Respiration III

Question 1

How do the pulmonary capillaries compare to the systemic capillaries?

Answer 1

The walls of the pulmonary capillaries are thinner than those of similar vessels in the systemic circulation.

Question 2

How does the pressure drop of the systemic circulation compare to the pressure of the pulmonary circulation? Include specific values.

Answer 2

The pressure of the systemic circulation drops from around 100 mm Hg (mean) to nearly 0, so that is a drop of 100 mm Hg. The pressure of the pulmonary circulation drops from around 15 mm Hg (mean) to around 5 mm Hg, so that is a drop of 10 mm Hg.

Question 3

Explain the anatomical reason why the pressure of the pulmonary circulation can be lower than the systemic circulation.

Answer 3

Because the apex of the lungs is close to the heart compared to the organs, the distance that the blood needs to travel in the pulmonary circulation is less.

Question 4

How does the pulmonary resistance compare to the systemic resistance?

Answer 4

The pulmonary resistance is about 1/10 of that of the systemic circulation.

Question 5

Explain how the pulmonary circulation can accommodate increases in cardiac output with little change in the pulmonary arterial pressure.

Answer 5

As pulmonary blood flow increases, pulmonary resistance falls. This fall in vascular resistance results from increasing the cross-sectional area of the vascular bed.

Question 6

Name the 2 ways in which the pulmonary blood vessels can accomodate a high cardiac output. Explain them.

Answer 6

1. Recruitment: recruit capillaries that are closed at rest, thus increasing surface area.
2. Distension: increase the diameter of the blood vessels to reduce resistance.

Question 7

What is the effect of serotonin on the accomodation of pulmonary blood vessels?

Answer 7

It will cause the contraction of smooth muscle, increasing pulmonary vascular resistance in large pulmonary arteries.

Question 8

What is the effect of histamine on the accomodation of pulmonary blood vessels?

Answer 8

It will cause the contraction of smooth muscle, increasing pulmonary vascular resistance in large pulmonary arteries.

Question 9

What is the effect of norepinephrine on the accomodation of pulmonary blood vessels?

Answer 9

It will cause the contraction of smooth muscle, increasing pulmonary vascular resistance in large pulmonary arteries.

Question 10

What is the effect of acetylcholine on the accomodation of pulmonary blood vessels?

Answer 10

It can relax smooth muscle, decreasing pulmonary vascular resistance by inducing the release of nitric oxide from endothelial cells.

Question 11

What is the effect of isoproteranol on the accomodation of pulmonary blood vessels?

Answer 11

It can relax smooth muscle, decreasing pulmonary vascular resistance.

Question 12

How does oxygenation affect the accomodation of pulmonary blood vessels?

Answer 12

There is a reflex vasoconstriction in regions of the lungs that are poorly oxygenated.

Question 13

How do endothelial cells affect the accomodation of pulmonary blood vessels?

Answer 13

Nitric oxide produced by endothelial cells relaxes vascular smooth muscle, leading to vasodilation.

Question 14

In the upright position, describe how gravity affects pulmonary blood flow and why.

Answer 14

Blood flow increases almost linearly from top to bottom of the lungs. The vessels are more distended towards the bottom of the lungs because the hydrostatic pressure of the blood causes an uneven distribution.

Question 15

Describe the effect of gravity on pulmonary blood flow according to the Startling Resistor concept.

Answer 15

The lungs can be looked at as consisting of 3 zones:
- Top: pulmonary arterial pressure < alveolar pressure, so capillaries are compressed. This only occurs in cases of low arterial pressure or positive ventilation
- Middle: pulmonary arterial prsesure > alveolar pressure > venous pressure
- Bottom: pulmonary arterial pressure > venous pressure > alveolar pressure

Question 16

Explain how gravity affects ventilation.

Answer 16

At rest, the alveoli at the top of the lungs are more opened than the bottom ones. Therefore, during breathing the alveoli from the bottom of the lungs are opened wider than those at the top - bottom alveoli receive preferential ventilation.

Question 17

Describe and draw the relationship between ventilation and perfusion in the lungs in normal gravity from bottom to top of the lung.

Answer 17

Va/Q increases from bottom to top. It is equal to 1 at around rib 3, which is when the 2 lines cross.

Ventilation increases slow from top to bottom of the lung but blood flow increases more rapidly. Therefore, the ventilation-perfusion ratio is abnormally high at the top and much lower at the bottom.

Question 18

What is Fick’s principle related to pulmonary blood flow?

Answer 18

Fick’s principle: O2 consumption per minute (VO2) is equal to the O2 taken up by the blood in the lungs in one minute. The [O2] in the blood entering the lungs is CvO2 and that leaving is CaO2.