h. Alveoli. Alveoli are tiny, grape-like clusters of microscopic air sacs. Air
enters the alveoli from the bronchioli. The wall of an alveolus is one cell layer thick.
The alveolus is surrounded by equally thin capillaries. Oxygen (O2) molecules from the
air inside the alveolus travel through the alveolus and capillary walls to the blood within
the capillary. The hemoglobin in the red blood cells captures the oxygen molecules and
release carbon dioxide (CO2) molecules. The carbon dioxide molecules and some
water molecules travel from the blood, through the walls, and into the alveoli.
i. Lungs. The alveoli, bronchioli, and associated blood vessels make up two
cone-like organs called lungs. The lungs are broad at their base (which rests on the
diaphragm) and narrow at the apex (top). Each lung is surrounded by pleural
membranes that prevent friction when the lung expands and contracts. The right lung is
divided into three lobes; the left lung is divided into two lobes. The left lung is smaller
than the right lung because the heart takes up space on the left side of the chest cavity.
MECHANICS OF BREATHING
Breathing refers to the process of moving air into and out of the lungs. The
process is usually performed automatically (without conscious thought) by the
respiratory control center located in the medulla oblongata of the brain stem. The
normal range of breathing rates (one cycle consists of one inspiration and one
exhalation) in an adult is 12 to 20 breaths per minute. Regular, easy breathing is
referred to as eupnea. Difficulty in breathing is referred to as dyspnea.
a. Inhalation. During the inhalation (inspiration) phase of breathing, the
diaphragm and the intercostal muscles contract. When the diaphragm muscle (located
at the base of the lungs) contracts, it is pulled downward toward the abdomen. This
flattening of the diaphragm enlarges the chest cavity. When the intercostal muscles
(located between the ribs) contract, they lift the rib cage up and out (chest rises). This
also enlarges the chest cavity. This expansion of the chest cavity causes the air
pressure in the alveoli to decrease. Air from the outside environment rushes in through
the nose or mouth to equalize the pressure.
b. Exhalation. During the exhalation (expiration) phase of breathing, the
diaphragm and the intercostal muscles relax. When the diaphragm muscle relaxes, it
resumes its dome-like shape (moves upward). When the intercostal muscles relax, they
let the rib cage return to its original position (moves down and inward). Both of these
actions cause the air pressure in the alveoli to increase and force air out of the lungs,
through the airways, and out the nose or mouth.