Have a Heart
CARDIOVASCULAR SYSTEM
The cardiovascular system transports materials throughout the body that cells and tissue could not survive without. Conveying oxygen to the tissues and relaying carbon dioxide back to the lungs is an essential function of this system. However, other critical materials are also conveyed in the blood, such as hormones and immune system cells. The cardiovascular system is much more than merely a pump and plumbing to transport gases.
Heart Structure
The driving force of the cardiovascular system is the heart. This muscular organ begins pumping even before the heart is fully formed in the embryo and consistently and spontaneously beats for a lifetime. Divided into four chambers and separated into right and left portions, the heart functions as a pump for two circulations: the systemic circulation for the body and the pulmonary circulation for the lungs. Using both of these removes carbon dioxide from the body while supplying fresh oxygen to the tissues.
Chambers
The two superior (upper) chambers of the heart are the atria, which actually begin as a single chamber during embryonic development and later form a partition (interatrial septum) to divide into right and left chambers. These thin-walled chambers are the primary holding areas for blood returning from the body (right atrium) and from the lungs (left atrium). Movement of the blood from the atria downward into the respective ventricles (lower chambers) is accomplished primarily from low pressure pulling the blood downward.
Like the atria, the ventricles begin as a single chamber that is separated into right and left chambers by a thick muscular partition (interventricular septum).
How the heart’s pump system works
Oxygen-depleted blood in the right atrium is pulled into the right ventricle as its walls expand during the relaxation phase (diastole) of a cardiac cycle. Likewise, oxygen-rich blood in the left atrium is pulled into the left ventricle. The left ventricle has the thickest of the muscular walls because the blood in this side must be pumped throughout the body and back to the heart, while blood in the right ventricle must only have sufficient pressure to make it to the lungs and back to the heart.
Cardiac Valves
The cardiac valves regulate the passage of blood from one chamber to the next as well as ensure that the blood only flows in one direction. Atrioventricular valves regulate the movement of blood between the atria and ventricles. The structure of these valves is similar in both the right and left atrioventricular canals. Formed from thin connective tissue sheets (cusps), the tricuspid valve (on the right) and the bicuspid valve (on the left) are pushed against the wall of the ventricles as the blood is pulled into the ventricles from the atria. When the ventricles contract (systole), blood is compressed and forced upward in the ventricles. The most open path out of the ventricle is back up into the atrium.
As the blood moves up, it is forced under the cusps, which are inflated, and moved upward toward the atrium. If the cusps were the only structural portion of the valves, they would simply be pushed aside into the atrium as the blood rushes upward. This does not happen because attached to each cusp are cords (chordae tendineae) made from tough connective tissue. These cords are anchored into large bundles of cardiac muscle (papillary muscle) present in the ventricle that contract as the ventricle as a whole contracts.
Thus, as the blood moves upward because of the ventricular contraction, the papillary muscles pull downward on the cords with the same force as the upward moving blood. Because these forces are equal, they allow the cusps to move upward until they are aligned at the atrioventricular canal. This closes off the canal, preventing the blood from moving backward (regurgitation). The closing of these valves produces the first heart sound, or the “lub” that is heard for the heartbeat.
Another set of valves, the semilunar valves, prevent blood from flowing back into the heart from major vessels. These valves are present in the great vessels just outside of the heart. Blood from the right ventricle is forced into the pulmonary trunk (a major blood vessel that divides into the two pulmonary arteries) and is destined for the lungs. Blood from the left ventricle is moved into the aortic trunk (the root of the aorta that attaches to the heart) and is pushed to the rest of the body. Located within each of these vessels are three cusps that resemble pockets. These cusps inflate when the blood falls downward in an attempt to return to the ventricles, effectively closing off the passageway and keeping the blood in the vessel. The semilunar valves allow blood to exit the ventricles, but not return.