Respiration

Respiration

• Is the vital exchange of oxygen and carbon dioxide that occurs in the lungs. The air-blood barrier consists of alveolar type I cells, basal lamina, and capillary endothelial cells. The alveolar type II cells secrete surfactant.

Inspiration
Occurs when the ribs and sternum (or thoracic cage) are elevated by the following muscles: the diaphragm; external, internal (interchondral part), and innermost intercostal muscles; sternocleidomastoid; levator costarum; serratus anterior; scalenus; pectoralis major and minor; levator costarum; and serratus posterior superior muscles.

Involves the following processes:

1. Contraction of the diaphragm

• Pulls the dome inferiorly into the abdomen, thereby increasing the vertical diameter of the thorax.

2. Enlargement of the pleural cavities and lungs

• Reduces the intrapulmonary pressure (creates a negative pressure ), thus allowing air to rush into the lungs passively because of atmospheric pressure.

3.Forced inspiration

• Involves contraction of the intercostal muscles and elevation of the ribs (superolateral movement),
  with the sternum moving anteriorly like a bucket handle . (When the handle is raised, the convexity
  moves laterally.)
• Results in increased transverse and anteroposterior diameters of the thoracic cavity. The abdominal
  volume is decreased with an increased abdominal pressure.
  

Expiration
Involves the following muscles: the muscles of the anterior abdominal wall, internal intercostal
(costal part) muscles , and serratus posterior inferior muscles.
Involves the following processes:

1. Overall process

• Involves relaxation of the diaphragm, the internal intercostal muscles (costal part), and other muscles; decrease in thoracic volume; and increase in the intrathoracic pressure. The abdominal pressure is decreased , and the ribs are depressed.

2. Elastic recoil of the lungs

• Produces a subatmospheric pressure in the pleural cavities. Thus, much of the air is expelled. (Quiet expiration is a passive process caused by the elastic recoil of the lungs, whereas quiet inspiration results from contraction of the diaphragm.)

3. Forced expiration

• Requires contraction of the anterior abdominal muscles and the internal intercostals (costal part).

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