6.5

The Heart and Circulatory System

• The heart is a muscular organ that pumps blood throughout the body.
• The heart has multiple layers, including the pericardium (made of inflexible connective tissue).
• The heart is mainly composed of cardiac muscle tissue. The pericardium secretes lubricating fluids allowing the heart to slide within the thoracic cavity.
• Cardiac muscle fibers contract involuntarily through electrical impulses.

Heart Structures

• Atria act as collecting chambers (right atrium collects deoxygenated blood from the vena cava, left atrium collects oxygenated blood from pulmonary veins.
• Ventricles are pumping chambers (right ventricle sends blood to the lungs, left ventricle sends blood to the body).
• The left ventricle has a thicker wall than the right because it needs to generate more force to pump blood further throughout the body.
• Atrioventricular (AV) valves prevent backflow of blood from ventricles to atria. They are one way valves.
• Tricuspid valve (right AV valve): with 3 flaps. Bicuspid valve (left AV valve): with 2 flaps.

Heart Valves

• Chordae tendineae (fibrous strings) support valves.
• Semilunar valves prevent backflow between ventricles and the arteries leaving the heart. Two sets exist, Pulmonary and Aortic.

Other Structures

• Pulmonary trunk branches from the right ventricle to the pulmonary arteries leading to the lungs.
• Aorta branches from the left ventricle to the body.
• Septum separates the left and right sides of the heart to keep oxygenated and deoxygenated blood separate to prevent mixing.

Cardiac Cycle

• The cardiac cycle involves two steps: systole (contraction) and diastole (relaxation).
• The heart rate is approximately 70 beats per minute.
• The cycle takes approximately 0.85 seconds.
• Atria contract first, then ventricles, with a small delay to allow the atria to refill before the ventricles contract.

Heartbeat Regulation

• Sinoatrial (SA) node (pacemaker) initiates the heartbeat.
• Impulse travels through the the atria and then the atrioventricular (AV) node, which delays the signal and allows the heart to refill with blood.
• Electrical impulse travels to the ventricles through the Bundle of His and Purkinje fibers.
• Specialized electrical nodes generate electrical impulses to coordinate the contracting muscle of the heart; these nodes are a part of the intrinsic cardiac conduction system.
• Heartbeat sounds (lub-dub) are due to the closing of heart valves.

Premature Ventricular Contraction (PVC)

• PVC occurs when Purkinje fibers fire without input from the rest of the heart.
• This generates a premature ventricular contraction.
• This can feel like skipping a heart beat, can be common but it can life threatening in some circumstances.

Test your knowledge on the heart and circulatory system with this quiz. Learn about the structure and function of different heart components, including atria, ventricles, and valves. Challenge yourself to understand how the heart pumps blood and maintains circulation in the body.