heart anatomy

Cardiac muscle cells, also known as cardiomyocytes, are distinct from skeletal muscle cells in several key aspects.

  • Structure: Cardiac muscle cells are single nucleated and branched, which allows them to connect with one another and form a functional syncytium, enabling coordinated contractions. In contrast, skeletal muscle cells are multinucleated due to the fusion of myoblasts (stem cells) during development, forming long, cylindrical fibers.

  • Striation: Like skeletal muscle, cardiac muscle cells are striated due to the organized arrangement of actin and myosin filaments. This striation is essential for the muscle's ability to contract efficiently.

  • Regulation: Two key characteristics distinguish cardiac muscle from skeletal muscle:

    • They are involuntary, meaning they contract autonomously without conscious control, regulated primarily by the autonomic nervous system and hormones.

    • They possess intercalated discs, specialized structures that facilitate cell-to-cell communication through gap junctions and desmosomes, allowing for rapid electrical conduction and mechanical stability, which is vital for continuous heart contractions.

Heart Function

The heart is a marvel of physiological engineering, contracting approximately 70 times per minute under normal resting conditions to pump blood throughout the entire body.

  • Mechanism of Contraction: The process of contraction is initiated by electrical signals that involve sodium influx, creating action potentials that propagate through the cardiac muscle cells.

    • Cardiac muscle cells can create a graded potential similar to neurons; however, these potentials are specifically adapted for muscle contraction, allowing for varying levels of contraction based on stimulus strength.

    • Sodium influx increases the membrane potential until the threshold is reached, resulting in depolarization and contraction.

  • Location and Protection: The heart is anatomically located in the mediastinum, bordered by pericardial layers that offer protection and lubrication.

    • The parietal pericardium serves as an outer layer composed of fibrous connective tissue, which protects the heart and anchors it in the thoracic cavity.

    • The visceral pericardium, also known as the epicardium, is the inner layer that closely adheres to the heart’s surface, providing even more protection.

    • The endocardium lines the interior chambers of the heart, providing a smooth surface that reduces friction during blood flow and ensuring efficient hemodynamics.

Heart Chambers and Blood Flow

The heart is divided into four primary chambers: two atria (upper chambers) and two ventricles (lower chambers), each with distinct roles in circulation.

  • Atria:

    • The right atrium receives deoxygenated blood from the systemic circulation via the superior and inferior vena cavae.

    • The left atrium receives oxygenated blood from the lungs through the pulmonary veins.

    • Atrial walls are thinner than ventricular walls, facilitating passive blood flow into the ventricles.

  • Ventricles:

    • The right ventricle pumps deoxygenated blood to the lungs via the pulmonary trunk for gas exchange.

    • The left ventricle, which is thicker-walled due to its role in pumping oxygenated blood to the entire body through the aorta, is crucial for sustaining systemic circulation and maintaining blood pressure.

    • Auricles, flaps of tissue on top of each atrium, assist in increasing the blood capacity in the atria, enhancing the heart's ability to manage variations in blood return.

Blood Vessels and Coronary Circulation

Coronary circulation is vital for supplying oxygenated blood to the heart muscle itself, ensuring its functionality.

  • The right coronary artery primarily supplies blood to the right ventricle, whereas

  • The left coronary artery serves the left ventricle, branching into the left anterior descending artery (LAD) which nourishes major portions of the heart muscle.

  • The coronary sinus collects venous blood from the myocardium and drains it into the right atrium, facilitating efficient removal of the heart's metabolic waste.

Heart Valves

The heart includes four crucial valves that regulate blood flow and prevent backflow:

  • The right AV valve (tricuspid valve) sits between the right atrium and right ventricle, ensuring unidirectional blood flow.

  • The left AV valve (bicuspid or mitral valve) is located between the left atrium and left ventricle, playing a similar role.

  • The pulmonary semilunar valve is positioned between the right ventricle and pulmonary trunk, while the aortic semilunar valve regulates blood flow from the left ventricle into the aorta.

Electrical Conduction System

The heart possesses its intrinsic conduction system responsible for initiating and regulating the heartbeat:

  • The SA node (sinoatrial node), known as the heart's natural pacemaker, generates electrical impulses that initiate heartbeats.

  • The AV node (atrioventricular node) receives these signals and transmits them to the ventricles, allowing for a delay that ensures the atria contract before the O branch into Purkinje fibers located in the ventricular walls.

  • These action potentials result in the synchronized contraction of muscle cells, leading to the heartbeat cycle: systole (contraction phase) and diastole (relaxation phase).

Blood Flow Sequence

Blood flow through the heart and body follows a specific sequence:

  1. Deoxygenated blood returns from the body via superior and inferior vena cava into the right atrium.

  2. Blood flows through the right AV valve into the right ventricle.

  3. Right ventricle contracts, and blood is pumped through the pulmonary semilunar valve into the pulmonary trunk to the lungs for oxygenation.

  4. Oxygenated blood returns from the lungs via the pulmonary veins to the left atrium.

  5. Blood flows through the left AV valve into the left ventricle.

  6. Left ventricle contracts, sending blood out through the aortic semilunar valve into the aorta, which distributes oxygenated blood throughout the body.

Cardiac Output

Cardiac output represents the volume of blood pumped by the heart in one minute, which is crucial for meeting the body’s metabolic demands.

  • Each heartbeat pumps a specific volume known as stroke volume, with typical values around 70 mL.

  • Cardiac output can be calculated by multiplying the stroke volume by heart rate (approximately 70 beats per minute), resulting in a total cardiac output of about 5 liters per minute, which is roughly equivalent to the total blood volume of an average adult human being.