Acid base physiology

Question 1

How does the body maintain pH

Answer 1

Buffers: weak acids and their conjugate bases that can limit the change in pH with the addition of more acid or base

Question 2

Normal human pH

Answer 2

7.4. 6.8-7.8 is compatible with life

Question 3

Two types of buffers in body

Answer 3

intracellular: organic phosphates, proteins. The most important intracellular protein is hemoglobin. Extracellular: phosphate, bicarbonate and plasma proteins (mostly albumin). Bicarbonate is the most important buffer here

Question 4

Know the Henderson-Hasselbalch Equation for Bicarbonate/CO2

Answer 4

pH= pKa + log ([A-] / [HA]). For bicarbonate: pH = 6.1 + log ([HCO3]/ 0.03 x PCO2)

Question 5

What is Pka

Answer 5

characteristic of a specific buffer and is the pH at which the concentration of HA equals A-

Question 6

When is buffering best

Answer 6

when pH of solution is closest to pKa of the buffering pair

Question 7

What is the conjugate acid of bicarbonate?

Answer 7

Effectively it is CO2 b/c H2CO3 is rapidly coverted to CO2 by carbonic anhydrase. H2O + CO2 <>H2CO3 <> H + HCO3

Question 8

2. Know the normal arterial blood gas values for pH, PaCO2, and [HCO3–]

Answer 8

pH= 7.40. PaCO2= 40 torr. HCO3= 24mM (sea level)

Question 9

Describe hemoglobin as a buffer

Answer 9

Deoxyhemoglobin has a pK of 7.9, so is a very good buffer. CO2 can diffuse into RBCs then be converted to bicarbonate and the protons are buffered by deoxyhemoglobin. This is why venous pH is only slightly lower than arterial pH despite all of the CO2 it carries.

Question 10

Define acidemia and alkalemia

Answer 10

Acidemia: more acid in the blood than normal, resulting in lower pH (7.40)

Question 11

How does the body compensate for abnormal pH

Answer 11

Lungs can regulate CO2 levels (minutes) and
kidneys can regulate bicarbonate (hours to days). Compensation will never completely correct to normal pH, nor will it overcompensateLungs can regulate CO2 levels (minutes) and
kidneys can regulate bicarbonate (hours to days). Compensation will never completely correct to normal pH, nor will it overcompensate

Question 12

What is respiratory acidosis and what causes it generally

Answer 12

Too much CO2 results in lower pH. Virtually always due to ineffective ventilation. Can be either acute (before kidneys can compensate) or chronic

Question 13

Which are specific causes of respiratory acidosis

Answer 13

Acute: CNS depressants (opiates, benzodiazepines, alcohol most common) and Respiratory muscle fatigue (increased work of breathing). Chronic: Central hypoventilation (e.g. obesity hypo-ventilation syndrome), Neuromuscular disease (e.g. ALS), Chronic lung diseases (emphysema, bronchiectasis, etc) and
HypothyroidismAcute: CNS depressants (opiates, benzodiazepines, alcohol most common) and Respiratory muscle fatigue (increased work of breathing). Chronic: Central hypoventilation (e.g. obesity hypo-ventilation syndrome), Neuromuscular disease (e.g. ALS), Chronic lung diseases (emphysema, bronchiectasis, etc) and
Hypothyroidism

Question 14

Compensation mechanism for respiratory acidosis

Answer 14

conservation of bicarbonate by the kidneys. The kidneys excrete protons in the form of NH4Cl, while retaining bicarbonate. This compensatory mechanism is relatively slow, taking two to three days to complete

Question 15

What is respiratory alkalosis and what causes it generally

Answer 15

•Too little CO2 results in higher pH due to increased ventilation

Question 16

Which are specific causes of respiratory alkalosis

Answer 16

Acute: pain, anxiety, fever, mechanical ventilation. Chronic causes: living at altitude, brain injury, chronic ASA toxicity, pregnancy

Question 17

Compensation mechanism for respiratory alkalosis

Answer 17

takes place through the kidneys, which increase the excretion of bicarbonate and lower the pH toward its normal value. Renal compensation for respiratory alkalosis is slow, taking hours or days.

Question 18

What is metabolic acidosis and what causes it generally

Answer 18

•Too much acid results in decreased HCO3- and lower pH. Respiratory compensation with increased ventilation causes decreased CO2.

Question 19

Two types of metabolic acidosis

Answer 19

anion gap and non anion gap.

Question 20

How to calculate anion gap and what it means when it is elevated vs. normal

Answer 20

[Na+] – ([Cl-] + [HCO3-]). Normal is 12-14. Elevated anion gap indicates presence of additional acid as the cause of metabolic acidosis which is buffered by bicarbonate and thereby increases the amount of unmeasured anions. A normal anion gap in metabolic acidosis occurs due to GI or renal losses

Question 21

Which are specific causes of metabolic acidosis

Answer 21

anion gap related: MUDPILES - methanol, uremia, Diabetic KetoAcidosis (or ketoacids such as EtOH), propylene glycol, isoniazid, lactate, ethylene glycol, salicylates. Non anion gap related: GI losses (diarrhea), renal losses, too much IV saline

Question 22

Compensation mechanism for metabolic acidosis

Answer 22

Increased ventilation removes CO2 (quickly) and increases pH.

Question 23

Winters formula

Answer 23

Used to calculate the expected CO2 pressure in metabolic acidosis. Expected pCO2 = 1.5[HCO3] + 8 + or - 2. If pCO2 measured on the blood gas is in the range of the expected value then the compensation is considered complete. If the pCO2 is higher than expected then it is incomplete, or there is also a respiratory acidosis component.

Question 24

What is metabolic alkalosis and what causes it generally

Answer 24

•Too much HCO3- (or any other base) results in higher pH. pCO2 is increased with respiratory compensation (hypoventilation)

Question 25

Which are specific causes of metabolic alkalosis

Answer 25

Vomiting (loss of gastric acid), ingestion of bicarb or other alkali substance, hypovolemia(causes reabsorption of bicarb by kidneys), diuretics

Question 26

Compensation mechanism for metabolic alkalosis

Answer 26

Decreased ventilation will increase PCO2 to bring pH back towards normal. This compensation is often small b/c it will reduce PO2.

Question 27

Acute vs chronic respiratory disturbances

Answer 27

An acute change in PaCO2 of 10 Torr yields a pH change of about 0.08. A chronic change in PaCO2 of 1 Torr should lead to compensatory change in [HCO3–] of 0.4 meq/L in the same direction