Unit 7 - Acid Base Physiology

Question 1

how does the Henderson-Hasselbach relate to bicarbonate?

Answer 1

pH = 6.1 * log[HCO-3/0.03PCO2]

Question 2

what are isobars on Davenport graphs?

Answer 2

“metabolic lines” that describe changes in pH and bicarbonate that occur when acid or base is added at constant PCO2

Question 3

what does the Davenport graph show?

Answer 3

relationship between pH bicarbonate at a series of constant values of PCO2 for normal humans at sea level

• each line at constant PCO2 is a metabolic line (isobar)
• there is a hexagon on the PCO2 = 40 mmHg that represents the range of values for normal, healthy individuals

Question 4

what does the CO2 absorption curve show?

Answer 4

respiratory line represents effect of pulmonary ventilation on blood pH and bicarbonate
-when Hb is greater, more HCO3 is produced per change in pH

Question 5

what does respiratory acidosis show? how is it compensated? what is the adjustable parameter?

Answer 5

decreased pH with increased PCO2 (from 40 to 80 mmHg) caused by hypoventilation

• pH initially decreases (7.4 to 7.1, but then compensates up to 7.3)
• bicarbonate is adjustable parameter; can change as much as possible as long as pH is within 6.8 to 7.8
• respiratory compensations are immediate, but renal mechanisms take more time to develop

Question 6

what are some causes of hypoventilation? what secondary disease do these cause?

Answer 6

hypoventilation causes primary respiratory acidosis

• COPD: emphysema
• asthma
• narcotics/anesthesia
• airway obstruction
• lung collapse
• muscular dystrophy/paralysis
• pneumonia
• bronchitis
• severe pulmonary edema

Question 7

what happens to pH and bicarbonate during respiratory acidosis? what does pH depend on?

Answer 7

bicarbonate increases, and pH falls

• during renal compensation to respiratory acidosis, bicarboate is increasing further while pH rises (so pH rises or falls when bicarb increases)
• pH of blood depends on proton concentration, NOT bicarbonate

Question 8

what happens during respiratory alkalosis? how is it compensated?

Answer 8

increased pH with decreased PCO2 (from 40 mmHg to 20 mmHg) caused by hyperventilation

• causes decrease in cerebral blood flow causing lightheadedness, tingling of hands/feet with increase in neuromuscular excitability
• compensated by decreasing pH and bicarbonate, but still not to normal values

Question 9

what are causes of hyperventilation? what disease does this cause?

Answer 9

causes of respiratory alkalosis:

• ventilator set too high in ICU
• anxiety attack
• trauma to respiratory center in brain
• brain tumor

Question 10

a blood pH above what causes cardiac arrhythmias and seizures? when is treatment required?

Answer 10

CA and seizures at 7.7, but should be treated if exceeds 7.55

Question 11

what is the pulmonary and renal response to respiratory alkalosis?

Answer 11

pulm: cease hyperventilation (remove source of anxiety)
renal: decrease reabsorption of HCO3- and retain acid (decreased excretion of NH4Cl)
- only acid/base disorder where renal compensation completely normalizes pH

Question 12

what is the pulmonary and renal response to respiratory acidosis?

Answer 12

pulm: increase in ventilation to extent possible (increase frequency or depth of breathing)
renal: increase reabsorption of HCO3 and excretion of acid
- excretion of NH4Cl causes hypochloremia and increased plasma bicarbonate, leaving arterial blood mildly acidic

Question 13

what is the most common acid/base abnormality in hospital patients?

Answer 13

metabolic alkalosis

Question 14

what happens in metabolic alkalosis? how is it compensated for pulmonary and renal? which is more effective?

Answer 14

increased pH with increased bicarbonate (PCO2 still equal to 40 mmHg)

• pulm: decrease ventilation (ineffective b/c need O2)
• renal: decrease reabsorption of HCO3-

Question 15

what are causes of metabolic alkalosis?

Answer 15

ECF volume contraction (Cl- responsive) and ECF volume expansion (Cl- resistant)

• responsive: vomiting, gastric tubes, or excess diuretics
• resistant: hypoaldosteronism

Question 16

how can excess diuretics cause metabolic acidosis?

Answer 16

through volume contraction

• increased loss of NaCl and fluid
• increased RAAS
• increased renal H+ secretion
• increased plasma bicarbonate

Question 17

what happens in metabolic acidosis?

Answer 17

decreased pH with decreased bicarbonate

Question 18

what are causes of metabolic acidosis?

Answer 18

1. excessive overproduction/exogenous overload causing accumulation of acids in blood
   - poorly controlled DM or prolonged starvation (ketoacidosis)
2. underexcretion of acids in kidney
3. excessive renal excretion of bicarbonate
   - hypercholeremic increased [Cl] and decreased [bicarbonate]
4. GI loss of bicarbonate
5. anion gap acidosis

Question 19

what is the anion gap? how is it changed in ketoacidosis? what can cause it?

Answer 19

[Na+] - ([Cl-] + [HCO3-])

• increases 12 +/- 4 (increased organic anion, decreased HCO3-)
• caused by overdoses of methanol, ethylene glycol, aspirin; diabetes mellitus (acetoacetic and beta-hydroxybutyric acids), and exercise/ischemic stress (lactic acids)

Question 20

what does “hyperchloremic” mean and what does it cause/what causes it?

Answer 20

increased [Cl-] and decreased [HCO3-]
-caused by severe diarrhea and uremias (proximal and distal renal tubular acidosis; of blood, NOT urine) that causes metabolic acidosis

Question 21

what is base excess VS base deficit?

Answer 21

BE: amount of [HCO3-] that should be added or subtracted to bring patient back to normal
-positive in alkalosis and negative in acidosis
=actual [HCO3-] - 24

BD: indication of how serious is the abnormality in acid/base status
-negative base excess in acidosis