage-Wadsworth Acid-Base Buffers Physiologically important buffers –Proteins Most potent, amphoteric Especially hemoglobin in blood –Bicarbonate (HCO 3 - )-carbonic acid (H 2 CO 3 ) system System used for respiratory & renal pH regulation H 2 CO 3  H + + HCO 3 -

 2009 Cengage-Wadsworth Respiratory Regulation of pH Increased plasma CO 2  more H 2 CO 3 formed –Results in fall in pH Hypoventilation increases CO 2 loss –Results in restoration of normal pH Rise in plasma pH triggers slowed respiration to increase CO 2

 2009 Cengage-Wadsworth Renal Regulation of pH Kidneys regulate by: –Controlling secretion of H + ions –Conserving/producing HCO 3 - –Synthesizing ammonia from glutamate to form ammonium ions Normal pH of urine = Directed at maintenance of normal ratio of [HCO 3 - ] to [H 2 CO 3 ]

 2009 Cengage-Wadsworth Renal Regulation of pH Alkalosis –Net increase in excretion of HCO3- Acidosis –H + secretion increases –Most filtered HCO 3 - converted to H 2 CO 3 and reabsorbed as CO 2 –Excess H + secreted in urine

 2009 Cengage-Wadsworth Perspective 14 Fluid Balance & the Thermal Stress of Exercise

 2009 Cengage-Wadsworth Fluid Balance & Thermal Stress Thermoregulation mechanisms Dangers of loss of >2% body weight via sweating Dehydration & fluid replacement Electrolyte losses in sweat Need (?) for electrolyte replacement