D Question 1 2 pts Which is correct about the contraction sequence of the heart?

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D Question 1 2 pts Which is correct about the contraction sequence of the heart? The contraction and relaxation of all four chambers happen at the same time. OThe right atrium and ventricle contract at the same time, and as they are relaxing, the left atrium and left ventricle begin to contract. Contraction begins in the order: right atrium, then right ventricle, then left atrium, then left ventricle. The two atria contract at the same time, and as they are relaxing, the two ventricles contract together. The atria and ventricles begin their contractions at the exact same time, but the atrial contraction only lasts half as long as the ventricle contraction. Question 2 2 pts Pacemaker cells located in thegenerate the autorhythmic electrical activity that ultimately results in a cardiac cycle ("heart beat") SA node AV node Bundle of His Bundle branches Purkinje fibers

Explanation / Answer

Answers:

1) Option D

Explanation:

The events which include the contraction and relaxation of the heart chambers constitute the cardiac cycle. As blood enters from the pulmonary vein into the left atria and from the superior and inferior vena cavae into the right atria respectively, the two chambers passively receive blood from the corresponding vessels. But once the pressure exceeds than those in the ventricles or the lower chambers, both the atria immediately contracts and passes the blood into the corresponding ventricles during systole( contraction). Once the entire blood is passed onto the ventricles, the atria relax before it further receives blood again and this relaxation period is known as diastole.

The ventricles also have episodes of systole and diastole before it pumps blood into the aorta or the pulmonary artery. The entire cardiac cycle takes around 8 seconds and is under strict regulation and rhythmic in nature.

2)Option A

The contraction and relaxation of the heart chambers are under rhythmic control by electrical impulses. The heart muscles are auto-rhythmic in nature; that is, they can generate their own impulses. Although, the heart has a specialized excitatory structure from which discharge of impulses rapidly takes place. This structure is called the pacemaker. In mammals, the pacemaker is the sino-arterial or the SA node.

It is located in the superior part of the lateral wall of the right atrium, just below the opening of the superior vena cava. The rate of production of impulses is around 70 - 80 per minute. From here, the impulse is conducted on to the AV node, then the bundle of his, to the Purkinje fibers and finally to the ventricular muscles.

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