Gas exchange and breathingIB MYP Sciences: Revision notes

The respiratory system brings oxygen into the body and removes carbon dioxide. Its main parts are:

• Trachea (windpipe): a tube that carries air from the nose and mouth towards the lungs
• Bronchi (one is a bronchus): the two tubes that branch from the trachea, one to each lung
• Bronchioles: smaller tubes that branch from the bronchi
• Alveoli (one is an alveolus): tiny air sacs at the ends of the bronchioles, where gas exchange happens
• Lungs: the organs that contain the airways and alveoli

Air travels: nose or mouth, then trachea, then bronchi, then bronchioles, then alveoli.

The lungs sit inside the chest (thorax), which is formed by the ribcage, diaphragm and intercostal muscles.

• The ribcage is a cage of bones that protects the lungs and heart. It can move up and out and down and in.
• The intercostal muscles are the muscles between the ribs. They move the ribcage.
• The diaphragm is a dome-shaped sheet of muscle underneath the lungs. It separates the thorax from the abdomen and moves down when it contracts.

These structures work together to change the volume of the chest, which moves air in and out (see Ventilation, smoking and exercise).

Gas exchange is the swapping of gases between the air and the blood. In the lungs:

• Oxygen moves from the air in the alveoli into the blood
• Carbon dioxide moves from the blood into the air in the alveoli, and is breathed out

Body cells need oxygen for respiration, and respiration makes carbon dioxide as a waste product. Gas exchange keeps the blood supplied with oxygen and removes the carbon dioxide.

Gases move across the alveolus wall by diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration. The difference in concentration is called the concentration gradient.

• The air in the alveoli has more oxygen than the blood arriving at the lungs, so oxygen diffuses into the blood.
• The blood arriving at the lungs has more carbon dioxide than the air in the alveoli, so carbon dioxide diffuses into the alveoli.

Diffusion is passive: it needs no energy from the cell.

Your lungs have millions of alveoli. Their structure makes diffusion fast and efficient:

• Large surface area: there are millions of alveoli, so a lot of gas can diffuse at once
• Thin walls: the wall is one cell thick, so the diffusion distance is short
• Good blood supply: a dense network of capillaries carries oxygen away and brings carbon dioxide, which keeps the concentration gradient steep
• Moist lining: gases dissolve in the thin layer of moisture before they diffuse across

In an exam, name the feature and say how it helps, for example 'thin walls, so a short diffusion distance'.