Unit III: HomeostasisAcid-Base Balance

Chapter 24: pp 895-903

Acid-Base Balance

• Important part of homeostasis

– metabolism depends on enzymes, and enzymes are sensitive to pH

Active tissues continuously generate acids: carbonic acid, lactic acid, fatty acids, phosphoric acids.

Tissue cells

Buffer Systems

Normalplasma pH(7.35–7.45)

short-term pHstability.

Eliminatescarbon dioxide

Hydrogen ions are secreted into the urine causing an average pH of 6.0

Acids and Bases

• Determined by the solution’s hydrogen (H+) ions.

• Acids

– strong acids ionize freely, markedly lower pH

• Bases

– strong bases bind H+ ions, markedly raise pHNormal pH of the ECF:

7.35 to 7.45.

pH

Extremelyacidic

Extremelybasic

pH falls below 7.35Acidosis

pH rises above 7.45Alkalosis

(+) H+ ions (-) to solution

− Buffer Systems

Intracellular fluid (ICF) Extracellular fluid (ECF)

occur in

Phosphate BufferSystem

Protein Buffer Systems Carbonic Acid–Bicarbonate Buffer System

Found in ICF and urine

Found in ECF and ICFFound in the ECF

Hemoglobinbuffer system(RBCs only)

Amino acidbuffers

(All proteins)

Plasmaproteinbuffers

Buffers

• Resist changes in pH

• Physiological buffer

– regulates output of acids, bases or CO2

• Chemical buffer

– restore normal pH rapidly

Bicarbonate Buffer System

• Optimum pH = 6.1

• Kidneys and lungs have a pH of 7.4

– to raise pH, kidneys excrete H+ and lungs excrete CO2

– to lower pH, kidneys excrete HCO3-

Additionof HStart

CO2 CO2 H2O H2CO3 H HCO3

Lungs

HCO3 Na NaHCO3

Generationof HCO3

BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE

Otherbuffer

systemsabsorb H

KIDNEYS

Secretionof H

Alkalosis:↓respiratory rate = ↑CO2 carbonic acid

Response to Alkalosis:Conserve H+

Secrete HCO3-

Response to Acidosis

Phosphate Buffer System

• H2PO4- HPO4

2- + H+

• Optimum pH of 6.8

Protein Buffer System

• Acidic side groups can release H+

-COOH -COO- + H+

• Amino side groups can bind H+

-NH2 + H+ -NH3+

Start

Normal pH(7.35–7.45)

Increasing acidity (decreasing pH)

Respiratory Control of pH

• Neutralizes 2 to 3 times as much acid as chemical buffers

• Collaborates with bicarbonate system

• ↑CO2 and ↓pH stimulate pulmonary ventilation

• While an ↑pH inhibits pulmonary ventilation

Tubularfluid

Renal tubule cells ECF

H

H

H

H

Na

Na

CO2 CO2

HCO3

HCO3

H2CO3

HCO3

CO2

H2O

Cl

Cl

Carbonicanhydrase

Renal Control of pH

• Most powerful buffer system (but slow response)

1. Renal tubules secrete H+ into tubular fluid

• Bicarbonate, ammonia, and phosphate buffers

2. Excreted in urine

• Limiting pH of 4.5

Acid-Base Balance

IncreasedPCO2

↑ PCO2 = ↓ plasma pH

Respiratory Acidosis

Responses to Acidosis

Combined Effects

Respiratory compensation

Renal compensation

↓ PCO2

↓H and ↑HCO3

↑ respiratory rate

H ions are secreted andHCO3

ions are generated.

Buffer systems other than thecarbonic acid–bicarbonatesystem accept H ions.

HOMEOSTASISDISTURBED

HOMEOSTASISRESTORED

Hypoventilationcauses ↑ PCO2

Plasma pHreturns to normalStart

Normal acid-base balance

HOMEOSTASIS

Disorders of Acid-Base Balances

•Acidosis – causes confusion, disorientation, and coma

–Respiratory acidosis - rate of alveolar ventilation falls behind CO2 production

–Metabolic acidosis - production of organic acids, ingestion of acidic drugs, or loss of base

•Alkalosis – causes muscle spasms, convulsions, or respiratory paralysis

–Respiratory alkalosis - CO2 eliminated faster than it is produced

–Metabolic alkalosis – overuse of bicarbonates or loss of acid

Compensation for pH Imbalances

• Respiratory compensation– hypercapnia ( CO2) stimulates pulmonary ventilation– hypocapnia reduces pulmonary ventilation

• Renal compensation– effective for imbalances of a few days or longer– acidosis causes in H+ secretion – alkalosis causes bicarbonate secretion