Heart Anatomy Physiology

Introduction to the Heart and Its Functions

  • The heart is a muscle that functions as a pump within the cardiovascular system.

  • Fascination with the heart has persisted through history, as illustrated by a reference to a scene from "Indiana Jones and the Temple of Doom," linked to ancient Mayan rituals involving heart sacrifices.

  • The heart pumps approximately 7,200 liters of blood daily.

  • This study leads into understanding the pathophysiology of the heart.

Anatomy of the Heart

Location of the Heart

  • The heart is located within the mediastinum, surrounded by the lungs.

  • It consists of the apex (pointed end) and the base (wider part).

  • Major blood vessels associated with the heart include:

    • Superior vena cava

    • Aorta

    • Pulmonary trunk

Pericardium

  • The heart is encased in a protective sac called the pericardium, composed of three major layers:

    • Fibrous Pericardium:

    • Outermost layer.

    • Composed of dense irregular connective tissue with abundant collagen fibers.

    • Anchors the heart to surrounding structures like the diaphragm and major blood vessels.

    • Parietal Layer of Serous Pericardium:

    • Composed of simple squamous epithelium and areolar connective tissue.

    • Lines the fibrous pericardium.

    • Visceral Layer of Serous Pericardium (Epicardium):

    • Attaches directly to the heart muscle.

    • Continuous with the parietal layer.

  • The pericardial cavity between these layers contains serous fluid for lubrication, reducing friction during heart contractions.

Layers of the Heart Wall

  • The heart wall consists of three additional layers:

    • Epicardium: Same as the visceral pericardium.

    • Myocardium:

    • Thickest layer made of cardiac muscle tissue that contracts to pump blood.

    • Endocardium:

    • Inner layer covering the heart's chambers and valves, composed of simple squamous epithelium and areolar connective tissue.

    • Continuous with the lining of blood vessels.

Function of the Cardiovascular System

Overview of Blood Circulation

  • The cardiovascular system comprises the heart and blood vessels, responsible for transporting blood and nutrients to tissues and removing waste products such as CO2.

  • Perfusion:

    • Defined as the delivery of blood per time per gram of tissue, crucial for maintaining cell health.

Blood Vessels

Arteries

  • Arteries carry blood away from the heart, usually oxygen-rich blood.

  • Exception: Pulmonary arteries carry deoxygenated blood to the lungs.

Veins

  • Veins transport blood back to the heart and generally carry deoxygenated blood rich in CO2.

  • Exception: Pulmonary veins transport oxygenated blood from the lungs.

Capillaries

  • Capillaries are the sites of gas exchange between the blood and surrounding cells.

Valves of the Heart

  • The heart contains four main valves:

    • Atrioventricular Valves:

    • Right AV valve (Tricuspid valve): Connects the right atrium to the right ventricle.

    • Left AV valve (Bicuspid or Mitral valve): Connects the left atrium to the left ventricle.

    • Semilunar Valves:

    • Pulmonary semilunar valve: Connects the right ventricle to the pulmonary trunk.

    • Aortic semilunar valve: Connects the left ventricle to the aorta.

  • Valves ensure unidirectional blood flow through the heart.

Blood Flow Through the Heart

Systemic Circulation

  • Deoxygenated blood returns from systemic circulation:

    • Blood from the upper body enters the heart via the superior vena cava.

    • Blood from the lower body enters via the inferior vena cava.

    • Blood from the heart muscle drains into the right atrium through the coronary sinus.

Pulmonary Circulation

  • Oxygenated blood returns from the lungs:

    • Pulmonary veins carry oxygen-rich blood to the left atrium.

  • Blood moves from the atria to the ventricles, primarily through passive flow aided by atrial contraction.

  • Valves prevent backflow, ensuring the efficiency of circulation.

Electrical Conduction System of the Heart

Overview

  • The heart's electrical conduction system initiates and propagates action potentials necessary for contractions.

  • Main components:

    • Sinoatrial Node (SA Node):

    • Located in the right atrium; acts as the primary pacemaker.

    • Exhibits intrinsic rhythmicity (autorythmicity), generating action potentials spontaneously.

    • Atrioventricular Node (AV Node):

    • Located between the atria and ventricles; delays transmission to allow complete ventricular filling before contraction.

    • Bundle of His (Atrioventricular Bundle):

    • Conducts impulses to the ventricles via bundle branches.

    • Purkinje Fibers:

    • Distribute electrical signals throughout the ventricles, causing contraction.

Action Potentials in Nodal and Cardiac Muscle Cells

  • SA Nodal Cell Action Potential:

    • Threshold is reached through spontaneous depolarization, opening voltage-gated sodium and calcium channels.

    • Depolarization leads to an action potential, followed by repolarization via the outflux of potassium.

  • Cardiac Muscle Cell Action Potential:

    • Characterized by a plateau phase due to simultaneous calcium influx and potassium efflux, preventing tetany (sustained contraction).

Cardiac Cycle

Phases of Cardiac Cycle

  1. Atrial Systole: Atria contract to push blood into ventricles.

  2. Ventricular Systole: Ventricles contract, ejecting blood to the pulmonary trunk and aorta.

  3. Diastole: Heart muscle relaxes, allowing ventricles to fill passively before the next contraction.

Important Concepts

  • Stroke Volume (SV): Amount of blood ejected from each ventricle per heartbeat; approximately 70 mL.

  • End Systolic Volume (ESV) and End Diastolic Volume (EDV):

    • ESV: Volume remaining in the ventricle post-systole (~60 mL).

    • EDV: Volume in the ventricle post-diastole (~130 mL).

Cardiac Output (CO)

  • Cardiac Output is the amount of blood pumped by a ventricle per minute (L/min).

  • Calculated as:
    CO=HRimesSVCO = HR imes SV
    where HR = heart rate (beats/min) and SV = stroke volume (mL/beat).

  • Typically, a normal resting CO is about 5.25 L/min (75 beats/min × 70 mL).

Electrocardiogram (ECG)

Purpose of ECG

  • An ECG measures the electrical activity of the heart, providing insights into heart health.

  • Electrodes are placed on the thorax to capture the electrical signals as they propagate through the heart.

Interpretation of ECG Tracing

  • P Wave: Represents atrial depolarization.

  • QRS Complex: Represents ventricular depolarization; atrial repolarization occurs simultaneously but is overshadowed.

  • T Wave: Represents ventricular repolarization.

Heart Rate Regulation

  • Influenced by:

    • The autonomic nervous system (sympathetic and parasympathetic).

    • Hormones (e.g., thyroid hormones).

Summary

  • The heart's structure and function, including its unique electrical conduction system, are essential for proper circulation and respiration.

  • Understanding these systems is crucial for diagnosing and treating cardiovascular diseases.