Figure 8.6: The external structure of the heart: the major part of the heart consists of muscles and is
known as the myocardium. The region in which the heart is found is known as the pericardial cavity,
which is enclosed by the pericardium.
Internal structure of the heart
As previously mentioned, the heart is made up offour chambers. There aretwo atriaat the
top of the heart which receive blood andtwo ventriclesat the bottom of the heart which
pump blood out of the heart. Theseptumdivides the left and right sides of the heart. In order
to make sure that blood flows in only one direction (forward) to prevent backflow of blood,
there are valves between the atria and ventricles (atrioventricular valves). These valves only
open in one direction, to let blood into the ventricles, and are flapped shut by the pressure
of the blood when the ventricles contract.
Thetricuspid valveis situated between the right atrium and the right ventricle while thebi-
cuspid/mitral valveis found between the left atrium and the left ventricle. Strong tendinous
cords (chordae tendineae) attached to valves prevent them from turning inside out when they
close. Thesemi-lunar valvesare located at the bottom of the aorta and pulmonary artery,
and prevent blood from re-entering the ventricles after it has been pumped out of the heart.
Chapter 8. Transport systems in animals 227