Ventilation and perfusion

Question 1

Define ventilation

Answer 1

Exchange of air between the atmosphere and the lungs to allow for gas exchange in the alveoli (oxygen in and carbon dioxide out)

Question 2

Define diffusion

Answer 2

The passive movement of particles down a concentration gradient

[Nb. active diffusion requires energy and moves particles against a concentration gradient]

Question 3

Define perfusion

Answer 3

The passage of fluid through tissues

Question 4

Define dead space

Answer 4

The areas of the lungs that do not permit gas exchange (ventilation but no perfusion)

Question 5

Define conducting airways

Answer 5

Conducting airways are airways where no gas exchange occurs. Gases move down these airways through bulk movement down a pressure gradient.
These airways include trachea, main bronchi, bronchioles and terminal bronchioles.

Question 6

Define hypoxia

Answer 6

Deficiency in the amount of oxygen reaching tissue which can lead to cell/ tissue death

Question 7

What is the process of ventilation and its importance?

Answer 7

Increased volume (and thus reduced pressure) in the lungs caused by expansion of the thoracic wall and depression of the diaphragm allows for movement of gases down a pressure gradient (oxygen in and CO2 out) which allows for gas exchange to take place in the alveolus

Question 8

What is anatomical dead space?

Answer 8

Anatomical dead space refers to the conducting airways of the lungs as the air inspired into these airways is also exhaled and therefore no gas exchange takes place

Question 9

What is physiological dead space?

Answer 9

Physiological dead space is the sum of anatomical dead space and distributive (or alveolar) dead space which refers to the parts of the lungs that do not support gas exchange due to alveoli being damaged or having poor perfusion

Question 10

What is the process of perfusion and its importance?

Answer 10

The passage of fluid through tissues.
In terms of respiration, this allows for oxygen to be carried from the lungs to tissues and CO2 to be carried from the lungs to tissues for removal.

Question 11

What factors determine a gases ability to dissolve in a liquid?

Answer 11

Temperature (solubility higher at higher temperatures)

Pressure (solubility higher at increased pressures)

Question 12

What is the relationship between ventilation and perfusion?

Answer 12

In ideal circumstances, V/Q ratio would be 1 (i.e. rates of ventilation and perfusion would be equal)

[Nb. In reality, rates of ventilation and perfusion occur at different rates in different areas of the lungs causing a mismatch - not normally an issue in healthy individuals]

Question 13

What happens when the relationship between ventilation and perfusion goes wrong?

Answer 13

At high V/Q ratio - Hb is fully saturated and therefore cannot increase any further.
At low V/Q ratio - low oxygen saturation

Shunting occurs as blood from areas of high and low V/Q ratios mix together - this cannot fully increase saturation levels to normal as even a saturation of 98% cannot fully cancel out a low saturation level.

Question 14

How can gravity affect ventilation and perfusion?

Answer 14

Gravity alters blood flow to different areas of the lungs (related to distance from heart):

Zone 1 (Apex) - absent perfusion
Zone 2 (Body) - sporadic perfusion 
Zone 3 (Base) - constant perfusion

Question 15

What is hypoxic vasoconstriction?

Answer 15

Occurs when low partial pressure of oxygen is sensed by capillaries leading to constriction of the airways in order to divert blood flow to alveolar with higher partial pressures of oxygen in order to maximise gas exchange

Question 16

What makes up the diffusion barrier between the alveoli and Hb for transport?

Answer 16

Gas in the alveolus - Epithelial cells in alveolus - Tissue fluid and connective tissue - Endothelial cell of capillary - Plasma - Red blood cell membrane and cytoplasm

Question 17

What four factors affect diffusion rate?

Answer 17

1. Surface area
2. Solubility, molecular weight (i.e. what is diffusing)
3. Thickness of barrier
4. Partial pressure gradient

Question 18

Name a lung condition that results in an increased V/Q ratio

Answer 18

Pulmonary Embolism - prevents effective perfusion thus increasing V/Q ratio

Question 19

Name a lung condition that results in a decreased V/Q ratio

Answer 19

Pneumonia - collection of fluid and pus prevents effective ventilation in affected lung areas thus decreasing V/Q ratio