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Homeostasis. Section 2: Acid-Base Balance Acid-base balance (H + production = loss) – Normal plasma pH: 7.35–7.45 – H + gains: many metabolic activities.

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Presentation on theme: "Homeostasis. Section 2: Acid-Base Balance Acid-base balance (H + production = loss) – Normal plasma pH: 7.35–7.45 – H + gains: many metabolic activities."— Presentation transcript:

1 Homeostasis

2 Section 2: Acid-Base Balance Acid-base balance (H + production = loss) – Normal plasma pH: 7.35–7.45 – H + gains: many metabolic activities produce acids CO 2 (to carbonic acid) from aerobic respiration Lactic acid from glycolysis – H + losses and storage Respiratory system eliminates CO 2 H + excretion from kidneys Buffers temporarily store H +

3 Figure 24 Section 2 1 The major factors involved in the maintenance of acid-base balance Active tissues continuously generate carbon dioxide, which in solution forms carbonic acid. Additional acids, such as lactic acid, are produced in the course of normal metabolic operations. Tissue cells Buffer Systems Normal plasma pH (7.35–7.45) Buffer systems can temporarily store H  and thereby provide short-term pH stability. The respiratory system plays a key role by eliminating carbon dioxide. The kidneys play a major role by secreting hydrogen ions into the urine and generating buffers that enter the bloodstream. The rate of excretion rises and falls as needed to maintain normal plasma pH. As a result, the normal pH of urine varies widely but averages 6.0—slightly acidic.

4 Section 2: Acid-Base Balance Classes of acids – Fixed acids Do not leave solution – Remain in body fluids until kidney excretion Examples: sulfuric and phosphoric acid – Generated during catabolism of amino acids, phospholipids, and nucleic acids – Organic acids Part of cellular metabolism – Examples: lactic acid and ketones Most metabolized rapidly so no accumulation

5 Section 2: Acid-Base Balance Classes of acids (continued) – Volatile acids Can leave body by external respiration Example: carbonic acid (H 2 CO 3 )

6 Module 24.5: Buffer systems pH imbalance – ECH pH normally between 7.35 and 7.45 Acidemia (plasma pH <7.35): acidosis (physiological state) – More common due to acid-producing metabolic activities – Effects » CNS function deteriorates, may cause coma » Cardiac contractions grow weak and irregular » Peripheral vasodilation causes BP drop Alkalemia (plasma pH >7.45): alkalosis (physiological state) – Can be dangerous but relatively rare

7 Figure

8 Figure The narrow range of normal pH of the ECF, and the conditions that result from pH shifts outside the normal range The pH of the ECF (extracellular fluid) normally ranges from 7.35 to pH When the pH of plasma falls below 7.5, acidemia exists. The physiological state that results is called acidosis. When the pH of plasma rises above 7.45, alkalemia exists. The physiological state that results is called alkalosis. Severe acidosis (pH below 7.0) can be deadly because (1) central nervous system function deteriorates, and the individual may become comatose; (2) cardiac contractions grow weak and irregular, and signs and symptoms of heart failure may develop; and (3) peripheral vasodilation produces a dramatic drop in blood pressure, potentially producing circulatory collapse. Severe alkalosis is also dangerous, but serious cases are relatively rare. Extremely acidic Extremely basic

9 Module 24.5: Buffer systems CO 2 partial pressure effects on pH – Most important factor affecting body pH – H 2 O + CO 2  H 2 CO 3  H + + HCO 3 – Reversible reaction that can buffer body pH – Adjustments in respiratory rate can affect body pH

10 Figure When carbon dioxide levels rise, more carbonic acid forms, additional hydrogen ions and bicarbonate ions are released, and the pH goes down. When the P CO 2 falls, the reaction runs in reverse, and carbonic acid dissociates into carbon dioxide and water. This removes H  ions from solution and increases the pH. If P CO 2 rises If P CO 2 falls P CO 2 40–45 mm Hg pH 7.35–7.45 The inverse relationship between the P CO 2 and pH HOMEOSTASIS H 2 O  CO 2 H 2 CO 3 H   HCO 3  H 2 CO 3 H 2 O  CO 2 P CO 2 pH

11 Module 24.5: Buffer systems Buffer – Substance that opposes changes to pH by removing or adding H + – Generally consists of: Weak acid (HY) Anion released by its dissociation (Y – ) HY  H + + Y – and H + + Y –  HY

12 Figure The reactions that occur when pH buffer systems function HY H   Y  HH H   HY HH H   Y  A buffer system in body fluids generally consists of a combination of a weak acid (HY) and the anion (Y  ) released by its dissociation. The anion functions as a weak base. In solution, molecules of the weak acid exist in equilibrium with its dissociation products. Adding H  to the solution upsets the equilibrium and results in the formation of additional molecules of the weak acid. Removing H  from the solution also upsets the equilibrium and results in the dissociation of additional molecules of HY. This releases H .

13 Module 24.5 Review a. Define acidemia and alkalemia. b. What is the most important factor affecting the pH of the ECF? c. Summarize the relationship between CO 2 levels and pH.

14 Module 24.6: Major body buffer systems Three major body buffer systems – All can only temporarily affect pH (H + not eliminated) 1.Phosphate buffer system Buffers pH of ICF and urine 2.Carbonic acid–bicarbonate buffer system Most important in ECF Fully reversible Bicarbonate reserves (from NaHCO 3 in ECF) contribute

15 Module 24.6: Major body buffer systems Three major body buffer systems (continued) 3.Protein buffer systems (in ICF and ECF) Usually operate under acid conditions (bind H + ) – Binding to carboxyl group (COOH – ) and amino group (—NH 2 ) Examples: – Hemoglobin buffer system » CO 2 + H 2 O  H 2 CO 3  HCO 3 – + Hb-H + » Only intracellular system with immediate effects – Amino acid buffers (all proteins) – Plasma proteins

16 Figure The body’s three major buffer systems Buffer Systems Intracellular fluid (ICF)Extracellular fluid (ECF) occur in Phosphate Buffer System Protein Buffer Systems Carbonic Acid– Bicarbonate Buffer System Has an important role in buffering the pH of the ICF and of urine Contribute to the regulation of pH in the ECF and ICF; interact extensively with the other two buffer systems Is most important in the ECF Hemoglobin buffer system (RBCs only) Amino acid buffers (All proteins) Plasma protein buffers

17 Figure The reactions of the carbonic acid–bicarbonate buffer system CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE Start CO 2 CO 2  H 2 O H 2 CO 3 (carbonic acid) HH  HCO 3  (bicarbonate ion) NaHCO 3 (sodium bicarbonate) HCO 3   Na  Body fluids contain a large reserve of HCO 3 , primarily in the form of dissolved molecules of the weak base sodium bicarbonate (NaHCO 3 ). This readily available supply of HCO 3  is known as the bicarbonate reserve. Addition of H  from metabolic activity The primary function of the carbonic acid–bicarbonate buffer system is to protect against the effects of the organic and fixed acids generated through metabolic activity. In effect, it takes the H  released by these acids and generates carbonic acid that dissociates into water and carbon dioxide, which can easily be eliminated at the lungs. Lungs

18 Figure The events involved in the functioning of the hemoglobin buffer system Tissue cells Plasma Lungs Red blood cells Released with exhalation CO 2 H2OH2O H 2 CO 3 HCO 3   Hb HH H   HCO 3  Hb H 2 CO 3 H2OH2O CO 2

19 Figure The mechanism by free amino acids function in protein buffer systems Start Normal pH (7.35–7.45) Increasing acidity (decreasing pH) At the normal pH of body fluids (7.35– 7.45), the carboxyl groups of most amino acids have released their hydrogen ions. If pH drops, the carboxylate ion (COO  ) and the amino group (—NH 2 ) of a free amino acid can act as weak bases and accept additional hydrogen ions, forming a carboxyl group (—COOH) and an amino ion (—NH 3  ), respectively. Many of the R-groups can also accept hydrogen ions, forming RH .

20 Module 24.6: Major body buffer systems Disorders – Metabolic acid-base disorders Production or loss of excessive amounts of fixed or organic acids Carbonic acid–bicarbonate system works to counter – Respiratory acid-base disorders Imbalance of CO 2 generation and elimination Must be corrected by depth and rate of respiration changes

21 Module 24.6 Review a. Identify the body’s three major buffer systems. b. Describe the carbonic acid–bicarbonate buffer system. c. Describe the roles of the phosphate buffer system.

22 Module 24.7: Metabolic acid-base disorders Metabolic acid-base disorders – Metabolic acidosis Develops when large numbers of H + are released by organic or fixed acids Accommodated by respiratory and renal responses – Respiratory response » Increased respiratory rate lowers P CO2 » H + + HCO 3 –  H 2 CO 3  H 2 O + CO 2 – Renal response » Occurs in PCT, DCT, and collecting system » H 2 O + CO 2  H 2 CO 3  H + + HCO 3 –  H + secreted into urine  HCO 3 – reabsorbed into ECF

23 Figure The responses to metabolic acidosis Addition of H  Start CO 2 CO 2  H 2 O H 2 CO 3 (carbonic acid) HH  HCO 3  Lungs (bicarbonate ion) HCO 3   Na  NaHCO 3 (sodium bicarbonate) Generation of HCO 3  CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE Respiratory Response to Acidosis Renal Response to Acidosis Other buffer systems absorb H  KIDNEYS Secretion of H  Increased respiratory rate lowers P CO 2, effectively converting carbonic acid molecules to water. Kidney tubules respond by (1) secreting H  ions, (2) removing CO 2, and (3) reabsorbing HCO 3  to help replenish the bicarbonate reserve.

24 Figure The activity of renal tubule cells in CO 2 removal and HCO 3  production Tubular fluid Renal tubule cells ECF HH HH HH HH Na  CO 2 HCO 3  H 2 CO 3  HCO 3  CO 2  H 2 O Cl  Carbonic anhydrase CO 2 generated by the tubule cell is added to the CO 2 diffusing into the cell from the urine and from the ECF. Steps in CO 2 removal and HCO 3  production Carbonic anhydrase converts CO 2 and water to carbonic acid, which then dissociates. The chloride ions exchanged for bicarbonate ions are excreted in the tubular fluid. Bicarbonate ions and sodium ions are transported into the ECF, adding to the bicarbonate reserve.

25 Module 24.7: Metabolic acid-base disorders Metabolic alkalosis – Develops when large numbers of H + are removed from body fluids – Rate of kidney H + secretion declines – Tubular cells do not reclaim bicarbonate – Collecting system transports bicarbonate into urine and retains acid (HCl) in ECF

26 Module 24.7: Metabolic acid-base disorders Metabolic alkalosis (continued) – Accommodated by respiratory and renal responses Respiratory response – Decreased respiratory rate raises P CO2 – H 2 O + CO 2  H 2 CO 3  H + + HCO 3 – Renal response – Occurs in PCT, DCT, and collecting system – H 2 O + CO 2  H 2 CO 3  H + + HCO 3 – » HCO 3 – secreted into urine (in exchange for Cl – ) » H + actively reabsorbed into ECF

27 Figure The responses to metabolic alkalosis Start Lungs Removal of H  CO 2  H 2 O HH  HCO 3  H 2 CO 3 (carbonic acid) HCO 3   Na  NaHCO 3 (sodium bicarbonate) (bicarbonate ion) CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE Generation of H  KIDNEYS Secretion of HCO 3  Other buffer systems release H  Respiratory Response to Alkalosis Renal Response to Alkalosis Decreased respiratory rate elevates P CO 2, effectively converting CO 2 molecules to carbonic acid. Kidney tubules respond by conserving H  ions and secreting HCO 3 .

28 Figure The events in the secretion of bicarbonate ions into the tubular fluid along the PCT, DCT, and collecting system Tubular fluid Renal tubule cells ECF H 2 CO 3  CO 2  H 2 O Carbonic anhydrase HH CO 2 HCO 3  HH CO 2 Cl  CO 2 generated by the tubule cell is added to the CO 2 diffusing into the cell from the tubular fluid and from the ECF. Carbonic anyhydrase converts CO 2 and water to carbonic acid, which then dissociates. The hydrogen ions are actively transported into the ECF, accompanied by the diffusion of chloride ions. HCO 3  is pumped into the tubular fluid in exchange for chloride ions that will diffuse into the ECF.

29 Module 24.7 Review a. Describe metabolic acidosis. b. Describe metabolic alkalosis. c. lf the kidneys are conserving HCO 3 – and eliminating H + in acidic urine, which is occurring: metabolic alkalosis or metabolic acidosis?

30 CLINICAL MODULE 24.8: Respiratory acid-base disorders Respiratory acid-base disorders – Respiratory acidosis CO 2 generation outpaces rate of CO 2 elimination at lungs Shifts bicarbonate buffer system toward generating more carbonic acid H 2 O + CO 2  H 2 CO 3  H + + HCO 3 – – HCO 3 – goes into bicarbonate reserve – H + must be neutralized by any of the buffer systems » Respiratory (increased respiratory rate) » Renal (H + secreted and HCO 3 – reabsorbed) » Proteins (bind free H + )

31 Figure The events in respiratory acidosis CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE Lungs CO 2 CO 2  H 2 O H 2 CO 2 (carbonic acid) HH  HCO 3  (bicarbonate ion) HCO 3   Na  NaHCO 3 (sodium bicarbonate) When respiratory activity does not keep pace with the rate of CO 2 generation, alveolar and plasma P CO 2 increases. This upsets the equilibrium and drives the reaction to the right, generating additional H 2 CO 3, which releases H  and lowers plasma pH. As bicarbonate ions and hydrogen ions are released through the dissociation of carbonic acid, the excess bicarbonate ions become part of the bicarbonate reserve. To limit the pH effects of respiratory acidosis, the excess H  must either be tied up by other buffer systems or excreted at the kidneys. The underlying problem, however, cannot be eliminated without an increase in the respiratory rate.

32 Figure The integrated homeostatic responses to respiratory acidosis Increased P CO 2 Elevated P CO 2 results in a fall in plasma pH Respiratory Acidosis Responses to Acidosis Combined Effects Respiratory compensation Renal compensation Decreased P CO 2 Decreased H  and increased HCO 3  Stimulation of arterial and CSF chemoreceptors results in increased respiratory rate. H  ions are secreted and HCO 3  ions are generated. Buffer systems other than the carbonic acid–bicarbonate system accept H  ions. HOMEOSTASIS DISTURBED HOMEOSTASIS RESTORED Hypoventilation causing increased P CO 2 Plasma pH returns to normal Start Normal acid- base balance HOMEOSTASIS

33 CLINICAL MODULE 24.8: Respiratory acid-base disorders Respiratory alkalosis – CO 2 elimination at lungs outpaces CO 2 generation rate – Shifts bicarbonate buffer system toward generating more carbonic acid – H + + HCO 3 –  H 2 CO 3  H 2 O + CO 2 H + removed as CO 2 exhaled and water formed – Buffer system responses – Respiratory (decreased respiratory rate) – Renal (HCO 3 – secreted and H + reabsorbed) – Proteins (release free H + )

34 Figure The events in respiratory alkalosis If respiratory activity exceeds the rate of CO 2 generation, alveolar and plasma P CO 2 decline, and this disturbs the equilibrium and drives the reactions to the left, removing H  and elevating plasma pH. CO 2 CO 2  H 2 O H 2 CO 2 (carbonic acid) HH  HCO 3  (bicarbonate ion) HCO 3   Na  NaHCO 3 (sodium bicarbonate) Lungs CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE As bicarbonate ions and hydrogen ions are removed in the formation of carbonic acid, the bicarbonate ions— but not the hydrogen ions—are replaced by the bicarbonate reserve.

35 Figure The integrated homeostatic responses to respiratory alkalosis Start Normal acid- base balance HOMEOSTASIS Decreased P CO 2 Lower P CO 2 results in a rise in plasma pH Respiratory Alkalosis HOMEOSTASIS DISTURBED Hyperventilation causing decreased P CO 2 Plasma pH returns to normal HOMEOSTASIS RESTORED Increased P CO 2 Combined Effects Increased H  and decreased HCO 3  Responses to Alkalosis Respiratory compensation Renal compensation Inhibition of arterial and CSF chemoreceptors results in a decreased respiratory rate. H  ions are generated and HCO 3  ions are secreted. Buffer systems other than the carbonic acid–bicarbonate system release H  ions.

36 CLINICAL MODULE 24.8 Review a. Define respiratory acidosis and respiratory alkalosis. b. What would happen to the plasma P CO2 of a patient who has an airway obstruction? c. How would a decrease in the pH of body fluids affect the respiratory rate?


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