Acid base physiology Flashcards
How does the body maintain pH
Buffers: weak acids and their conjugate bases that can limit the change in pH with the addition of more acid or base
Normal human pH
7.4. 6.8-7.8 is compatible with life
Two types of buffers in body
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
Know the Henderson-Hasselbalch Equation for Bicarbonate/CO2
pH= pKa + log ([A-] / [HA]). For bicarbonate: pH = 6.1 + log ([HCO3]/ 0.03 x PCO2)
What is Pka
characteristic of a specific buffer and is the pH at which the concentration of HA equals A-
When is buffering best
when pH of solution is closest to pKa of the buffering pair
What is the conjugate acid of bicarbonate?
Effectively it is CO2 b/c H2CO3 is rapidly coverted to CO2 by carbonic anhydrase. H2O + CO2 <>H2CO3 <> H + HCO3
- Know the normal arterial blood gas values for pH, PaCO2, and [HCO3–]
pH= 7.40. PaCO2= 40 torr. HCO3= 24mM (sea level)
Describe hemoglobin as a buffer
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.
Define acidemia and alkalemia
Acidemia: more acid in the blood than normal, resulting in lower pH (7.40)
How does the body compensate for abnormal pH
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
What is respiratory acidosis and what causes it generally
Too much CO2 results in lower pH. Virtually always due to ineffective ventilation. Can be either acute (before kidneys can compensate) or chronic
Which are specific causes of respiratory acidosis
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
Compensation mechanism for respiratory acidosis
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
What is respiratory alkalosis and what causes it generally
•Too little CO2 results in higher pH due to increased ventilation
Which are specific causes of respiratory alkalosis
Acute: pain, anxiety, fever, mechanical ventilation. Chronic causes: living at altitude, brain injury, chronic ASA toxicity, pregnancy
Compensation mechanism for respiratory alkalosis
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.
What is metabolic acidosis and what causes it generally
•Too much acid results in decreased HCO3- and lower pH. Respiratory compensation with increased ventilation causes decreased CO2.
Two types of metabolic acidosis
anion gap and non anion gap.
How to calculate anion gap and what it means when it is elevated vs. normal
[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
Which are specific causes of metabolic acidosis
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
Compensation mechanism for metabolic acidosis
Increased ventilation removes CO2 (quickly) and increases pH.
Winters formula
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.
What is metabolic alkalosis and what causes it generally
•Too much HCO3- (or any other base) results in higher pH. pCO2 is increased with respiratory compensation (hypoventilation)
