b. Right Atrium. The right atrium, the upper chamber of the right side of the
heart, receives blood from the superior vena cava (blood from the upper body) and from
the inferior vena cava (blood from the lower body). As the right atrium contracts, the rig
ventricle relaxes. Contraction of the right atrium forces blood through the triscupid valve
into the relaxed right ventricle. The triscupid valve is so formed that blood cannot flow
back into the right atrium.
c. Right Ventricle. When the right ventricle is filled with blood, it receives an
impulse from the sinoatrial (SA) node. This impulse causes the muscles of the right
ventricle to contract. The contraction causes the inside of the ventricle (the space
where the blood is) to become smaller. 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 its carbon dioxide and
picks up oxygen from the air in the alveolus. The blood, now rich in oxygen, returns to
the left atrium through the pulmonary veins.
e. Left Atrium. The left atrium receives blood from the lungs though two
pulmonary veins. When the left ventricle relaxes after having contracted, 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 which causes it to contract and pump blood into the large artery
called the aorta. When 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). Arteries going to the head and upper body
branch off the aorta. Two of these arteries (the right and left coronary arteries) supply
the heart muscles with blood. After the aorta turns down, it divides into smaller arteries
which go to the lower parts of the body. Arteries go to all parts of the body and divide
into smaller blood vessels that eventually become capillaries. Blood in the capillaries
give nutrients and oxygen to the cells and remove wastes. Capillaries then group to
form veins that return blood to the right atrium. Before we leave the systemic circulatory
system, though, let us take a brief look at the blood that flows to the intestines and to