Cardiovascular System: The Heart

Overview

The heart is a muscular organ responsible for pumping blood throughout the body via the circulatory system. Its structure and function are essential for maintaining homeostasis and supporting cellular metabolism.

Heart Anatomy

Cardiac Muscle Cells

• Short and Branched Cells: Cardiac muscle cells (cardiomyocytes) are shorter and more branched than skeletal muscle fibers, allowing for a strong, interconnected network.

• Sarcomeres: Like skeletal muscle, cardiac muscle contains sarcomeres, the basic contractile units responsible for muscle contraction.

• More Mitochondria: Cardiac cells have a high density of mitochondria to meet the energy demands of continuous contraction.

• Less Ca2+ Storage: Cardiac muscle stores less calcium than skeletal muscle, relying more on extracellular calcium for contraction.

• Intercalated Disks: Specialized junctions that connect cardiac cells, allowing rapid transmission of electrical impulses and synchronized contraction.

• Automaticity & Autorhythmicity: Cardiac cells can generate their own action potentials without external stimulation, enabling the heart to beat independently.

Cardiac Tissue Layers

• Epicardium: The outermost layer of the heart wall, also known as the visceral pericardium.

• Myocardium: The thick, muscular middle layer responsible for the heart's contractile force.

• Endocardium: The smooth, inner lining of the heart chambers and valves, minimizing friction as blood flows through the heart.

Pericardium and Pericardial Cavity

• Pericardium: A double-walled sac surrounding the heart, consisting of:

  • Parietal Pericardium: The outer layer, attached to the diaphragm and great vessels.

  • Visceral Pericardium (Epicardium): The inner layer, directly covering the heart.

• Pericardial Cavity: The space between the parietal and visceral layers, containing pericardial fluid that reduces friction during heartbeats.

Circulatory System Circuits

• Pulmonary Circuit: Carries deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side of the heart.

• Systemic Circuit: Delivers oxygenated blood from the left side of the heart to the body and returns deoxygenated blood to the right side.

• Coronary Circulation: Supplies blood to the heart muscle itself via coronary arteries and veins.

• Blood Vessels:

  • Arteries: Carry blood away from the heart (usually oxygenated, except pulmonary arteries).

  • Veins: Carry blood toward the heart (usually deoxygenated, except pulmonary veins).

  • Capillaries: Microscopic vessels where exchange of gases, nutrients, and wastes occurs between blood and tissues.

Heart Chambers and Septa

• Atria: The two upper chambers (right and left) that receive blood returning to the heart.

• Ventricles: The two lower chambers (right and left) that pump blood out of the heart.

• Interatrial Septum: Wall separating the right and left atria.

• Fossa Ovalis/Foramen Ovale: A depression in the interatrial septum, remnant of a fetal opening (foramen ovale) that allowed blood to bypass the fetal lungs.

• Interventricular Septum: Wall separating the right and left ventricles.

Heart Valves

• Atrioventricular (AV) Valves: Separate the atria from the ventricles, preventing backflow during ventricular contraction.

  • Right AV Valve (Tricuspid): Between right atrium and right ventricle.

  • Left AV Valve (Bicuspid/Mitral): Between left atrium and left ventricle.

• Semilunar Valves: Prevent backflow from arteries into ventricles.

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and ascending aorta.

Major Blood Vessels Associated with the Heart

• Right Atrium: Receives deoxygenated blood from:

  • Superior Vena Cava: Drains blood from upper body.

  • Inferior Vena Cava: Drains blood from lower body.

  • Coronary Sinus: Drains blood from the heart's own circulation.

• Right Ventricle: Pumps blood through the pulmonary valve into the pulmonary trunk and pulmonary arteries to the lungs.

• Left Atrium: Receives oxygenated blood from the pulmonary veins.

• Left Ventricle: Pumps blood through the aortic valve into the ascending aorta, arch of aorta, and descending aorta to the systemic circulation.

• Coronary Arteries: Branch from the ascending aorta to supply the heart muscle.

• Ductus Arteriosus/Ligamentum Arteriosum: In the fetus, the ductus arteriosus connects the pulmonary trunk to the aorta, bypassing the lungs. After birth, it closes and becomes the ligamentum arteriosum.

Summary Table: Heart Chambers, Valves, and Major Vessels

Example: Blood Flow Through the Heart

1. Deoxygenated blood enters the right atrium from the superior and inferior vena cava and coronary sinus.

2. Passes through the tricuspid valve into the right ventricle.

3. Pumped through the pulmonary valve into the pulmonary trunk and arteries to the lungs.

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

5. Passes through the bicuspid (mitral) valve into the left ventricle.

6. Pumped through the aortic valve into the aorta and systemic circulation.

Additional info: The heart's structure is closely related to its function as a dual pump, ensuring separation of oxygenated and deoxygenated blood and efficient circulation throughout the body.