Anatomy & Physiology Study Notes: The Heart (Chapter 19)

The Heart: Structure and Function

Overview of the Heart

The heart is a muscular organ that functions as a double pump, essential for circulating blood throughout the body. It is central to both the pulmonary and systemic circuits, ensuring oxygen delivery and waste removal.

• Pulmonary circuit: The right side of the heart receives oxygen-poor blood from the body and pumps it to the lungs for oxygenation.

• Systemic circuit: The left side of the heart receives oxygen-rich blood from the lungs and pumps it throughout the body.

• Atria: The upper chambers that receive blood from the pulmonary and systemic circuits.

• Ventricles: The lower chambers that pump blood into the pulmonary and systemic circuits.

Example: Blood enters the right atrium from the body, moves to the right ventricle, and is pumped to the lungs. Oxygenated blood returns to the left atrium, moves to the left ventricle, and is pumped to the rest of the body.

Location and Orientation of the Heart

The heart is located within the thoracic cavity, specifically in the mediastinum, and is positioned between the lungs. Its anatomical features are important for understanding its function and clinical assessment.

• Weight: A healthy heart typically weighs 250–350 grams.

• Mediastinum: The heart is the largest organ in this central thoracic compartment.

• Apex: The pointed end of the heart, lying to the left of the midline.

• Base: The broad posterior surface of the heart.

Additional info: The heart's position is clinically relevant for auscultation and imaging.

Heart Chambers and Circulation

Chambers of the Heart

The heart consists of four chambers: two atria and two ventricles, separated by septa and marked externally by sulci.

• Right and Left Atria: Superior chambers that receive blood.

• Right and Left Ventricles: Inferior chambers that pump blood.

• Interatrial septum: Separates the atria.

• Interventricular septum: Separates the ventricles.

• Coronary sulcus: External groove marking the division between atria and ventricles.

• Anterior and Posterior interventricular sulci: Grooves marking the position of the interventricular septum.

Pathway of Blood Through the Heart

Blood flows through the heart in a specific sequence, ensuring separation of oxygen-poor and oxygen-rich blood.

1. Oxygen-poor blood enters the right atrium via the superior and inferior vena cava.

2. Passes through the right ventricle and is pumped to the lungs via the pulmonary trunk.

3. Oxygen-rich blood returns to the left atrium via pulmonary veins.

4. Passes into the left ventricle and is pumped to the body via the aorta.

Additional info: The atria contract together, followed by the ventricles contracting together.

Structure of the Heart Wall and Coverings

Pericardium and Heart Wall Layers

The heart is enclosed by the pericardium and composed of three main layers, each with distinct functions.

• Fibrous pericardium: Strong, dense connective tissue layer providing protection and anchorage.

• Serous pericardium: Double-layered membrane (parietal and visceral layers) that reduces friction.

• Epicardium: Visceral layer of the serous pericardium.

• Myocardium: Cardiac muscle layer responsible for contraction; muscle fibers arranged in circular and spiral patterns.

• Endocardium: Endothelial layer lining the internal heart chambers.

Additional info: The myocardium is the thickest layer and is essential for the heart's pumping action.

Heart Valves and Cardiac Skeleton

Valve Structure and Function

Heart valves ensure unidirectional blood flow and prevent backflow. They are composed of endocardium with a connective tissue core.

• Atrioventricular (AV) valves: Located between atria and ventricles (right AV = tricuspid, left AV = bicuspid/mitral).

• Semilunar valves: Located at the junction of ventricles and great arteries (pulmonary and aortic valves).

Cardiac Skeleton

The cardiac skeleton is a framework of dense connective tissue that surrounds all four valves.

• Anchors valve cusps.

• Prevents overdilation of valve openings.

• Main point of insertion for cardiac muscle.

• Blocks direct spread of electrical impulses between atria and ventricles.

Heart Sounds and Auscultation

Heart Sounds

Heart sounds are produced by the closing of valves and are important for clinical diagnosis.

• First sound (“lub”): Closing of AV valves.

• Second sound (“dup”): Closing of semilunar valves.

Each valve sound is best heard at specific locations on the thoracic surface:

• Pulmonary valve: Superior left corner

• Aortic valve: Superior right corner

• Mitral (bicuspid) valve: Apex

• Tricuspid valve: Inferior right corner

Cardiac Cycle and Heart Wall Structure

Cardiac Cycle

The cardiac cycle consists of alternating contraction and relaxation phases.

• Systole: Contraction of a heart chamber.

• Diastole: Expansion (relaxation) of a heart chamber.

• Normal resting heart rate: ~80 beats per minute.

Heart Wall Thickness

The thickness of the heart wall varies between chambers, reflecting their functional demands.

• Atria: Thin walls

• Ventricles: Thick walls

• Left ventricle: Three times thicker than right ventricle; exerts greater pumping force for systemic circulation.

Additional info: The systemic circuit is longer and offers greater resistance than the pulmonary circuit.

Cardiac Muscle Tissue

Microscopic Anatomy

Cardiac muscle tissue forms the myocardium and is specialized for rhythmic contraction.

• Striated muscle, similar to skeletal muscle.

• Cells are short, branching, and have one or two nuclei.

• Cells are joined at intercalated discs, which contain:

  • Fasciae adherens: Desmosome-like junctions for mechanical strength.

  • Gap junctions: Allow electrical coupling between cells.

• Numerous large mitochondria (25–35% of cell volume).

Additional info: The sarcoplasmic reticulum is simpler than in skeletal muscle, and T-tubules are wider but less numerous.

Conducting System and Innervation

Intrinsic Conducting System

The heart has an intrinsic ability to generate and conduct electrical impulses, coordinating contraction.

• Specialized cardiac muscle cells form the conducting system.

• Sinoatrial (SA) node: Sets the inherent rate of contraction (pacemaker).

Extrinsic Innervation

Heart rate and contraction strength are modulated by the autonomic nervous system.

• Parasympathetic fibers: Decrease heart rate; restricted to SA node, AV node, and coronary arteries.

• Sympathetic fibers: Increase heart rate and contraction strength; innervate SA node, AV node, and coronary arteries.

• Cardiac centers in the medulla oblongata regulate autonomic input:

  • Cardioinhibitory center: Influences parasympathetic neurons.

  • Cardioacceleratory center: Influences sympathetic neurons.

Coronary Circulation

Arterial Supply

The heart receives its blood supply from the coronary arteries, which branch from the base of the aorta.

• Right coronary artery (RCA): Descends in the coronary sulcus; branches into marginal artery and posterior interventricular artery (posterior descending artery).

• Left coronary artery (LCA): Branches into anterior interventricular artery (left anterior descending artery, LAD) and circumflex artery.

Venous Drainage

Cardiac veins return deoxygenated blood from the heart wall to the right atrium.

• Coronary sinus: Main vein running in the posterior coronary sulcus; returns most venous blood to the right atrium.

• Tributaries: Great cardiac vein, middle cardiac vein, small cardiac vein.

Disorders of the Heart

Coronary Artery Disease

Coronary artery disease is a major cause of heart dysfunction and includes several conditions:

• Atherosclerosis: Fatty deposits in coronary arteries.

• Angina pectoris: Chest pain due to reduced blood flow.

• Myocardial infarction: Blocked coronary artery leading to heart attack.

• Silent ischemia: Reduced blood flow without pain or warning.

Heart Failure

Heart failure is a progressive weakening of the heart's ability to pump blood.

• Congestive heart failure (CHF): Heart enlarges, pumping efficiency declines.

• Pulmonary arterial hypertension: Enlargement and potential failure of the right ventricle.

Fetal and Newborn Circulation

Developmental Pathways

Fetal circulation includes unique structures that bypass nonfunctional organs, adapting to prenatal life.

• Umbilical vein: Delivers oxygenated blood from mother to fetus.

• Ductus venosus: Shunt bypassing the liver to the inferior vena cava.

• Foramen ovale: Opening between atria, allowing blood to bypass fetal lungs.

• Ductus arteriosus: Shunt from pulmonary artery to aorta, bypassing fetal lungs.

Additional info: These structures close after birth, establishing normal adult circulation.

Summary Table: Heart Valves

Valve	Location	Function	Alternative Name
Right AV Valve	Between right atrium and right ventricle	Prevents backflow into right atrium	Tricuspid valve
Left AV Valve	Between left atrium and left ventricle	Prevents backflow into left atrium	Bicuspid (Mitral) valve
Pulmonary Valve	At opening of right ventricle and pulmonary trunk	Prevents backflow into right ventricle	Semilunar valve
Aortic Valve	At opening of left ventricle and aorta	Prevents backflow into left ventricle	Semilunar valve

Key Equations

• Cardiac Output: The volume of blood pumped by each ventricle per minute. $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$