Comprehensive Study Notes: The Heart (Anatomy & Physiology)

The Heart

Overview and Location

The heart is a muscular organ responsible for pumping blood throughout the body via the pulmonary and systemic circuits. It is located in the thoracic cavity, slightly left of the midline, and protected by the rib cage.

• Pulmonary circuit: Blood vessels carry blood to and from the lungs for gas exchange.

• Systemic circuit: Blood vessels carry blood to and from the rest of the body.

• Base of heart: Superior border, where major veins/arteries are attached. Located posterior to sternum at the 3rd costal cartilage, 1.2 cm to the left.

• Apex of heart: Pointed tip, reaches the 5th intercostal space, about 7.5 cm left of midline.

Chambers of the Heart

The heart consists of four chambers: two atria and two ventricles. These chambers work together to ensure unidirectional blood flow.

• Atria: Receive blood from systemic and pulmonary circuits.

• Ventricles: Pump blood into pulmonary and systemic circuits.

• Left atrium: Collects oxygenated blood from pulmonary circuit.

• Right atrium: Collects deoxygenated blood from systemic circuit.

• Left ventricle: Pumps oxygenated blood into systemic circuit.

• Right ventricle: Pumps deoxygenated blood into pulmonary circuit.

Heart Wall Structure

The heart wall is composed of three layers, each with distinct functions and structures.

• Epicardium: Outer layer, also called visceral pericardium. Consists of mesothelium and areolar tissue.

• Myocardium: Middle layer, composed of cardiac muscle tissue. Responsible for contraction and pumping action.

• Endocardium: Inner layer, consists of simple squamous epithelium and underlying areolar tissue. Continuous with endothelium of blood vessels.

Pericardium and Heart Coverings

The heart is enclosed in a double-walled sac called the pericardium, which provides protection and reduces friction.

• Fibrous pericardium: Dense connective tissue, anchors heart and prevents overfilling.

• Serous pericardium: Two layers (parietal and visceral) with pericardial cavity containing serous fluid for lubrication.

Cardiac Muscle Cells

Cardiac muscle cells are specialized for continuous rhythmic contraction and are interconnected for efficient electrical conduction.

• Cell structure: Small size, single central nucleus, branching interconnections (intercalated discs).

• Intercalated discs: Join adjacent cells via gap junctions and desmosomes, allowing rapid electrical communication.

• Syncytium: Cardiac muscle cells function as a coordinated unit.

• Metabolism: High aerobic metabolism, requires abundant oxygen.

Boundaries and Surface Features

The heart has distinct external boundaries and surface features that help identify its chambers and major vessels.

• Sulci: Grooves on the heart surface marking boundaries between chambers (e.g., coronary sulcus, anterior/posterior interventricular sulcus).

• Auricles: Flap-like extensions of atria.

Coronary Circulation

The heart has its own blood supply via the coronary arteries and veins, which ensure oxygen and nutrient delivery to cardiac tissue.

• Coronary arteries: Originate at the base of the ascending aorta.

• Left coronary artery: Supplies left atrium, left ventricle, and interventricular septum.

• Right coronary artery: Supplies right atrium, right ventricle, and portions of the conducting system.

• Coronary veins: Drain deoxygenated blood from myocardium into the coronary sinus, which empties into the right atrium.

Coronary Artery	Supplied Region
Left Coronary Artery	Left atrium, left ventricle, interventricular septum
Right Coronary Artery	Right atrium, right ventricle, conducting system

Heart Valves

Valves ensure unidirectional blood flow through the heart and prevent backflow.

• Atrioventricular (AV) valves: Separate atria from ventricles (tricuspid on right, bicuspid/mitral on left).

• Semilunar valves: Separate ventricles from major arteries (pulmonary and aortic valves).

• Valve structure: Folds of fibrous tissue supported by the cardiac skeleton.

• Chordae tendineae: Tendinous cords that anchor AV valve cusps to papillary muscles, preventing prolapse.

Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) in the atria and ventricles.

• Systole: Contraction phase, blood is ejected from chambers.

• Diastole: Relaxation phase, chambers fill with blood.

• Typical durations:

  • Atrial systole: 100 msec

  • Ventricular systole: 370 msec

  • Heart cycle: 800 msec

Electrical Conduction System

The heart's rhythmic contractions are coordinated by its intrinsic conduction system, which generates and distributes electrical impulses.

• Sinoatrial (SA) node: Pacemaker, initiates action potential (60-100 bpm).

• Atrioventricular (AV) node: Delays impulse, allows atria to contract before ventricles.

• AV bundle (Bundle of His): Conducts impulses from atria to ventricles.

• Bundle branches: Carry impulses through interventricular septum.

• Purkinje fibers: Distribute impulse to ventricular myocardium.

Regulation of Heart Rate

Heart rate is regulated by autonomic nervous system input and circulating hormones.

• Sympathetic stimulation: Increases heart rate and contractility via beta-1 adrenergic receptors.

• Parasympathetic stimulation: Decreases heart rate via vagus nerve and acetylcholine release.

• Hormones: Epinephrine, norepinephrine, and thyroid hormones increase heart rate.

Stroke Volume and Cardiac Output

Stroke volume is the amount of blood ejected by a ventricle during each contraction. Cardiac output is the total volume of blood pumped by the heart per minute.

• Stroke volume (SV): $SV = EDV - ESV$ (End-diastolic volume minus end-systolic volume)

• Cardiac output (CO): $CO = SV \times HR$ (Stroke volume times heart rate)

• Factors affecting SV: Preload, contractility, afterload

• Frank-Starling law: Greater EDV leads to greater stroke volume due to increased stretch of cardiac muscle fibers.

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart and is used to diagnose arrhythmias and other cardiac conditions.

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

• Common arrhythmias: Premature atrial/ventricular contractions, atrial fibrillation, ventricular fibrillation

ECG Feature	Event
P wave	Atrial depolarization
QRS complex	Ventricular depolarization
T wave	Ventricular repolarization

Clinical Considerations

Several conditions can affect heart function, including coronary artery disease, valve disorders, and arrhythmias.

• Coronary artery disease (CAD): Blockage of coronary arteries reduces blood supply to myocardium.

• Valve disorders: Stenosis (narrowing) or regurgitation (backflow) can impair blood flow.

• Arrhythmias: Abnormal heart rhythms can affect cardiac output and efficiency.

Summary Table: Heart Structures and Functions

Structure	Function
Atria	Receive blood from veins
Ventricles	Pump blood into arteries
Valves	Ensure unidirectional blood flow
Coronary vessels	Supply blood to heart tissue
Conduction system	Coordinate heart contractions

Additional info:

• Cardiac output can increase up to 6-7 times during exercise due to increased heart rate and stroke volume.

• Heart sounds ("lub-dub") are produced by closure of AV and semilunar valves.

• ECG abnormalities can indicate specific cardiac pathologies, such as myocardial infarction or conduction blocks.