ACID-BASE BALANCE

Question 1

Define acidemia and alkalemia

Answer 1

Acidemia: pH <7.35
Alkalemia: pH >7.45

Question 2

Describe the relationship between blood pH, CO2, and HCO3-

Answer 2

• blood pH is affected by amount of dCO2 and HCO3-
• HCO3- will counteract decreased pH by neutralizing dCO2

Question 3

Formula for pH

Answer 3

pH = pKa + log [base/acid] = pKa + log [(HCO3-) / (a x pCO2)]

Question 4

List the 4 blood buffering systems

Answer 4

1. Bicarbonate (HCO3-)
2. Hemoglobin
3. Phosphate
4. Plasma proteins

Question 5

Ratio of bicarbonate to dissolved CO2

Answer 5

20:1

Question 6

List 3 reasons why the bicarbonate buffering system is the most important

Answer 6

• bicarbonate neutralizes CO2
• lungs can modify respiration rate, and thus pCO2
• kidneys can control bicarbonate reabsorption/ excretion

Question 7

How is CO2 regulated in the tissues ?

Answer 7

• 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

Question 8

How is CO2 regulated in the lungs ?

Answer 8

• Hgb binds O2 which releases H+
• H+ binds HCO3- to form H2CO3 = dissociates into CO2 and H2O
• CO2 is exhaled through alveoli

Question 9

How does hypoventilation affect CO2 levels and pH ?

Answer 9

increases amount of CO2 in the blood and decreases pH

Question 10

How does hyperventilation affect CO2 levels and pH ?

Answer 10

decreases amount of CO2 in blood and increases pH

Question 11

How do the kidneys respond to metabolic acidosis via bicarbonate ?

Answer 11

• 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

Question 12

How do the kidneys respond to metabolic acidosis ?

Answer 12

• 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

Question 13

The primary cause of metabolic acidosis is a __

Answer 13

The primary cause of metabolic acidosis is a BICARBONATE DEFECIT

Question 14

What happens when bicarbonate to CO2 ratio is <20:1 ?

Answer 14

pH decreases

Question 15

List 4 causes of metabolic acidosis

Answer 15

• 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

Question 16

Primary vs secondary compensation for metabolic acidosis

Answer 16

Primary: LUNGS
- hyperventilation; more CO2 is excreted

Secondary: KIDNEYS
- excrete H+ through ammonia formation
- increase bicarbonate reclamation from glomerular filtrate

Question 17

The primary cause of respiratory acidosis is __

Answer 17

The primary cause of respiratory acidosis is EXCESS CO2

Question 18

Causes of respiratory acidosis

Answer 18

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

Question 19

Primary vs secondary compensation for respiratory acidosis

Answer 19

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

Question 20

The primary cause of metabolic alkalosis is __

Answer 20

EXCESS BICARBONATE

Question 21

Causes of metabolic alkalosis

Answer 21

• 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)

Question 22

Primary vs secondary compensation for metabolic alkalosis

Answer 22

Primary: LUNGS
- hypoventilation raises pCO2

Secondary: KIDNEYS
- decrease bicarbonate reclamation

Question 23

The primary cause of respiratory alkalosis is __

Answer 23

The primary cause of respiratory alkalosis is HYPERVENTILATION

Question 24

Causes of respiratory alkalosis

Answer 24

• hysteria, fever, CNS infections
• drugs (salicylates, nicotine)
• hypoxia (pneumonia, congestive heart failure)

Question 25

Primary vs secondary compensation for respiratory alkalosis

Answer 25

PRIMARY: KIDNEYS
- decrease reclamation of bicarbonate
- increase reclamation of H+
- decrease formation of ammonia

Question 26

Why are salicylates problematic ?

Answer 26

• an unmeasured anion ie. EXOGENOUS ACID = increased anion gap
• also stimulates hyperventilation = DECREASES pCO2
• this results in a mixed metabolic acidosis and respiratory alkalosis

Question 27

Ref: pH

Answer 27

7.35 - 7.45

Question 28

Ref: pCO2

Answer 28

35 - 45 mm Hg

Question 29

Ref: pO2

Answer 29

70 - 90 mm Hg

Question 30

Ref: HCO3-

Answer 30

20 -27 mmol/L