What is an explanation of how the heart beats?
How the Heart Beats
The heart is a vital organ that works continuously to pump blood throughout the body. This process, known as the cardiac cycle, involves four distinct phases:
1. Diastole (Relaxation)
The cardiac cycle begins with the heart in a state of relaxation, known as diastole. During this phase, the heart's chambers (atria and ventricles) fill with blood:
- Atrial systole: This is a weak and short contraction of the atria, the upper chambers of the heart. It helps direct blood into the ventricles, the lower chambers of the heart.
- Ventricular filling: The ventricles continue to fill with blood passively as the atrial systole occurs. Blood flows from the atria to the ventricles without any further muscle contractions during diastole.
2. Atrial systole (Atrial Contraction)
Once the ventricles are filled with blood, the atria contract simultaneously, giving the ventricles a final boost of blood. This contraction is coordinated by electrical signals from the sinoatrial (SA) node, which is the heart's natural pacemaker.
3. Ventricular Systole (Ventricular Contraction)
This is the main phase of the cardiac cycle where the ventricles contract forcefully to pump blood out of the heart and into the major arteries. The SA node generates electrical impulses that travel down to the atrioventricular node (AV node), which delays the signal briefly to ensure that the atria empty before the ventricles contract. The electrical signal is then transmitted throughout the ventricles, causing the myocardial cells (muscle cells of the heart) to contract.
- Isovolumetric ventricular contraction: Initially, the ventricles contract without significantly changing the volume of blood within them. This is because the aortic and pulmonary valves (valves connecting the heart to major arteries) are closed and ventricular pressure is rapidly building up.
- Ventricular ejection: Once the ventricular pressure exceeds the pressure in the arteries, the aortic and pulmonary valves open. Blood is rapidly ejected out of the ventricles into the aorta and pulmonary artery, respectively.
4. Ventricular Relaxation
After the ejection of blood, the ventricular contraction stops and the ventricles begin to relax. This allows the pressure within the ventricles to drop. As the ventricular pressure falls below the pressure in the aorta and pulmonary artery, the aortic and pulmonary valves close, preventing backflow of blood into the ventricles. The heart then moves back to the diastole phase, and the cycle repeats itself.
The rhythm and coordination of the cardiac cycle are crucial for maintaining proper circulation and ensuring that oxygen and nutrients are continuously delivered to the body's tissues and organs.