The respiratory system, also known as the pulmonary system, is used for breathing. The windpipe and lungs are the two main parts of the respiratory system. When you inhale, or breathe air in through your mouth or nose, oxygen travels down your windpipe and into your lungs. Your lungs then filter the oxygen and send it through your blood stream to all your body parts. When you breathe out, or exhale, you send the left over carbon dioxide out of your body, through your windpipe and out of your mouth or nose.
Exhaling does not require any effort from the body's muscles. However, in order to you breathe air into your body, a combination of four respiratory muscle groups are used. The diaphragm, a strong, dome-shaped muscle that separates the abdominal and chest cavities, is normally the main muscle that you use when you inhale. The intercostal muscles are located between the ribs. These muscles help to expand your ribs as you inhale. The neck muscles normally work to expand your upper chest when inhaling. The abdominal muscles work with these other muscle groups to help you breath deeply and cough.
Role of the Spinal Cord
The brain normally sends signals through nerves in the spinal cord to control the four respiratory muscle groups. When everything is working properly, the pulmonary system and respiratory muscles work together allowing you to breath in and out without much effort. In fact, most people breath without ever thinking about it.
After SCI the windpipe and lungs are not typically affected by a spinal cord injury. However, respiratory problems may occur when the signals sent from the brain can no longer flow through the spinal cord to control the respiratory muscles. The amount of muscle control that is lost after a spinal cord injury depends on the level of the injury along with the completeness of the injury. Individuals with injuries below the T12 level do not usually lose any control of the four respiratory muscle groups needed for breathing. This means the respiratory system is not usually affected by injuries in the lumbar or sacral regions of the spinal cord.
Individuals with complete thoracic or cervical injuries do experience a loss of their respiratory muscle control. The higher the level of injury, the greater the loss to the respiratory muscle control. Complete injuries in the thoracic or cervical regions usually result in the permanent loss of respiratory muscle function below the level of injury. However, if that injury is incomplete, it is impossible to predict whether the individual will regain some or all of their respiratory function below the level of injury. Injuries in the thoracic area (T1-T12) of the spinal cord affect the control of the intercostal and abdominal muscles. A lower level of injury, such as a T10, results in the individual losing a small amount of muscle control. With a higher level of injury, such as a T2, individuals will lose most of their intercostal and abdominal muscle control.
Complete injuries in the cervical region usually result in a total loss of intercostal and abdominal muscle control. Again, the higher the level of injury, the greater the loss of additional muscle control. For example, a complete injury between levels C3 and C5 loses all control of the diaphragm muscles. With a complete injury at level C3 and higher the individual loses control of all four muscle groups that are needed for breathing. A ventilator is then needed to assist in breathing.
The ventilator does the work of the absent muscles and forces air into the lungs. Many people with a C4 level of injury, and even some people with a C3 level of injury, can eventually breathe without the aid of a ventilator or may only need it for part-time assistance. Those individuals with complete injuries above C3 use a ventilator for full-time assistance.