Pulmonary Gas Exchange

Question 1

what affects CO2 in alveolar air?

Answer 1

• CO2 production is directly proportional to CO2 in alveolar air (if CO2 production increases, so does the CO2 in alveolar air)
• alveolar ventilation is inversely proportional to CO2 in alveolar air (if alveolar ventilation increases, CO2 in alveolar air decreases)
• this is why hyperventilation results in drop in CO2 and more basic pH
• sample questions 223-224

Question 2

what are the determinants of alveolar PCO2 (=PA CO2)?

Answer 2

• PA CO2 is directly proportional to the metabolic CO2 production rate
• PA CO2 is indirectly proportional to alveolar ventilation

Question 3

what determinants of alveolar PO2 (=PA O2)?

Answer 3

the partial pressure of inspired oxygen and the alveolar partial pressure of CO2
- PA O2 is basically the amount of oxygen diffused to tissue replaced by CO2

Question 4

what is the respiratory exchange ratio? what can alter it?

Answer 4

• the ratio describing the exchange of O2 and CO2
• depends on tissue metabolism
• can be altered by what you eat – if you eat more glucose its about 1
• normally ≈ 0.8
• sample questions 233-234

Question 5

what happens to PA O2 during:
- hypoventilation?
- hyperventilation?
- oxygen therapy?

Answer 5

• increase CO2 in blood so overall decrease in PA O2
• decrease CO2 in blood so overall increase in PA O2, but not dramatic
• increase inspired O2 so overall increase in PA O2

Question 6

what factors affect gas transfer across alveolar-capillary membrane?

Answer 6

• PRESSURE GRADIENT (delta P)
• alveolar surface area (A)
• thickness of membrane (T)
• solubility of gas (S)
• rate of diffusion in gas phase (MW)

Question 7

why is high intravascular pressure dangerous in the capillaries?

Answer 7

because pulmonary capillaries have little structural support

Question 8

how is the blood flow and artery pressure different in pulmonary circulation?

Answer 8

• lower flow rate and lower artery pressure but same cardiac output

Question 9

what are the features of pulmonary circulation?

Answer 9

1. receives entire cardiac output
2. low intravascular pressure
3. low resistance (1/10 of systemic resistance)
4. less smooth muscle in vessel walls and no muscular arterioles

Question 10

what happens when there is increased blood flow?

Answer 10

1. recruitment – increase blood flow to the vessels that aren’t used all the time to maintain blood pressure
2. distension – the stretch of the vessels to increase flow rate and maintain pressure

Question 11

what is hypoxic pulmonary vasoconstriction (HPV)? what are the physiological effects of it?

Answer 11

= when there are regions of the lungs that are poorly ventilated so the blood flow is directed away from that region and toward another region that is better ventilated
- improves the ventilation-perfusion matching
- HPV is responsible for pulmonary hypertension in people residing at high altitudes

Question 12

what is the ventilation-perfusion ratio?

Answer 12

the ratio of alveolar ventilation compared to blood flow
- there needs to be a balance of these values
- normal = 0.8-1.2

Question 13

what is hypoxemia? what are the causes of hypoxemia?

Answer 13

= low blood oxygen levels
1. diffusion abnormalities
2. hypoventilation – low arterial PO2
3. shunt – blood bypasses the lungs and goes straight to arterial blood (mix of O2 blood and non-O2 blood)
4. low V/Q ratio – MOST COMMON – blocked ventilation area

Question 14

what is the effected of blocked VENTILATION on the alveoli?

Answer 14

Normal:
- PAO2 = 100mmHg
- PACO2 = 40mmHg
- normal ventilation and pulmonary blood flow
- V/Q = 1
Blocked ventilation:
- PAO2 = 40mmHg
- PACO2 = 45mmHg
- flow is the same
- V/Q = 0

Question 15

what is the effected of blocked PULMONARY VESSELS on the alveoli?

Answer 15

Normal:
- PAO2 = 100mmHg
- PACO2 = 40mmHg
- normal ventilation and pulmonary blood flow
- V/Q = 1
Blocked ventilation:
- PAO2 = 150mmHg
- PACO2 = 0mmHg
- flow is blocked
- V/Q = infinite because Q = 0