Cardiovascular System: Heart, Blood, and Blood Vessels

The Cardiovascular System: The Heart, Blood, and Blood Vessels Anatomy & Physiology

Introduction to the Cardiovascular System

  • The cardiovascular system consists of a network of vessels that circulates blood throughout the body, powered by the heart's action.

  • Specifics regarding the heart will be addressed in subsequent lessons; however, this guide will focus on the circulatory system.

Objectives

  • Describe the heart’s location, shape, its four chambers and the pulmonary and systemic circuits.

  • Describe the general features of the heart's location.

  • Identify the major vessels supplying the heart and cite their locations.

  • Distinguish between the pulmonary and systemic circuits, and identify afferent and efferent blood vessels.

  • Identify and distinguish among the types of blood vessels based on structure and function.

  • Determine the formed elements present in blood.

Anatomy of the Heart

  • Size and Shape: Approximately the size of a fist.

  • Location:

    • Superior surface of the diaphragm

    • Left of the midline

    • Anterior to the vertebral column and posterior to the sternum.

Heart Anatomical Features

  • Midsternal Line: An important reference line for anatomical positioning.

  • Point of Maximal Intensity (PMI): Where the heartbeat is most easily heard, usually located at the apex of the heart.

  • Surrounding Structures: Right lung (anterior), left lung (anterior), aorta, superior vena cava, inferior vena cava, pulmonary trunk, and the parietal pericardium (cut).

Coverings of the Heart

  • Pericardium: A double-walled sac surround the heart comprised of:

    • Superficial Fibrous Pericardium: Protective outer layer.

    • Deep Serous Pericardium: Divided into two layers:

    • Parietal Layer: Lines the internal surface of the fibrous pericardium.

    • Visceral Layer (Epicardium): Lines the surface of the heart.

    • Pericardial Cavity: Space between the parietal and visceral layers filled with serous fluid, functioning to reduce friction.

Function of the Pericardium

  • Protection: Acts as a barrier to infection.

  • Anchoring: Secures heart to surrounding structures.

  • Prevents Overfilling: Regulates the amount of blood filling the heart.

  • Friction-Free Environment: Allows heart movements to occur smoothly during contractions.

Structure of the Heart Wall

  • Epicardium: The outermost layer; also known as the visceral layer of the serous pericardium.

  • Myocardium: Thick layer of cardiac muscle that contracts to pump blood.

  • Fibrous Skeleton: Framework of connective tissue forming a support structure for vessels and valves.

  • Endocardium: Inner layer lining the heart chambers and covering the heart valves.

Major Vessels of the Heart

Anterior View of the Heart

  • Returning Blood Vessels:

    • Superior Vena Cava: Drains deoxygenated blood from the upper body.

    • Inferior Vena Cava: Drains deoxygenated blood from the lower body.

    • Right and Left Pulmonary Veins: Carry oxygenated blood from the lungs to the left atrium.

  • Conveying Blood Away Vessels:

    • Pulmonary Trunk: Splits into right and left pulmonary arteries for deoxygenated blood to lungs.

    • Ascending Aorta: Major artery leaving the heart, divides into:

    • Brachiocephalic Artery

    • Left Common Carotid Artery

    • Left Subclavian Artery

Blood Supply and Drainage to the Heart

  • Coronary Arteries:

    • Right and Left Coronary Arteries: Located in the atrioventricular groove supply blood to the heart muscle.

    • Additional Arteries: Marginal artery, circumflex artery, anterior interventricular artery.

  • Coronary Veins:

    • Great Cardiac Vein: Drains blood from the anterior heart.

    • Middle Cardiac Vein: Drains blood from the posterior heart.

    • Small Cardiac Vein: Drains blood from the right margin of the heart.

Chambers of the Heart

Atria of the Heart

  • Function: Receiving chambers for blood.

  • Structures: Each atrium possesses a protruding auricle and is lined with pectinate muscles.

  • Blood Entrance:

    • Right atrium receives blood from the superior and inferior venae cavae and the coronary sinus.

    • Left atrium receives blood from the pulmonary veins.

Ventricles of the Heart

  • Function: Discharging chambers that pump blood out of the heart.

  • Structures: Marked by papillary muscles and trabeculae carneae, providing structural support.

  • Pumping Mechanism:

    • Right ventricle pumps blood through the pulmonary valve to the pulmonary trunk.

    • Left ventricle pumps blood through the aortic semilunar valve into the aorta.

Myocardial Thickness and Function

  • Thickness Variation: Myocardium thickness varies according to chamber function:

    • Atria: Thin walls for blood delivery to ventricles.

    • Ventricles: Thicker walls for stronger contractions; right ventricle pumps to lungs (low resistance), while left ventricle has the thickest wall to sustain systemic circulation.

Pathway of Blood Through the Heart and Lungs

  1. Right Atrium: Blood enters through tricuspid valve into the right ventricle.

  2. Right Ventricle: Pumps blood through pulmonary semilunar valve into the pulmonary arteries towards the lungs.

  3. Lungs: Blood receives oxygen and releases carbon dioxide.

  4. Pulmonary Veins: Oxygenated blood returns to the left atrium.

  5. Left Atrium: Blood is transferred through the bicuspid valve into the left ventricle.

  6. Left Ventricle: Pumps blood through the aortic semilunar valve into aorta for systemic circulation.

Circulatory Divisions

  • Pulmonary Circuit: Carries CO₂-rich blood to lungs and returns O₂-rich blood to the heart.

  • Systemic Circuit: Distributes O₂-rich blood to body tissues and returns CO₂-rich blood to the heart.

  • Key Connection: Capillary Beds: Sites for gas exchange within lungs and tissues.

Coronary Circulation

Functional Blood Supply

  • Definition: The functional blood supply mechanism to the heart muscle includes collateral routes to ensure adequate perfusion.

  • Arterial Supply Composition:

    • Right Coronary Artery: Supplies right heart structures.

    • Left Coronary Artery: Supplies left heart structures.

    • Branching includes circumflex artery and anterior interventricular artery.

  • Venous Supply Composition:

    • Great Cardiac Vein: Draining blood from the heart muscle back to venous circulation.

    • Coronary Sinus: Collects blood from cardiac veins.

Heart Valves

Functionality

  • Heart Valves: Ensure unidirectional blood flow through the heart. Composed of:

    • Atrioventricular (AV) Valves:

    • Right (Tricuspid) Valve: Prevents backflow into the right atrium during ventricular contraction.

    • Left (Bicuspid/Mitral) Valve: Prevents backflow into the left atrium during ventricular contraction.

    • Semilunar Valves:

    • Aortic Valve: Prevents backflow into the left ventricle.

    • Pulmonary Valve: Prevents backflow into the right ventricle.

  • Anchoring Mechanism: Chordae tendineae are connective tissues anchoring valves to the papillary muscles, ensuring proper closing.

Atrioventricular Valve Action

  1. Atria filling causes pressure against AV valves, forcing them open.

  2. Filling ventricles allow AV valve flaps to hang relaxed.

  3. Atria contract to push additional blood into ventricles.

  4. Ventricles contract, forcing blood against valve cusps, causing them to close.

  5. Contracted papillary muscles tighten chordae tendineae to prevent valve flaps from reverting into atria.

Semilunar Valve Action

  1. Ventricles contract creating increased intraventricular pressure against semilunar valves, forcing them open.

  2. Ventricles relax, lowering pressure, allowing backflow from the aorta or pulmonary arteries to fill cusps of the semilunar valves, forcing them to close.

Microscopic Anatomy of Cardiac Muscle

  • Characteristics: Cardiac muscle is:

    • Striated, short, branched, and interlinked tissue structure.

  • Functional Syncytium: Cardiac muscle cells behave as a single functional unit due to intercalated discs which facilitate ion passage.

Cardiac Muscle Contraction

  • Automaticity: Cardiac muscle is self-excitable, can contract without external stimuli.

  • Refractory Period: The absolute refractory period lasts approximately 250 ms to prevent tetany in heart muscle.

Heart Physiology: Conduction System

  • Autorhythmic Cells: Initiate action potentials, characterized by unstable resting potentials (pacemaker potentials).

  • Pacemaker Mechanism: Calcium influx predominantly regulates action potential rise.

Sequence of Excitation

  1. Sinoatrial (SA) Node: Generates impulses about 75 times per minute.

  2. Atrioventricular (AV) Node: Delays impulse transmission by 0.1 seconds, allowing atrial contraction.

  3. Atrioventricular Bundle (Bundle of His): Carries impulses to ventricles.

  4. Purkinje Fibers: Conduct impulses throughout the ventricular walls and apex, resulting in coordinated contraction.

Electrocardiography (ECG)

  • ECG Waveforms:

    • P Wave: Corresponds to SA node depolarization (atrial contraction).

    • QRS Complex: Reflects ventricular depolarization (ventricular contraction).

    • T Wave: Associated with ventricular repolarization.

  • Atrial repolarization is masked by the QRS complex.

Heart Sounds

  • Heart Sounds (Lub-Dup): Associated with valve closures:

    • First sound signifies closure of AV valves and initiation of systole.

    • Second sound marks closure of SL valves at the start of diastole.

Cardiac Cycle

  • Definition: The sequence of events involved with blood flow through the heart.

  • Stages:

    • Systole: Contraction phase of the cardiac muscle.

    • Diastole: Relaxation phase of the cardiac muscle.

    • Phases include ventricular filling, ventricular systole (ejection), and isovolumetric relaxation.

Cardiac Output (CO) and Regulation

  • Definition: The amount of blood pumped by each ventricle in one minute.

  • Formula: (CO = HR imes SV) where:

    • HR = Heart Rate (beats per minute).

    • SV = Stroke Volume (amount of blood pumped per beat).

    • Example Calculation:

    • For HR = 75 ext{ beats/min} and SV = 70 ext{ ml/beat}:

    • CO = 75 imes 70 = 5250 ext{ ml/min} (5.25 ext{ L/min})

Stroke Volume Regulation

  • Stroke Volume (SV) Calculation: SV = EDV - ESV where:

    • EDV: End Diastolic Volume (blood volume in ventricles before contraction).

    • ESV: End Systolic Volume (blood volume remaining in ventricles after contraction).

Factors Affecting Stroke Volume

  • Preload: Degree of stretch of ventricles during diastole.

  • Contractility: Strength of heart muscle contraction independent of EDV.

  • Afterload: Pressure the ventricles must overcome to eject blood.

Regulation of Heart Rate

  • Positive Chronotropic Factors: Increase heart rate (e.g., caffeine).

  • Negative Chronotropic Factors: Decrease heart rate (e.g., sedatives).

  • Autonomic Nervous System:

    • Sympathetic activation increases heart rate via cardioacceleratory center.

    • Parasympathetic activity decreases heart rate via cardioinhibitory center and vagal tone.

Chemical Regulation of Heart Rate

  • Hormonal influences include:

    • Epinephrine: Increases heart rate.

    • Thyroxine: Modulates heart activity.

  • Ion balance (sodium, calcium, potassium) is critical for normal function.

Clinical Considerations

Congestive Heart Failure (CHF)

  • Causes:

    • Coronary atherosclerosis, hypertension, multiple myocardial infarcts, dilated cardiomyopathy.

  • Symptoms:

    • Left-sided failure leads to pulmonary congestion and edema; exhibit shortness of breath

    • Right-sided failure results in peripheral edema.

Coronary Artery Disease (CAD)

  • Mechanism: Insufficient blood supply to heart often from atherosclerosis or spasm.

  • Treatment Options: Drugs, bypass surgery, balloon angioplasty, stenting.

Common Cardiac Problems

  • Myocardial Infarction (MI): Death of cardiac muscle tissue due to oxygen deprivation; treated by medications like thrombolytics.

  • Angina Pectoris: Chest pain from ischemia; often a precursor of heart attack.

Developmental Aspects of the Heart

  • Fetal Heart Structures:

    • Foramen Ovale: Connects atria, bypassing pulmonary circuit.

    • Ductus Arteriosus: Connects pulmonary trunk to aorta bypassing the lungs.

  • Congenital Heart Defects: Typically occur in 1 in every 500 births, examples include:

    • Ventricular Septal Defect: Blood mixing between ventricles.

    • Coarctation of the Aorta: Narrowing of the aorta, increasing heart workload.

    • Tetralogy of Fallot: Combination of defects affecting normal blood flow.

Age-Related Changes in the Heart

  • Increased sclerosis and thickening of valve flaps.

  • Declined cardiac reserve.

  • Development of cardiac fibrosis leading to decreased elasticity.

  • Increased incidence of atherosclerosis.

Summary of Key Concepts

  • Blood pressure variations occur between systolic and diastolic pressures, affecting pulse rates and cardiovascular health.

  • Understanding the structures and functions of the cardiovascular system is crucial for assessing cardiac health and diagnosing conditions.