Anatomy and Physiology of the Heart and Cardiovascular System

Chapter 20: The Heart

Overview and Learning Objectives

This chapter covers the anatomy of the heart, its vascular supply, pericardium structure, and the flow of blood through the heart. It also explains the events of cardiac muscle action potentials, the role of calcium ions, the heart's conducting system, and the interpretation of electrocardiograms. The cardiac cycle, heart sounds, cardiac output, and factors influencing heart rate and stroke volume are discussed, with emphasis on physiological coordination during physical activity.

Cardiovascular System Components

Main Components

• Heart: Muscular organ that pumps blood throughout the body.

• Blood: Fluid connective tissue that transports oxygen, nutrients, and wastes.

• Blood Vessels: Tubular structures that carry blood to and from the heart and tissues.

Types of Blood Vessels

• Arteries: Carry blood away from the heart.

• Veins: Return blood to the heart.

• Capillaries (Exchange Vessels):

  • Interconnect smallest arteries and veins.

  • Exchange dissolved gases, nutrients, and wastes between blood and surrounding tissues.

Circulation Pathways

Pulmonary Circuit

• Carries blood to and from the gas exchange surfaces of the lungs.

Systemic Circuit

• Carries blood to and from the rest of the body.

Each circuit begins and ends at the heart, and blood travels through these circuits in sequence.

Anatomy of the Heart

External Features

• Base: Superior portion where great vessels connect.

• Apex: Pointed inferior tip of the heart.

• Mediastinum: Central compartment in the thoracic cavity between the lungs where the heart sits.

Pericardium

• Surrounds the heart and consists of:

  • Outer fibrous pericardium

  • Inner serous pericardium:

    • Outer parietal layer

    • Inner visceral layer (epicardium)

  • Pericardial cavity: Space between parietal and visceral layers, contains pericardial fluid for lubrication.

Pericarditis

• Inflammation of pericardial surfaces, often due to infection.

• Can produce a distinctive scratching sound and restrict heart movement (cardiac tamponade) due to excess fluid.

Heart Wall Structure

Layers of the Heart Wall

• Epicardium: Visceral layer of serous pericardium covering the heart surface.

• Myocardium: Cardiac muscle tissue responsible for contraction.

• Endocardium: Inner lining of the heart, composed of simple squamous epithelium and areolar tissue.

Heart Chambers and Valves

Chambers

• Atria: Two thin-walled upper chambers that receive blood.

• Ventricles: Two lower chambers that pump blood out of the heart.

Valves

• Atrioventricular (AV) Valves:

  • Tricuspid valve (right)

  • Bicuspid/Mitral valve (left)

• Semilunar Valves:

  • Pulmonary valve

  • Aortic valve

Blood Flow Through the Heart

1. Blood enters right atrium from systemic veins.

2. Passes through tricuspid valve to right ventricle.

3. Pumped through pulmonary valve to pulmonary arteries and lungs.

4. Returns via pulmonary veins to left atrium.

5. Passes through bicuspid/mitral valve to left ventricle.

6. Pumped through aortic valve to systemic arteries.

Cardiac Muscle and Electrical Activity

Action Potential in Cardiac Muscle

• Initiated by pacemaker cells, spreads through conducting system.

• Calcium ions (Ca2+): Essential for contraction; extracellular Ca2+ entry triggers further Ca2+ release from sarcoplasmic reticulum.

Conducting System of the Heart

• Sinoatrial (SA) Node: Pacemaker in right atrium wall.

• Atrioventricular (AV) Node: Junction between atria and ventricles.

• Conducting Cells: Internodal pathways, AV bundle, bundle branches, Purkinje fibers.

Electrocardiogram (ECG/EKG)

• Records electrical events in the heart using electrodes.

• Key features: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).

• Intervals and segments help diagnose heart function and rhythm disturbances.

Cardiac Cycle and Heart Sounds

Cardiac Cycle

• Consists of alternating periods of contraction (systole) and relaxation (diastole) in atria and ventricles.

Heart Sounds

• S1: Loud sound as AV valves close.

• S2: Loud sound as semilunar valves close.

• S3, S4: Soft sounds from blood flow and atrial contraction.

• Heart Murmur: Abnormal sounds due to regurgitation through valves.

Cardiac Output and Regulation

Definition and Formula

• Cardiac Output (CO): Volume of blood pumped by left ventricle per minute.

Formula:

$CO = HR \times SV$

• $CO$ = cardiac output (mL/min)

• $HR$ = heart rate (beats/min)

• $SV$ = stroke volume (mL/beat)

Stroke Volume (SV)

• End-Diastolic Volume (EDV): Blood in ventricle at end of diastole.

• End-Systolic Volume (ESV): Blood remaining after systole.

• Ejection Fraction: Percentage of EDV ejected during contraction.

Factors Affecting Heart Rate

• Autonomic Activity: Sympathetic and parasympathetic nervous system influence.

• Circulating Hormones: Epinephrine, norepinephrine, thyroid hormone increase heart rate.

Cardiovascular Regulation

• Cardiac centers in the brain control heart rate and vessel diameter.

• System responds to activity changes and emergencies to maintain adequate tissue perfusion.

Summary Table: Heart Wall Layers

Layer	Location	Main Function
Epicardium	Outer surface	Protection, lubrication
Myocardium	Middle layer	Contraction, pumping blood
Endocardium	Inner surface	Lines chambers, reduces friction

Summary Table: Heart Valves

Valve	Location	Function
Tricuspid	Right AV	Prevents backflow to right atrium
Bicuspid/Mitral	Left AV	Prevents backflow to left atrium
Pulmonary	Right ventricle	Prevents backflow from pulmonary artery
Aortic	Left ventricle	Prevents backflow from aorta

Additional info:

• Cardiac muscle cells are highly sensitive to extracellular calcium levels, and energy for contraction is primarily derived from aerobic metabolism using fatty acids and glucose.

• Disturbances in heart rhythm include bradycardia, tachycardia, arrhythmias, and ectopic pacemakers.