Overview of acid/base

Question 1

Normal physiological pH

Answer 1

7.35-7.45

Question 2

Acidemia pH

Answer 2

< 7.35

Question 3

Alkalemia pH

Answer 3

> 7.45

Question 4

Carbonic acid/bicarbonate buffer system

Answer 4

H+ + HCO3- <–> H2CO3 <–> CO2 + H2O

Think of the right side of the equation as occurring in the lungs and the left in the kidneys

The lungs compensate metabolic disorders while the kidneys compensate respiratory disorders

Question 5

Henderson-Hasselbach equation

Answer 5

pH= pka + log (base/acid)

carbonic anhydrase pka= 6.1

Most of the carbonic acid in plasma is in the form of carbon dioxide gas; therefore, the concentration of carbonic acid can be estimated as the partial pressure of CO2 (pCO2) multiplied by 0,03

pH= 6.1 + log (HCO3-/ 0.03 x pCO2)

Question 6

Normal blood gas values

Answer 6

PaCO2= 35-45 mmHg (40)
HCO2= 22-26 mEq/L (24)
PaO2= 95-100 mmHg
SaO2= > or equal to 95%

Question 7

Adverse consequences

Answer 7

Acidemia
Alkalemia

Question 8

Acidemia

Answer 8

Cardiovascular:
- decrease cardiac output
- Impairment of cardiac contractibility
- Increase pulmonary vascular resistance and arrhythmia’s

Metabolic:
- Insulin resistance
- Inhibition of glycolysis
- Hyperkalemia

CNS: coma or altered mental status

Others: decreased respiratory muscle strength, hyperventilation, dyspnea

Question 9

Alkalemia

Answer 9

Cardiovascular:
- Decrease coronary blood flow
- Arteriolar constriction
- Decrease anginal threshold
- Arrhythmias

Metabolic:
- Decrease K+, Ca, and Mg
- Stimulation of glycolysis

CNS: decrease cerebral blood flow, seizures

Others: hypoventilation

Question 10

Acid generation

Answer 10

Diet: 1 mEq/kg/day of acid consumed per day; comes from oxidation of protein and fats

Aerobic metabolism of glucose produces 15-20K mmol of CO2 each day

Anaerobic metabolism produces lactic and pyruvic acid

Question 11

Three standard mechanisms of acid regulation

Answer 11

1. Buffering
2. Renal regulation
3. Ventilatory regulation

Question 12

Buffering

Answer 12

First line of defense

Buffer: ability of a weak acid and its anion to resist change in pH with addition of a strong acid or base

Question 13

Bicarbonate

Answer 13

principle buffer

rapid onset with intermediate capacity

HCO3- buffer present in largest concentration extracellularly over any other buffers

Ability of the kidneys and lungs to excrete and retain HCO3- and CO2, respectively

Question 14

Phosphates

Answer 14

Intermediate onset and capacity

Extracellular inorganic phosphates limited activity

Intracellular organic phosphates

Calcium phosphates in bone relatively inaccessible

Question 15

Proteins

Answer 15

Albumin/hemoglobin: rapid onset, limited capacity

More effective from intracellular than extracellular

Question 16

Renal regulation

Answer 16

Kidney serves 2 purposes:
1. Reabsorb filtered HCO3-
2. Excrete H+ ions released from nonvolatile acids

Question 17

Bicarbonate reabsorption

Answer 17

85-90% reabsorbed by proximal tubule

1. Filtered HCO3- combines with a secreted H+ to form H2CO3
2. Through the action of carbonic anhydrase disassociated occurs to form H2O and CO2
3. H2O and CO2 are reabsorbed into the tubular cell
4. H2CO3 dissociates into HCO3-, which is reabsorbed into the capillary, and H+, is secreted into the urine in exchange for Na+

Question 18

Net effect

Answer 18

filtered HCO3- is reabsorbed without any net loss of H+

Anything limiting H+ secretion into the proximal tubule lumen results in urinary bicarbonate losses

Ex: carbonic anhydrase inhibitors

Question 19

Bicarbonate genration/H+ excretion

Answer 19

Delayed onset but large capacity

Reclamation of all filtered HCO3- is not sufficient to maintain a normal blood pH

H+ excretion take place primarily in the distal tubule

Question 20

Ammonium excretion

Answer 20

1. Secreted H+ combines with NH3 to form NH4+, NH4+ cannot cross membranes and is excreted
2. HCO3- is formed during the process and is also reabsorbed in the capillaries
3. New HCO3- is formed (40 mEq/day)

Question 21

Titratable acidity

Answer 21

1. Filtered HPO4- combines with H+ and is excreted as H2PO4-
2. The intracellular HCO3- formed from the dissociation of H2CO3- is reabsorbed as “new” HCO3-
3. Accounts for 30 mEq/day of “new” HCO3-

Question 22

Distal tubular hydrogen ion excretion

Answer 22

Comprises 50% of net acid secretion

CO2 combines with water in the presence of carbonic anhydrase to form H2CO3, which breaksdown to H+ and HCO3-

The H+ is transported back into the tubular lumen by ATPase

Question 23

Ventilatory regulation

Answer 23

Rapid onset and LARGE capacity

Chemoreceptors detect an increase in the PaCO2 and increase the rate and depth of ventilation

Peripheral chemoreceptors in carotid arteries and aorta: activated by arterial acidosis, hypercapnia, and hypoxia

Central chemoreceptors in medulla: activated by CSF acidosis