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

6

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

7

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

9

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)

10

Summary of Acid-Base Balance

Know this slide!

Figure from: Hole’s Human A&P, 12th edition, 2010

(Seconds)

(Minutes)

(Hours-Days)

11

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

12

Acidosis

Respiratory acidosis Metabolic acidosis

Nervous system depression, coma, death

(hypopnea)

Figure from: Hole’s Human A&P, 12th edition, 2010

13

Alkalosis

Respiratory alkalosis Metabolic alkalosis

Nervousness, tetany, convulsions, death

Figure from: Hole’s Human A&P, 12th edition, 2010

14

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

15

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