Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Diaphragm
What is Diaphragm?
A sheet of muscle and fibrous tissue that separates the thorax from the abdomen and forms the floor of the chest cavity. At rest it is dome-shaped, curving upward into the thorax; because the liver lies beneath it the right dome sits higher than the left. Its muscle fibres run from the lower ribs, the sternum and the lumbar vertebrae inward to a central tendon, so that when they contract the domes are pulled flatter and downward. That increases the volume of the thoracic cavity and lowers the pressure inside the lungs, drawing air in. When the fibres relax the diaphragm returns to its dome shape, the volume of the thorax decreases, the pressure inside the lungs rises, and air is forced out. It is therefore an active muscle during inhalation and a passive one during normal exhalation.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Common mistakes with Diaphragm
- 3. Skipping the pressure step. “The diaphragm contracts so air rushes in” is a two-mark answer written as a one-mark answer. Volume changes, therefore pressure changes, therefore air moves.
- 4. Getting the diaphragm backwards. When the diaphragm contracts it becomes flatter and moves down, which increases the volume of the thorax. A relaxed diaphragm is the dome-shaped one. Many students write the reverse because a relaxed muscle sounds like it should be flat.
Questions students ask about Diaphragm
Why does air move into the lungs?
Because the pressure inside the lungs has become lower than the pressure of the atmosphere outside. The external intercostal muscles contract and raise the ribs, and the diaphragm contracts and flattens; both increase the volume of the thorax. The same quantity of air then occupies a larger space, so its pressure falls, and air moves in down that pressure difference. The lungs themselves contain no muscle and do not pull.
What does the diaphragm do when you breathe in?
It contracts, which makes it flatter and moves it downward. That increases the volume of the thoracic cavity. When it relaxes it returns to its dome shape and moves upward, which decreases the volume. Many students reverse this, because a relaxed muscle sounds as though it should be the flat one.
Do the internal intercostal muscles work during every breath out?
No. In quiet breathing, exhalation happens because the external intercostal muscles and the diaphragm relax and the tissues recoil, so the internal intercostals stay relaxed too. They contract only during forced exhalation, such as coughing, blowing hard or breathing out during vigorous exercise, when they pull the ribs downwards and inwards more strongly.