Anatomy and Physiology of the Heart: Structure, Circulation, and Valves

The Pulmonary and Systemic Circuits

Overview of Heart Circulation

The heart functions as a double pump, supplying blood to two major circuits: the pulmonary and systemic circuits. Each side of the heart serves a distinct role in blood oxygenation and distribution.

• Right side: Receives oxygen-poor blood from body tissues and pumps it to the lungs via the pulmonary circuit to eliminate CO2 and absorb O2.

• Left side: Receives oxygen-rich blood from the lungs and pumps it to body tissues via the systemic circuit.

Receiving and Pumping Chambers

• Right atrium: Receives blood returning from the systemic circuit.

• Left atrium: Receives blood returning from the pulmonary circuit.

• Right ventricle: Pumps blood through the pulmonary circuit.

• Left ventricle: Pumps blood through the systemic circuit.

Diagrammatic Representation

The systemic and pulmonary circuits are often depicted with blue (deoxygenated) and red (oxygenated) pathways, showing the flow of blood through the heart, lungs, and body tissues.

Location, Size, and Orientation of the Heart

Position in the Mediastinum

The heart is located in the mediastinum, the central compartment of the thoracic cavity, between the lungs.

• Midsternal line: The heart lies slightly left of this line.

• 2nd rib and diaphragm: The heart extends from the 2nd rib down to the diaphragm.

• Apical impulse: Palpated between the fifth and sixth ribs, just below the left nipple.

Size and Orientation

• Base: The posterior surface leans toward the right shoulder.

• Apex: Points toward the left hip.

Coverings of the Heart

Pericardium

The heart is enclosed in a double-walled sac called the pericardium, which consists of two main layers:

• Fibrous pericardium: Superficial layer that protects, anchors the heart, and prevents overfilling.

• Serous pericardium: Deep, two-layered membrane:

  • Parietal layer: Lines the internal surface of the fibrous pericardium.

  • Visceral layer (epicardium): Covers the external surface of the heart.

• The two serous layers are separated by the pericardial cavity, filled with fluid to reduce friction.

Clinical Note: Pericarditis and Cardiac Tamponade

• Pericarditis: Inflammation of the pericardium, causing roughened membrane surfaces and a pericardial friction rub (creaking sound).

• Cardiac tamponade: Excess fluid in the pericardial space compresses the heart, impairing its ability to pump. Treatment involves fluid removal.

Layers of the Heart Wall

Three Layers

• Epicardium: Visceral layer of serous pericardium.

• Myocardium: Composed of circular or spiral bundles of contractile cardiac muscle cells.

  • Cardiac skeleton: Crisscrossing, interlacing layer of connective tissue that anchors muscle fibers, supports vessels and valves, and limits the spread of action potentials.

• Endocardium: Innermost layer, continuous with endothelial lining of blood vessels; lines heart chambers and covers cardiac skeleton of valves.

Arrangement of Cardiac Muscle Bundles

Cardiac muscle fibers are arranged in circular and spiral bundles, which facilitate efficient contraction and ejection of blood.

Chambers and Associated Great Vessels

Internal Features

• Four chambers: Two superior atria and two inferior ventricles.

• Interatrial septum: Separates atria; contains the fossa ovalis, a remnant of the fetal foramen ovale.

• Interventricular septum: Separates ventricles.

Surface Features

• Coronary sulcus: Encircles the junction of atria and ventricles.

• Anterior and posterior interventricular sulci: Mark the position of the interventricular septum on the heart's surface.

Atria: The Receiving Chambers

• Auricles: Appendages that increase atrial volume.

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

• Pectinate muscles: Ridges found in the atrial walls.

• Left atrium: Receives oxygenated blood from the lungs via four pulmonary veins.

Ventricles: The Discharging Chambers

• Right ventricle: Most of the anterior surface; pumps blood into the pulmonary trunk.

• Left ventricle: Forms the posteroinferior surface; pumps blood into the aorta.

• Trabeculae carneae: Irregular ridges of muscle on ventricular walls.

• Papillary muscles: Project into the ventricular cavity and anchor the chordae tendineae attached to heart valves.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow and open/close in response to pressure changes.

• Atrioventricular (AV) valves: Located between atria and ventricles.

  • Tricuspid valve: Right AV valve, three cusps.

  • Mitral valve: Left AV valve (bicuspid), two cusps.

  • Chordae tendineae: Anchor valve cusps to papillary muscles, preventing valve inversion.

• Semilunar (SL) valves: Located between ventricles and major arteries.

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

  • Aortic SL valve: Between left ventricle and aorta.

  • Each SL valve has three half-moon-shaped cusps.

Valve Function

• AV valves: Open when atrial pressure exceeds ventricular pressure; close when ventricles contract, preventing backflow into atria.

• SL valves: Open when ventricular pressure exceeds arterial pressure; close when ventricles relax, preventing backflow from arteries.

Clinical Note: Valve Disorders

• Incompetent valve: Blood backflows, causing the heart to repump the same blood repeatedly.

• Valvular stenosis: Stiff valve flaps restrict opening, requiring more force to pump blood.

• Defective valves may be replaced with mechanical, animal, or cadaver valves.

Pathway of Blood Through the Heart

Right Side of the Heart

1. Superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus

2. Right atrium

3. Tricuspid valve

4. Right ventricle

5. Pulmonary semilunar valve

6. Pulmonary trunk

7. Pulmonary arteries

8. Lungs

Left Side of the Heart

1. Four pulmonary veins

2. Left atrium

3. Mitral valve

4. Left ventricle

5. Aortic semilunar valve

6. Aorta

7. Systemic circulation

Comparative Features

• Equal volumes of blood are pumped to pulmonary and systemic circuits.

• Pulmonary circuit: Short, low-pressure circulation.

• Systemic circuit: Long, high-friction circulation.

• Left ventricle walls are approximately three times thicker than right, allowing for higher pressure generation.

Summary Table: Heart Chambers and Valves

Chamber	Function	Associated Valve
Right Atrium	Receives deoxygenated blood from body	Tricuspid (AV) valve
Right Ventricle	Pumps blood to lungs	Pulmonary semilunar valve
Left Atrium	Receives oxygenated blood from lungs	Mitral (AV) valve
Left Ventricle	Pumps blood to body	Aortic semilunar valve

Key Terms and Definitions

• Pericardium: Double-walled sac enclosing the heart.

• Epicardium: Outer layer of the heart wall.

• Myocardium: Muscular middle layer responsible for contraction.

• Endocardium: Inner lining of the heart chambers.

• Atria: Upper receiving chambers of the heart.

• Ventricles: Lower pumping chambers of the heart.

• Chordae tendineae: Tendinous cords anchoring AV valve flaps.

• Papillary muscles: Muscles that attach to chordae tendineae.

• Semilunar valves: Valves between ventricles and arteries.

• Fossa ovalis: Depression in the interatrial septum, remnant of fetal circulation.

Equations and Additional Info

• Pressure-Driven Valve Function: $\text{Valve opens when}~P_{\text{upstream}} > P_{\text{downstream}}$ $\text{Valve closes when}~P_{\text{downstream}} > P_{\text{upstream}}$

Additional info: The notes above are expanded and clarified for academic completeness, including definitions and clinical context for key terms and conditions.