In a normal heartbeat the ventricles begin to contract ______. A at exactly the same time as the atria B immediately before the atria because they are larger C at the same time the atria begin to relax D when the second heart sound is heard
| A |
at exactly the same time as the atria
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| B |
immediately before the atria because they are larger
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| C |
at the same time the atria begin to relax
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| D |
when the second heart sound is heard
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The blood circulating through the circulatory system is responsible for the transport of oxygen and nutrients to the cells, and removal of carbon dioxide and toxic metabolic waste from the cells. The part of the circulatory system that is responsible for pumping the blood into the blood vessels and subsequently to all the body parts is the heart. The heart is a muscular organ located slightly toward the left side of the breastbone. The size of the heart is approximately equivalent to the size of fist. The heart is a four chambered structure that is responsible for pumping the blood into the arteries and veins of the cardiovascular system. The specialized cells of the heart include cardiomyocytes and pacemaker cells. The rhythm at which the heart is pumped is controlled by the pacemaker cells. These cells have the ability to get excited and generate current that causes the contraction of the heart, thereby pumping the blood into the vessels.
The human heart consists of four chambers. The two upper chambers are called right atrium and left atrium, whereas the two lower chambers are called right ventricle and left ventricle. The flow of blood in each chamber is regulated by the presence of the valves in the chamber. These valves are flap-like structures which present the backward flow of the blood in the heart. They act as one-way inlets and outlets for blood entering into and leaving form the ventricles, respectively. The human heart comprises four valves: tricuspid valve which is located between the right atrium and the right ventricle; pulmonary valve which is located between the right ventricle and the pulmonary artery; mitral valve which is located between the left atrium and the left ventricle; aortic valve which is located between the left ventricle and the aorta.
The rhythmic pumping of the heart is represented by a cardiac cycle. Thus, the cardiac cycle depicts activity of the heart from the ending of one heartbeat to the beginning of the next. The cycle consists of two phases: diastole and systole. The diastole is the part of the cardiac cycle during which the heart relaxes and is filled with blood. The systole is the part of the cardiac cycle during which the heart contracts after getting refilled with blood. The upper chambers of the heart receive the blood whereas lower chambers discharge the blood from the heart. The cardiac cycle begins with relaxation and expansion of the heart such that it receives blood into both ventricles through both atria. This phase of cycle is called early ventricular diastole. The next phase is late ventricular diastole the atrial chambers contract and they send blood down lower ventricle chambers. Once the ventricles are filled with blood, the valves of ventricles are closed. The next phase is the ventricular systole during which the ventricle contracts and pumps the blood to the other body parts. The last phase occurs simultaneously with the ventricular systole phase and is called atrial diastole during which the upper atrial chambers are relaxed. Therefore, when the ventricles are contracting during systole, the atria are relaxed and collecting returning blood.
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