Published: June 3, 2006

The purpose of the respiratory system is to supply oxygen to body tissues and to remove carbon dioxide, which is a waste product, from the body tissues. Breathing is the process by which oxygen in the air is brought into the lungs and into close contact with the blood, which absorbs it and carries it to all parts of the body. At the same time, the blood gives up carbon dioxide, which is carried out of the lungs with air breathed out.

Upper Respiratory System
Upper Respiratory System

The parts of the upper respiratory system are illustrated above.

The Nasal cavity or Nose is the preferred entrance for outside air to enter into the respiratory system.

The Oral cavity or Mouth is an alternative way for air to enter the respiratory system.

The Pharynx or Throat collects incoming air from the nose and mouth and passes it downward to the trachea (windpipe).

The Epiglottis is a flap of tissue that guards the entrance to the trachea, closing when anything is swallowed in order to prevent food or fluid from entering the lungs.

The Larynx or Voice Box contains two vocal cords. Air moving through the larynx creates voice sounds.

The Esophagus is the passage leading from the mouth and throat to the stomach.

The Trachea or Windpipe is the passage leading from the pharynx to the lungs.

The Bronchi or Tubes are the two main tubes into the lung that divide from the trachea. The bronchi subdivide into the lobar bronchi — three on the right side and two on the left. These, in turn, subdivide further.

Lower Respiratory System
Lower Respiratory System

The parts of the lower respiratory system are illustrated above.

The Bronchioles are the smallest subdivisions of the bronchi, at the end of which are the alveoli (plural of alveolus).

The Alveoli are the very small air sacs that are the destination of inhaled air. The capillaries are blood vessels that are imbedded in the walls of the alveoli. The blood discharges carbon dioxide into the alveoli and takes up oxygen from the air in the alveoli.

The Lungs are elastic organs with sponge-like tissue. Inhalation requires an active effort, whereas exhalation occurs automatically. The right lung is divided into three lobes and the left is divided into two lobes.

The Pleura are the two membranes (actually one continuous membrane folded on itself) that surround each lobe of the lungs and separate the lungs from the chest wall. The pleural space is the space between the two pleura.

Diaphragm is the strong wall of muscle that separates the chest cavity from the abdominal cavity. By moving downwards, it creates suction to draw in air and expand the lungs.

Ribs are bones separating and protecting the chest cavity. They move to a limited degree, helping the lungs to expand and contract.

Nerve and Muscles of the Respiratory System

When a spinal cord injury occurs, the lungs and the alveoli remain the same. However, just as the injury affects the nerves that control the muscles of the arms and legs, it also affects the nerves that activate the muscles used for breathing.

There are four sets of muscles that control the respiratory system.

* The Diaphragm
* The Intercostal Muscles
* The Abdominal Muscles, and
* The Accessory Muscles

The Diaphragm is a dome-shaped muscle, which separates the chest from the abdominal cavity. It is the major muscle and the one that begins the inhalation process. During inhalation, the diaphragm contracts and moves downwards, pushing out the abdomen and creating suction which draws in the air and expands the lungs.

The nerves responsible for stimulating and diaphragm and causing it to contract are called phrenic nerves. These nerves originate at the spinal cord at the C-3, C-4 and C-5 level (Cervical 3-5) and travel downward, attaching themselves to the diaphragm. Therefore, spinal cord injury, which occurs at the C-1 through C-5 levels, may result in partial or complete paralysis of the diaphragm.

The Intercostal muscles are located between the ribs, and are activated by the intercostal nerve, T-1 through T-11 (Thoracic 1-11). These muscles act to increase and decrease the diameter of the chest cage. The ability to take a deep breath and cough is affected by the loss of the intercostal muscles.

The Abdominal muscles originate at the T-6 through L-1 (Lumbar) levels, and are the most essential for an effective cough. When the abdominal muscles contract, the diaphragm is forced upward and coughing or forcefully blowing out air results.

The Accessory muscles are located in the neck and receive their stimulation from the spinal cord nerves in the C-1 through C-3 region. They act to elevate the rib cage and can assist in deep respiration. However, they alone are not sufficient to support deep ventilation.

For further information on the cervical, thoracic, and lumbar areas of the spinal cord, see Movement Disorders in SCI.


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