Pulmonary Blood Flow, Gas Exchange & Transport 1

Question 1

Where does the largest change in volume take place in the lung for a given change in intrapleural pressure?

Answer 1

The base

Question 2

What happens to alveolar ventilation from lung base to apex?

Answer 2

It declines

Question 3

What happens to compliance within the lung from base to apex?

Answer 3

It declines

Question 4

Why does the change in compliance between lung base and apex exist?

Answer 4

The alveoli are more inflated at FRC.

Due to forces of gravity, the base of the lung is more compressed and the apex is already somewhat stretched. On inspiration, the base is able to inflate and stretch more, thus greater compliance.

Question 5

What are the bronchial arteries?

Answer 5

Arteries branching from the systemic circulation which provide nutritive circulation to the lung tissues, smooth muscle and nerves.

They are part of the bronchial circulation as opposed to the pulmonary circulation.

Question 6

What is the pulmonary circulation?

Answer 6

The circulation from the right ventricle to the left atrium.

It carries entire cardiac output from right ventricle and supplies the dense capillary network surrounding the alveoli and returns oxygenated blood to the left atrium via the pulmonary vein.

Question 7

What type of system is the pulmonary circulation?

Answer 7

Unique system

Question 8

Describe the pulmonary circulation in terms of pressure and blood flow.

Answer 8

It is a low pressure, high flow system.

Question 9

Explain the blood flow in the pulmonary circulation and why it is different to the systemic circulation.

Answer 9

It is high flow as the pulmonary circulation is much shorter than the systemic circulation but must still pass the entire blood volume in the body at the same rate as the systemic circulation. (i.e. shorter distance, same time therefore increased speed).

Question 10

Explain the blood pressure in the pulmonary circulation and why it is different to the systemic circulation.

Answer 10

It is a low pressure system compared with system circulation because:

1. It is a shorter distance than systemic circulation and blood returns to heart quicker.
2. It reduces resistance to blood flow.

Question 11

What is the systolic pressure in pulmonary circulation?

Answer 11

around 25 mm Hg

Question 12

What is the systolic pressure in systemic circulation?

Answer 12

around 120 mm Hg

Question 13

What can the partial pressure of gases in the arterial circulation tell us about partial pressures elsewhere in the body?

Answer 13

Partial pressures in alveoli are in equilibrium with the partial pressures in systemic arterial blood.

Therefore it can tell us the exact partial pressures in the alveoli (in healthy patients).

Question 14

What can the partial pressure of gases in the venous circulation tell us about partial pressures elsewhere in the body?

Answer 14

Partial pressures in venous blood (pulmonary arterial blood) are in equilibrium with partial pressures in peripheral tissues.

Therefore it can tell us the partial pressures of gases in cells (i.e. in peripheral tissues).

Question 15

What is the partial pressures of oxygen in the arterial circulation and in the alveoli?

Answer 15

100 mm Hg (13.3kPa)

Question 16

What is the partial pressures of carbon dioxide in the arterial circulation and in the alveoli?

Answer 16

40 mm Hg (5.3kPa)

Question 17

What is the partial pressures of oxygen in the venous circulation and in the peripheral tissues?

Answer 17

40 mm Hg (5.3 kPa)

Question 18

What is the partial pressures of carbon dioxide in the venous circulation and in the peripheral tissues?

Answer 18

46 mm Hg (6.2kPa)

Question 19

What are the factors that affect rate of diffusion?

Answer 19

1. Partial pressure gradients (directly proportional)
2. Solubility (directly proportional)
3. Surface area available for gas exchange (directly proportional)
4. Membrane thickness (inversely proportional)
5. Diffusion distance (inversely proportional)

Question 20

Why is rate of gas exchange for oxygen and carbon dioxide roughly the same even though the partial pressure gradient of oxygen is much greater?

Answer 20

Oxygen has a lower solubility in water than carbon dioxide.

Diffusion must take place where their is an air-water interface in the alveoli and plasma in blood, hence solubility is important.

Question 21

What are the gas exchange values for oxygen and carbon dioxide?

Answer 21

O2 = 250ml/min
CO2 = 200ml/min

Question 22

Which anatomical lung feature ensures short diffusion distance for gases?

(There are two points)

Answer 22

The respiratory membrane, i.e. the fused basement membranes of the alveoli and capillaries.

There are never any elastic fibres between capillaries and type I cells to ensure short diffusion distance.

Question 23

How does emphysema affect gas exchange?

Answer 23

Emphysema destroys alveolar surfaces and elastic fibres surrounding alveoli. This reduces the surface area available for gas exchange.

P(A) O2 - normal or low
P(a) O2 - low (due to surface area)

Question 24

How does lung fibrosis affect gas exchange?

Answer 24

With lung fibrosis, the membrane wall thickens which increases the distance for diffusion. Fibrosis may also lead to low compliance and therefore a decrease in alveolar ventilation.

P(A) O2 - normal or low
P(a) O2 - low (due to membrane thickness)

Question 25

How does pulmonary oedema affect gas exchange?

Answer 25

With oedema, there comes an increase in diffusion distance between alveoli and capillaries due to the fluid.

P(A) O2 - normal or low
P(a) O2 - low (due to diffusion distance/non-solubility)
P(a) Co2 - normal (due to solubility)

Question 26

How does asthma affect gas exchange?

Answer 26

Asthma cause self-restrictive constriction of the bronchial smooth muscle, this increased resistance leads to decrease in ventilation.

P(A) O2 - low
P(a) O2 - low

Question 27

How does the body respond to changes in partial pressures of oxygen and carbon dioxide?

Answer 27

The body can tolerate moderate changes to partial pressures of oxygen.

However, the body is not good at tolerating any changes in partial pressure of carbon dioxide.

Question 28

What is the most common cause of emphysema?

Answer 28

Smoking

It increases levels of elastase which breaks down elastic fibres and destroys alveoli surfaces.

Question 29

What should be the ideal relationship between ventilation and perfusion?

Is this relationship always possible?

Answer 29

Optimal conditions would be that they ideally match and compliment one another (i.e. ventilation = perfusion).

This is not always possible because blood flow and alveolar pressure are not the same across the lungs.

Question 30

How does blood vary between the apex and base of the lung?

Answer 30

Blood flow is higher at the base of the lung and lower at the apex.

Arterial pressure is greater than alveolar pressure at the base, therefore there is lower vascular resistance and greater alveoli compression.

At the apex, alveolar pressure is greater than arterial pressure (low compliance) and therefore there is greater vascular resistance due to arterial compression.

Question 31

What can you say about the decline of blood flow and ventilation across the lung?

Answer 31

Both blood flow and ventilation decline from base to apex of lung.

However, blood flow declines faster thus at the apex:

P(a)O2 < P(A)O2

Question 32

What is the optimal ventilation-perfusion ratio?

Where does this occur anatomically?

Answer 32

1

3rd rib

Question 33

What happens to the ventilation-perfusion ratio below rib 3?

Answer 33

It decreases, but does not deviate far from 1 (i.e. somewhat efficient gas exchange).

Question 34

What happens to the ventilation-perfusion ratio above rib 3?

Answer 34

It increases rapidly, there is considerably low blood flow and ventilation and the least efficient gas exchange (i.e. biggest mismatch between ventilation and perfusion).

Question 35

How does the body auto-regulate when perfusion is greater than ventilation (i.e. base of the lungs)?

Answer 35

Pulmonary vasoconstriction - diverts blood flow to better ventilated alveoli (this is particular to pulmonary vessels. This creates a shunt.

Mild bronchodilation - response to increases in alveolar CO2 partial pressures.

Question 36

What is a shunt?

Answer 36

A term used to describe the passage of blood through areas of the lung that are poorly ventilated.

This results in poorly oxygenated blood being diluted with oxygenated blood.

Question 37

How do systemic vessels behave in response to hypoxia?

Answer 37

Vasodilation (i.e. more blood flow to tissues)

Question 38

How does the body auto-regulate when perfusion is less than ventilation (i.e. apex of the lungs)?

Answer 38

Pulmonary vasodilation - increases alveolar O2

Mild bronchoconstriction - reduces alveolar CO2, decreases ventilation slightly

Question 39

What is alveolar dead space?

Answer 39

Refers to alveoli that are ventilated but not perfused e.g. pulmonary embolism

Question 40

What is the purpose of auto-regulation?

Answer 40

It makes up for the imbalances between ventilation and perfusion between the apex and base of the lungs. It brings the ventilation-perfusion ratio back towards 1.