Elastic fibres contain elastin that allows alveoli to stretch when they are full of air and to revert, or recoil, back to their original shape when air leaves alveoli.

An alveolus is a small air sac with a thin wall comprising one layer of squamous epithelial cells, elastic fibres, and collagen fibres.

Each alveolus is also surrounded by a capillary network.

Collagen fibres contain strong collagen that prevents alveoli from bursting and limits overstretching.

Oxygen diffuses from atmospheric air in the alveoli cavity across the alveoli epithelium into the blood in the pulmonary capillaries. There, it binds to haemoglobin in red blood cells.

Carbon dioxide dissociates from haemoglobin in red blood cells and diffuses from the blood in the pulmonary capillaries across the alveoli epithelium into the cavity of the alveoli. From there, it can be exhaled.

1. Thin walls - the alveolar epithelium is one cell thick so gases only travel a short distance
2. Steep diffusion gradient maintained by ventilation of the lungs - increases diffusion speed of gases
3. Large surface area - increases diffusion speed of gases
4. Partially permeable - only specific gases (carbon and oxygen) can move across the alveolar epithelium
5. Moist inner surface - water allows gases to dissolve

1. Thin walls - capillary walls are one cell thick so gases only travel a short distance
2. Steep diffusion gradient maintained by the movement of blood - increases diffusion speed of gases
3. Large surface area - increases diffusion speed of gases
4. Slow movement of blood - speed of blood reduces in the capillaries giving more time for diffusion
5. Red blood cells are flat against capillary wall - reduces gas diffusion distance between the red blood cells and the alveoli

Pulmonary artery - delivers deoxygenated blood (rich in carbon dioxide) from the heart to the pulmonary capillaries

Capillaries - carbon dioxide diffuses from the capillary blood into the alveoli, and oxygen diffuses from the alveoli into the capillary blood

Pulmonary vein - delivers oxygenated blood (rich in oxygen) from the pulmonary capillaries to the heart

The role of the human gas exchange system is to exchange oxygen and carbon dioxide between the body and the environment.

Oxygen diffuses from the environment into the body, specifically into the alveoli in the lungs from which it enters blood. Carbon dioxide diffuses from the blood into the alveoli and then out of the body.

1. Air is not dense enough to support the delicate lung tissues
2. To reduce water loss

Gas exchange occurs across the alveolar membrane of alveoli.

The alveoli are small air sacs located at the end of the bronchioles.

1. Air enters the trachea (the windpipe)
2. Air moves into the two bronchi (there is one bronchus per lung)
3. Air moves into the bronchioles (smaller airways)
4. Air moves into the alveoli (small air sacs at the end of each bronchiole)

1. Cartilage - supports the airways and prevents them from collapsing
2. Smooth muscle -  contracts or relaxes to constrict or dilate the airway and change airflow
3. Goblet cells - produce mucus to trap dirt, dust, and microorganisms
4. Ciliated epithelial cells - lined with cilia to waft mucus away from the alveoli

The bronchioles also contain smooth muscle, goblet cells and ciliated epithelial cells in their walls, but do not contain cartilage.

The lungs are located inside the chest in the thoracic cavity, protected by the ribcage.

The thoracic cavity is sometimes called the thorax, and the lungs are also surrounded by the pleural membranes.