Question

Physiology:

Describe what happens during a heartbeat. Include what muscles contract when and what causes those contractions, what happens to blood within the heart during different parts of the beat, and what you would observe on an ECG during each part of the beat.

Answer 1

Heartbeat: Heartbeat is a two-step rhythmic process that includes relaxation (diastole) and contraction (systole) of the heart chambers (atria and ventricles).  

During diastole, both the atria and ventricles are relaxed. In contrast, during systole both these parts contact one by one.

The heartbeat has two sounds; first sound (lub) and the second sound (dub or dup). The closing of the mitral and tricuspid valve produces a sound as lub, and the closing of the semilunar valve produces a sound as dup.

As the blood flows into the atria, and the atria are full of blood, an action potential is generated in the SA node (sinoatrial node), which is further transmitted to the atrial muscles to contract. As a result, the mitral and tricuspid valve, which are present between the atria and ventricles get opened and the blood flows from the atria to the ventricles.

When the ventricles are full of blood, the reverse pressure causes the mitral and the tricuspid valve to be closed. The electrical signals are sent from the SA node to the ventricle muscles and cause them to contract. As a result, the semilunar valve present between the ventricles and the blood vessels gets opened and the blood flows from the ventricle to the blood vessels.

ECG: ECG is the graphical representation of the electrical activities of the heart with the help of an instrument called electrocardiogram.

During a normal heartbeat, the ECG graph is shown below:

Parts of a normal ECG:

P wave: It is a relatively small hump of low magnitude. Atrial depolarization or contraction causes this wave to generate.

QRS complex: Ventricle contraction or depolarzation causes this complex to form.

T wave: The repolarization of ventricles causes the formation of this wave.

Answer 2

During a heartbeat, a series of events occur to pump blood through the heart and circulate it throughout the body. The heartbeat is controlled by electrical impulses that stimulate the heart muscles to contract and relax rhythmically.

1. Atrial Contraction (Atrial Systole): The heartbeat starts with the contraction of the atria, the upper chambers of the heart. The electrical impulse originates in the sinoatrial (SA) node, also known as the heart's natural pacemaker. The SA node sends out electrical signals that cause the atria to contract, pushing blood into the ventricles.

On an electrocardiogram (ECG or EKG), the atrial contraction is represented by the P-wave. The P-wave shows the electrical activity associated with the depolarization (contraction) of the atria.

1. Ventricular Contraction (Ventricular Systole): After the atria contract and fill the ventricles with blood, the electrical impulse travels to the atrioventricular (AV) node, located between the atria and ventricles. The AV node delays the impulse slightly to allow the ventricles to fill completely.

Once the ventricles are filled, the electrical signal continues along the Purkinje fibers, stimulating the ventricles to contract. This powerful contraction forces blood out of the heart into the arteries.

On an ECG, the ventricular contraction is represented by the QRS complex. The QRS complex shows the electrical activity associated with the depolarization (contraction) of the ventricles.

1. Ventricular Relaxation (Ventricular Diastole): After the ventricles contract and expel blood, they relax to fill with blood from the atria once again. The relaxation phase is called ventricular diastole. At this time, the atria are also relaxed, and they begin to fill with blood from the veins.

On an ECG, ventricular relaxation is represented by the T-wave. The T-wave shows the electrical activity associated with the repolarization (relaxation) of the ventricles.

Throughout the heartbeat, there is a specific pattern of blood flow within the heart:

• Atrial Contraction: Blood flows from the veins (superior and inferior vena cava and pulmonary veins) into the atria. As the atria contract, blood is pushed into the ventricles through the open atrioventricular valves (tricuspid and mitral valves).

• Ventricular Contraction: As the ventricles contract, the atrioventricular valves close to prevent backflow of blood into the atria. The semilunar valves (aortic and pulmonary valves) open, allowing blood to be ejected from the ventricles into the aorta and pulmonary artery.

• Ventricular Relaxation: After ventricular contraction, the semilunar valves close to prevent backflow into the ventricles. The atrioventricular valves open again, allowing blood to flow from the atria into the ventricles, preparing for the next heartbeat.

In summary, during a heartbeat, the heart undergoes a complex series of contractions and relaxations driven by electrical impulses. These actions ensure that blood is efficiently pumped and circulated throughout the body. On an ECG, the different phases of the heartbeat are represented by specific waveforms (P-wave, QRS complex, and T-wave), reflecting the electrical activity associated with each part of the cardiac cycle.