How Does the Human Gas Exchange System Work?

The human gas exchange system brings air into the lungs so oxygen can diffuse from the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli to be breathed out.

How does air reach the alveoli?

Air travels down the trachea, through the bronchi and then through smaller bronchioles to the alveoli. Alveoli are tiny air sacs surrounded by capillaries, which are narrow blood vessels. Gas exchange takes place between the air in the alveoli and the blood in these capillaries.

When you breathe in, the diaphragm contracts and flattens, and the external intercostal muscles contract to move the ribs up and out. This increases the volume of the chest, lowers the pressure in the lungs and draws air in. During normal breathing out, these muscles relax. Chest volume decreases, pressure rises and air moves out. The internal intercostal muscles can help during forced breathing out.

How are the gases exchanged?

Fresh air in an alveolus has a higher oxygen concentration than the blood arriving at its capillaries. Oxygen therefore diffuses into the blood, where much of it binds to haemoglobin in red blood cells. The arriving blood has a higher carbon dioxide concentration than the air in the alveolus, so carbon dioxide diffuses in the opposite direction and is exhaled. Diffusion is the net movement of particles from a higher to a lower concentration.

The many alveoli provide a large surface area. Their walls and the capillary walls are each one cell thick, giving the gases a short diffusion distance. Ventilation refreshes the air, while blood flow carries oxygen away and brings more carbon dioxide. Together, these maintain the concentration differences needed for diffusion.

Exam mistake to avoid: the diaphragm does not pump oxygen directly into the blood. Breathing moves air; diffusion moves gases across the alveolar and capillary walls.