Cardiovascular System: The Heart – Structure, Function, and Circulation

Cardiovascular System: Heart

Overview

The cardiovascular system is essential for transporting blood, nutrients, and gases throughout the body. The heart acts as a muscular double pump, driving blood through two major circuits: the pulmonary and systemic circuits. Understanding the heart's anatomy and physiology is fundamental for students of anatomy and physiology.

Thoracic Cavity: Mediastinum

Location and Boundaries

• Mediastinum: Central compartment of the thoracic cavity, containing all thoracic viscera except the lungs and pleural sacs.

• Bounded by the sternum and costal cartilage anteriorly, thoracic vertebrae posteriorly, and extends from the superior thoracic aperture to the diaphragm.

• Houses the heart, great vessels, trachea, esophagus, and thymus.

Heart Structure and Function

General Characteristics

• The heart typically weighs 250–350 grams.

• Largest organ of the mediastinum.

• Apex: Lies to the left of the midline.

• Base: Broad posterior surface, covered by ribs 2–5.

Pulmonary and Systemic Circuits

• Pulmonary circuit: Right side receives oxygen-poor blood from the body and pumps it to the lungs.

• Systemic circuit: Left side receives oxygenated blood from the lungs and pumps it throughout the body.

Chambers of the Heart

• Atria: Superior chambers; receive blood from pulmonary and systemic circuits.

• Ventricles: Inferior chambers; pumping chambers of the heart.

Dextrocardia

Congenital Anomaly

• Dextrocardia: Condition where the apex of the heart is misplaced to the right side of the thorax.

• May be associated with other congenital anomalies.

Pericardium and Pericardial Cavity

Layers of the Pericardium

• Fibrous pericardium: Tough outer layer covering the heart and the beginning of its great vessels.

• Serous pericardium: Composed of parietal and visceral layers (epicardium).

Pericardial Cavity

• Space inside the pericardial sac, between the parietal and visceral pericardium.

• Contains a small amount of serous fluid to reduce friction during heartbeats.

Clinical Procedures and Conditions

• Pericardiocentesis: Procedure to drain fluid from the pericardial cavity.

• Pericarditis: Inflammation of the pericardium.

• Hemopericardium: Presence of blood in the pericardial cavity.

• Cardiac tamponade: Life-threatening condition where fluid accumulation compresses the heart, impairing its function.

Heart Wall

Layers of the Heart Wall

• Epicardium: Also known as visceral pericardium; outermost layer.

• Myocardium: Middle layer; composed of cardiac muscle arranged in spiral and circular patterns.

• Endocardium: Innermost layer; lines the internal walls of the heart, made of simple squamous epithelium.

Myocardium Structure

• Consists of cardiac muscle and forms the bulk of the heart.

• Connective tissues surround and bind muscle cells into spiral and circular bundles, enabling efficient blood ejection.

• Cells join at intercalated discs (complex junctions):

  • Fascia adherens: Binds adjacent cells and transmits contractile force.

  • Gap junctions: Transmit contractile signals to adjacent cells.

• Cells are separated by endomyium: Loose connective tissue containing blood vessels and nerves.

• Cardiac muscle is involuntarily innervated and exhibits inherent rhythmicity.

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Innervation	Voluntary	Involuntary	Involuntary
Rhythmicity	No	Yes	Yes
Intercalated Discs	No	Yes	No
Location	Attached to bones	Heart	Walls of hollow organs

Heart Chambers: Details

Right Atrium

• Forms the right border of the heart.

• Receives oxygen-poor blood from systemic circuit via:

  • Superior vena cava

  • Inferior vena cava

  • Coronary sinus

• Opens to the right ventricle through the tricuspid valve.

• Pectinate muscles: Ridges on the inside anterior wall.

• Fossa ovalis: Depression in interatrial septum; remnant of foramen ovale (fetal circulation).

Right Ventricle

• Receives blood from the right atrium.

• Pumps blood into pulmonary circuit through pulmonary trunk.

• Pulmonary semilunar valve: Between right ventricle and pulmonary trunk.

• Tricuspid valve: Right atrioventricular valve.

• Trabeculae carneae: Irregular ridges of muscle along inner surface.

• Papillary muscles: Finger-like muscle projections.

• Chordae tendineae: Thin bands anchoring valve cusps to papillary muscles.

Left Atrium

• Makes up the heart's posterior surface (base).

• Receives oxygen-rich blood from lungs via four pulmonary veins (2 per lung).

• Opens into the left ventricle through the bicuspid (mitral) valve.

• Floor of fossa ovalis.

Left Ventricle

• Forms the apex of the heart.

• Internal walls contain:

  • Trabeculae carneae

  • Papillary muscles

  • Chordae tendineae

• Pumps blood through the aortic semilunar valve into the ascending aorta and systemic circuit.

• Thicker wall, larger cavity, larger papillary muscles than right ventricle.

Septal Defects

Types and Clinical Significance

• Atrial septal defects (ASD): Patent foramen ovale (PFO) occurs when the foramen ovale fails to close after birth, allowing abnormal blood flow between atria.

• Ventricular septal defects (VSD): Account for 25% of congenital heart disease; abnormal opening in interventricular septum.

Heart Valves

Structure and Function

• Composed of endocardium with a connective tissue core.

• Open to allow blood flow, close to prevent backflow.

• Atrioventricular (AV) valves:

  • Right AV valve = tricuspid valve

  • Left AV valve = bicuspid (mitral) valve

• Aortic and pulmonary semilunar valves: Located at junction of ventricles and great arteries.

Cardiac Skeleton

Functions

• Dense connective tissue surrounds all four valves.

• Anchors valve cusps and prevents over-dilation of valve openings.

• Main point of insertion for cardiac muscles.

• Blocks direct spread of electrical impulses between atria and ventricles.

Pathway of Blood Through the Heart

Sequential Flow

• Blood passes through all heart structures in sequence.

• Atria contract together, followed by simultaneous ventricular contraction.

Pathway Steps

1. Right atrium → right ventricle → pulmonary trunk → lungs

2. Lungs → left atrium → left ventricle → aorta → systemic circulation

Heart Sounds

Mechanism and Clinical Relevance

• "Lub-dub" sound is produced by valve closure.

• First sound ("lub"): AV valves closing.

• Second sound ("dub"): Semilunar valves closing.

• Heard at specific auscultatory areas of the chest (second and fifth intercostal spaces).

Heartbeat

Phases and Measurement

• Normal rate: 70–80 beats per minute at rest.

• Systole: Contraction of a heart chamber.

• Diastole: Relaxation and expansion of a heart chamber.

• Terms often refer to ventricular contraction and relaxation.

• Left ventricle is the systemic pump; its wall is at least three times thicker than the right ventricle.

• Blood pressure: Measured as left ventricular systolic pressure over diastolic pressure.

• Equation: $\text{Blood Pressure} = \frac{\text{Systolic Pressure}}{\text{Diastolic Pressure}}$

Conduction System of the Heart

Intrinsic Conducting System

• Cardiac muscle tissue can generate and conduct electrical impulses independently.

• Specialized cardiac muscle cells (not nervous tissue) transmit impulses throughout the heart.

Key Components

• Sinoatrial (SA) node: Pacemaker, located in right atrium wall.

• Atrioventricular (AV) node: Located in inferior interatrial septum.

• Atrioventricular (AV) bundle: Conducts impulses from AV node to bundle branches.

• Bundle branches: Right and left branches in interventricular septum.

• Purkinje fibers (subendocardial branches): Distribute impulses to ventricular walls.

Sequence of Conduction

1. SA node generates impulse; atria contract first (top-down).

2. Impulse delayed 0.1 seconds at AV node, allowing atria to empty.

3. Impulse travels down AV bundle and bundle branches to Purkinje fibers.

4. Ventricles contract in a wave from bottom up.

Cardiac Control Centers

Autonomic Regulation

• Heart rate is altered by external controls.

• Parasympathetic motor branches of the vagus nerve decrease heart rate.

• Sympathetic motor fibers from cervical and upper thoracic chain ganglia increase heart rate.

Fetal Circulation

Unique Features

• Fetus supplies blood to the placenta for oxygen and nutrients.

• Umbilical arteries: Carry deoxygenated blood to placenta; become medial umbilical ligaments after birth.

• Umbilical vein: Carries oxygenated blood from placenta; becomes ligamentum teres (round ligament of liver).

• Ductus venosus: Bypasses most of the liver, empties oxygenated blood into inferior vena cava; becomes ligamentum venosum.

• Foramen ovale: Bypasses pulmonary circuit by shunting blood from right to left atrium; becomes fossa ovalis.

• Ductus arteriosus: Bypasses pulmonary circuit by shunting blood from pulmonary trunk to aorta; becomes ligamentum arteriosum.

Fetal Structure	Postnatal Structure	Function in Fetus
Foramen ovale	Fossa ovalis	Shunts blood from right to left atrium
Ductus arteriosus	Ligamentum arteriosum	Shunts blood from pulmonary trunk to aorta
Ductus venosus	Ligamentum venosum	Bypasses liver, delivers blood to inferior vena cava
Umbilical vein	Ligamentum teres	Delivers oxygenated blood from placenta
Umbilical arteries	Medial umbilical ligaments	Carry deoxygenated blood to placenta

Newborn Circulation

Transition After Birth

• Blood is oxygenated in the lungs after birth.

• Fetal shunts close, and the heart becomes functionally divided with the first breaths.

• Right side receives and pumps poorly oxygenated blood; left side receives and pumps highly oxygenated blood.

Additional info: The notes above expand on the original lecture slides with definitions, clinical context, and comparative tables for muscle types and fetal vs. postnatal circulation. All major anatomical and physiological features of the heart relevant to a college-level Anatomy & Physiology course are covered.