The internal structure of the mammalian heart is characterized by four chambers: two atria and two ventricles. These chambers are separated by muscular walls, and their arrangement facilitates the efficient flow of blood.

Atria: The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. Atrioventricular (AV) valves are located between the atria and ventricles. The tricuspid valve is between the right atrium and right ventricle, and the mitral valve (bicuspid valve) is between the left atrium and left ventricle. These valves ensure unidirectional blood flow from the atria to the ventricles.

Ventricles: The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery. The left ventricle pumps oxygenated blood to the body via the aorta. The ventricles are separated by the interventricular septum, a thick muscular wall. Semilunar valves are located at the exit of each ventricle: the pulmonary semilunar valve is between the right ventricle and the pulmonary artery, and the aortic semilunar valve is between the left ventricle and the aorta. These valves prevent backflow of blood from the arteries into the ventricles.

Major Blood Vessels within the Heart:

The cardiac cycle is initiated by the sinoatrial node (SAN), often referred to as the heart's natural pacemaker. The SAN spontaneously generates electrical impulses, initiating atrial contraction. These impulses then spread throughout the atria, causing them to contract and effectively pump blood into the ventricles.

The electrical impulse then reaches the atrioventricular node (AVN). The AVN acts as a gatekeeper, delaying the impulse slightly. This delay is crucial because it allows the atria to fully contract and empty their contents into the ventricles before ventricular contraction begins. The AVN then transmits the impulse down the bundle of His.

The Purkinje tissue is a network of fibres that rapidly conducts the electrical impulse throughout the ventricles. This rapid conduction ensures that the ventricles contract in a coordinated manner, effectively pumping blood out to the pulmonary artery and aorta. The Purkinje fibres allow for a rapid and widespread depolarization of the ventricular muscle cells, leading to a powerful and efficient contraction.

Sequence of events:

1. SAN firing: Electrical impulse generated.
2. Atrial contraction: Impulse spreads through atria, causing contraction.
3. AVN delay: Impulse is delayed at the AVN.
4. Ventricular contraction: Impulse travels down the bundle of His and Purkinje fibres, causing ventricular contraction.

The arrangement of the heart's chambers and valves is crucial for ensuring efficient and unidirectional blood flow, which is essential for both the pulmonary and systemic circuits. The separation of oxygenated and deoxygenated blood into distinct circulatory pathways is a key aspect of this efficiency.

The right side of the heart handles the pulmonary circuit, which pumps deoxygenated blood to the lungs for oxygenation. The right atrium receives deoxygenated blood, which then passes through the tricuspid valve into the right ventricle. The right ventricle pumps this blood through the pulmonary artery to the lungs. The pulmonary veins then return the oxygenated blood to the left atrium.

The left side of the heart handles the systemic circuit, which pumps oxygenated blood to the rest of the body. Oxygenated blood enters the left atrium via the pulmonary veins, passes through the mitral valve into the left ventricle, and is then pumped through the aorta to the body. The aortic semilunar valve prevents backflow into the left ventricle.

The valves play a vital role in maintaining unidirectional flow. They open to allow blood to flow forward and close to prevent backflow. The muscular walls of the ventricles ensure that the pressure generated during contraction is sufficient to pump blood to the respective circuits. The thickness of the ventricular walls reflects the pressure required for each circuit; the left ventricle has thicker walls because it pumps blood to the entire body, requiring greater force.

In essence, the four-chambered design with its associated valves creates two separate, yet interconnected, circulatory loops. This prevents the mixing of oxygenated and deoxygenated blood, maximizing the efficiency of oxygen delivery to the tissues and removal of carbon dioxide. The coordinated contraction and relaxation of the atria and ventricles, controlled by the sinoatrial (SA) node and atrioventricular (AV) node, further contribute to the efficient circulation.