The heart
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Describe the external structure of the mammalian heart.
The mammalian heart has four chambers: two atria and two ventricles. Major blood vessels include the vena cava (entering right atrium), pulmonary artery (leaving right ventricle), pulmonary vein (entering left atrium), and aorta (leaving left ventricle). Coronary arteries supply the heart muscle itself with blood.
Describe the internal structure of the mammalian heart.
The atria are separated from the ventricles by atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left). Semilunar valves are located at the exits of the ventricles into the pulmonary artery and aorta. The septum divides the left and right sides of the heart.
Explain why the walls of the ventricles are thicker than the walls of the atria.
Ventricles pump blood to the lungs and the rest of the body, requiring more force and therefore thicker muscular walls. Atria only pump blood to the ventricles, a shorter distance, so their walls are thinner.
Explain why the left ventricle wall is thicker than the right ventricle wall.
The left ventricle pumps blood to the entire body, against higher pressure, requiring more force and a thicker wall. The right ventricle only pumps blood to the lungs, a shorter distance with lower pressure.
Describe what happens during atrial systole.
The atria contract, increasing pressure and forcing blood through the open atrioventricular valves into the ventricles. The ventricles are relaxed (diastole) at this stage.
Describe what happens during ventricular systole.
The ventricles contract, increasing pressure and forcing the atrioventricular valves to close (preventing backflow). As pressure rises further, the semilunar valves open, and blood is ejected into the pulmonary artery and aorta.
Describe what happens during diastole.
Both the atria and ventricles are relaxed. Blood flows passively from the veins into the atria and then into the ventricles through the open atrioventricular valves. The semilunar valves are closed.
Explain the role of the sinoatrial node (SAN) in the cardiac cycle.
The SAN is the heart's natural pacemaker. It initiates electrical impulses that spread across the atria, causing them to contract simultaneously. This starts each cardiac cycle.
Explain the role of the atrioventricular node (AVN) in the cardiac cycle.
The AVN receives the electrical impulse from the SAN. It delays the impulse briefly, allowing the atria to finish contracting before the ventricles contract. It then passes the impulse to the bundle of His.
Explain the role of the Purkyne tissue in the cardiac cycle.
Purkyne fibres carry the electrical impulse rapidly throughout the ventricular walls. This ensures a coordinated and simultaneous contraction of the ventricles, efficiently pumping blood out of the heart.
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