Mammalian Heart Function and Cardiac Cycle Mechanics

Functioning of the Mammalian Heart

Blood circulates through blood vessels to the heart, driven by coordinated movements of systolic contraction and diastolic dilation. Both movements occur simultaneously.

Circulatory Pathway

• Left side: Blood from the lungs enters the heart through the left atrium, passes into the left ventricle, and is distributed throughout the body via the aorta.
• Return: Blood returns to the heart through the superior and inferior vena cava, which flow into the right atrium.
• Right side: Blood passes into the right ventricle and travels to the lungs through the pulmonary artery.
• Completion: Blood returns to the heart through the pulmonary veins, which empty into the left atrium.

Heart Valves

The heart has four valves that open in only one direction, determining the flow of blood:

• Mitral valve: Between the left atrium and left ventricle.
• Tricuspid valve: Between the right atrium and right ventricle.
• Aortic valve: Between the left ventricle and the aorta.
• Pulmonary valve: Between the right ventricle and the pulmonary artery.

The atria and ventricles differ in size and wall thickness because the ventricles must pump blood throughout the body, whereas the atria only pump blood to the ventricles.

The Cardiac Cycle

The cardiac cycle is the sequence of processes occurring in the heart to produce a full beat. Each heartbeat consists of:

• Atrial and ventricular diastole: Prevents the return of blood from arteries to the heart.
• Atrial systole and ventricular diastole: The muscles of the atria contract, pumping blood into the ventricles, which are in diastole.
• Ventricular systole and atrial diastole: The ventricles contract and push blood through the arteries. The mitral and tricuspid valves close, preventing blood from flowing back into the atria.

Origin and Spread of the Heartbeat

The heartbeat in a mammal begins in a group of specialized muscle cells in the right atrium called the sinoatrial node (or pacemaker), which contract rhythmically on their own. The impulse spreads rapidly through the walls of the atria, causing them to contract.

These pulses arrive at the atrioventricular node, where the beat is retained for 0.1 seconds to allow the atria to empty completely. The impulse is then transmitted via the bundle of His to the walls of the left and right ventricles, which contract simultaneously. Note that heart rate can vary based on factors such as physical activity.