ACID-BASE BALANCE Flashcards
Define acidemia and alkalemia
Acidemia: pH <7.35
Alkalemia: pH >7.45
Describe the relationship between blood pH, CO2, and HCO3-
- blood pH is affected by amount of dCO2 and HCO3-
- HCO3- will counteract decreased pH by neutralizing dCO2
Formula for pH
pH = pKa + log [base/acid] = pKa + log [(HCO3-) / (a x pCO2)]
List the 4 blood buffering systems
- Bicarbonate (HCO3-)
- Hemoglobin
- Phosphate
- Plasma proteins
Ratio of bicarbonate to dissolved CO2
20:1
List 3 reasons why the bicarbonate buffering system is the most important
- bicarbonate neutralizes CO2
- lungs can modify respiration rate, and thus pCO2
- kidneys can control bicarbonate reabsorption/ excretion
How is CO2 regulated in the tissues ?
- CO2 diffuses out of tissues into the blood
- in the blood, CO2 diffuses into RBCs
- inside RBCs, CO2 is converted to H2CO3 by carbonic anhydride = carbonic acid dissociates into HCO3- and H+
- bicarbonate leaves RBCs and chloride enters RBCs = chloride shift
- free H+ binds to deoxygenated Hgb
How is CO2 regulated in the lungs ?
- Hgb binds O2 which releases H+
- H+ binds HCO3- to form H2CO3 = dissociates into CO2 and H2O
- CO2 is exhaled through alveoli
How does hypoventilation affect CO2 levels and pH ?
increases amount of CO2 in the blood and decreases pH
How does hyperventilation affect CO2 levels and pH ?
decreases amount of CO2 in blood and increases pH
How do the kidneys respond to metabolic acidosis via bicarbonate ?
- Na+ in the lumen of PCT are exchanged for H+ in the tubule cell
- in the lumen, H+ and HCO3- = carbonic acid = CO2 and H2O by carbonic anhydrase
- CO2 diffuses into PCT cells and recombines to form carbonic acid
- carbonic acid dissociates into HCO3- and H+
- HCO3- diffuses back into blood
*BUT in response to alkalosis, the kidney decreases the amount of HCO3- that is reclaimed = increases bicarbonate excretion
How do the kidneys respond to metabolic acidosis ?
- increased H+ excretion
- H+ can combine with ammonia in the filtrate (from deamination of glutamine) = ammonium ions
- production of ammonia is increased in acidosis to buffer excess H+
- NOT compensation for acute acidosis; increased ammonia production takes ~3 days
The primary cause of metabolic acidosis is a __
The primary cause of metabolic acidosis is a BICARBONATE DEFECIT
What happens when bicarbonate to CO2 ratio is <20:1 ?
pH decreases
List 4 causes of metabolic acidosis
- endogenous acids (lactic, keto acids) = increased ANION gap
- exogenous acids (ethanol, methanol, ethylene glycol, salicylates) = increased ANION and OSMOLAL gap
- inability to excrete acid (renal failure) = decreased ammonia formation/ H+ excretion = increased ANION gap
- loss of bicarbonate (diarrhea, pancreatitis) = NORMAL ANION gap
Primary vs secondary compensation for metabolic acidosis
Primary: LUNGS
- hyperventilation; more CO2 is excreted
Secondary: KIDNEYS
- excrete H+ through ammonia formation
- increase bicarbonate reclamation from glomerular filtrate
The primary cause of respiratory acidosis is __
The primary cause of respiratory acidosis is EXCESS CO2
Causes of respiratory acidosis
ANY CONDITION THAT CAUSES HYPOVENTILATION:
- drugs (barbituates, morphine, alcohol poisoning)
- CNS trauma, tumors
- mechanical obstruction of airways (COPD*, pulmonary fibrosis)
- obesity, sleep disorders
*COPD - chronic obstructive pulmonary disease
Primary vs secondary compensation for respiratory acidosis
Primary: BLOOD BUFFERS
- bicarbonate, hemoglobin, and proteins neutralize CO2
Secondary: KIDNEYS*
- excrete H+ through ammonia formation
- increase bicarbonate reclamation from glomerular filtrate
*TAKES HOURS TO BE EFFECTIVE
The primary cause of metabolic alkalosis is __
EXCESS BICARBONATE
Causes of metabolic alkalosis
- hypochloremic; loss of chloride (diarrhea, vomitting) = loss of gastric HCl causes renal retention of bicarbonate
- excess corticoids (hyperaldosteronism) = electrolyte imbalance
- excess administration/ ingestion of bicarbonate (IV therapy, transfusion, bicarbonate antacids)
Primary vs secondary compensation for metabolic alkalosis
Primary: LUNGS
- hypoventilation raises pCO2
Secondary: KIDNEYS
- decrease bicarbonate reclamation
The primary cause of respiratory alkalosis is __
The primary cause of respiratory alkalosis is HYPERVENTILATION
Causes of respiratory alkalosis
- hysteria, fever, CNS infections
- drugs (salicylates, nicotine)
- hypoxia (pneumonia, congestive heart failure)
Primary vs secondary compensation for respiratory alkalosis
PRIMARY: KIDNEYS
- decrease reclamation of bicarbonate
- increase reclamation of H+
- decrease formation of ammonia
Why are salicylates problematic ?
- an unmeasured anion ie. EXOGENOUS ACID = increased anion gap
- also stimulates hyperventilation = DECREASES pCO2
- this results in a mixed metabolic acidosis and respiratory alkalosis
Ref: pH
7.35 - 7.45
Ref: pCO2
35 - 45 mm Hg
Ref: pO2
70 - 90 mm Hg
Ref: HCO3-
20 -27 mmol/L
