Acid base homeostasis

ACID-BASE HOMEOSTASISACID-BASE HOMEOSTASIS

A. Relative constancy of A. Relative constancy of the body’s pH is the body’s pH is essential, because essential, because metabolism requires metabolism requires enzymes that operate at enzymes that operate at an optimal pH. an optimal pH.

B. Hydrogen ion B. Hydrogen ion concentration is concentration is measured as pH.measured as pH.

C. An acid can donate a C. An acid can donate a hydrogen ion, and a base hydrogen ion, and a base can accept a hydrogen can accept a hydrogen ion.ion.

D. Buffers are D. Buffers are combinations of salts and combinations of salts and weak acids that prevent weak acids that prevent major changes in pH.major changes in pH.

E. Hemoglobin and E. Hemoglobin and bicarbonate are the most bicarbonate are the most important blood buffers.important blood buffers.

F. Intracellular buffering F. Intracellular buffering is provided by proteins is provided by proteins and organic phosphates.and organic phosphates.

ACID-BASE ACID-BASE DISTURBANCESDISTURBANCES

G. The first defense against G. The first defense against a change in blood pH is a change in blood pH is provided by the blood provided by the blood buffers, but it is the lungs buffers, but it is the lungs and the kidneys that must and the kidneys that must ultimately correct the ultimately correct the hydrogen load.hydrogen load.

H. Changes in ventilation H. Changes in ventilation can rapidly change Pcan rapidly change PCO2CO2 and therefore alter pH.and therefore alter pH.

I. Metabolic production of I. Metabolic production of fixed acids requires that fixed acids requires that the kidneys eliminate the kidneys eliminate hydrogen ions and hydrogen ions and conserve HCOconserve HCO33..

J. Acid-base J. Acid-base abnormalities accompany abnormalities accompany many diseases, and the many diseases, and the restoration of normal restoration of normal blood pH should be a blood pH should be a consideration in the consideration in the treatment of any disease.treatment of any disease.

K. Respiratory acidosis is K. Respiratory acidosis is caused by the caused by the accumulation of carbon accumulation of carbon dioxide, which decreases dioxide, which decreases blood pH. blood pH.

Respiratory AcidosisRespiratory Acidosis(hypoventilation)(hypoventilation)

• Head trauma which damages the Head trauma which damages the respiratory control centersrespiratory control centers

• Drugs which depress the respiratory Drugs which depress the respiratory control centerscontrol centers

• Fractured ribsFractured ribs

• Airway obstructionAirway obstruction

H+ + HCO3- H+ + HCO3- H2CO3 H2CO3 H2O + CO2 H2O + CO2

H+ + Hb- H+ + Hb- HHb HHb

Respiratory Respiratory AcidosisAcidosis

24

L.L. Respiratory Respiratory alkalosis is caused by alkalosis is caused by the loss of carbon the loss of carbon dioxide, which dioxide, which increases blood pH.increases blood pH.

Respiratory AlkalosisRespiratory Alkalosis(hyperventilation)(hyperventilation)

• Stimulation of the chemoreceptors by Stimulation of the chemoreceptors by hypoxiahypoxia

• Stimulation of the intrapulmonary Stimulation of the intrapulmonary receptors by lung injury or receptors by lung injury or inflammationinflammation

• Excessive use of a ventilator while Excessive use of a ventilator while anesthetizedanesthetized

• Psychogenic causes (pain, fear)Psychogenic causes (pain, fear)

H+ + HCO3- H+ + HCO3- H2CO3 H2CO3 H2O + H2O + CO2CO2H+ + Hb- H+ + Hb- HHb HHb

Respiratory Respiratory AlkalosisAlkalosis

24

M.M. Metabolic Metabolic acidosis is caused acidosis is caused by the by the accumulation of accumulation of fixed acids or the fixed acids or the loss of buffer base, loss of buffer base, which decreases which decreases blood pH.blood pH.

Metabolic AcidosisMetabolic Acidosis(Fixed Acids Increased)(Fixed Acids Increased)

• Anaerobic metabolism and lactic Anaerobic metabolism and lactic acidosisacidosis

• Grain overload and increased acid Grain overload and increased acid production in the rumenproduction in the rumen

• Exogenous toxins such as ethylene Exogenous toxins such as ethylene glycolglycol

• Ketone production in diabetes Ketone production in diabetes mellitusmellitus

Metabolic AcidosisMetabolic Acidosis(Increased Loss of Buffer)(Increased Loss of Buffer)

• Choke in cows with loss of saliva Choke in cows with loss of saliva (HCO3-)(HCO3-)

• Diarrhea with excessive loss of Diarrhea with excessive loss of HCO3- in fecesHCO3- in feces

H+ + HCO3- H+ + HCO3- H2CO3 H2CO3 H2O + H2O + CO2CO2

Metabolic Metabolic AcidosisAcidosis

24

N. Metabolic alkalosis is N. Metabolic alkalosis is caused by the excessive caused by the excessive elimination of hydrogen elimination of hydrogen ions or by the intake of ions or by the intake of base, such as HCObase, such as HCO33

--, ,

which increases blood pH.which increases blood pH.

Metabolic AlkalosisMetabolic Alkalosis

• Excessive accumulation of Excessive accumulation of HCO3-HCO3-– Iatrogenic HCO3- administrationIatrogenic HCO3- administration

• Excessive loss of H+Excessive loss of H+– VomitingVomiting– Abomasal TorsionAbomasal Torsion– HypokalemiaHypokalemia

H+ + HCO3- H+ + HCO3- H2CO3 H2CO3 H2O + H2O + CO2CO2

Metabolic Metabolic AlkalosisAlkalosis

24

O. Respiratory O. Respiratory compensations for acid-compensations for acid-base abnormalities occur base abnormalities occur rapidly; renal rapidly; renal compensations occur over compensations occur over several hours.several hours.

NAE = net acid excretion

P. Hydrogen and P. Hydrogen and potassium ions are potassium ions are interrelated in acid-base interrelated in acid-base

homeostasis.homeostasis.

H+ and K+ Ion Inter-H+ and K+ Ion Inter-relationshipsrelationships• 1. Hypokalemia can cause metabolic alkalosis.1. Hypokalemia can cause metabolic alkalosis.

• 2. More commonly, metabolic alkalosis causes 2. More commonly, metabolic alkalosis causes hypokalemia.hypokalemia.

• 3. Acute changes in pH influence K+ secretion 3. Acute changes in pH influence K+ secretion by the DT and CD.by the DT and CD.

• 4. Metabolic acidosis also affects K+ ion 4. Metabolic acidosis also affects K+ ion movement because H+ ions move into cells to movement because H+ ions move into cells to be buffered.be buffered.

Q.Q. The diagnosis of The diagnosis of acid-base acid-base disturbances depends disturbances depends on interpretation of on interpretation of measurements of measurements of arterial blood pH and arterial blood pH and PPCO2CO2 from which HCO from which HCO33

-- concentration and concentration and total buffer base are total buffer base are calculated.calculated.

R. Over the years, a large R. Over the years, a large number of terms have number of terms have been used to explain acid-been used to explain acid-base balance.base balance.

Acid-Base Acid-Base Derangements Constant Derangements Constant PcoPco22/Hco/Hco33 ratio ratio

Acid/Base Acid/Base DisorderDisorder

Primary Primary changechange

Compensatory Compensatory changechange

RespiratoryRespiratory acidosisacidosis

Increase Increase pcopco22

Increase HcoIncrease Hco3 3

RespiratoryRespiratory alkalosisalkalosis

Decrease Decrease pcopco22

Decrease HcoDecrease Hco3 3

MetabolicMetabolic acidosisacidosis

DecreaseHcoDecreaseHco

33

Decrease pcoDecrease pco22

MetabolicMetabolic alkalosisalkalosis

Increase Increase HcoHco3 3

Increase pcoIncrease pco22

Basic ConceptsBasic Concepts

HH+ + concconc. in ECF is . in ECF is determineddetermined

by balance between by balance between PcoPco22 partial pr.partial pr.

and and HCOHCO3 3 conc.conc. in the fluid. in the fluid.

• HH++ in n eq/L =24 in n eq/L =24 × Pco2/Hco3 .× Pco2/Hco3 .• Using arterial pcoUsing arterial pco22 of 40mmHg and HCO of 40mmHg and HCO33 conc. conc.

of 24meq/L, the normal Hof 24meq/L, the normal H++ in the arterial blood in the arterial blood is is

24 × (40/24) = 40neq/L24 × (40/24) = 40neq/L• Since nano-equivalent is a cumbersome term Since nano-equivalent is a cumbersome term

HH++ is routinely expressed as pH units . is routinely expressed as pH units .• A normal HA normal H++= of 40neq/L corresponds to a pH = of 40neq/L corresponds to a pH

of 7.4of 7.4