Study Notes: The Heart – Structure, Function, and Circulation

Chapter 20 – The Heart

Overview of the Circulatory System

The circulatory system is essential for transporting substances throughout the body, maintaining homeostasis, and supporting cellular function. It consists of the heart, blood vessels, and blood.

• Purpose: To deliver oxygen and nutrients to tissues, remove waste products, and transport hormones and immune cells.

• Main Components:

  • Heart – the pump

  • Blood vessels – arteries, veins, capillaries

  • Blood – the transport medium

• Transported Substances: Oxygen, carbon dioxide, nutrients, hormones, metabolic wastes.

• Closed System: Blood is contained within vessels, preventing direct contact with tissues except at capillary beds.

Blood Pressure and Flow

Blood pressure is the force exerted by blood on vessel walls, generated by the heart's contractions. A pressure gradient is necessary to move blood efficiently through the closed circulatory system.

• Hydrostatic Pressure: The pressure of blood against vessel walls; drives blood flow.

• Pressure Gradient: Blood moves from areas of higher pressure (arteries) to lower pressure (veins).

• Necessity: Ensures continuous circulation and delivery of substances.

Location, Size, and Shape of the Heart

The heart is a muscular organ located in the thoracic cavity, slightly left of the midline, within the mediastinum.

• Location: Between the lungs, behind the sternum.

• Size: Approximately the size of a closed fist (about 250–350 grams in adults).

• Shape: Cone-shaped, with a pointed apex and broad base.

• Function: Central pump of the circulatory system, necessary for maintaining blood flow.

Pulmonary and Systemic Circuits

The heart pumps blood through two main circuits: pulmonary (to the lungs) and systemic (to the body).

• Pulmonary Circuit: Right side of the heart pumps deoxygenated blood to the lungs for oxygenation.

• Systemic Circuit: Left side of the heart pumps oxygenated blood to the rest of the body.

• Anatomical Differences: The left ventricle has a thicker myocardium to generate higher pressure for systemic circulation.

Pericardium: Structure and Function

The pericardium is a double-walled sac that surrounds and protects the heart.

• Structure:

  • Fibrous pericardium – outer layer, tough connective tissue

  • Serous pericardium – inner layer, subdivided into parietal and visceral layers

• Function: Protects the heart, anchors it in the chest, prevents overfilling.

Layers of the Heart Wall

The heart wall consists of three layers, each with distinct functions.

• Epicardium: Outer layer; provides protection and contains blood vessels.

• Myocardium: Middle layer; composed of cardiac muscle, responsible for contraction.

• Endocardium: Inner layer; lines the chambers and valves, reduces friction.

Cardiac Muscle and Specialized Structures

The myocardium is made of cardiac muscle cells, which are interconnected for efficient contraction.

• Cardiac Muscle: Striated, involuntary muscle unique to the heart.

• Intercalated Discs: Specialized junctions between cells; contain gap junctions and desmosomes.

• Gap Junctions: Allow electrical signals to pass rapidly between cells, enabling coordinated contraction.

• Location: Found throughout the myocardium.

• Function: Synchronize heart muscle contraction.

Muscle Bundles in Atria and Ventricles

The atria and ventricles have different muscle thicknesses due to their distinct functions.

• Atria: Thinner walls; pump blood into ventricles.

• Ventricles: Thicker walls, especially the left ventricle; pump blood to lungs (right) and body (left).

• Myocardium Thickness: Thickest in the left ventricle due to higher pressure needed for systemic circulation.

Structure and Function of Atria and Ventricles

The heart has four chambers: two atria and two ventricles, each with unique structural and functional roles.

• Atria: Superior chambers; receive blood from veins.

• Ventricles: Inferior chambers; eject blood into arteries.

• External Features: Auricles (ear-like flaps) on atria; thicker muscular walls in ventricles.

• Functional Differences: Atria act as receiving chambers; ventricles as pumping chambers.

Definitions: Veins and Arteries

Veins and arteries are the main types of blood vessels, distinguished by the direction of blood flow.

• Vein: Vessel that carries blood toward the heart.

• Artery: Vessel that carries blood away from the heart.

Major Vessels Entering the Atria

Several major vessels deliver blood to the heart's atria, with distinct oxygenation states.

• Right Atrium: Receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus.

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

Major Vessels Leaving the Ventricles

The ventricles pump blood into major arteries, each carrying blood with specific oxygenation.

• Right Ventricle: Pumps deoxygenated blood into the pulmonary trunk (to lungs).

• Left Ventricle: Pumps oxygenated blood into the aorta (to body).

Atrioventricular (AV) Valves

AV valves separate the atria from the ventricles, ensuring one-way blood flow.

• Structure: Flaps (cusps) anchored by chordae tendineae.

• Location: Between atria and ventricles.

• Types:

  • Tricuspid valve – right side, three cusps

  • Bicuspid (mitral) valve – left side, two cusps

• Function: Prevent backflow of blood into atria during ventricular contraction.

Semilunar Valves

Semilunar valves prevent backflow from arteries into the ventricles.

• Aortic Valve: Between left ventricle and aorta.

• Pulmonary Valve: Between right ventricle and pulmonary trunk.

• Structure: Three crescent-shaped cusps.

• Function: Prevent blood from re-entering ventricles after contraction.

Circulation Patterns

Blood flows through the heart in a specific sequence, involving pulmonary and systemic circuits.

• Through the Heart: Body → right atrium → right ventricle → lungs → left atrium → left ventricle → body

• Pulmonary Circulation: Right ventricle → pulmonary trunk → lungs → left atrium

• Systemic Circulation: Left ventricle → aorta → body → right atrium

Coronary Circulation

The myocardium requires its own blood supply, provided by the coronary arteries and veins.

• Coronary Arteries: Branch from the base of the aorta; right and left coronary arteries supply the heart muscle.

• Major Branches:

  • Right coronary artery – marginal and posterior interventricular branches

  • Left coronary artery – anterior interventricular (LAD) and circumflex branches

• Coronary Veins: Drain deoxygenated blood from myocardium into the coronary sinus.

Anastomoses in Coronary Circulation

Anastomoses are connections between blood vessels that provide alternate routes for blood flow.

• Definition: Direct connections between arteries or veins.

• Importance: Ensure continued blood supply if one route is blocked.

• Location: Found in coronary circulation to protect heart tissue.

Coronary Sinus

The coronary sinus is the main vein that collects blood from the heart's veins and returns it to the right atrium.

• Function: Drains most of the deoxygenated blood from the myocardium.

• Location: Posterior aspect of the heart, opening into the right atrium.

Summary Table: Heart Chambers, Valves, and Major Vessels

Chamber	Receives Blood From	Pumps Blood To	Associated Valve	Blood Oxygenation
Right Atrium	Body (via vena cavae), coronary sinus	Right Ventricle	Tricuspid (AV)	Deoxygenated
Right Ventricle	Right Atrium	Lungs (via pulmonary trunk)	Pulmonary (semilunar)	Deoxygenated
Left Atrium	Lungs (via pulmonary veins)	Left Ventricle	Bicuspid (mitral, AV)	Oxygenated
Left Ventricle	Left Atrium	Body (via aorta)	Aortic (semilunar)	Oxygenated

Key Equations

• Blood Pressure: $P = F / A$ (Pressure = Force / Area)

• Flow Rate: $Q = \Delta P / R$ (Flow = Pressure difference / Resistance)

Additional info: Academic context and definitions have been expanded for clarity and completeness.