(2) The aorta turns down and divides into smaller arteries which go to the
lower parts of the body. Some of the blood picks up fluids and nutrients from the
intestines. Some of the blood passes through the liver and kidneys which remove
bacteria and other unwanted substances from the blood. The blood returns to the heart
from these areas through the inferior vena cava.
THE RESPIRATORY SYSTEM
The respiratory system consists of two lungs and the respiratory tract that carries
air to and from the lungs (figure 1-3). When a person inhales, the air enters the nose or
mouth, travels down the trachea, and into the two bronchi. Each bronchus divides into
smaller and smaller air tubes. Finally, the air reaches the alveoli (air sacs). The red
blood cells in the capillaries surrounding the alveoli absorb oxygen from the air and give
off carbon dioxide, which passes into the alveoli. When a person exhales, the air
travels from the alveoli through the air tubes, up the trachea, and out the nose or mouth.
Of course, not all of the air inhaled reaches the alveoli nor is all of the oxygen removed
from the air in the alveoli. The average adult takes in about 500 milliliters (ml) of air
each time he inhales and he exhales the same amount. Even after the person exhales,
the lungs still contain about 2300 ml of air. The anatomy (structures) and the
physiology (functions) of the respiratory system are briefly discussed below.
a. Nose. The nose is composed of two nostrils (openings) and two nasal
cavities (air chambers above the roof of the mouth and below the cranium). A structure
called the nasal septum separates the right nostril and nasal cavity from the left nostril
and nasal cavity. The nose warms, moistens, and filters the inhaled air. Special nerve
endings in the upper part of the nasal cavities provide the sense of smell.
b. Pharynx. The pharynx is a part of the throat that is part of both the
respiratory system and the digestive system. The pharynx is divided into three parts.
The nasopharynx (upper part) connects with the nasal chambers. The oropharynx
(middle part) connects with the oral cavity (mouth). The laryngopharynx (lower part)
connects with the larynx (respiratory system) and the esophagus (digestive system).
c. Epiglottis. The epiglottis is a flap that covers the entrance to the larynx when
a person swallows. This prevents food from entering the larynx instead of the
esophagus. When a person inhales, the entrance to the larynx is not covered and air
enters the larynx. If a foreign object enters the airway, it can block the airway and
cause breathing to stop.
d. Larynx. The larynx is a box-like structure composed of cartilage, ligaments,
and muscles that sits on top of the trachea. The larynx contains the vocal cords, which
produce the voice; therefore, it is sometimes called the voice box. It is also called the
Adam's apple because of the bulge it causes in the throat.