Topic 11: Gas exchange in humans

Every cell in your body needs oxygen to release energy, and every cell produces carbon dioxide that must be removed. The lungs are where this swap happens. In this topic you will see how the lungs are beautifully adapted for fast, efficient gas exchange, how the muscles of breathing pump air in and out, and how the air you breathe out differs from the air you breathe in.

The breathing system

Air travels through a series of structures to reach the lungs, where gas exchange takes place.

• Air enters through the nose and passes down the trachea (windpipe).
• The trachea divides into two bronchi, one to each lung.
• Each bronchus branches into many smaller bronchioles.
• Bronchioles end in tiny air sacs called alveoli, where gas exchange happens.

Rings of cartilage hold the trachea and bronchi open so air can always flow. The lungs sit inside the thorax, surrounded by the ribs and separated from the abdomen by the diaphragm, a sheet of muscle.

Adaptations of the alveoli

Gas exchange in the alveoli works by diffusion: oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be breathed out. The alveoli are highly adapted to make this fast.

• There are millions of alveoli, giving a very large surface area.
• Each alveolus has a thin wall (one cell thick), giving a short diffusion distance.
• Alveoli have a rich blood supply from a network of capillaries, keeping concentration gradients steep.
• The surface is moist, so gases dissolve before diffusing.
• A good supply of fresh air maintains steep concentration gradients for oxygen and carbon dioxide.

Ventilation: breathing in and out

Ventilation means moving air in and out of the lungs. It is brought about by the diaphragm and the intercostal muscles between the ribs.

• During inspiration (breathing in), the diaphragm contracts and flattens, and the external intercostal muscles contract to pull the ribs up and out. This increases the volume of the thorax, lowering the pressure so air is drawn in.
• During expiration (breathing out), the diaphragm and external intercostal muscles relax, the ribs move down and in, and the thorax volume decreases. Pressure rises, so air is pushed out.

So changes in volume cause changes in pressure, and air always moves from higher to lower pressure.

Inspired air, expired air and exercise

The air you breathe out is different from the air you breathe in because gas exchange has taken place.

• Inspired air contains more oxygen and less carbon dioxide.
• Expired air contains less oxygen, more carbon dioxide, and more water vapour.
• The percentage of nitrogen stays roughly the same.

You can show expired air contains more carbon dioxide using limewater, which turns milky (cloudy) faster when breathed-out air bubbles through it. During exercise, muscles respire faster and need more oxygen, so the rate and depth of breathing increase to supply more oxygen and remove more carbon dioxide. Goblet cells make mucus to trap pathogens and dust, and ciliated cells sweep this mucus away from the lungs.