Chapter 26
Fluid, Electrolytes, and Acid-Base BalanceLecture 17
Part 2b: Acid-Base Balance
Marieb’s HumanAnatomy and
PhysiologyNinth Edition
Marieb Hoehn
2
Strengths of Acids and Bases
• Weak bases ionize less completely and bind fewer H+
• Strong bases ionize more completely and bind more H+
• Weak acids ionize less completely and release fewer H+ (**allows them to act as buffers)
• Strong acids ionize more completely and release more H+
3
Sources of Hydrogen Ions
Some H+ is also absorbed from the digestive tract
Figure from: Hole’s Human A&P, 12th edition, 2010
4
Regulation of Hydrogen Ion Concentration
1. chemical acid-base buffer systems (physical buffers) • first line of defense• can tie-up acids or bases, but cannot eliminate them• act in seconds
2. respiratory excretion of carbon dioxide• a physiological buffer (can eliminate excess acid indirectly via CO2)• minutes
3. renal excretion of hydrogen ions• a physiological buffer (can eliminate excess metabolic acids directly, e.g., keto-, uric, lactic, phosphoric)• hours to a day
5
Acid-Base Buffer Systems
Bicarbonate System• the bicarbonate ion converts a strong acid to a weak acid• carbonic acid converts a strong base to a weak base• an important buffer of the ECF (~ 25 mEq/L)
H+ + HCO3- ↔ H2CO3 ↔ CO2 + H2O
Phosphate System• the monohydrogen phosphate ion converts a strong acid to a weak acid• the dihydrogen phosphate ion converts a strong base to a weak base
H+ + HPO4-2 ↔ H2PO4
-
Strong acid Weak acid
Strong acid Weak acid
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Acid-Base Buffer Systems
Protein Buffer SystemICF, plasma proteins, Hb
NH2 group accepts hydrogen ions when pH falls
COOH group releases hydrogen ions when pH rises
-
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Most plentiful and powerful chemical buffer system
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Respiratory Excretion of Carbon Dioxide
A physiological buffer system
Figure from: Hole’s Human A&P, 12th edition, 2010
8
Renal Excretion of Hydrogen Ions
*The kidney is most powerful and versatile acid-base regulating system in the body
Figure from: Hole’s Human A&P, 12th edition, 2010
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Note that secretion of H+ relies on carbonic anhydrase activity within tubular cells
Net result is secretion of H+ accompanied by the (1) retention of HCO3
-
FigureS from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Buffering Mechanisms in the Kidney
Production of new HCO3
-
(2)
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Summary of Acid-Base Balance
Know this slide!
Figure from: Hole’s Human A&P, 12th edition, 2010
(Seconds)
(Minutes)
(Hours-Days)
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Acidosis and Alkalosis
If the pH of arterial blood drops to 6.8 or rises to 8.0 for more than a few hours, survival is jeopardized
Classified according to:
1. Whether the cause is respiratory (CO2), or metabolic (other acids, bases)
2. Whether the blood pH is acid or alkaline
Figure from: Hole’s Human A&P, 12th edition, 2010
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Acidosis
Respiratory acidosis Metabolic acidosis
Nervous system depression, coma, death
(hypopnea)
Figure from: Hole’s Human A&P, 12th edition, 2010
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Alkalosis
Respiratory alkalosis Metabolic alkalosis
Nervousness, tetany, convulsions, death
Figure from: Hole’s Human A&P, 12th edition, 2010
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Acidosis and Alkalosis
• What would be the indications of acidosis and alkalosis in terms of changes in pH and PCO2? pH and HCO3
-?
• How would the body try to compensate for – Acidosis
• Respiratory• Metabolic
– Alkalosis• Respiratory• Metabolic
See Handout: Marieb, Human Anatomy & Physiology, Pearson, 2004
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Review
• Acidosis (pH < 7.35)– Excessive H+ in the plasma – Respiratory acidosis– Metabolic acidosis
• Alkalosis (pH > 7.45)– Insufficient H+ in the plasma– Respiratory alkalosis– Metabolic alkalosis
• Compensations