CHAPTER 1 Cardiovascular System^3
How the Cardiovascular System Works
The cardiovascular system is responsible for delivery of blood, which carries oxy-
gen and other nutrients, to the tissues of the body. The heart pumps the blood to
the body where it delivers nutrients and oxygen, picks up waste products, and then
returns to the heart.
The heart has four chambers. The upper chambers are the atria; the lower cham-
bers are the ventricles. In the middle, there is a septum, a wall that separates the
right side of the heart from the left side of the heart. Atrioventricular (AV) valves
control the blood flow between the upper and lower chambers of the heart. The tri-
cuspid valve is on the right side, while the mitral valve is on the left side between
the atria and the ventricle. The pulmonic valve controls the flow between the right
ventricle and the pulmonary artery, while the aortic valve controls the flow between
the left ventricle and the aorta.
Unoxygenated blood empties into the right atrium from the systemic circulation
via the inferior vena cava and superior vena cava. As the right atrium contracts, the
tricuspid valve opens, allowing the blood to flow into the right ventricle. With con-
traction of the right ventricle, the pulmonic valve opens, allowing the unoxygenated
blood to enter the pulmonary artery to go to the lungs to pick up oxygen. Once
oxygenated, the blood returns to the heart via the pulmonary vein and enters the
left atrium. As the left atrium contracts, the mitral valve opens, allowing the blood
to flow into the left ventricle. As the left ventricle contracts, the aortic valve opens,
allowing the blood to flow into the aorta and systemic circulation. The blood will
return to the heart from the lower body via the inferior vena cava and from the
upper body via the super vena cava. The actions on the right side and left side of
the heart happen simultaneously. So when we listen to a normal heartbeat, the
sounds we hear are the sounds of the valves closing. The mitral and tricuspid valves
create the first heart sound (S1), while aortic and pulmonic valves create the sec-
ond heart sound (S2).
The electrical conduction system of the heart starts at the sino-atrial (SA) node,
which is located in the right atrium. It initiates the heart beat, ranging between 60
to 100 beats per minute, every day, for a lifetime. The electrical current travels
across both atria, then converges on the atrio-ventricular (AV) node, where the cur-
rent slows, allowing the atria to repolarize. The AV node is located in the superior
portion of the ventricular septum. In the bottom portion are located the right and left
Bundle of His, which is a group of special cardiac muscles that sends an electrical
impulse to the ventricle to begin cardiac contractions. These end in the Purkinje
fibers and spread out through the ventricles. The current passing through these
fibers causes ventricular contraction, forcing the blood from the right ventricle to
the lungs and from the left ventricle to the aorta, and thus, the systemic circulation.