To maintain diffusion of gases across alveolar epithelium air must be constantly moved in and out of lungs = ventilation.
Air pressure of atomosphere = greater than that of lungs, air forced in = inspiration.
Air pressure in lungs greater than that of atomosphere, air forced out = expiration.
Pressure change brought about by two sets of muscles:
- Diaphragm = sheet of muscle that seperates thorax from abdomen.
- Intercostal muscles = lie between ribs, two sets: internal intercostal muscles (contraction leads to expiration) and external intercostal muscles (contraction leads to inspiration).
NB: Volume of blood pumped by heart in one minute = cardiac output.
Breathing = active process (uses energy).
- External intercostal muscles contract, internal intercostal muscles relax.
- Ribs pulled upwards and outwards…