Respiration is the process of exchanging O2 with CO2. In order to get the oxygen into the lungs, all of the structures that you have just learned act together to increase the area of the thoracic cavity. The ribs and diaphragm move in such a way that three dimensions of the thoracic cavity are increased:
During inspiration, the lateral dimensions of the thoracic cavity are increased by the 7-10th ribs moving laterally (similar to bucket handles). The anteroposterior dimension is increased by the sternum being pushed forward by the true ribs (1-6). The superoinferior dimension is increased by the diaphragm contracting and becoming lower. During restful breathing, the diaphragm probably does most of the work, although small movement in all directions probably occur. During increased need for oxygen (exercise, pathology), the lateral and anterioposterior movements will be increased. When the thoracic muscles can no longer do the job, other muscles attaching to the thorax will be called into action (pectoralis major and minor, sternomastoid, etc.)
During expiration, the intercostal muscles and the diaphragm relax and the elastic fibers of the lung and the costal cartilages recoil to their original state before inspiration. The automatic nature of the respiratory cycle is controlled in the respiratory centers of the brain stem.
These various actions are demonstrated here:
Respiratory movements of thorax
If we take a look at the thoracic wall in more detail, we can see just how the lung enlarges to draw in air.
- the dimensions of the thoracic cavity increase
- the parietal pleura follows it because it is firmly attached by subpleura fascia and, thereby, increases its surface area.
- the visceral pleura follows the parietal pleura because it adheres to it by a thin layer of pleural fluid, thereby increasing its surface area
- the surface of the lung tissue is firmly attached to the visceral pleura and therefore, follows it, increasing its surface area.
- when the surface area of the lung increases, the lung inflates, the air sacs increase in size and air is brought into the lungs by way of the nose or mouth, pharynx, larynx, trachea, bronchi, bronchioles, air ducts and finally into the alevolae.
- O2 is exchanged with CO2
Table of Contents for Thorax
||This is copyrighted©1999 by Wesley Norman, PhD, DSc