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

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Heart Anatomy and Location

Position and Structure of the Heart

The heart is a muscular organ located in the thoracic cavity, slightly left of the midline, behind the sternum. It is protected by the rib cage and rests on the diaphragm.

  • Base of the heart: The superior surface, where major vessels attach.

  • Apex: The pointed inferior tip, directed toward the left hip.

  • Layers of the heart wall:

    • Epicardium: Outer layer, also known as the visceral layer of the serous pericardium.

    • Myocardium: Middle, muscular layer responsible for contraction; composed of cardiac muscle fibers.

    • Endocardium: Inner lining of the heart chambers and valves.

Example: The apex is where the apical impulse (heartbeat) can be felt most strongly.

Heart Chambers and Valves

Internal Anatomy

The heart consists of four chambers and four main valves that regulate blood flow and prevent backflow.

  • Chambers:

    • Right atrium and left atrium: Receive blood returning to the heart.

    • Right ventricle and left ventricle: Pump blood out of the heart.

  • Valves:

    • Atrioventricular (AV) valves: Tricuspid (right) and mitral/bicuspid (left) valves separate atria from ventricles.

    • Semilunar valves: Pulmonary (right) and aortic (left) valves separate ventricles from major arteries.

Example: The mitral valve prevents backflow from the left ventricle to the left atrium during ventricular contraction.

Blood Flow Through the Heart

Pulmonary and Systemic Circulation

The heart pumps blood through two main circuits: pulmonary (to the lungs) and systemic (to the body).

  • Pulmonary circulation:

    • Deoxygenated blood flows from the right ventricle to the lungs via the pulmonary arteries.

    • Blood is oxygenated in the lungs and returns to the left atrium via pulmonary veins.

  • Systemic circulation:

    • Oxygenated blood is pumped from the left ventricle to the body through the aorta.

    • Deoxygenated blood returns to the right atrium via the superior and inferior vena cava.

Key Terms: Arteries carry blood away from the heart; veins carry blood toward the heart.

Major Blood Vessels of the Heart

Connections and Pathways

The heart is connected to major vessels that facilitate blood flow to and from the lungs and body.

  • Aorta: Main artery carrying oxygenated blood to the body.

  • Pulmonary trunk: Splits into right and left pulmonary arteries to the lungs.

  • Superior and inferior vena cava: Bring deoxygenated blood from the body to the right atrium.

  • Pulmonary veins: Return oxygenated blood from the lungs to the left atrium.

Heart Valves and Blood Flow Direction

Valve Function and Heart Sounds

Valves ensure unidirectional blood flow and produce characteristic heart sounds during the cardiac cycle.

  • AV valves: Close during ventricular contraction (systole) to prevent backflow into atria.

  • Semilunar valves: Close during ventricular relaxation (diastole) to prevent backflow from arteries.

  • Heart sounds: "Lub" (AV valves closing), "Dup" (semilunar valves closing).

Example: Valve dysfunction, such as regurgitation or stenosis, can lead to abnormal heart sounds and impaired blood flow.

Coronary Circulation

Blood Supply to the Heart Muscle

The heart muscle (myocardium) receives its own blood supply via the coronary arteries.

  • Right coronary artery (RCA) and left coronary artery (LCA) branch from the aorta.

  • Major branches: Left anterior descending (LAD) artery, circumflex artery.

  • Cardiac veins: Collect deoxygenated blood and drain into the coronary sinus, which empties into the right atrium.

Coronary Arteries

Supplied Regions

Right Coronary Artery (RCA)

Right atrium, right ventricle, SA and AV nodes

Left Coronary Artery (LCA)

Left atrium, left ventricle, interventricular septum

Left Anterior Descending (LAD)

Anterior wall of left ventricle, septum

Circumflex Artery

Lateral wall of left ventricle

Clinical Note: Blockage of coronary arteries can cause angina pectoris or myocardial infarction (heart attack).

Cardiac Muscle vs. Skeletal Muscle

Structural and Functional Differences

Cardiac muscle fibers differ from skeletal muscle fibers in several key ways.

Feature

Skeletal Muscle

Cardiac Muscle

Striations

Yes

Yes

Cell Shape

Long, cylindrical

Short, branched

Nuclei per Cell

Multinucleate

One or two

Intercalated Discs

No

Yes

Gap Junctions

No

Yes (syncytium)

Contraction Control

Voluntary

Involuntary

Regeneration

Limited

Very limited

Energy Use

Aerobic/anaerobic

Aerobic only

Additional info: Cardiac muscle cells are connected by intercalated discs, allowing rapid electrical communication and coordinated contraction.

Electrical Conduction System of the Heart

Pacemaker Cells and Conduction Pathway

The heart's intrinsic conduction system coordinates rhythmic contractions.

  • Sinoatrial (SA) node: Primary pacemaker, initiates each heartbeat.

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

  • Bundle of His, bundle branches, Purkinje fibers: Distribute impulse through ventricles.

Example: Damage to the SA node can result in arrhythmias or require artificial pacemakers.

Electrocardiogram (ECG/EKG)

Electrical Events and Heart Function

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

  • P wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization.

  • T wave: Ventricular repolarization.

  • Intervals: PR interval, ST segment, QT interval—used to assess conduction and rhythm.

Example: Abnormal ECG patterns can indicate arrhythmias, heart block, or myocardial infarction.

Cardiac Cycle and Heart Sounds

Phases of the Cardiac Cycle

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

  • Atrial systole: Atria contract, pushing blood into ventricles.

  • Ventricular systole: Ventricles contract, ejecting blood into arteries.

  • Diastole: Chambers relax and fill with blood.

Heart sounds: "Lub" (AV valves close), "Dup" (semilunar valves close).

Cardiac Output and Regulation

Stroke Volume and Heart Rate

Cardiac output (CO) is the volume of blood pumped by one ventricle per minute. It is determined by stroke volume (SV) and heart rate (HR).

  • Stroke Volume (SV): Volume of blood ejected by one ventricle in one contraction.

  • End Diastolic Volume (EDV): Volume in ventricle after filling.

  • End Systolic Volume (ESV): Volume remaining after contraction.

Formula:

Example: If EDV = 125 mL, ESV = 65 mL, then SV = 60 mL. If HR = 60 bpm, CO = 3600 mL/min = 3.6 L/min.

Regulation of Cardiac Function

Intrinsic and Extrinsic Controls

Cardiac function is regulated by both intrinsic mechanisms (within the heart) and extrinsic mechanisms (nervous and hormonal).

  • Intrinsic: Frank-Starling law (greater stretch = stronger contraction).

  • Extrinsic: Autonomic nervous system (sympathetic increases HR and contractility; parasympathetic decreases HR), hormones (epinephrine, thyroxine).

  • Baroreceptors: Detect changes in blood pressure and adjust heart rate accordingly.

Additional info: Cardiac output can increase significantly during exercise due to increased HR and SV.

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