Gas Exchange in the Lungs

Question 1

What do we use to quantify gases?

Answer 1

Partial pressures

Question 1

What does partial pressure refer to?

Answer 1

Partial pressure refers to a single gas in a mixture of gases and what it is contributing to the total pressure within a system

Question 2

State the percentages of each gas in the air

Answer 2

Air is around 78% nitrogen, 21% oxygen, 3% carbon dioxide and the water partial pressure varies depending on the humidity in the air

Question 3

State how we can calculate partial pressure of a total system

Answer 3

Partial pressure of the total system = partial pressure of water + the sum of the partial pressures of the other constituent gases

Question 4

State how to find the partial pressure of an individual constituent gas

Answer 4

Partial pressure of an individual constituent gas = (atmospheric pressure - water vapour partial pressure) x the mole fraction (the % of total moles represented by the individual gas)

Question 5

How is partial pressure calculated

Answer 5

Partial pressure is calculated by multiplying total pressure by mole fraction

Question 6

What is the concentration of a gas dissolved in a liquid determined by?

Answer 6

The partial pressure and solubility of a gas -

Concentration = partial pressure x solubility

Partial pressure of a gas dissolved in a liquid reflects the amount of gas that would dissolve (at equilibrium) if the liquid was placed in contact with a gas phase equivalent partial pressure- e.g. if pure water was placed in oxygen at 21 kPa then oxygen would enter that liquid until the partial pressure in the water was 21 kPa (until equilibrium was reached)

Question 7

Describe the series of structures oxygen has to diffuse through to get to the blood

Answer 7

1. The oxygen enters the alveolar airspace from the atmosphere
2. The oxygen dissolves in the alveolar air fluid
3. Oxygen diffuses through the alveolar epithelium, basement membrane and capillary endothelial cells
4. Oxygen then diffuses into the blood plasma and binds to haemoglobin molecules

Question 8

Explain why it is important that the blood gas barriers are thin

Answer 8

• Oxygenation of blood must occur during the brief time period (approx 0.75 s) that it takes for red blood cells to flow through the pulmonary capillary
• In a healthy situation oxygen is able to diffuse fast enough to ensure adequate oxygenation
• When the blood- gas barrier becomes thickened it means that full oxygenation is not achieved as it takes too long for oxygen to diffuse into the blood plasma
• This is further exacerbated during exercise when the rate of blood flow increases - red blood cells flow through the lungs at a faster rate so oxygenation is even lower

Question 9

State the factors that impact gas exchange

Answer 9

• Alveolar surface area
• Partial pressure gradient between alveolar air and capillary blood (Pa - Pc)
• Epithelial and endothelial cell thickness + basement membrane thickness + fluid layer depth (distance squared)

Question 10

State an equation that describes the factors that impact diffusion rate

Answer 10

Rate of diffusion is proportional to (surface area/distance squared) x (Pa - Pc)

Question 11

State what is needed for a maximal diffusion rate

Answer 11

• A large partial pressure gradient - high oxygen / low carbon dioxide in the alveolar air and low oxygen / high carbon dioxide in the capillary blood
• A high surface area
• A small diffusion distance

Question 12

State some pathologies that impact diffusion rate

Answer 12

• Hypoventilation e.g. from type II respiratory failure causes a reduced partial pressure gradient
• Emphysema decreases the surface area
• Fibrosis increases the basement membrane thickness , pulmonary oedema increases the thickness of the fluid layer

Question 13

Describe how the alveoli are adapted for gas exchange

Answer 13

• Large surface area - each individual alveoli has a small surface area but all alveoli form a large surface area to volume ratio
• The epithelium is one cell thick and the basement membrane is fused to the capillary
• The alveoli are richly innervated by capillaries so have a good perfusion

Question 14

Describe why ventilation perfusion coupling is needed (1)

Answer 14

Blood flow through pulmonary capillaries (perfusion) needs to be matched to alveolar ventilation to enable efficient gas exchange

Question 15

Describe why ventilation perfusion coupling is needed (2) and what a healthy value is

Answer 15

Each unit of blood has a finite amount of oxygen it is able to carry and so demand must be met through changing the rate of blood flow

The alveolar ventilation / perfusion ratio (V/Q) describes this relationship - 1L of blood can carry 200 ml of oxygen and 1L of dry air has 200ml of oxygen so V / Q should be 1 and in a healthy individual it is

Question 16

Describe what causes the V/Q ratio to change

Answer 16

When V/Q rises above one it is because of hypoperfusion (lack of perfusion) ‘dead space effect’

When V/Q falls below one it is because of hypoventilation (lack of ventilation) ‘shunt’

Question 17

Describe why we dont talk about CO2 in V/Q ratios

Answer 17

In theory the inequality between V and Q affects both oxygen and carbon dioxide exchange however in most cases increasing the partial pressure of carbon dioxide will induce a reflex hyperventilation that clears most of the excess carbon dioxide but does not increase the partial pressure of oxygen

Question 18

Describe how ventilation perfusion coupling is maintained

Answer 18

• Homeostatic mechanisms exist to reduce ventilation perfusion mismatching
• Hypoxic vasoconstriction of capillaries diverts blood flow from poor to well ventilated alveoli

1. Under normal condition blood flow and ventilation are matched
2. If ventilation of specific alveoli decreases the partial pressure of carbon dioxide will increase and partial pressure of oxygen will decrease (decreases the partial pressure gradients) therefore there is decreased oxygenation of blood flowing through capillaries
3. A low partial pressure of oxygen induces vasoconstriction which decreases blood flow - hence blood flow is diverted to alveoli with increased ventilation

Question 19

State what can cause pulmonary dead space (ventilation without perfusion)

Answer 19

• Heart failure (cardiac arrest)
• Blocked vessels (pulmonary embolism)
• Loss/damage to capillaries (emphysema)

Question 20

Describe how pulmonary embolism affects V/Q ratio

Answer 20

• In a pulmonary embolism pulmonary arteries can be occluded meaning the blood supply to that region of the lung is prevented
• This creates an unperfused alveoli (dead space) where the V/Q ratio is increased
• Perfusion to the remaining alveoli then increases as cardiac output is diverted to other alveoli which decreases the V/Q ratio
• Unless ventilation can increase to match perfusion, hypoxaemia and hypercapnia will occur
• Can be treated by using oxygen therapy

Question 21

What can cause a pulmonary shunt (perfusion without ventilation)

Answer 21

• Pneumonia
• Acute lung injury
• Respiratory distress syndrome
• Atelectasis (lung collapse)

Question 22

Explain why a shunt cannot be treated with oxygen therapy

Answer 22

• When one alveoli is damaged e.g. from alveolar oedema from a lung injury it can no longer have gas exchange so the blood passing past the alveoli remain at the same low oxygen saturation
• Other alveoli that are still oxygenating the blood - these can be fully saturated using oxygen therapy but because oxygenation is already at 98% usually, pushing it to 100% for example is not much of an improvement
• Also by the time the fully saturated blood mixes with the underperfused blood from the other alveoli the overall blood perfusion is still lower than normal as the oxygenated alveoli cannot fully compensate