The diaphragm is a large, dome-shaped sheet of muscle located at the base of the chest cavity. It functions as the primary muscle responsible for breathing, separating the chest cavity, which contains the heart and lungs, from the abdominal cavity below. This muscle’s rhythmic contraction and relaxation facilitate air movement into and out of the lungs.
Understanding the Diaphragm’s Structure
The diaphragm is a musculotendinous sheet, composed of muscle fibers and a strong, flat tendon. It forms a partition between the thoracic and abdominal cavities. The muscle fibers originate from the lower ribs, sternum, and lumbar vertebrae, converging and inserting into a central tendon. The central tendon is a thin, robust layer forming the crest of the diaphragm’s dome.
The diaphragm is typically a double-domed structure, with the right dome slightly higher than the left due to the liver beneath it. It is not a solid barrier; it contains several openings that allow structures to pass between the chest and abdomen. These include the esophagus, which carries food to the stomach, and major blood vessels like the aorta and inferior vena cava.
The Mechanics of Breathing: Inhalation
Inhalation is an active process initiated by the contraction of the diaphragm. When the diaphragm contracts, its muscle fibers shorten, causing the dome-shaped structure to flatten and move downwards towards the abdominal cavity. This downward movement increases the vertical dimension of the thoracic cavity. The expansion of the thoracic cavity increases lung volume.
As lung volume increases, air pressure within them decreases. This creates a pressure difference between the lower pressure in the lungs and the higher atmospheric pressure outside. Air then flows from the higher atmospheric pressure into the lower pressure in the lungs until pressures equalize. This inflow constitutes inhalation.
The Mechanics of Breathing: Exhalation
Exhalation is a passive process, relying on the relaxation of the inspiratory muscles. During exhalation, the diaphragm relaxes and returns to its original dome-shaped position, moving upwards into the thoracic cavity. This upward movement, combined with the natural elastic recoil of the lung tissue and chest wall, reduces the volume of the thoracic cavity.
As the volume of the thoracic cavity and lungs decreases, the air pressure inside the lungs increases. This internal pressure becomes higher than atmospheric pressure. Driven by this pressure gradient, air is passively forced out of the lungs. While exhalation is passive, other muscles can assist in forceful exhalation, such as during exercise or coughing.