Ventilation and Perfusion

Question 1

What are the roles of the lungs?

Answer 1

• Gas exchange
• Reservoir for blood and oxygen
• Metabolism of some circulating compounds
• Filter blood

Question 2

How is the respiratory system organised?

Answer 2

Conducting airways:

• Anatomical dead space
• Bulk flow
  
  • Trachea
  • Bronchi
  • Bronchioles
  • Terminal bronchioles

Respiratory airways:

• Gas exchange
• Gases move down their individual pressure gradients
  
  • Respiratory bronchioles
  • Alveolar ducts
  • Alveolar sacs

Question 3

What are the different airways made up of?

Answer 3

Bronchi: cartilage

Bronchioles: smooth muscle and

Question 4

What type of blood does the pulmonary artery carry?

Answer 4

Deoxygenated

Question 5

What type of blood does the pulmonary vein carry?

Answer 5

Oxygenated

Question 6

What does the distensibility of the pulmonary capillaries allow?

Answer 6

Allows for low resistance throughout the pulmonary circulation allowing blood flow to the lungs to increase without resistance and pressure increasing.

Question 7

How does the respiratory system regulate blood flow?

Answer 7

Hypoxic vasoconstriction:

• Shunting blood to areas of the lung with high partial pressures of oxygen and away from alveoli with low partial pressures of oxygen.

Question 8

Define diffusion

Answer 8

The movement of gases across the alveolar membrane

Question 9

If perfusion is increased? What will happen to the partial pressure of O₂ in the alveoli?

Answer 9

Decrease

Question 10

If ventilation is increased, what will happen to the partial pressure of O₂ in the alveoli?

Answer 10

Increase

Question 11

What is the diffusion barrier made up of?

Answer 11

• Epithelial cell of alveolus
• Tissue fluid and connective tissue
• Endothelial cell of capillary
• Plasma
• Red blood cell membrane
• Red blood cell cytoplasm

Question 12

Under normal circumstances, what is Hb saturation limited by?

At what point along the pulmonary capillaries are Hb completely saturated with oxygen?

Answer 12

Perfusion limited- perfusion limitation occurs with both oxygen and carbon dioxide under normal circumstances.

25%

Question 13

In disease states, how may Hb saturation be limited?

Answer 13

Diffusion limited- full equilibration may not occur until the end of capillaries or not at all

Question 14

What is the effect of disease processes on the oxygen reserve?

Answer 14

Hb may not be able to fully saturated (reach full equilibration) therefore there is just enough to provide tissues with oxygen but not enough to cover increased demand (e.g. exercise)

Question 15

Define ventilation

Answer 15

Movement of gas in and out of the lungs

Question 16

What are the 4 lung volumes?

Answer 16

Tidal volume: Volume of air moved in and out of lungs in normal (quiet) breathing

Inspiratory reserve volume: Extra voolume that can be breathed in over that at rest

Expiratory reserve volume: Extra volume that can be breathed out over that at rest

Residual volume: Volume of air left in the lungs after maximum expiration

Lung volumes change with breathing patterns

Question 17

What are the 4 lung capacities?

Answer 17

Total lung capacity: Inspiratory reserve volume + expiratory reserve volume + tidal volume + residual volume

Inspiratory capacity: Tidal volume + inspiratory reserve volume

Expiratory capacity: Tidal volume + expiratory reserve capacity

Functional residual capacity: Reserve of air left in lungs at the end of passive exhalation (resting expiratory level)

Lung capacities do not change with breathing patterns

Question 18

Define dead space airways

Answer 18

Areas of the airways not involved in gas exchange

Air here last in and first out: enters and leaves via same airways, does not enter alveoli.

2 types:

• Serial: conducting airways (anatomical dead space)
• Distributive: damaged alveoli or those with poor perfusion

Question 19

Define total/minute ventilation

What is a normal minute ventilation at rest and in exercise?

Answer 19

The amount of air being moved in and out of the lungs per minute

Normal = 6-8L/min at rest, can reach 80L/min during exercise

MV = tidal volume x respiratory rate

Question 20

Define the alveolar ventilation rate

How is it calculated?

Answer 20

The amount of air that reaches the alveoli per minute

Need to account for ‘dead space’ (air that sits in conducting airways and does not reach alveoli). 2 types:

• Serial: volume of conducting airways (anatomical dead space)
• Distributive: parts of the lungs that are not conducting airways but cannot take part in gas exchange (damaged alveoli or those with poor perfusion)

Serial + distributive = physiological dead space

AVR = (V_t - V_ds) x RR

Question 21

What effect does shallow breathing have on dead space?

Answer 21

Amplifies the effect of dead space- less air reaches alveoli

Question 22

What effect does alveolar ventilation rate have on diffusion?

Answer 22

Increasing AVR will remove CO₂ from the alveoli, increasing the partial pressure gradient of CO₂ between pulmonary capillaries and alveoli, thus increasing its diffusion from the blood into the lungs and its removal from the body.

Question 23

What is the ideal V/Q (ventilation/perfusion) ratio?

Answer 23

1 (equal)

Question 24

What does a V/Q ratio less than 1 mean?

What does a V/Q ratio more than 1 mean?

Answer 24

Less than 1 = perfusion > ventilation

More than 1 = ventilation > perfusion

Question 25

What is the main reason for defective gas exchange in respiratory diseases?

Answer 25

Exacerbated V/Q mismatches: * Increased V/Q ratio= blood saturated, little O₂ added by ventilation * Decreased V/Q ratio= O₂ reduction due to low ventilation compared to perfusion.

Question 26

How do ventilation and perfusion change from the apex to the base of the lung?

Answer 26

Both increase from the apex to the base of the lung * Perfusion increases more rapidly therefore V/Q ratio decreases towards the base of the lung * Increase in blood flow at the base of the lung due to gravity. * Ventilation increases as the base of the lung is more compressed therefore more room for expansion

Question 27

What happens to the V/Q ratio in: Pneumonia P.E

Answer 27

P.E: Increased V/Q ratio (decreased perfusion) Pneumonia: Decreased V/Q ratio (decreased ventilation)

Question 28

Why is there increased ventilation towards the base of the lung?

Answer 28

The base of the lung is more compressed due to gravity therefore has more room for expansion

Question 29

What happens to apical blood vessels during exercise?

Answer 29

Blood vessels at the apex of the lung which are normally closed open during exercise to accommodate increased cardiac output. This increases oxygen perfusion at the apex of the lung.

Question 30

Why can high and low V/Q mismatch areas not cancel eachother out?

Answer 30

Areas with high V/Q ratios cannot add anymore oxygen to blood as it is already saturated with oxygen. Areas with low V/Q will therefore result in a significant oxygen decrement in mixed capillary blood.

Question 31

How do V/Q ratios explain the oxygen dissociation curve?

Answer 31

At the top of the curve where blood is fully saturated, there is a high V/Q ratio. At the bottom of the curve, where blood is poorly saturated, there is a low V/Q ratio.

Question 32

How can pneumonia produce low oxygen saturations

Answer 32

Well saturated blood from high V/Q ratio areas of the lung mixes with poorly saturated blood from low V/Q ratio areas of the lung to produce overall lower than normal oxygen saturation Blood from low V/Q area is effectively shunted as it does not pick up oxygen as it passes through the lung.