Chapter 18

Water, electrolyte,and acid-base balance help maintain homeostasis Electrolyte – molecules that release ions in water Water and electrolytes lost must equal water and electrolytes gained Body requires mechanisms that: Replace lost water and electrolytes Excrete waste Electrolytes are dissolved in water, making their concentrations interdependent

Water Balance Exists when total water intake equals total water output Average adult takes in ~2.5-L of water a day 60% from liquid refreshments (milk) 30% from moist foods (chocolate cake) 10% from by-product of metabolism

Water Balance Thirst regulates water intake Osmotic pressure of extracellular fluid stimulates osmoreceptors in the hypothalamus Mechanism triggered when water concentration of body decreases by as little as 1.0% Drinking water stretches the stomach wall triggering nerve impulses that inhibit the thirst mechanism Drinking stops before swallowed water is absorbed Prevents “over-hydration”

Water Balance Water enters through the mouth Exits through urine, feces, sweat, evaporation from skin and lungs during breathing 2.5-L in 2.5-L out 60% lost by urine 6% lost by feces 6% lost by sweat 28% lost by evaporation from skin and lungs Vary with temperature, humidity and exercise

Water Balance Primary controlling mechanism is urine Nephrons in collecting ducts of the kidneys regulate the volume of water excreted in urine Antidiuretic Hormone (ADH) Increases permeability of collecting ducts This increases water reabsorption and decreases urine output Diuretics Substances that promote urine production Caffeine in tea and coffee Alcohol Drugs used clinically to reduce volume of body fluids

Electrolyte Balance Exists when the quantities of electrolytes the body gains equal those lost Electrolytes dissociate to release ions: Sodium Potassium Calcium Magnesium Chloride Sulfate Phosphate Bicarbonate Hydrogen

Electrolyte Balance Food provides most of our electrolytes Salt Craving – result of electrolyte deficiency, produces strong desire to eat salty foods Electrolytes may lost by sweating, feces and urine output Precise concentrations of Na +, K +, and Ca 2+ are required for nerve impulse conduction and muscle fiber contraction Aldosterone and the kidneys regulates Na + and K + concentrations Parathyroid hormone regulates Ca 2+ concentrations

Acid-Base Balance Acid – electrolytes that dissociate in water and release hydrogen ions Base – electrolytes that release ions that combine with hydrogen ions Sources of hydrogen ions Aerobic respiration of glucose CO 2 diffuses out of cells and reacts with H 2 O in extracellular fluids to produce carbonic acid (H 2 CO 3 ) Carbonic acid ionizes to produce hydrogen ions (H + ) and bicarbonate ions (HCO 3 - ) H 2 CO 3 → H + + HCO 3 -

Acid-Base Balance Sources of hydrogen ions Anaerobic respiration of glucose Anaerobically metabolized glucose produces lactic acid, which adds hydrogen ions to body fluids Incomplete oxidation of fatty acids Produces acidic ketone bodies, which increase hydrogen ion concentration Oxidation of sulfur-containing amino acids Yields sulfuric acid (H 2 SO 4 ), ionizes to release hydrogen ions Breakdown of phosphoproteins and nucleic acids Oxidation of these compounds prod. phosphoric acid (H 3 PO 4 ) Ionizes to release hydrogen ions

Strong Acid Dissociate to release hydrogen ions more completely HCl found in gastric juices Weak Acid Dissociate to release hydrogen ions less completely H 2 CO 3 produced when carbon dioxide reacts with water Sources of Bases Produce hydroxide ions when dissolved in water NaOH, NaHCO 3 Strong bases dissociate to release more OH - ions

Acid-Base buffer systems Combine with strong acid/base to produce weak acid/base Minimize pH changes to body fluids Bicarbonate buffer system HCO 3 - combines with H + ions to produce H 2 CO 3 Phosphate buffer system During acid conditions H + + HPO 4 2- → H 2 PO 4 - During alkaline conditions H 2 PO 4 - → H + + HPO 4 2- Protein buffer system Amino groups (-NH 2 ) accepts hydrogen ions when the pH of a solution falls: -NH 2 + H + → NH 3 + Carboxyl groups (-COOH) ionize to release hydrogen ions when the pH of a solution rises: -COOH → -COO - + H +

The Medullary Respiratory Center Located in the brainstem Regulates concentration of hydrogen ions in body fluids by controlling the rate and depth of breathing The Kidneys Nephrons help regulate the hydrogen ion concentration of body fluids by excreting hydrogen ions in urine

Buffer systems maintain hydrogen ion concentrations of body fluids within very narrow pH ranges Arterial blood: Values below 7.35 produce Acidosis Values above 7.45 produce Alkalosis pH values below 6.8 or above 8.0 are not survivable

Acidosis Results from an accumulation of acids or loss of bases Two major types Respiratory Acidosis Symptoms: Drowsiness, disorientation, stupor, labored breathing, and cyanosis Causes: Injury to the respiratory center of the brain stem Obstruction of air passages Diseases that decrease gas exchange (pneumonia) or decrease surface area of the respiratory membrane (emphysema) In uncompensated acidosis patient may become comatose and die

Acidosis Metabolic acidosis Due to accumulation of non-respiratory acids or loss of bases Causes: Kidney disease Diabetes mellitus Prolonged vomiting with loss of alkaline contents Prolonged diarrhea with loss of excess alkaline intestinal secretions

Alkalosis Results from a loss of acids or accumulation of bases Two major types Respiratory alkalosis Results from hyperventilation in which too much CO 2 is lost Decreasing CO 2 decreases carbonic acid and hydrogen ion concentration Causes of hyperventilation: Anxiety, high fever, poisoning from salicylates and high altitude Symptoms: Lightheadedness, agitation, dizziness and tingling sensations

Alkalosis Metabolic alkalosis Causes: Great losses of hydrogen ions or from a gain in bases Ingesting to much antacid (sodium bicarbonate) Symptoms: Decreased breathing rate and depth, increased blood carbon dioxide concentration Alkalemia – increase in blood pH