b. Right Atrium. The right atrium receives blood from the superior vena cava
and the inferior vena cava. When the right ventricle relaxes (that is, after it has
contracted and pumped blood), blood flows from the right atrium into the right ventricle
through the tricuspid valve. The tricuspid valve is formed so that blood cannot flow back
into the right atrium when the right ventricle contracts.
c. Right Ventricle. When the right ventricle is filled with blood, it receives an
impulse from the sinoatrial node. This impulse causes the muscles of the right ventricle
to contract. This contraction causes the inside of the ventricle (the space where the
blood is) to become smaller. The increased pressure forces blood out of the ventricle
and into the pulmonary artery. The pulmonary valve located at the beginning of the
pulmonary artery keeps blood from flowing back into the right ventricle when the
ventricle relaxes and returns to its normal size.
d. Lungs (Pulmonary System). The pulmonary artery divides into two arteries.
One artery travels to the right lung while the other artery travels to the left lung. The
arteries divide until they reach the capillary stage. The capillaries surround the alveoli
(air sacs) of the lungs. There the oxygen-poor blood gets rid of carbon dioxide and
picks up oxygen from the air in the alveolus. The blood, now high in oxygen content,
then returns to the left atrium through the pulmonary veins.
e. Left Atrium. The left atrium receives blood from the lungs through two
pulmonary veins. When the left ventricle relaxes after having contracted, the blood
flows from the left atrium into the left ventricle through the mitral valve. The mitral valve
keeps blood from flowing back into the left atrium when the left ventricle contracts.
f. Left Ventricle. After the left ventricle is filled with oxygen-rich blood, it
receives an impulse from the sinoatrial node, which causes it to contract and pump
blood into the large artery call the aorta. When the blood enters the aorta, it passes
through the aortic valve. This valve keeps the blood from flowing back into the heart
once the left ventricle relaxes.
g. Body (Systemic System).
(1) Some arteries branch off the aorta to provide the brain, upper body, and
heart with blood. Blood returns to the heart from these areas through the superior vena
If blood flow to the brain stops and is not restored (either the casualty's heart
starts beating on its own or cardiopulmonary resuscitation is administered),
the brain will begin to die in six to ten minutes.