Ventilation and perfusion

Question 1

What is the usual value of tidal volume?

Answer 1

500mls

Question 2

How do you calculate minute ventilation?

Answer 2

Tidal volume x respiratory rate

Question 3

What is the typical value of minute ventilation?

Answer 3

6-8 L/min at rest

Question 4

Define anatomical dead space.

Answer 4

Volume of air in the conducting airways

Question 5

What is the typical value of anatomical dead space?

Answer 5

150mls

Question 6

Define physiological dead space.

Answer 6

The total volume of air that does not participate in gas exchange

Question 7

Give the calculation for physiological dead space.

Answer 7

Physiological dead space = anatomical dead space + alveolar dead space

Question 8

Define hyperventilation.

Answer 8

An increase in the rate and depth of breathing

Question 9

Describe how increased alveolar ventilation affects the partial pressure of CO2 in the alveoli and how this would affect diffusion of CO2 from the blood.

Answer 9

When alveolar ventilation increases, it helps remove CO2 from the alveoli and brings in more oxygen. Removing CO2 reduces the partial pressure of CO2 in the alveoli. This lowers alveolar pCO2 which increased the pressure gradient between the alveoli and the blood. With a greater pressure gradient, CO2 will diffuse more easily from the blood into the alveoli to be exhaled.

Question 10

What are some common causes of low V/Q ratio?

Answer 10

Airway obstruction such as asthma or pneumonia

Question 11

What could be a cause of high V/Q ratio?

Answer 11

Pulmonary embolism

Question 12

The respiratory control centre is in which area of the brain?

Answer 12

Medulla

Question 13

The brainstem responds to changes in the lungs via which 2 mechanisms?

Answer 13

Change in lung volume via mechanoreceptors

Change in blood gas composition via chemoreceptors

Question 14

The central chemoreceptors are located in which area of the medulla?

Answer 14

Ventral surface of the medulla

Question 15

What is the function of the central chemoreceptors in relation to CO2?

Answer 15

Maintain pCO2 within normal range

Question 16

What 2 things do the central chemoreceptors respond to and how?

Answer 16

Directly to changes in pH of CSF

Indirectly to changes in arterial pCO2

Question 17

Which 2 areas are the peripheral chemoreceptors located?

Answer 17

Carotid bodies – at the bifurcation of the common carotid artery

Aortic bodies – in the aortic arch

Question 18

What 3 things do the peripheral chemoreceptors respond to?

Answer 18

Arterial pO2

Arterial pCO2

Arterial H+ and thus pH

Question 19

The peripheral chemoreceptors respond dramatically when pO2 is below what value?

Answer 19

Respond dramatically when pO2 <8kPa

Question 20

How do the peripheral chemoreceptors signal for a change in ventilation?

Answer 20

Signal to the respiratory control centre to modulate ventilation via the vagus nerve and glossopharyngeal nerve

Question 21

Which 2 mechanisms do the lungs use to prevent a large increase in pressure when blood flow increases?

Answer 21

Capillary distension

Capillary recruitment – previously closed capillaries open up to accommodate more blood flow

Question 22

Name a local lung mediator which causes vasoconstriction.

Answer 22

Thromboxane A2

Question 23

Name a local lung mediator which causes vasodilation.

Answer 23

Prostacyclin

Question 24

What effect does O2 have on pulmonary arteriole smooth muscle?

Answer 24

Relaxes pulmonary arteriole smooth muscle

Question 25

What is the normal V/Q value?

Answer 25

0.8

Question 26

Describe what a shunt is and what value V/Q ratio this would lead to.

Answer 26

An area of lung with perfusion but no ventilation, resulting in a V/Q ratio of zero

Question 27

Describe what dead space is and what value V/Q ratio this would lead to.

Answer 27

An area of lung with ventilation but no perfusion, resulting in an infinite V/Q ratio

Question 28

In which area of the lung is perfusion greater, apex or base? Why?

Answer 28

Base, due to the vessels at the base having less gravity to work against

Question 29

In which area of the lung is ventilation greater, apex or base? Why?

Answer 29

Base

Due to gravity, the intrapleural pressure at the apex of the lung is more negative compared to the base. This means the alveoli at the apex are more expanded at rest. In contrast, alveoli at the base are relatively compressed. Since the alveoli at the apex are already more expanded, they are less compliant and don’t expand as much as we inhale. The alveoli at the base, being less expanded to start with, are more compliant and can expand more with each breath. As a result, a larger proportion of inhaled air goes to the base of the lungs, making ventilation greater there

Question 30

Are gravitational effects greater on ventilation or perfusion?

Answer 30

Perfusion

Question 31

How does V/Q ratio change as you move from the apex to the base of the lung?

Answer 31

V/Q is high at the apex of the lung and low at the base of the lung

Question 32

Describe why other areas of the lung can’t compensate when certain areas of the lung have a low V/Q ratio, for example in pneumonia?

Answer 32

In areas with a normal V/Q ratio, the haemoglobin quickly becomes saturated with oxygen, meaning increased ventilation will not increase the oxygen content of the blood