Respiratory Physiology III

Question 1

What is anatomic dead space and what happens as a result of it?

Answer 1

This is essentially the total volume of the conducting airways from the nose or mouth down to the terminal bronchioles.

As a result, some inspired air remains in this space during inspiration and is therefore exhaled unexchanged.

Question 2

How is pulmonary ventilation calculated?

Answer 2

Pulmonary ventilation (L) = tidal volume (L/breath) x respiratory rate (breath/min).

e.g. under resting conditions, this would be 0.5 L x 12 breath/min = 6 L/min.

Question 3

How does alveolar ventilation compare to pulmonary ventilation? Why is this?

Answer 3

It is less than pulmonary ventilation.

This is due to the presence of anatomic dead space.

Question 4

How is alveolar ventilation calculated?

Answer 4

Alveolar ventilation = (tidal volume - anatomic dead space volume) x respiratory rate.

e.g. under resting conditions, this would be (0.5 x 0.15) x 12 = 4.2 L/min.

Question 5

What is pulmonary ventilation?

Answer 5

The volume of air breathed in and out of the lungs per minute.

Remember - this includes the volume remaining in the airways which is exhaled unexchanged.

Question 6

What is alveolar ventilation?

Answer 6

The volume of air exchanged between the atmosphere and alveoli per minute.

Remember - this is more important than pulmonary ventilation as it represents the new air available for gas exchange.

Question 7

What two changes increase pulmonary ventilation?

Answer 7

1. Increased depth of breathing (tidal volume).

2. Increased rate of breathing (respiratory rate - RR).

Question 8

Is it more advantageous to increase the depth or rate of breathing to increase alveolar ventilation? Why?

Answer 8

It is more advantageous to increase the depth of breathing.

This is due to dead space.

Question 9

What two factors does the transfer of gases between the body and atmosphere depend upon?

Answer 9

1. Ventilation - the rate at which gas is passing through the lungs.
2. Perfusion - the rate at which blood is passing through the lungs.

Question 10

Is blood flow greater at the bottom or top of the lung?

Answer 10

The bottom of the lung.

Question 11

Is ventilation better at the bottom or top of the lung?

Answer 11

The bottom of the lung.

Question 12

What can be calculated from knowing the ventilation and perfusion?

Answer 12

The V/Q ratio.

Question 13

What happens as a result of ventilation and perfusion varying to different degrees from the bottom to the top of the lung? Is this significant?

Answer 13

The average arterial and alveolar partial pressure of oxygen are not exactly the same.

This effect is usually not significant but can be in disease.

Question 14

What makes up alveolar dead space?

Answer 14

Alveoli which are ventilated but are not adequately perfused with blood.

Question 15

What makes up the physiological dead space?

Answer 15

The combined volume of both the anatomic dead space and the alveolar dead space.

Question 16

Is the alveolar dead space important in healthy people?

Answer 16

No, it is very small and of little importance, but it can increase significantly in disease.

Question 17

What does the body do as a result of alveolar accumulation of carbon dioxide due to increased perfusion?

Answer 17

It decreases airway resistance to increase airflow.

Question 18

What response does the body make when alveolar oxygen concentration is increased due to increased ventilation?

Answer 18

The pulmonary vessels dilate such that the blood flow increases to match the larger airflow.

Question 19

What happens to the following in an area in which perfusion is greater than ventilation?

1. Oxygen and carbon dioxide concentrations in the area.
2. Dilatation/constriction of airways and blood vessels.
3. Airflow and blood flow.

Answer 19

1. Carbon dioxide concentration will increase in the area while oxygen concentration will drop.
2. The local airway will dilate to increase airflow whilst the local blood vessels will constrict.
3. The airflow will increase as a result of airway dilatation while blood flow will decrease due to vasoconstriction.

Question 20

What happens to the following in an area in which ventilation is greater than perfusion?

1. Oxygen and carbon dioxide concentrations in the area.
2. Dilatation/constriction of airways and blood vessels.
3. Airflow and blood flow.

Answer 20

1. Carbon dioxide concentration will decrease in the area whilst oxygen concentration will increase.
2. The local airway will constrict whilst the local blood vessels will dilate.
3. The result is that airflow will decrease whilst blood flow will become increased.

Question 21

In pulmonary arterioles, what is the effect of i) increased oxygen concentration and ii) decreased oxygen concentration on the state of the vessel?

Answer 21

i) Increased oxygen concentration results in vasodilatation of pulmonary arterioles.
ii) Decreased oxygen concentration results in vasoconstriction of pulmonary arterioles.

Question 22

In systemic arterioles, what is the effect of i) increased oxygen concentration and ii) decreased oxygen concentration on the state of the vessel?

Answer 22

i) Increased oxygen concentration results in vasoconstriction of systemic arterioles.
ii) Decreased oxygen concentration results in vasodilatation of systemic arterioles.

Question 23

What are the four factors that influence the rate of gas exchange across the alveolar membrane?

Answer 23

1. Partial pressure gradient of oxygen and carbon dioxide.
2. Diffusion coefficient for oxygen and carbon dioxide.
3. Surface area of the alveolar membrane.
4. Thickness of the alveolar membrane.

Question 24

What does the partial pressure of a gas in a mixture of gases determine for that gas?

Answer 24

Its pressure gradient.

Question 25

What is the partial pressure of a gas defined as?

Answer 25

The pressure that one gas would exert if it occupied the total volume for the mixture in the absence of other components at a given temperature.

e.g. if the total pressure of a gas mixture was 100 kPa, if half of the mixture is one gas, then the partial pressure for that gas would be 50 kPa.

Question 26

What is Dalton’s Law of partial pressures?

Answer 26

This law states the following:

“the total pressure exerted by a gaseous mixture = the sum of each individual component in the gas mixture.”

i.e. P(total) = P(1) + P(2) + … + P(n)

Question 27

What is the alveolar gas equation?

Answer 27

PAO2 = PiO2 - [PaCO2/0.8]

where PAO2 = partial pressure of oxygen in alveolar air;
PiO2 = partial pressure of oxygen in inspired air;
PaCO2 = partial pressure of CO2 in arterial blood;
0.8 is the respiratory exchange rate (RER) (i.e. ratio of CO2 produced/oxygen consumed) for someone eating a mixed diet.

Question 28

How does the partial pressure gradient of oxygen across pulmonary capillaries compare with that of carbon dioxide?

Answer 28

It is much larger - 60 mmHg vs. 6 mmHg.

Question 29

How does the partial pressure gradient of oxygen across systemic capillaries compare with that of carbon dioxide?

Answer 29

It is much larger - > 60 mmHg vs > 6 mmHg.

Question 30

What offsets the difference in partial pressure gradient between carbon dioxide and oxygen such that they are exchanged equally?

Answer 30

Carbon dioxide is more soluble in membranes than oxygen.

Question 31

What is the solubility of a gas in a membrane known as?

Answer 31

Its diffusion coefficient.

Question 32

How many times greater is the diffusion coefficient of carbon dioxide compared with that of oxygen?

Answer 32

20 times greater.

Question 33

Is a small gradient between PAO2 and PaO2 normal? Why is this?

Answer 33

Yes, it is normal.

This is because ventilation-perfusion match is usually not perfect so exchange is not 100% efficient.

Question 34

What two problems could a big gradient between PAO2 and PaO2 be indicative of?

Answer 34

1. Problems with the gas exchange in the lungs.

2. A right to left shunt of the heart.

Question 35

What is Fick’s Law of diffusion?

Answer 35

The amount of gas that moves across a sheet of tissue in unit time is proportional to the area of the sheet but inversely proportional to its thickness.

Question 36

Describe the walls of alveoli in terms of the cells mainly involved in gas exchange and the way these are organised.

Answer 36

The walls consist of a single layer of flattened type I alveolar cells.

Question 37

How do the partial pressure gradients of oxygen and carbon dioxide influence the rate of gas transfer?

Answer 37

The rate of transfer increases as partial pressure increases.

Question 38

How does the diffusion coefficient influence the rate of gas transfer?

Answer 38

The rate of transfer increases as diffusion coefficient increases.

Question 39

How does the surface area of the alveolar membrane influence the rate of gas transfer?

Answer 39

The rate of transfer increases as surface area increases.

Question 40

How does the thickness of the alveolar membrane influence the rate of gas transfer?

Answer 40

The rate of transfer increases as thickness decreases.

Question 41

What are some non-respiratory functions of the respiratory system?

Answer 41

1. Route for water loss and heat elimination.
2. Enhances venous return.
3. Helps maintain normal acid-base balance.
4. Enables speech, singing, and other vocalisations.
5. Defends against inhaled foreign matter.
6. Removes, modifies, activates, or inactivates various materials passing through the pulmonary circulation.
7. Nose serves as the organ of smell.

Question 42

What happens to the state of blood vessels (in terms of constriction/dilatation) as a result of i) decreased oxygen and ii) increased oxygen in the following?

1. Pulmonary arterioles.
2. Systemic arterioles.

Answer 42

1. i) Vasoconstriction.
   ii) Vasodilatation.
2. i) Vasodilation
   ii) Vasoconstriction.