cells send electrical messages calledimpulsesto other muscle cells. The impulses then reach
the AV node. The AV node is found in the lower part of the right atrium. The AV node
conducts the impulses that come from the SA node through the atria to the ventricles. The
impulses then spread around the ventricles and they contract.
The frequency of the heart’s contractions, called theheart rate,can be changed by nervous
or hormonal signals. Activities such as exercise or getting frightened can make the heart
rate increase. After the exercise is over, or the fright has passed, the heart rate returns to
normal.
Blood Circulation and Blood Vessels
There are actually two separate circulation systems within the heart. Both of these together
make up the complete circulatory system of humans and other animals. Neither system can
work alone. These are thepulmonary circulationand thesystemic circulation. The
human heart is made up of two separate pumps, the right side which pumps deoxygenated
blood into the pulmonary circulation, and the left side which pumps oxygenated blood into
the systemic circulation. Blood in one circuit has to go through the heart to enter the other
circuit.
The blood vessels are an important part of the cardiovascular system. They connect the
heart (the pump), to every cell in the body. Arteries carry blood away from the heart, while
veins return blood to the heart. The main arteries and veins of the heart are shown in
Figure18.13.
The veins that return oxygen- poor blood to the heart are the superior vena cava and
theinferior vena cava. Thepulmonary veinsreturn oxygen-rich blood to the heart. The
pulmonary veinsare the only veins that carry oxygen-rich blood all other veins carry oxygen-
poor blood.
Thepulmonary arteriescarry oxygen-poor blood away from the heart to the lungs. These
are the only arteries that carry oxygen-poor blood. The aorta is the largest artery in the
body. It carries oxygen-rich blood away from the heart. Further away from the heart, the
aorta branches into smaller arteries.
Pulmonary Circulation
Thepulmonary circulationis the part of the cardiovascular system which carries oxygen-
poor blood away from the heart and brings it to the lungs. Oxygen-poor blood returns
to the heart from the body and leaves the right ventricle through the pulmonary arteries,
which carry the blood to each lung. Once at the lungs, the red blood cells release carbon
dioxide and pick up oxygen duringrespiration.The oxygen-rich blood then leaves the lungs
through the pulmonary veins which return it to the left side of the heart. This completes