Breathing - Mechanics

Mechanics

In Mammals, breathing in, or inhaling, is due to the contraction and flattening of the diaphragm, a domed muscle that separates thorax and abdomen. If the abdomen is relaxed, this contraction causes the abdomen to bulge outwards, expanding the volume of the body. This increased volume causes a fall in pressure in the thorax, which causes the expansion of the lungs. When the diaphragm relaxes air leaves largely by elasticity of the lung. This is quiet, relaxed breathing needing little energy. When need increases the abdominal muscles resist expansion. The increased abdominal pressure then tilts the diaphragm and ribcage upwards with an increase in volume and the entry of air. Expiration follows relaxation of diaphragm and abdominal muscles, but can be increased by downward action of abdominal muscles on the rib cage. This forced expiration increases pressure across the airway's walls and may lead to narrowing and perhaps to wheezing. Intercostal muscles are auxiliary, stiffening and shaping the rib cage. Speech depends on the balance between the two forms of breathing, and in man conscious change often modifies autonomous reaction to need. The pattern can vary with fear in anticipation of need, and so with anxiety, and may be conditioned to experience such as the loss of an inhaler. It is also affected by loss of lung elasticity in age or pulmonary disease, of abdominal expansion from obesity, or of muscle power to resist expansion or to pull the ribcage down. Ten muscles are used for inspiration:

Diaphragm, Intercostal Muscles, Scalenes, Pectoralis Minor, Serratus Anterior, Sternocleidomastoid, Levator Costarum, Upper Trapezius, Latissmus Dorsi, and Subclavis.

Eight are used for forced expiration:

Internal intercostal, Obliquus Internus, Obliquus Externus, Levator Ani, Triangularis Sterni, Transversalis, Pyramidalis, and Rectus Abdominus.

In amphibians, the process used is positive pressure breathing. Muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils (which uses the same mechanics - pressure, volume, and diffusion - as a mammalian lung). With the nostrils and mouth closed, the floor of the oral cavity is forced up, which forces air down the trachea into the lungs.