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Moving air into and out of the lungs effectively facilitates gas exchange
Problems with ventilation means that air is not moving into and out of the lungs effectively, reducing gas exchange
Pathosphysiology: In acute asthma, bronchospasm and mucus secretion narrows and constricts the airways, making effective ventilation difficult, which reduces gas exchange as air is not moved in and out of the lungs effectively.

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The alveoli have a rich blood supply of capillaries
Pathophysiology: circulatory issues such as hypovolaemia or a blockage such as a pulmonary embolism, could lead to reduced blood flow to the lungs, which decreases gas exchange as oxygen and carbon dioxide diffuse into and out of the blood.

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Increase the size of respiratory membranes (amount of alveoli available for gas exchange), increase the rate of gas exchange
Pathophysiology: COPD, particularly emphysema which causes destruction of the alveolar walls, reducing the surface area available for gas exchange

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Increasing the diffusion distance between the air in the alveoli and the blood in the capillaries, decreases the rate of gas exchange
The diffusion distance between the alveoli and blood is small - 4 cells thick, so diffusion of gases is fast
Pathophysiology: Pneumonia causes fluid and mucus to accumulate in the lungs, increasing the diffusion distance between the alveoli and the blood, decreasing the rate of diffusion, slowing down gas exchange

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Partial Pressures of O2 and Carbon DIoxide 

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Partial Pressures of O2 and Carbon DIoxide 

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A difference (concentration gradient) in partial pressures of gases between air and blood needs to be maintained for gas exchange to take place
A larger gradient increases the diffusion rate
High level of oxygen in the inspired air, low level in the blood returning from systemic circulation (used up by respiring cells), increases diffusion of oxygen into the blood
Low level of carbon dioxide in inspired air, high level in blood returning from systemic circulation (produced by respiring cells), increases the diffusion of carbon dioxide into the alveoli to be expired.

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Gases have different solubility coefficients, rate of gas exchange depends on how soluble the gas is.
CO2 is 24 x more soluble than O2 and therefore, diffuses faster.
Pathophysiology: Pulmonary Oedema (build up of fluid in the lungs) affects oxygen diffusion more than carbon dioxide as carbon dioxide is more soluble