(2) The atrioventricular node. In the lower part of the inner wall of the right
atrium and above the valves opening into the right ventricle, there is a special tissue
called the atrioventricular (AV) node. This node also functions in contracting the heart.
The AV node acts as a relay station for the impulses originated by the SA node.
f. Cardiac Cycle. The action of the heart occurs as a cycle, repeated
continuously and in regular rate and rhythm. The cycle consists of alternate contraction
and relaxation of the heart, the wave of contraction beginning in the atria and spreading
to the ventricles. This phase of contraction is known as systole; that of relaxation is
called diastole. Several actions occur simultaneously during each phase. During the
systolic phase, both atria contract at the same time, followed in an instant by the
simultaneous contraction of the ventricles. At the beginning of ventricular contraction,
the tricuspid and bicuspid valves close; at the end of ventricular contraction, the aortic
and pulmonary semilunar valves close. Immediately upon closure of the semilunar
valves, the diastolic phase, or rest period, begins. During this phase, the atria and then
the ventricles relax as blood flows into the atria and ventricles. As the atria become
filled, the AV valves open, the atria contract, and the systolic phase begins again.
THE LYMPHATIC SYSTEM
(1) Immunity. The lymphatic system (figure 25) plays an important role in
immunity. The lymph nodes, which are small, oval bodies of lymphoid tissue occurring
at strategic places along the course of lymph vessels, are a line of defense against
different types of foreign matter, whether of living organisms or inert particles. These
particles are carried by the lymph to the lymph nodes where they come into contact with
lymphatic phagocytes. These phagocytes prevent many foreign materials from entering
the bloodstream. Inert particles accumulate in the lymph nodes and will finally cause a
type of fibrous proliferation. If the particles entering the lymphatics happen to be
particularly pathogenic bacteria, they may overcome the lymphatic barrier, pass into the
bloodstream, and cause a general sepsis.
(2) Intestinal absorption. The lymphatic system is one of the major
channels for absorption from the intestinal tract. Fats of more than 12 carbon atoms are
absorbed through the intestinal villi into lymphatic vessels called central lacteals;
smaller fats are absorbed directly into the bloodstream. Fats in the lymphatic system
pass through the thoracic duct to the bloodstream. During digestion, the lymph in the
lacteals is milky in appearance and is called chyle. Chyle contains the fats absorbed
from the intestine. After a fatty meal, the lymph in the thoracic duct may contain as
much as two percent fat.