Biology: The Heart and Cardiac Cycle

Understanding Biology: The Heart and Cardiac Cycle

The heart works by creating pressure differences. Muscle contraction raises pressure inside a chamber, so blood moves toward an area with lower pressure. When a ventricle is relaxed, its pressure falls below the pressure in the atrium above it.

The valve between them opens and blood enters. As the ventricle contracts, its pressure rises sharply. That rise closes the inlet valve before blood can move backward.

When ventricular pressure becomes greater than the pressure in the artery leaving it, the outlet valve opens. This simple pressure rule explains every valve movement. Valves do not need muscles or nerves to open and shut.

One complete heartbeat has several closely timed stages. First, both atria fill while the ventricles are relaxing. Most blood enters the ventricles without an atrial squeeze.

Near the end of filling, the atria contract and add a small extra amount. This is more important when the heart beats quickly because there is less time for passive filling. The ventricles then contract.

At first, all four valves are closed for a brief moment. Pressure rises without a change in blood volume. Next, blood is pushed into the pulmonary artery and aorta.

After ejection, ventricular muscle relaxes. The outlet valves close, then pressure falls until the inlet valves can open again.

The timing comes from specialised heart cells that produce and carry electrical signals. A signal starts in the sinoatrial node, spreads across the atria, then pauses briefly at the atrioventricular node. This delay gives the ventricles time to fill before they contract.

The signal then travels through fast conducting fibres in the ventricular walls. Both ventricles squeeze from the lower part upward, which helps direct blood into the large arteries above them. An electrocardiogram, often called an ECG, records these electrical events from the skin.

It records the signal, not the force of the heartbeat. A regular electrical pattern does not always guarantee normal pumping, but it provides important clues about rhythm problems.

Stroke volume is the amount of blood ejected by one ventricle in one beat. Cardiac output equals heart rate times stroke volume. During exercise, muscles need more oxygen and release more carbon dioxide.

The heart responds by beating faster and pumping more blood per beat. Veins return extra blood from active muscles, which stretches the ventricles slightly and can strengthen their next contraction. Blood pressure is highest when ventricles eject blood and lower while they relax.

A pulse is the pressure wave that travels along an artery after the left ventricle contracts. It is not a packet of blood racing from the heart to the wrist.

When learning this topic, trace blood flow separately from electrical flow. They are connected but they are not the same process. Pay attention to the reason for each valve action by comparing pressures on its two sides.

Remember that the left ventricle has a thicker wall because it must push blood through the entire body, while the right ventricle only pushes blood to the nearby lungs. Heart murmurs can occur when blood flows through a narrowed valve or a valve that does not seal fully. Blocked coronary arteries are different because they reduce oxygen delivery to the heart muscle itself, which can weaken its pumping ability.