Fluid, Electrolyte, and Acid-Base Homeostasis A. Fluid compartments and fluid balance B. Water 1. Regulation of fluid intake (gain) 1. Regulation of fluid intake (gain) 2. Regulation of fluid output (loss) 2. Regulation of fluid output (loss) C. Electrolytes 1. Distribution 1. Distribution D. Movement of body fluids 1. Exchange between plasma and interstitial fluid 1. Exchange between plasma and interstitial fluid 2. Exchange between interstitial and intracellular fluids 2. Exchange between interstitial and intracellular fluids E. Acid-base balance 1. Buffer systems 1. Buffer systems 2. Exhalation of carbon dioxide 2. Exhalation of carbon dioxide 3. Kidney excretion of H+ 3. Kidney excretion of H+

Body Fluid Compartments 1. intracellular fluid (67%) 2. extracellular fluid (33% a. interstitial fluid (80%) a. interstitial fluid (80%) b. plasma (20%) b. plasma (20%) c. other fluids c. other fluids

Selectively permeable cell membranes separate body fluids into the fluid compartments. Which cells do this? plasma endothelium tissue cells arteriolar end filtrationreabsorption venular end interstitial fluid osmosis Fluids are in constant motion between the three compartments.

Water is the main component of all body fluids. 1. What does the term fluid balance mean? 2. What is the primary mechanism by which water moves from one compartment to the next? 3. What controls osmosis and therefore fluid balance? 4. Fluid balance, then, means water balance, but it implies electrolyte balance as well; the two are inseparable in this regard.

Water makes up % of total body weight. The exact percentage is dependent upon: Age- Age- newborn = 75%, lean adult man = 60%, old age = 45% Body Size- Body Size- obese = as little as 45%, very lean as much as 75% Gender- Gender- lean adult man = 60%, lean adult women = 50%

Water gain and loss (2,500 ml/day) 1. water gain a. preformed vs b. metabolic 2. water loss sensible vs. insensible sensible vs. insensible

Dehydration, Thirst, and Rehydration

Regulation of fluid intake (Thirst) decreased salivation decreased blood volume increased blood osmotic pressure dry mouth and throat decreased blood pressure stimulation of hypothalamic osmoreceptors conscious awareness of thirst increased water intake stimulation of tactile receptors increased angiotensin II stimulation of hypothalamic thirst center

Regulation of Fluid Output (adjusted by 3 hormones) dehydration ANTIDIURETIC HORMONE increased blood osmolarity stimulation of hypothalamic osmoreceptors rehydration secretion of ADH from posterior pituitary gland increased water reabsorption increased thirst ALDOSTERONE dehydration rehydration increased water reabsorption increased Na+ reabsorption increased aldosterone increased angiotensin II ATRIAL NATRIURETIC PEPTIDE increased blood volume decreased Na+ reabsorption decreased water reabsorption decreased blood volume stretch of right atrium secretion of ANP

Electrolytes versus Non-electrolytes Electrolytes have a greater effect on osmosis than do nonelectrolytes. Consider the following: C 6 H 12 O 6 in water = C 6 H 12 O 6 C 6 H 12 O 6 in water = C 6 H 12 O 6 NaCl in water = Na+ and Cl- NaCl in water = Na+ and Cl- CaCl 2 in water = Ca++ and Cl- and Cl- CaCl 2 in water = Ca++ and Cl- and Cl- Which of these three compounds will exert the greater effect on osmosis? Just as important, once the electrolyte dissociates, its ions can attract other ions of the opposite charge, creating an electrochemical gradient.

Electrolyte Distribution

Electrolytes serve four general functions: 1. essential minerals 2. exert a greater effect on osmosis 3. help maintain acid-base balance 4. carry electrical current

Exchange of body fluids between plasma and interstitial fluid occurs across capillary membranes in what three ways? 1. diffusion 2. vesicular transport 3. bulk flow Bulk flow is dependent on what four pressures that determine the net filtration pressure? NFP = (BHP + IFOP) - (BCOP + IFHP) NFP = (BHP + IFOP) - (BCOP + IFHP) = +10 mm Hg (net filtration) = +10 mm Hg (net filtration) = - 9 mm Hg (net reabsorption) = - 9 mm Hg (net reabsorption)

Exchange between ICF and interstitial fluid 1. based on osmosis 2. different ionic compositions 3. net flow = 0 4. Na+ and K+ most important 5. example -- decreased [Na+]

Relationship Between Sodium Balance and Water Balance intake of plain water RELATIONSHIP BETWEEN SODIUM BALANCE AND WATER BALANCE decreased interstitial sodiumdecreased interstitial osmotic pressure excessive sweating, vomiting, diarrhea net flow of water into cells from interstitial space decreased blood volume increased filtration from blood capillaries + overhydration of cells convulsions  coma decreased blood pressure DEATH circulatory shock

Acid-base balance Normal body fluid pH is This range is maintained by three major mechanisms: 1. buffer systems 2. exhalation of carbon dioxide (respiratory compensation) (respiratory compensation) 3. kidney excretion of H+ (renal compensation) (renal compensation) Metabolic acidosis vs. respiratory acidosis Metabolic alkalosis vs. respiratory alkalosis

In a buffer system, a strong acid is converted to a weaker one, using a weak base, or, a strong base is converted to a weaker one using a weak acid. The carbonic acid-bicarbonate system: H 2 CO 3 HCO H + HCO3- (bicarbonate ion acts as a weak base.) H2CO3 (carbonic acid acts as a weak acid.) H+ + HCO3- H2CO3 H+ + HCO3- H2CO3 (Used if there is an excess of H+.) (Used if there is an excess of H+.) H2CO3 H+ + HCO3- H2CO3 H+ + HCO3- (Used if there is a shortage of H+) (Used if there is a shortage of H+)

The phosphate buffer system is an important regulator of pH in the intracellular fluid. H2PO 4 - HPO H+ H2PO 4 - (dihydrogen phosphate acts as a weak acid) HPO 4 2- (monohydrogen phosphate acts as a weak base) OH- + H2PO 4 - H2O + HPO 4 2- (used as a weak acid to buffer strong bases) H+ + HPO 4 2- H2PO 4 - (used as a weak base to buffer a strong acid)

The protein buffer system is the most abundant buffer in cells and plasma. Proteins act as both acidic and basic buffers because they have a free carboxyl group and a free amine group.

Inside red blood cells, hemoglobin is an especially good buffer. In the tissues, the Bohr effect ensures that oxygen is delivered to the tissues as hydrogen ions are buffered.

Exhalation of Carbon Dioxide What would be the net effect of hyperventilation? reaction shifts to the left increased H+ used to reform carbonic acid used to reform CO2 pH increases increased CO2 lost from the body What would be the net effect of hypoventilation? reaction shifts to the right as CO2 accumulates in the body H+ accumulate in the body pH decreases CO 2 + H 2 O H 2 CO3 H+ + HCO 3 -

Kidney excretion of hydrogen ions pH is adjusted by changing the rate of H+ secretion by the renal tubules. Kidneys slow to respond to imbalances Imbalances need to last for a few days or longer More permanent

Respiratory Acidosis/Alkalosis versus Metabolic acidosis/alkalosis