How does oxygen and carbon dioxide diffuse both between the alveoli and the blood, and blood and the body tissues?

Oxygen breathed into the lungs diffuses into the alveoli, and then passes through into the capillary walls. Here it turns into oxyhaemoglobin. Carbon dioxide is carried by blood into the alveoli and is breathed out of the lungs.

Gases diffuse along pressure gradients. In other words, gases spread from high concentration areas to low-concentration areas.

The blood in the capillaries arriving at the alveoli is low in oxygen and high in carbon dioxide. Air in the alveoli is high in oxygen and low in carbon dioxide.

Therefore oxygen diffuses into the blood (attaching itself to the haemoglobin), and carbon dioxide diffuses into the alveoli, where it is breathed out.

The reverse process occurs at the capillaries that surround the muscle fibres