lubricates the surfaces. Between the two lungs is the mediastinum, the central thoracic
cavity containing the heart, the great vessels, the esophagus, and the lower trachea.
2-4.
PHYSIOLOGY OF RESPIRATION
a. The walls of the alveoli are composed of a thin, permeable membrane. It is
here that oxygen passes from the alveoli into the tiny capillaries that surround each
alveoli. Carbon dioxide in the blood passes from the capillaries into the alveoli to be
exhaled. This exchange of oxygen and carbon dioxide in the lungs is called external
respiration.
b. The oxygen that enters the capillaries is carried by the red blood cells in a
chemical combination with hemoglobin. This blood, now oxygenated, returns to the
heart to be pumped out to the body through the arteries. The oxygen passes part of the
blood into the cells of the body, while carbon dioxide waste from the cells is passed into
the blood that returns to the heart. This exchange of gases between the capillary blood
and the cells of the body is called internal respiration.
2-5.
MECHANICS OF RESPIRATION
a. The act of breathing, the cycle of inspiration and expiration, is repeated about
16-20 times per minute in a resting adult. Breathing is regulated by the respiratory
center in the medulla of the brain. The level of carbon dioxide (CO) in the circulating
blood is one of the major influences upon the respiratory reflex. The respiratory center
is sensitive to changes in blood composition, temperature, and pressure, and will adjust
the rate and depth of breathing to accommodate the body's needs.
b. The physical conditions that control the flow of air into and out of the lungs
are referred to as the mechanics of ventilation. Air flows from an area of higher
pressure to an area of lower pressure. During inspiration, contraction of the diaphragm
and intercostal muscles increases the size of the thoracic cavity. This causes the
pressure within the thoracic cavity to become less than that of the atmosphere, and air
is drawn through the air passages into the alveoli. During normal expiration, relaxation
of the same muscles will cause the thoracic cavity to decrease in size, thereby
increasing the pressure within the thoracic cavity to that which is greater than
atmospheric pressure. Air will then flow out of the lungs into the atmosphere.
Section II. PHYSICAL EXAMINATION
2-6.
NOSE AND SINUSES
a. Physical examination of the nose and sinuses should begin with an overall
observation for general deformity or irregularity. Watch the patient breathe through both
the mouth and nose. Do the nostrils flare with inspiration? Is the sense of smell intact?
The nasal passages should be clear and unobstructed, with no discharge present.
Observe the nasal septum. It is normally straight and un-perforated, dividing the nasal
cavity into two chambers of relatively equal size and shape. The mucous membranes
should be pink.
MD0917
2-6
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