Anatomy and Physiology of the Heart: Structure, Function, and Electrophysiology

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Overview of the Heart

The Cardiovascular System

The cardiovascular system consists of the heart, blood vessels, and blood. Its primary function is to transport oxygen, nutrients, and waste products throughout the body.

Heart: Pumps blood through the vessels.
Blood vessels: Tubes (arteries, veins, capillaries) that distribute blood.
Blood: The liquid medium carrying oxygen, nutrients, and waste.

Location and Basic Structure of the Heart

The heart is a muscular organ located in the thoracic cavity, posterior to the sternum, and rests on the diaphragm. It is slightly left of the midline.

Apex: The pointed end of the heart, directed toward the left hip.
Base: The broad, superior portion facing the right shoulder.
Chambers: Four chambers—right and left atria (superior), right and left ventricles (inferior).

Functions of the Heart

The heart pumps blood through two main circuits:

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.

Why is the right side of the heart called the pulmonary pump? Because it sends blood to the lungs (pulmonary circuit).

Why is the left side called the systemic pump? Because it sends blood to the systemic circulation (the body).

Gas Exchange and Circuits

In the pulmonary capillaries, carbon dioxide diffuses from the blood into the alveoli, and oxygen diffuses from the alveoli into the blood.
In the systemic capillaries, oxygen diffuses from the blood into tissues, and carbon dioxide diffuses from tissues into the blood.
The pulmonary circuit is a low-pressure circuit; the systemic circuit is a high-pressure circuit.

Other Functions of the Heart

Maintains homeostasis of blood pressure.
Produces atrial natriuretic peptide (ANP), a hormone that lowers blood pressure by promoting sodium and water loss in the kidneys.

Heart Anatomy and Blood Flow Pathway

The Pericardium, Heart Wall, and Heart Skeleton

The heart is surrounded by a double-walled sac called the pericardium:

Fibrous pericardium: Tough outer layer that protects and anchors the heart.
Serous pericardium: Thin inner membrane producing serous fluid.
Parietal pericardium: Fused to the fibrous pericardium.
Visceral pericardium (epicardium): Covers the heart surface.
Pericardial cavity: Space between parietal and visceral layers, filled with pericardial fluid to reduce friction.

Heart Wall Structure

Epicardium: Outer layer (visceral pericardium).
Myocardium: Middle, muscular layer responsible for contraction.
Endocardium: Inner lining of the heart chambers.

The fibrous skeleton of the heart provides structural support, anchors cardiac muscle fibers, and insulates electrical impulses.

Chambers and Valves of the Heart

Four chambers: two atria (receive blood), two ventricles (pump blood).
Valves prevent backflow: atrioventricular (AV) valves (tricuspid and bicuspid/mitral) between atria and ventricles; semilunar valves (pulmonary and aortic) between ventricles and arteries.

Table: Heart Valves and Their Locations

Valve	Location	Function
Tricuspid (right AV)	Between right atrium and right ventricle	Prevents backflow into right atrium
Bicuspid/Mitral (left AV)	Between left atrium and left ventricle	Prevents backflow into left atrium
Pulmonary semilunar	Between right ventricle and pulmonary trunk	Prevents backflow into right ventricle
Aortic semilunar	Between left ventricle and aorta	Prevents backflow into left ventricle

Major Blood Vessels Associated with the Heart

Superior and inferior vena cava: Return deoxygenated blood from the body to the right atrium.
Pulmonary trunk and arteries: Carry deoxygenated blood from right ventricle to lungs.
Pulmonary veins: Return oxygenated blood from lungs to left atrium.
Aorta: Carries oxygenated blood from left ventricle to the body.

Coronary Circulation

The myocardium is supplied with oxygen and nutrients by the coronary arteries. Venous blood is collected by the coronary sinus and returned to the right atrium.

Left and right coronary arteries: Branch from the ascending aorta.
Coronary veins: Drain into the coronary sinus.
Coronary artery disease (CAD): Caused by plaque buildup, can lead to myocardial infarction (heart attack).

Cardiac Muscle Tissue Anatomy and Electrophysiology

Cardiac Muscle Tissue

Striated muscle cells with intercalated discs for synchronized contraction.
Exhibit autorhythmicity: can generate their own action potentials without nervous input.

Cardiac Electrophysiology: Pacemaker Cells and the Cardiac Conduction System

The heart's rhythm is set by specialized pacemaker cells forming the cardiac conduction system.

Sinoatrial (SA) node: Located in the right atrium; fastest intrinsic rate (~60-100 bpm); primary pacemaker.
Atrioventricular (AV) node: Located near the tricuspid valve; slower rate (~40-60 bpm).
Purkinje fiber system: Includes AV bundle, bundle branches, and Purkinje fibers; slowest rate (~20 bpm).

Table: Components of the Cardiac Conduction System

Component	Location	Intrinsic Rate (bpm)
SA node	Right atrium	60-100
AV node	Interatrial septum near tricuspid valve	40-60
AV bundle (Bundle of His)	Interventricular septum	20-40
Purkinje fibers	Ventricular walls	20-40

Pacemaker Cell Action Potential Phases

Slow initial depolarization: Nonspecific cation channels allow Na+ ions to leak in, causing slow depolarization.
Full depolarization: Voltage-gated Ca2+ channels open, Ca2+ enters, membrane depolarizes.
Repolarization: Voltage-gated K+ channels open, K+ exits, membrane repolarizes.
Minimum potential phase: K+ channels remain open until minimum potential is reached, then cycle repeats.

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart. It is used to assess heart rhythm and detect abnormalities.

P wave: Atrial depolarization
QRS complex: Ventricular depolarization
T wave: Ventricular repolarization

Key Equations

Cardiac Output (CO): The amount of blood pumped by each ventricle per minute.

HR: Heart rate (beats per minute)
SV: Stroke volume (mL per beat)

Summary Table: Heart Circuits and Functions

Circuit	Side of Heart	Destination	Blood Oxygenation
Pulmonary	Right	Lungs	Deoxygenated to oxygenated
Systemic	Left	Body tissues	Oxygenated to deoxygenated

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard Anatomy & Physiology content.