Acid Base Physiology Flashcards
Basic definition of acid and base.
Acid: substance that can donate hydrogen ions
Base: substance that can accept hydrogen ions
What are the two main types of acids?
1) Carbonic acid- Volatile acids that can be converted to CO2 from carbohydrate and fat metabolism (eliminated from lungs)
2) Noncarbonic acids- nonvolatile acids such as phosphoric and sulfuric acids that cannot be converted to CO2 and are made mostly from protein and phospholipids metabolism
How are Noncarbonic acids excreted?
these combine with buffers and are excreted by the kidneys
What are the pH extremes that are compatible with life (only for a short time)?
6.8-7.8
How is pH usually measured in the body?
arterial blood gas samples (also measures CO2 and hemoglobin and calculates HCO3- from the Henderson eqn)
How is HCO3- more directly measured?
venous samples as total CO2 conc (dissolved CO2 plus HCO3- conc)
so the venous estimation of HCO3- is always about 1-.1.5 me/l greater than actual HCO3 due to the extra dissolved CO2
So what is normal plasma HCO3-?
24 mEq/l (arterial)
Bicarbonate buffer system eqn
CO2+H2O - H2CO3 - H+ + HCO3-
Henderson eqn revised
(pH) H+ = 24(CO2/HCO3)
pH must be converted to H+ (nEq/L) pH= 7.4= 40 nEq/L
7.40=40=24(40/24)
Relationship between pH and [H+]
- 0=100
- 1=80
- 2=64
- 3=50
- 4=40
- 5=32
- 6=25
- 7=20
- 8= 16
[H+]= 80- decimal digits of pH
What is the definition of metabolic acidosis?
some process that lowers bicarbonate, while alkalosis is some process that increases HCO3-
What is the definition of respiratory acidosis?
increased CO2, while respiratory alkalosis is decreased CO2
T or F. HCO3- is slightly increased with respiratory acidosis (increased CO2)
T. Think of the eqn.
Do buffers act quickly or slowly?
very quickly- almost an immediate onset
What is the isohydric principle?
all buffers change in the same direction
Where is HCO3- used as a buffer?
ECFV
What buffers are used in the urine?
phosphate (HPO42- and H2PO4-) and ammonia (NH3 and NH4+)
How does the body compensate for metabolic disorders?
by altering CO2 (via the lungs, rapid onset, minutes)
likewise, respiratory disorders are compensated with HCO3- changes (via the kidney, slower onset, 1-2 days)
Why would there be a large pH change in acute respiratory disorders than chronic?
because the compensatory mechanisms haven’t kicked in yet
What happens to pH, HCO3- and pCO2 in metabolic acidosis?
all decrease, pCO2 as a compensatory mechanism
What happens to pH, HCO3- and pCO2 in metabolic alkalosis?
all increase, pCO2 as a compensatory mechanism
What happens to pH, HCO3- and pCO2 in respiratory acidosis?
pH decreases and HCO3- increases (compensatory) and pCO2 increases
What happens to pH, HCO3- and pCO2 in respiratory alkalosis?
pH increases and pCO2 and HCO3- decrease
How much does HCO3- compensate in acute respiratory acidosis?
HCO3- increases 1 mEq for each 10mm increase in PCO2
How much does HCO3- compensate in chronic respiratory acidosis?
HCO3- increases 4mEq for each 10 mm increase in PCO2 because the kidney has had time to react
How much does HCO3- compensate in acute respiratory alkalosis?
HCO3- decreases 2mEq for each 10mm decrease in PCO2
How much does HCO3- compensate in chronic respiratory alkalosis?
HCO3- decreases 5mEq for each 10mm decrease in PCO2
Golden rules of simple acid-base disorders
1) pCO2 and HCO3 always change in the same direction
2) the secondary physiologic mechanism must be present (if not, its mixed)
3) compensatory mechanisms never fully correct pH and they never over-shoot
What are some possible causes of metabolic acidosis?
- decreased renal acid excretion of H+ in the form of NH4+
- direct HCO3- losses (GI tract- diarrhea or intestinal fistulas (everywhere but stomach) or urine)
- Increased acid generation (exogenous or endogenous)
What are some causes of increased acid generation?
- lactic acidosis, -ketoacidosis,
- ingestion of acids (aspirin, ethylene glycol, methanol)
What is Type 1 renal tubular acidosis?
transport defect in H+ ATPases in the CD
What is Type 4 renal tubular acidosis?
low renin production- hypoaldosteronism
What is the main mechanism of respiratory acidosis?
-induced by hypercapnia (decreased alveolar ventilation)
What is the compensatory mechanism for respiratory acidosis?
HCO3- will increase rapidly but only about 1-2mEq/L
over time the kidney will work to increase acid excretion (NH4+) generating new bicarbonate ions (delayed response, 2-3 days)
What are some causes of acute respiratory acidosis?
- general anesthesia
- sedative overdose
- cardiac arrest
- pneumothorax
- aspiration of foreign body
- blockage of airways
What are some causes of chronic respiratory acidosis?
-COPD
-Brain tumor
-respiratory nerve damage
etc.
What are some causes of acute respiratory alkalosis?
-anxiety
-fever
-salicylate intoxication
-CNS diseases
-CHF
-Hypoxia
etc.
What are some causes of chronic respiratory alkalosis?
- pregnancy
- cirrhosis
T or F. In respiratory disorders, plasma Cl changes equally and inversely with plasma HCO3
T. The plasma anion gap does not change with respiratory disorders
NOTE: Plasma Na is not directly altered by acid base disorders
NOTE: Plasma Na is not directly altered by acid base disorders (and K+ levels usually)
What are the potential etiologies of metabolic alkalosis?
loss of H+ from the GI tract (vomiting- removes gastric secretions) or into the urine (diuretic therapy-mainly loop and thiazides)- will be hypotensive OR
if the kidney is excessively excreting acid into urine (primary hyperaldosteronism) - will have HTN
What is plasma anion gap?
only relevant to metabolic acidosis. If you add a strong acid to the system at pH=7.4, it will fully dissociate into H+ and A-. H+ will be buffered by HCO3- (thus, HCO3- goes down when an acid is added to the system)
What happens to the A-?
It is either excreted into the urine causing Cl- to increase to maintain electrical neutrality (normal anion gap)
or A- is reabsorbed by the kidney as an unmeasured anion (causes increased plasma anion gap, minimal change in plasma Cl- conc.)
What is the eqn for PAG?
PAG= Na+- (Cl- + HCO3-)
Normal PAG is ~12
What does increase in PAG suggest?
due to unmeasured negative charge. So if you see increase in PAG, you know the kidney is reabsorbing an unmeasured anion such as lactate or keto-anion
