The respiratory system

Air breathed in travels:

1. in through the mouth and nose
2. down the trachea (windpipe)
3. into the two bronchi, one to each lung
4. through the smaller bronchioles
5. to the alveoli, tiny air sacs where gas exchange happens.

Breathing in:

• the diaphragm contracts and flattens, moving down
• the intercostal muscles contract, lifting the ribs up and out
• the chest gets bigger, so air is drawn in.

Breathing out:

• the diaphragm relaxes and moves back up into a dome
• the intercostal muscles relax, so the ribs move down and in
• the chest gets smaller, so air is pushed out.

During exercise, these muscles work harder and faster, so a rower can take in more air.

• Breathing rate is the number of breaths taken per minute.
• Tidal volume is the volume of air breathed in or out in one breath.
• Minute ventilation is the volume of air breathed in or out in one minute.

Minute ventilation = tidal volume × breathing rate

Worked example at rest: tidal volume 0.5 litres × breathing rate 12 breaths per minute = 6 litres per minute.

During exercise: tidal volume 2 litres × 30 breaths per minute = 60 litres per minute. These are typical values: everyone's are different.

Gas exchange happens by diffusion: gases move from where there is a high concentration to where there is a low concentration.

• Oxygen diffuses from the air in the alveoli into the blood in the capillaries around them.
• Carbon dioxide diffuses from the blood into the alveoli, to be breathed out.

The alveoli are good at this because their walls are very thin, they have a huge surface area, and they are surrounded by capillaries.