Problem 82 - Objective Biology for NEET

Question

During systole of heart (a) Only atria contracts (b) Only ventricles contract (c) Atria and ventricles contract separately (d) Atria and ventricles contract simultaneously

Step-by-Step Solution

Answer

The answer to the exercise is (b) Only ventricles contract during systole. This is when the ventricles contract and pump blood out of the heart into the circulation.

Step 1: Understanding Systole

The systolic phase of the cardiac cycle involves the contraction of the heart muscles. In this phase, the heart pumps blood out into the circulation. But it is vital to understand that not all four chambers of the heart (two atria and two ventricles) contract simultaneously.

Step 2: Identifying Roles

To identify which parts of the heart contract during systole, recall the sequential process of a heartbeat. First, the atria contract to pump blood into the ventricles. This marks the end of diastole. Once the ventricles are full, the atria relax and then the ventricles contract, pumping blood out into the body and marking the onset of systole.

Step 3: Answer the Exercise

Therefore, based on this understanding of the stages of the cardiac cycle, it can be deduced that during systole, it's not the atria that contract, nor do the atria and ventricles contract separately or simultaneously. It is only the ventricles that contract to pump blood out of the heart into the circulation. Hence, the answer is (b) Only ventricles contract.

Key Concepts

Heart Systolic Phase

The heart systolic phase, a cornerstone concept in understanding cardiac physiology, marks the period during which the heart's muscular walls contract. This phase is critical in the cardiac cycle, as it is responsible for propelling blood out of the chambers of the heart and into the circulatory system. Specifically, during systole, the ventricles—which are the heart's main pumping chambers—tighten to force blood into the aorta and pulmonary artery, which leads to the systemic and pulmonary circulation, respectively.

Understanding the heart's systolic phase helps clarify how blood pressure and flow are maintained throughout the body. Systole results in the higher number documented in blood pressure readings, known as the systolic blood pressure. It is also during systole that the sound of a heartbeat is mostly generated, often heard through a stethoscope during medical exams. The significance of this phase in the overall function of the cardiovascular system cannot be overstated.

Ventricular Contraction

Ventricular contraction is the key event of the systolic phase and primarily means the tightening of the ventricular myocardium—the heart muscle. During this phase, the heart ventricles contract after being filled with blood during the diastolic phase, when the heart was relaxed and the ventricles were refilled with blood. This contraction closes the mitral and tricuspid valves, preventing backflow to the atria, and opens the aortic and pulmonary valves, allowing ejection of blood to the body and lungs.

The importance of understanding ventricular contraction lies in its role in determining cardiac output, which is the volume of blood the heart pumps in one minute. This process can be influenced by several factors, including heart rate and stroke volume—the amount of blood pumped by a ventricle in a single beat. Any disturbances in ventricular contraction can lead to cardiac conditions, making its adequate function crucial for maintaining systemic health.

Cardiac Cycle Physiology

Cardiac cycle physiology refers to the sequence of mechanical and electrical events that repeat with every heartbeat. It includes all phases of the heart's rhythm: the systole (contraction) and diastole (relaxation). The cycle begins with the atrial contraction, filling the relaxed ventricles with blood (atrial systole), followed by the ventricles contracting during the systolic phase to distribute blood to the lungs and body. After the ventricles contract, they relax, which is the start of the ventricular diastole phase, and the whole cycle repeats.

A clear grasp of the cardiac cycle is essential for students and medical practitioners as it underpins critical heart functions and serves as a foundation for diagnosing and treating heart-related ailments. It is a remarkable example of physiology, showing a synchronized rhythm ensuring efficient and uninterrupted blood flow to sustain life.