Breathing and gas exchange

Why do we breathe?

Breathing moves air into and out of the lungs. It brings in oxygen and removes carbon dioxide. The movement of air is also called ventilation.

Cells use oxygen in aerobic respiration, which releases energy from food. Respiration produces carbon dioxide. The blood carries oxygen from the lungs to cells and returns carbon dioxide to the lungs.

The route air takes

Air enters through the nose or mouth. Its route through the lungs becomes more finely branched:

• Trachea: the windpipe carrying air towards the lungs.
• Bronchi: the two branches of the trachea, one leading to each lung.
• Bronchioles: smaller branches inside the lungs.
• Alveoli: tiny air sacs at the ends of the bronchioles. One air sac is an alveolus.

Breathing in

The diaphragm is a sheet of muscle below the lungs. During breathing in:

• The diaphragm contracts, flattens and moves down.
• Muscles between the ribs move the ribcage up and out.
• The space in the chest increases, so the lungs expand.
• Air pressure inside the lungs falls below outside air pressure. Air flows in.

Breathing out

During normal, quiet breathing out:

• The diaphragm relaxes and moves upwards.
• The ribcage moves down and in, and the lungs spring back towards their smaller size.
• The space inside the lungs decreases.
• Air pressure rises above outside air pressure, so air flows out.

Gas exchange at the alveoli

Alveoli are surrounded by tiny blood vessels called capillaries. Oxygen moves from the air in the alveoli into the blood. Carbon dioxide moves from the blood into the alveoli and is breathed out.

These gases move by diffusion: overall, particles move from where their concentration is higher to where it is lower.

• Many alveoli provide a large surface area.
• The alveolar and capillary walls are very thin, so gases have only a short distance to cross.
• Blood flow carries oxygen away and brings more carbon dioxide.
• Breathing replaces the air, keeping oxygen available.

What changes during exercise?

Active muscles need more energy, so their rate of respiration increases.

• Muscles use more oxygen and produce more carbon dioxide.
• Breathing rate and heart rate usually rise to help meet this demand.
• Exhaled air still contains oxygen. It contains less oxygen and more carbon dioxide than inhaled air.

Breathing and respiration are different

Breathing is the physical movement of air. Respiration is a series of chemical reactions inside cells. Breathing, gas exchange and blood circulation work together to supply those cells with oxygen.