Chapter 18 Part A: The Cardiovascular System – Heart Anatomy and Physiology

Why This Matters

Clinical Relevance of Heart Anatomy and Physiology

The structure and function of the heart directly influence the sounds heard through a stethoscope, which are essential for clinical diagnosis. Understanding these relationships helps in interpreting heart sounds and detecting abnormalities.

• Heart sounds are produced by the closing of heart valves and the flow of blood through the heart chambers.

• Abnormal sounds (murmurs) may indicate valve dysfunction or other cardiac issues.

18.1 Heart Anatomy

The Pulmonary and Systemic Circuits

The heart is a dual pump system that circulates blood through two distinct circuits: pulmonary and systemic.

• Pulmonary circuit: Right side of the heart receives oxygen-poor blood from tissues and pumps it to the lungs for oxygenation and removal of carbon dioxide.

• Systemic circuit: Left side of the heart receives oxygen-rich blood from the lungs and pumps it to body tissues.

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.

Diagram: The Systemic and Pulmonary Circuits

Figure 18.1 illustrates the flow of blood through the heart and the two circuits, highlighting the separation of oxygen-poor and oxygen-rich blood.

Size, Location, and Orientation of the Heart

The heart is a muscular organ approximately the size of a fist and weighs less than one pound.

• Location:

  • In the mediastinum between the second rib and fifth intercostal space

  • On the superior surface of the diaphragm

  • Two-thirds of the heart is to the left of the midline

  • Anterior to the vertebral column, posterior to the sternum

• Base: Leans toward the right shoulder

• Apex: Points toward the left hip

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

Location of the Heart in the Mediastinum

Figures 18.2a and 18.2b show the anatomical position of the heart relative to the ribs, diaphragm, lungs, and vertebrae.

Coverings of the Heart

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

• Superficial fibrous pericardium: Protects, anchors the heart, and prevents overfilling.

• Deep two-layered serous pericardium:

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

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

  • Layers are separated by the pericardial cavity, filled with fluid to decrease friction.

Layers of the Heart Wall

The heart wall is composed of three layers:

• Epicardium: Visceral layer of the serous pericardium.

• Myocardium: 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 the endothelial lining of blood vessels; lines heart chambers and covers cardiac skeleton of valves.

Clinical – Homeostatic Imbalance 18.1

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

• Cardiac tamponade: Excess fluid in the pericardial space compresses the heart, impairing its pumping ability.

  • Treatment: Fluid is drawn out of the cavity, usually with a syringe.

Chambers and Associated Great Vessels

Internal Features

• Four chambers:

  • Two superior atria

  • Two inferior ventricles

• Interatrial septum: Separates atria; contains fossa ovalis (remnant of fetal foramen ovale).

• Interventricular septum: Separates ventricles.

Surface Features

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

• Anterior interventricular sulcus: Marks the anterior position of the interventricular septum.

• Posterior interventricular sulcus: Landmark on the posteroinferior surface.

Atria: The Receiving Chambers

• Auricles: Appendages that increase atrial volume.

• Right atrium: Receives deoxygenated blood from the body.

  • Anterior portion is smooth-walled.

  • Posterior portion contains ridges formed by pectinate muscles.

  • Regions separated by crista terminalis.

• Left atrium: Receives oxygenated blood from the lungs.

  • Pectinate muscles found only in auricles.

  • Four pulmonary veins return blood from lungs.

Ventricles: The Discharging Chambers

• Make up most of the heart's volume.

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

• Left ventricle: Posteroinferior surface; pumps blood into the aorta (largest artery in the body).

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

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

18.2 Heart Valves

Types and Functions of Heart Valves

Heart valves ensure unidirectional blood flow through the heart by opening and closing in response to pressure changes.

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

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

Atrioventricular (AV) Valves

• Tricuspid valve: Right AV valve; three cusps between right atrium and ventricle.

• Mitral valve: Left AV valve (bicuspid); two cusps between left atrium and ventricle.

• Chordae tendineae: Anchor valve cusps to papillary muscles, preventing valve flaps from inverting into atria during ventricular contraction.

Semilunar (SL) Valves

• Prevent backflow from major arteries into ventricles.

• Each valve has three cusps resembling a half moon.

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

• Aortic semilunar valve: Between left ventricle and aorta.

Valve Function and Blood Flow

• AV valves open when atrial pressure exceeds ventricular pressure, allowing blood to flow into ventricles.

• AV valves close when ventricular pressure exceeds atrial pressure, preventing backflow.

• SL valves open when ventricular pressure rises, allowing blood to flow into arteries; close when pressure falls, preventing backflow.

Absence of Valves Between Major Veins and Atria

• No valves are present between major veins and atria because:

  • Inertia of incoming blood prevents backflow.

  • Heart contractions compress venous openings.

Clinical – Homeostatic Imbalance 18.2

• Incompetent valve: Blood backflows so heart re-pumps the same blood repeatedly.

• Valvular stenosis: Stiff flaps constrict opening, causing the heart to exert more force to pump blood.

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

Table: Key Differences Between Skeletal and Cardiac Muscle

Feature	Skeletal Muscle	Cardiac Muscle
Cell Shape	Long, cylindrical, multinucleate	Short, branched, usually single nucleus
Striations	Present	Present
Intercalated Discs	Absent	Present
Control	Voluntary	Involuntary
Contraction Initiation	Neural stimulation	Intrinsic pacemaker cells
Source of Ca2+ for Contraction	Sarcoplasmic reticulum	Sarcoplasmic reticulum and extracellular fluid
Energy Supply	Aerobic and anaerobic	Primarily aerobic

Additional info:

• Cardiac muscle cells are highly specialized for continuous rhythmic contraction and are interconnected by intercalated discs, which facilitate synchronized contraction.

• Heart valves are critical for maintaining efficient circulation and preventing regurgitation of blood.

• Clinical conditions such as pericarditis and valvular disease can significantly impact cardiac function and patient health.