Lec 4- Respiration Ventilation Perfusion Balance

Question 1

How is the atmospheric pressure of 760 (above sea level) derived?

Answer 1

It is the summation of partial pressures of different gases.

PO2- 160 mmHg
PCO2- 0.3 mmHg
PN2-600 mmHg

Question 2

How to derive a partial pressure of a gas from its atmospheric %?

Answer 2

Multiply the gas atmospheric % with the Atmospheric pressure of 760 mmHg

Question 3

What are the pressure values of arterial oxygen compared to venous oxygen value?

Answer 3

arterial oxygen pressures- 100mmHg

Venous pressures of oxygen- 40 mmHg

Question 4

What are the arterial pressures of CO2 values compared to venous CO2 value?

Answer 4

Venous CO2- 46 mmHg

Arterial CO2- 40 mmHg

Question 5

What is the value of oxygen alveolar pressure?

Answer 5

110 mmHg

Question 6

What is CO2 alveolar pressure?

Answer 6

0.3 mmHg

Question 7

What is the value of ventilation (V) on normal bases?

Answer 7

4 L/min

Question 8

Perfussion value (Q)

Answer 8

5 L/min

Question 9

What is the ventilation-perfussion ratio? (V/Q)

Answer 9

4/5 = 0.8

Question 10

A V/Q ratio of > 0.8

Answer 10

perfusssion defect

Question 11

A V/Q ration of < 0.8

Answer 11

ventilation defect

Question 12

State the effect of gravity on alveolar pressure, precapillary, and postcapillary pressure when seated on an up-right position looking at the 3 zones of the lung

Answer 12

ZONE 1:

Alveolar pressure> precapillary> post capillary

• Aleoli in the apex are almost fully inflated and thus less ventilated. They have high volume and low pressures. However, their pressure is higher than that on precapillary and postcapillary region since these regions are also very poorly perfused. Blood flowing in them is reduced. Blood is pooled at the base of the lung by the force of gravity

ZONE 2:
Precapillary > alveolar> post capillary

• Precapilllary pressure is higher than alveolar. but alveolar is still higher than post capillary

ZONE 3:

Precapillary>Postcapillary>alveolar

• Both precapillary and postcapillary are higher than alveolar pressure due to increased blood flow (perfussion)

Question 13

Why are alveoli in the apex have increased volume?

Answer 13

Lung is pulled down by force of gravity, and that makes intrapleural pressure more negative at the apex. (Transpulmonary pressure will increase) Alveoli will inflate. So they will be less compliant with inhalation then. Can’t inhale further

Question 15

What are the 3 causes of ‘impairment of alveolar gas exchange’

Answer 15

1. Reduced blood flow along alveolar capillaries (perfussion defect)
2. Reduced alveolar ventilation e.g. bronchial obstruction (ventilation defect)
3. Thickened alveolar wall e.g. lung oedema (difussion defect)

Question 15

What is the body’s response mechanism to compensate for reduced ventilation but increased perfussion in order to match ventilation and perfusion ?

Answer 15

• Cause bronchodilation of airways (reduce contractility of alveolar smooth muscles which caused narrowing of the airways. With this there will now be a reduced air way resistance, and increased ventilation)
• Vasoconstriction of pulmonary arteries and limit blood flowing to areas with low oxygen or poorly ventilated.

Question 15

How to dertemine diffussion defect through calculations?

Answer 15

Determined by calculating the Alveolar-arterial oxygen difference

Question 15

What is the value of the pulmonary arteial pressure?

Answer 15

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