Cardiovascular System

Overview and Functions

The cardiovascular system is a closed system responsible for the transport and exchange of substances throughout the body. It consists of the heart, blood vessels, and blood.

• Transport and Exchange: Moves oxygen (O2), carbon dioxide (CO2), nutrients, hormones, and waste products between tissues and organs.

• Protection: Defends against disease (immune cells) and prevents blood loss (blood clotting mechanisms).

• Regulation: Maintains body temperature, fluid balance, pH, and electrolyte levels.

Example: Oxygen is transported from the lungs to tissues, while carbon dioxide is carried from tissues to the lungs for exhalation.

The Heart

Location and Structure

The heart is located in the mediastinum cavity of the thorax and is surrounded by a double-walled sac called the pericardium.

• Fibrous pericardium: Outermost layer, anchors heart to surrounding structures (e.g., diaphragm, aorta, vena cava).

• Serous pericardium: Inner layer, has two distinct layers:

  • Parietal pericardium: Fused to fibrous pericardium.

  • Visceral pericardium (epicardium): Fused to the heart wall.

• The space between parietal and visceral pericardium contains pericardial fluid to minimize friction.

Heart Wall Layers

The heart wall consists of three layers:

• Endocardium: Lines the inner surface of the heart and all blood vessels; made of simple squamous epithelium (endothelium) on connective tissue.

• Myocardium: The thick, muscular middle layer responsible for contraction; arranged in spiral patterns and reinforced with connective tissue.

• Epicardium: The outermost layer, also known as the visceral pericardium; consists of simple squamous epithelium and connective tissue.

Heart Chambers

The heart has four chambers: two atria and two ventricles.

• Atria are separated by the interatrial septum.

• Ventricles are separated by the interventricular septum.

• Right side pumps blood to the lungs; left side pumps blood to the body.

Cardiac (Fibrous) Skeleton

A plate of fibrous connective tissue located between the atria and ventricles:

• Forms solid rings around the heart valves, base of aorta, and pulmonary trunk, providing structural support.

• Creates a firm site for cardiac muscle attachment.

• Stops the spread of electrical signals from atria to ventricles, allowing for coordinated contraction.

Heart Valves

Valves ensure one-way blood flow through the heart:

• Atrioventricular (AV) valves: Sheet-like cusp valves between atria and ventricles.

  • Attached to papillary muscles via chordae tendineae.

  • Tricuspid valve: Between right atrium and right ventricle.

  • Bicuspid (mitral) valve: Between left atrium and left ventricle.

• Semilunar valves: Cup-like valves at the exits of the ventricles.

  • Pulmonary semilunar valve: Between right ventricle and pulmonary trunk.

  • Aortic semilunar valve: Between left ventricle and aorta.

Blood Vessels Entering and Exiting the Heart

• Veins: Superior and inferior vena cava, and coronary sinus drain into the right atrium.

• Pulmonary veins: Four veins drain oxygenated blood from the lungs into the left atrium.

• Arteries: Pulmonary trunk exits from the right ventricle and divides into pulmonary arteries; aorta exits from the left ventricle.

Cardiac Muscle Cells

Cardiac muscle cells (myocytes) form the majority of the myocardium and share similarities with skeletal muscle cells:

• Striated, contain myoglobin, myofibrils, thick and thin myofilaments, and are arranged into sarcomeres.

• Possess a sarcoplasmic reticulum and T-tubules.

• Short, branched, and uni-nucleated.

• Connected end-to-end by intercalated discs containing desmosomes (anchoring) and gap junctions (electrical coupling).

Conduction System Cells

Specialized non-contractile cardiac cells (1% of myocardial cells) generate and rapidly conduct electrical impulses:

• Sinoatrial (SA) node: Pacemaker cells located superior to the superior vena cava on the right atrial wall; initiates spontaneous action potentials.

• Atrioventricular (AV) node: Located at the base of the right atrium; receives impulses from the SA node.

• Atrioventricular bundle (Bundle of His): Rapidly conducts impulses from AV node to the apex of the ventricle.

• Bundle branches: Split from the bundle of His and run down the interventricular septum.

• Purkinje fibers: Terminal fibers that carry impulses upward and outward to the ventricles.

Example: The SA node initiates the heartbeat, causing the atria to contract downwards, while the ventricles contract upwards.

Circulatory Routes

Adult Circulation

The adult circulatory system is a closed, dual-circuit system with one-way blood flow:

• Pulmonary circuit: Right ventricle pumps deoxygenated blood to the lungs via pulmonary arteries; oxygenated blood returns to the left atrium via pulmonary veins.

• Systemic circuit: Left ventricle pumps oxygenated blood to all body tissues via the aorta; deoxygenated blood returns to the right atrium via veins.

Key Point: Arteries carry blood away from the heart; veins carry blood toward the heart, regardless of oxygenation status.

Coronary Circulation

Supplies the myocardium with oxygen and nutrients:

• Left and right coronary arteries: Originate at the base of the aorta.

• Branch into smaller arteries and capillaries.

• Cardiac veins drain into the coronary sinus, which empties into the right atrium.

Fetal Circulation

Fetal circulation exchanges nutrients, oxygen, and wastes with maternal circulation at the placenta. Blood supplies do not mix.

• Umbilical vein: Carries oxygenated blood from placenta to fetus.

• Umbilical arteries: Carry mixed fetal blood to the placenta.

• Three shunts: Allow blood to bypass the liver and lungs (non-functional before birth).

Example: The ductus arteriosus is a fetal shunt that diverts blood from the pulmonary artery to the aorta.

Summary Table: Heart Valves

Summary Table: Circulatory Circuits

Key Equations

• Cardiac Output: The amount of blood pumped by the heart per minute.

• Blood Pressure: The force exerted by circulating blood on vessel walls.

Additional info: The notes have been expanded with definitions, examples, and tables for clarity and completeness.