Page 38 - Critical Care Nursing Demystified

Chapter 2 CARE OF THE PATIENT WITH CRITICAL RESPIRATORY NEEDS 23

The lungs expand and contract in the following ways:
1. The downward and upward movement of the diaphragm lengthens and
shortens the chest area during inhalation and exhalation. The diaphragm
is a thin, dome-shaped muscle that is stimulated by the phrenic nerves.
When the diaphragm contracts during inspiration, abdominal contents are
forced downward and the chest expands upward to inflate the lungs. Dur-
ing expiration, which is primarily a passive process during normal breath-
ing, the diaphragm relaxes, the chest wall descends, and abdominal
contents return to their normal position.
2. Lung expansion and contraction also relies on the elevation and depres-
sion of the ribs, which increases and decreases the diameter of the chest
cavity during breathing.

How It All Occurs

Gas exchange in the lungs is known as external respiration. Gas exchange in the
body cells and tissues is known as internal respiration. The conducting or upper
airways consist of the nasopharynx, the oropharynx, the trachea, bronchi, and
bronchioles. Their job is to warm, humidify, and filter inhaled environmental Downloaded by [ Faculty of Nursing, Chiangmai University 5.62.158.117] at [07/18/16]. Copyright © McGraw-Hill Global Education Holdings, LLC. Not to be redistributed or modified in any way without permission.
air and channel it along the airways for further action. These structures down
to the respiratory bronchioles do not participate in actual gas exchange. This is
called the anatomic dead space and contains a volume of about 150 cc. of air.
The upper airway completes the job of warming, filtering, and humidifying
environmental air. External respiration takes place through the respiratory
bronchioles and the alveoli. The alveoli are the gas exchange units of the lungs.
The adult lungs contain several million alveoli. Gas exchange takes place
through the Type 1 alveolar cells, which are flat, squamous epithelial cells com-
prising 90% of the total alveolar surface area. Type 2 alveolar cells secrete sur-
factant—a very important lipoprotein that promotes alveolar inflation during
inspiration and prevents the collapse of the smaller airways during expiration.
Lungs have a dual blood supply. The first supply system is the bronchial circula-
tion, which does not participate in gas exchange but distributes blood to the air-
ways. Pulmonary circulation, the second supply system, contributes to gas exchange
by mixing with oxygenated blood that flows from the right side of the heart to the
lungs. Less-oxygenated blood leaves the right ventricle of the heart and enters the
pulmonary arteries. Blood passes from the pulmonary arteries through the pulmo-
nary capillary beds in the lungs, becomes oxygenated, flows back to the main pul-
monary veins, and finally flows into the left atrium of the heart. Oxygenated blood
is distributed throughout the body to create internal respiration through the