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2. 27-2 Chapter 27 Water, Electrolytes, and Acid-Base Balance

3. 27-3 27.1 Body Fluids Intracellular fluid compartment –All fluids inside cells of body –About 40% of total body weight Extracellular fluid compartment –All fluids outside cells –About 20% of total body weight –Subcompartments Interstitial fluid and plasma; lymph, CSF, synovial fluid Primary intracellular ions, interstitial fluid ions, and plasma ions –Intracellular cation = K + –Interstitial fluid cation = Na + –Plasma cation = Na + –Intracellular anion = Phosphate –Interstitial fluid = Cl - –Plasma anion = Cl -

4. 27-4 Body Fluid Compartments Approximate Concentration of major Solutes in Body Fluids* Intracellular Fluid † Phosphate (HPO 4 2− plus HPO 4 − ) * Expressed as milliequivalents per liter (meq/l). † Data are from skeletal muscle. Interstitial FluidPlasma TABLE 27.2 Solute Cations Sodium (Na + ) Potassium (K + ) Calcium (Ca 2+ ) Magnesium (Mg 2+ ) TOTAL Anions Chloride (Cl – ) Bicarbonate (HCO 3 – ) Protein Other TOTAL 153.2 4.3 3.8 1.4 145.1 4.1 3.4 1.3 12.0 150.0 4.0 34.0 153.9 118.0 27.0 2.3 0.0 6.6 153.9200.0 90.0 54.0 40.0 12.0 4.0 200.0 162.7 6.3 17.0 2.2 25.7 111.5 162.7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. 27-5 27.2 Regulation of Body Fluid Concentration and Volume Content regulated so total volume of water in body remains constant Kidneys are primary regulators of water excretion Regulation processes –Osmosis –Osmolality –Baroreceptors –Learned behavior Sources of water –Ingestion –Cellular metabolism Routes of water loss –Urine –Evaporation Perspiration Respiratory passages –Feces

6. 27-6 Extracellular Fluid Osmolality Osmolality –Measure of water vs. solute concentration; the higher the solute concentration, the higher the osmolality –Adding or removing water from a solution changes osmolality Increased osmolality: triggers thirst and ADH secretion Decreased osmolality: inhibits thirst and ADH secretion

7. 27-7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3 1 Baroreceptors in heart 2 Juxtaglomerular apparatuses in kidney 1 The baroreceptors in the carotid sinuses and aortic arch detect reduced blood pressure, which signals the hypothalamic thirst center. 2 Simultaneously, the juxtaglomerular apparatuses detect low blood pressure, which activates the renin-angiotensin system to produce angiotensin II. Angiotensin II stimulates the hypothalamic thirst center. 3 Osmoreceptors in the hypothalamus shrink when blood osmolality goes up, triggering action potentials that stimulate thirst. 4 The combination of these inputs activates thirst and promotes water consumption. Osmoreceptors in hypothalamus (increased osmolality) 4 Increased thirst

8. 27-8 Hormonal Regulation of Blood Osmolality Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ReactionsActions Reactions Osmoreceptors in the hypothalamus control center detect the increase in blood osmolality and signal the posterior pituitary to secrete ADH, which causes thirst. ADH also increases the permeability of the distal convoluted tubule and collecting ducts to water. Water reabsorption at the distal convoluted tubule and collecting duct increases; water consumption increases. Blood osmolality increases: Homeostasis Disturbed Blood osmolality decreases: Homeostasis Restored Blood osmolality (normal range) Blood osmolality (normal range) Blood osmolality decreases: Homeostasis Disturbed Blood osmolality increases: Homeostasis Restored Osmoreceptors in the hypothalamus detect the decrease in blood osmolality and signal the posterior pituitary to reduce ADH secretion, which decreases thirst. Water reabsorption at the distal convoluted tubule and collecting duct decreases; water consumption decreases. Start here 34 Control Center Effectors Activated: Control Center Effectors Activated: 2 1 5 6

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10. 27-10 Regulation of ECF Volume ECF can increase or decrease even if osmolality of extracellular fluid is maintained Carotid sinus and aortic arch baroreceptors monitor blood pressure, juxtaglomerular apparatuses monitor pressure changes, receptors in walls of atria and large vessels respond to small changes in BP These receptors activate these mechanisms –Neural: increase in BP recognized by baroreceptors. Decreased sympathetic stimulation of afferent arteriole leads to increased pressure in glomerulus leading to increased filtration and increased urine output. –Renin-angiotensin-aldosterone –Atrial natriuretic hormone (ANH) –Antidiuretic hormone (ADH)

11. 27-11 Regulation of Blood Volume Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Actions Pituitary: Baroreceptors stimulate posterior pituitary ADH secretion when blood volume decreases. Increased ADH also increases the sensation of thirst. Increased aldosterone and decreased ANH increase Na + reabsorption in the distal convoluted tubule and the collecting duct. Less Na+ and water are excreted in the urine. Increased ADH increases the permeability of the distal convoluted tubule and the collecting duct to water. Less water is excreted in urine. Decreased renal blood flow decreases filtrate formation, and less water is excreted in urine. Decreased aldosterone and increased ANH decrease Na + reabsorption into the distal convoluted tubule and collecting duct. More Na+ in urine, which decreases blood volume Decreased ADH decreases water reabsorption by the distal convoluted tubules and collecting ducts. Less water returns to the blood, and more water is excreted in the urine. Blood vessels: Sympathetic division baroreceptors detect increased blood volume, which causes vasodilation of renal arteries. Heart: Atrial cardiac muscle cells secrete ANH when blood volume increases. Kidney: Juxtaglomerular apparatuses inhibit renin release when blood volume increases, which decreases aldoster one secretion. Pituitary: Baroreceptors inhibit posterior pituitary ADH secretion when blood volume increases. Effectors Activated: Increased renal blood flow increases the rate of filtrate formation, and more water is excreted in the urine. High blood volume induces elevated blood pressure: Homeostasis Disturbed Start here Blood volume (normal range) Blood volume (normal range) Low blood volume induces lowered blood pressure: Homeostasis Disturbed Reduced blood volume due to loss of water and Na + in the urine lowers blood pressure: Homeostasis Restored Increased blood volume due to decreased Na+ and water loss in the urine raises blood pressure: Homeostasis Restored Blood vessels: Sympathetic division baroreceptors detect decreased blood volume, which causes vasoconstriction of renal arteries. Heart: Atrial cardiac muscle cells do not secrete ANH when blood volume decreases. Kidney: Juxtaglomerular apparatuses stimulate renin release when blood volume decreases, which increases aldosterone secretion. Reactions Effectors Activated: 34 2 1 5 6

12. 27-12 Regulation of ECF Volume Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Decreased BP Kidney Increased Na + and water reabsorption results in increased BP. Increased renin secretion (from kidney) Angiotensinogen Angiotensin I Angiotensin II Increased aldosterone secretion Increased BP in right atrium ANH Kidney Increased ANH Increased Na + excretion and increased water loss result in decreased BP.

13. 27-13 Regulation of ECF Osmolality Electrolytes –Molecules or ions with an electrical charge Ingestion adds electrolytes to body Kidneys, liver, skin, lungs remove from body –Concentration changes only when growing, gaining or losing weight Na + Ions –Dominant ECF cations –Responsible for 90-95% of osmotic pressure Regulation of Na + ions –Kidneys major route of excretion –Small quantities lost in sweat {sweat = (in decreasing amounts) water, Na +, urea, Cl - K +, NH 3 }. Insensible perspiration is water evaporating from skin. Sensible perspiration is secreted by the sweat glands. Contains solutes Terms –Hypernatremia: elevated plasma Na + –Hyponatremia: decreased Na +

14. 27-14 27.3 Regulation of ICF Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 2 3 4 1 2 3 4 Large organic molecules, such as proteins, which cannot cross the plasma membrane, are synthesized inside cells and influence the concentration of solutes inside the cells. The transport of ions, such as Na +, K +, and Ca 2+, across the plasma membrane influences the concentration of ions inside and outside the cell. An electrical charge difference across the plasma membrane influences the distribution of ions inside and outside the cell. The distribution of water inside and outside the cell is determined by osmosis. Large organic molecules Extracellular fluid Intracellular fluid Water moves by osmosis. Electrical charge difference Ion transport (e.g., Na +, K +, Ca 2+ ) H2OH2O (e.g., K + )

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16. 27-16 27.4 Regulation of Specific Electrolytes Chloride ions –Predominant anions in ECF Magnesium ions –Capacity of kidney to reabsorb is limited –Excess lost in urine –Decreased extracellular magnesium results in greater degree of reabsorption Potassium ions –Maintained in narrow range –Affect resting membrane potentials –Aldosterone increases amount secreted Terms –Hyperkalemia: abnormally high levels of potassium in extracellular fluid –Hypokalemia: abnormally low levels of potassium in extracellular fluid.

17. 27-17 Table 27.5 Homeostasis: Mechanisms Regulating Blood Sodium Response to Changes in Blood Osmolality Antidiuretic hormone (ADH); the most important regulator of blood osmolality Increased blood osmolality (e.g., increased Na + concentration) Increased ADH secretion from the posterior pituitary; mediated through cells in the hypothalamus Increased water reabsorption in the kidney; production of a small volume of concentrated urine Decreased blood osmolality as reabsorbed water dilutes the blood Decreased blood osmolality (e.g., decreased Na + concentration) Decreased ADH secretion from the posterior pituitary; mediated through cells in the hypothalamus Decreased water reabsorption in the kidney; production of a large volume of dilute urine Increased blood osmolality as water is excreted from the blood into the urine Response to Changes in Blood Pressure Renin-angiotensinaldosterone hormone mechanism Decreased blood pressure in the kidney’s afferent arterioles Increased renin release from the juxtaglomerular apparatuses; renin initiates the conversion of angiotensinogen to angiotensin; angiotensin I is converted to angiotensin II, which increases aldosterone secretion from the adrenal cortex Increased Na + reabsorption in the kidney (because of increased aldosterone); increased water reabsorption as water follows the Na + ; decreased urine volume Increased blood pressure as blood volume increases because of increased water reabsorption; blood osmolality is maintained because both Na + and water are reabsorbed* Increased blood pressure in the kidney’s afferent arterioles Decreased renin release from the juxtaglomerular apparatuses, resulting in reduced formation of angiotensin I; reduced angiotensin I leads to reduced angiotensin II, which causes a decrease in aldosterone secretion from the adrenal cortex Decreased Na + reabsorption in the kidney (because of decreased aldosterone); decreased water reabsorption as less Na + is reabsorbed; increased urine volume Decreased blood pressure as blood volume decreases because water is excreted in the urine; blood osmolality is maintained because both Na + and water are excreted in the urine* Atrial natriuretic hormone (ANH) Decreased blood pressure in the atria of the heart Decreased ANH released from the atria Increased Na + reabsorption in the kidney; increased water reabsorption as water follows the Na + ; decreased urinary volume Increased blood pressure as blood volume increases because of increased water reabsorption; blood osmolality is maintained because both Na + and water are reabsorbed* Increased blood pressure in the atria of the heart Increased ANH released from the atria Decreased Na + reabsorption in the kidney; decreased water reabsorption as water is excreted with Na + in the urine; increased urinary volume Decreased blood osmolality as blood volume decreases because water is excreted in the urine; blood osmolality is maintained because both Na+ and water are excreted in the urine* ADH—activated by significant decreases in blood pressure; normally regulates blood osmolality (see above) Decreased arterial blood pressure Increased ADH secretion from the posterior pituitary; mediated through baroreceptors Increased water reabsorption in the kidney; production of a small volume of concentrated urine Increased blood pressure resulting from increased blood volume; decreased blood osmolality Increased arterial blood pressure Decreased ADH secretion from the posterior pituitary; mediated through baroreceptors Decreased water reabsorption in the kidney; production of a large volume of dilute urine Decreased blood pressure resulting from decreased blood volume; increased blood osmolality Abbreviation: A DH = antidiuretic hormone. *Assumes normal levels of A DH. MechanismStimulusResponse to StimulusEffect of ResponseResult Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

18. 27-18 Consequences of Abnormal Plasma Levels of Sodium Ions Lethargy, confusion, apprehension, seizures, and coma When accompanied by reduced blood volume: reduced blood pressure, tachycardia, and decreased urine output When accompanied by increased blood volume: weight gain, edema, and distension of veins High dietary sodium (rarely causes symptoms) Administration of hypertonic saline solutions Oversecretion of aldosterone Thirst, fever, dry mucous membranes, and restlessness Most serious symptoms are convulsions and pulmonary edema When occurring with increased water volume: weight gain, edema, elevated blood pressure, and bounding pulse TABLE 27.6 Causes Inadequatedietary intake of sodium Extrarenal losses Dilution Hyperglycemia Symptoms HYPERNATREMIA Causes Symptoms Water loss Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. HYPONATREMIA

19. 27-19 Potassium Ion Regulation in ECF Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blood K + levels increase: Homeostasis Disturbed Blood K + levels decrease: Homeostasis Disturbed Blood K + levels decrease: Homeostasis Restored Blood K + levels increase: Homeostasis Restored Decreased blood levels of K + act on the adrenal cortex to decrease aldosterone secretion Decreased aldosterone reduces the rate of K + secretion from the distal convoluted tubules and collecting ducts of the kidneys into the urine. Increased blood levels of K + act on the adrenal cortex to increase aldosterone secretion. Elevated aldosterone increases the rate of K + secretion from the distal convoluted tubules and collecting ducts of the kidney into the urine. ReactionsActions ReactionsActions Start here Blood k + (normal range) Blood k + (normal range) Control CenterEffectors Activated: 34 2 1 5 6 Control CenterEffectors Activated:

20. 27-20 Consequences of Abnormal Concentrations of Potassium Ions Loss of intracellular K + due to cell trauma or reduced permeability of plasma membrane Reduced renal excretion Increased neuromuscular irritability Intestinal cramping and diarrhea Rapid cardiac repolarization Loss of muscle tone and paralysis Reduced rate of cardiac action potential conduction TABLE 27.7 HYPOKALEMIA Causes Symptoms Causes Symptoms Mild Severe Muscle weakness HYPERKALEMIA Atrioventricular block Delayed ventricular depolarization Bradycardia Decreased smooth muscle tone Decreased neuromuscular excitability Increased renalloss Reduced K – intake Alkalosis Insulin administration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

21. 27-21 Regulation of Calcium Ions Regulated within narrow range –Elevated extracellular levels prevent membrane depolarization –Decreased levels lead to spontaneous action potential generation Terms –Hypocalcemia –Hypercalcemia PTH increases Ca 2+ extracellular levels and decreases extracellular phosphate levels Vitamin D stimulates Ca 2+ uptake in intestines Calcitonin decreases extracellular Ca 2+ levels

22. 27-22 Consequences of Abnormal Concentrations of Calcium Ions Nutritional deficiencies Vitamin D deficiency Decreased parathyroid hormone secretion Malabsorption of fats (reduces vitamin D absorption) Bone tumors that increase Ca 2+ deposition Reduced cardiac ventricular depolarization TABLE 27.8 HYPOCALCEMIA Causes Symptoms Confusion Muscles pasmsa Hyperreflexi Intestinal cramping Convulsions Tetany Inadequate respiratory movements Prolonged cardiac ventricular depolarization HYPERCALCEMIA Excessive parathyroid hormone secretion Excess vitamin D Fatigue Weakness Causes Symptoms Anorexia Lethargy Nausea Constipation Kidney stones Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

23. 27-23 Regulation of Magnesium Ions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Decreased blood Mg 2+ levels cause the kidneys to reabsorb most of the Mg 2+ from the filtrate. Less Mg 2+ enters the urine, and the blood Mg 2+ level is maintained. Increased blood Mg 2+ levels in the filtrate exceed the kidney's capacity to reabsorb Mg 2+ from the filtrate. The Mg 2+ not reabsorbed from the filtrate enter the urine. Reactions Actions Reactions Blood Mg 2+ (normal range) Blood Mg 2+ (normal range) Blood Mg 2+ levels increase: Homeostasis Disturbed Start here Blood Mg 2+ levels decrease: Homeostasis Disturbed Blood Mg 2+ levels increase: Homeostasis Restored Blood Mg 2+ levels decrease: Homeostasis Restored Control CenterEffectors Activated: Control CenterEffectors Activated: 34 2 5 61

24. 27-24 Consequences of Abnormal Concentrations of Magnesium Ions Reduced magnesium intestinal absorption Renal tubular dysfunction TABLE 27.9 HYPOMAGNESEMIA (rare) Malnutrition Alcoholism Causes Symptoms Some diuretics Irritability Muscle weakness Tetany Convulsions HYPERMAGNESEMIA (rare) Renal failure Magnesium-containingant acids Nausea Vomiting Muscle weakness Hypotension Bradycardia Reduced respiration Causes Symptoms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

25. 27-25 Regulation of Blood Phosphate ion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Decreased blood PO 4 3– levels cause the kidneys to reabsorb most of the PO 4 3– from the filtrate. Less PO 4 3– enter the urine and the blood PO 4 3– level is maintained. Increased blood PO 4 3– levels cause the PO 4 3– levels in the filtrate to exceed the kidney’s capacity to reabsorb PO 4 3– from the filtrate. The PO 4 3– not reabsorbed from the filtrate enter the urine. Reactions Actions Reactions Blood PO 4 3– (normal range) Blood PO 4 3– levels increase: Homeostasis Disturbed Start here Blood PO 4 3– levels decrease: Homeostasis Disturbed Blood PO 4 3– levels decrease: Homeostasis Disturbed Blood PO 4 3– levels increase: Homeostasis Disturbed Control CenterEffectors Activated: Control CenterEffectors Activated: 34 2 1 5 6

26. 27-26 Regulation of Phosphate Ions Under normal conditions, reabsorption of phosphate occurs at maximum rate in the nephron An increase in plasma phosphate increases amount of phosphate in nephron beyond that which can be reabsorbed; excess is lost in urine –Hypophosphatemia: reduced absorption from intestine due to vitamin D deficiency or alcohol abuse. –Hyperphosphatemia: renal failure, chemotherapy, hyperparathyroidism (secondary to elevated plasma calcium levels)

27. 27-27 Reduced intestinal absorption due to vitamin D deficiency or alcohol abuse Hyperparathyroidism (reduced renal PO 4− excretion) Formation of calcium phosphate deposits in tissues of lungs, kidneys and joints TABLE 27.10 Consequences of Abnormal Concentrations of Phosphate Ions HYPOPHOSPHATEMIA Causes Symptoms Causes Symptoms Symptoms of reduced Ca 2+ related to formation of deposits Renal failure Tissue destruction from chemotherapy Reduced blood clotting Reduced white blood cell functions Reduced oxygentransport Reduced metabolicrate Hyperparathyroidism (elevatedrenal PO 4– excretion) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. HYPERPHOSPHATEMIA

28. 27-28 Comparison of Strong and Weak Acids Hydrochloric acid HCI Hydrogen ion H+H+ Chloride ion CI – (complete dissociation) + Carbonic acid H 2 CO 3 Hydrogen ion Bicarbonate ion + Equilibrium Strong base NaOH Na + OH – Sodium hydroxideSodium ion Hydroxide ion + (partial dissociation) (complete dissociation) Strong acid Weak acid H+H+ HCO 3 – Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

29. 27-29 27.5 Regulation of Acid-Base Balance Acids –Release H + into solution Bases –Remove H + from solution Acids and bases –Grouped as strong or weak Buffers: Resist changes in pH –When H + added, buffer removes it –When H + removed, buffer replaces it Types of buffer systems –Carbonic acid/bicarbonate –Protein –Phosphate

30. 27-30 Regulation of Acid-Base Balance Blood pH increases (H + decreases): Homeostasis Disturbed Blood pH decreases (H + increases): Homeostasis Restored Kidney: The distal convoluted tubules decrease H + secretion into the urine and decrease HCO 3 – reabsorption into the blood. Lungs: The respiratory center in the brain decreases the rate and depth of breathing, which increases blood CO 2. Kidney: Fewer H + are removed from the blood, and fewer HCO 3 – are available to bind to H + Lungs: Increased blood CO 2 reacts with water to produce carbonic acid, which dissociates to increase H +. H 2 O + CO 2 H 2 CO 3 H + + HCO 3 – Lungs: The respiratory center in the brain increases the rate and depth of breathing, which decreases blood CO 2. Actions Buffers: Buffers release H +. Reactions Buffers: The number of H + in the blood increases. H 2 O + CO 2 H 2 CO 3 H + + HCO 3 – Blood pH (normal range) Start here Blood pH decreases (H + increases): Homeostasis Disturbed Blood pH decreases (H + decreases): Homeostasis Disturbed H 2 O + CO 2 H 2 CO 3 H + + HCO 3 – Buffers: Buffers bind H. Kidney: The distal convoluted tubules increase H + secretion into the urine and increase HCO 3 – reabsorption into the blood. Lungs: Decreased blood CO 2 causes H + react with HCO 3 – to form carbonic acid, which decreases H+ in blood. H 2 O + CO 2 H 2 CO 3 H + + HCO 3 – Buffers: The number of H + in the blood decreases. Reactions Kidney: Fewer H + are removed from the blood, and fewer HCO 3 – are available to bind to H + Effectors Activated: 34 2 1 5 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

31. 27-31 Regulation of Acid/Base Balance Buffers: if pH rises, buffers bind H + ; if pH falls, buffers release H + –Protein buffer: Intracellular and plasma proteins absorb H +. Provide ¾ of buffering in body. E.g., hemoglobin. –Bicarbonate buffering system: Important in plasma –Phosphate buffer system: important as an intracellular buffer Respiratory center: if pH rises, respiratory rate decreases; if pH falls, respiratory rate increases Kidneys: if pH rises, distal tubule decreases H + secretion into the urine and decreases HCO 3 - absorption into the blood (more H 2 CO 3 will dissociate into H + and HCO 3 - ); if pH falls, distal tubule increases H + secretion into the urine and increases HCO 3 - absorption into the blood

32. 27-32 Carbonic acid/ bicarbonate buffer system Components of the carbonic acid/bicarbonate buffer system are not present in high enough concentrations in the extracellular fluid to constitute a powerful buffer system. However, the concentrations of the components of the buffer system are regulated. Therefore, it plays an exceptionally important role in controlling the pH of extracellular fluid. Intracellular proteins and plasma proteins form a large pool of protein molecules that can act as buffer molecules. Because of their high concentration, they provide approximately three-fourths of the body’s buffer capacity. Hemoglobin in red blood cells is an important intracellular protein. Other intracellular molecules, such as histone proteins and nucleic acids, also act as buffers. Components of the phosphate buffer system are low in the extracellular fluids, compared with the other buffer systems, but it is an important intracellular buffer system. Protein buffer system Phosphate buffer system Characteristics of Buffer Systems TABLE 27.11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

33. 27-33 Respiratory Regulation of Acid-Base Balance Achieved through carbonic acid/bicarbonate buffer system –As carbon dioxide levels increase, pH decreases –As carbon dioxide levels decrease, pH increases –Carbon dioxide levels and pH affect respiratory centers Hypoventilation increases blood carbon dioxide levels Hyperventilation decreases blood carbon dioxide levels

34. 27-34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H 2 O + CO 2 H 2 CO 3 H + + HCO 3 – Circulation Carbonic an hydrase Capillary 1 1 2 3 2 3 Carbon dioxide reacts with H 2 O to form H 2 CO 3. An enzyme, carbonic anhydrase, found in red blood cells and on the surface of blood vessel epithelium, catalyzed the reaction. Carbonic acid dissociates to form H + and HCO 3 –. An equilibrium is quickly established. Decreased pH in the extracellular fluid stimulates the respiratory center and causes an increased rate and depth of breathing. Increased rate and depth of breathing causes CO 2 to be expelled from the lungs, thus reducing the extracellular CO 2 levels. As CO 2 levels decrease,the extracellular concentration of H + decreases, and the extracellular fluid pH increases. Decreased pH Respiratory center in brainstem Lungs Increased respiratory rate and depth Increased CO 2 expelled from the lungs

35. 27-35 Renal Regulation of Acid-Base Balance Secretion of H + into filtrate and reabsorption of HCO 3 - into ECF cause extracellular pH to increase HCO 3 - in filtrate reabsorbed Rate of H + secretion increases as body fluid pH decreases or as aldosterone levels increase Secretion of H + inhibited when urine pH falls below 4.5

36. 27-36 Renal Regulation of Acid-Base Balance CO 2 Lumen Na + 4 When the filtrate or blood pH decreases, H + combine with HCO 3 – to form carbonic acid that is converted into CO 2 and H 2 O. The CO 2 diffuses into tubule cells. In the tubule cells, CO 2 combines with H 2 O to form H 2 CO 3 that dissociates to form H + and HCO 3. An antiport mechanism secretes H + into the filtrate in exchange for Na + from the filtrate. As a result, filtrate pH decreases. Bicarbonate ions are symported with Na + into the interstitial fluid. They then diffuse into capillaries. In capillaries, HCO 3– combine with H +. This decreases the H + concentration and increases blood pH. 1 2 5 3 1 2 3 4 5 Peritubular capillary Interstitial fluid Basal membrane Tubule cell cytoplasm Apical membrane CO 2 CO 2 + H 2 O H 2 CO 3 Na + H+H+ Symport Antiport H+H+ HCO 3 – + Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. + H 2 O H 2 CO 3 H + + HCO 3 – HCO 3 – +Na +

37. 27-37 Acidosis and Alkalosis Acidosis: pH body fluids below 7.35 –Respiratory: Caused by inadequate ventilation- reduced elimination of CO 2, asthma, damage to respiratory center in brain, emphysema. –Metabolic: Results from all conditions other than respiratory that decrease pH- diarrhea, vomiting, ingesting overdose of aspirin, untreated diabetes mellitus, anaerobic respiration Alkalosis: pH body fluids above 7.45 –Respiratory: Caused by hyperventilation, high altitude (reduced partial pressure of O 2 –Metabolic: Results from all conditions other than respiratory that increase pH- severe vomiting, too much aldosterone, ingestion of substances like bicarbonate of soda. Compensatory mechanisms

38. 27-38