Anatomy of the Heart

• The heart's importance has been recognized for centuries.
• Strokes (contractions) average about 72 per minute and continue unceasingly for a lifetime.
• Focus is on the heart's functions and disorders.

Heart Structure

• Slightly bigger than a person's fist.
• Located between the lungs, towards the left of the body's midline in the mediastinum of the thorax.
• The apex (pointed, inferior portion) points left.
• The broad, superior base points right and is the attachment for large blood vessels.

Tissue Layers of Heart Wall

• Wall consists of three layers;
  • Endocardium: Innermost layer of epithelial cells; Provides a smooth surface for blood flow; Extensions cover heart valve cusps.
  • Myocardium: Thickest, middle layer of heart muscle; Responsible for pumping blood.
  • Pericardium: Thin, outermost serous membrane; also considered the visceral layer of the pericardium.

The Pericardium

• Encloses/protects the heart.
• Fibrous pericardium is the sac's outermost, heaviest layer of connective tissue.
• Anchors the heart to the diaphragm, sternum, and surrounding structures.
• The serous membrane lines the fibrous sac, folding back to cover the heart's surface.
• Parietal layer: outer layer of serous membrane.
• Visceral layer (epicardium): the serous membrane's inner layer.
• The thin fluid film between layers reduces friction as the heart moves.
• Fluid accumulation in the pericardial cavity can occur under certain disease conditions.

Special Features of the Myocardium

• Lightly striated due to actin and myosin filaments.
• Contains a single nucleus per cell.
• Involuntarily controlled.
• Intercalated disks facilitate rapid electrical impulse transfer between cells.
• Intercalated disks are modified plasma membranes that firmly attach adjacent cells.
• Branched fibers are interwoven for coordinated contraction.

Divisions of the Heart

• The heart acts as a double pump; the right and left sides have different functions.

• The right side pumps oxygen-poor blood to the lungs through the pulmonary circuit.

• The left side pumps oxygen-rich blood to the body through the systemic circuit.

• Each side has two chambers.

• The Atria (upper chambers) receive blood.

• The Ventricles (lower chambers) pump blood forcefully.

• Blood flow sequence: Head/chest/arms > superior vena cava > right atrium > right ventricle > pulmonary trunk artery > to the lungs

• Blood flow sequence: Trunk/legs > inferior vena cava > right atrium > right ventricle > pulmonary trunk artery > to the lungs

• Blood flow sequence: Heart muscle > coronary sinus vein > right atrium > right ventricle > pulmonary trunk artery > to the lungs

• Blood flow sequence: Lungs > Pulmonary Veins > Left Atrium;

• Blood flow sequence: Left Atrium > Left Ventricle > Aorta Artery > Systemic Arteries to the Body

• The right atrium are thin-walled.

• The wall of left ventricle forms the heart's apex and are the thickest.

• The Septum divides the right and left chambers.

• The interatrial septumseparates atria, and the interventricular septum separates ventricles.

• Septa and the heart wall consist largely of myocardium.

Four Valves

• One-way valves direct blood flow at entrance and exit of each ventricle.
  • Atrioventricular (AV) valves located between atria and ventricles (entrance valves).
  • Semilunar valves located at exits of ventricles.
    • Named because flaps resemble a half moon.
• Specific valves:
  • Right atrioventricular (AV) valve (tricuspid): connects the right atrium to the right ventricle.
  • Left atrioventricular (AV) valve (bicuspid/mitral): connects the left atrium to the left ventricle.
  • Pulmonary valve (pulmonic): connects the right ventricle and the pulmonary trunk; a semilunar valve.
  • Aortic valve: connects the left ventricle and the aorta; a semilunar valve.
• Papillary muscles arising from the walls of the ventricles are attached to both the right and left AV valves by fibrous threads.
• Chordae tendineae stabilize valve flaps when ventricles contract, preventing backflow into the atria.
• Blood passes through the heart twice in a complete circuit.
  • From the right side > pulmonary circuit > to the lungs > back to the heart's left side > systemic circuit.

Blood Supply to the Myocardium

• The endocardium alone comes into contact with blood that flows through the heart chambers
• The myocardium requires its own blood vessels for oxygen, nourishment, and waste removal.
• Coronary circulation: describes the blood vessels
• Right and left coronary arteries supply blood to the heart muscle named such because they encircle like a crown.
• The arteries branch off the aorta, just above the cusps of the aortic valve, and extend to all regions of the heart muscle.
• Blood collects in the coronary sinus after passing through the capillaries, delivering blood to the right atrium near the inferior vena cava.

Heart Function

• The heart's two sides contract in partnership.

• Heart Contraction begins with the contraction of the Atria (upper chambers), then contraction follows in the Ventricles (lower chambers.)

• Systole: active phase, muscles contract forcing blood through the chambers.

• Diastole: resting phase, muscles relax refilling with blood.

• Cardiac Cycle: the complete sequence of heart contraction and relaxation; a single heartbeat.

  • The cycle at rest averages 0.8 seconds.

• The cycle starts with both Atria contracting together > Blood is forced though the Atrioventricular (AV) valves > to the Ventricles.

• After the Atria contract, the resting phase (diastole) begins at the same Time that the Atria Contraction (systole) begins in the Ventricles simultaneously

• Ventricles are contracting > forces blood through the Semilunar Valves > atria relax and again begin filling with blood.

Cardiac Output

• Contractions are more forceful as less blood enters the heart.
• Contractions are stronger as more blood enters the heart, muscles are contracted with greater strength so blood is pushed out to blood vessels.
• Pumping ALL the blood the heart receives prevents blood collecting within the chambers.
• Cardiac Output (CO): Volume of blood pumped by each ventricle per minute.
  • Product of stroke volume (SV): volume ejected per beat, and heart rate (HR).
  • CO = HR * SV

Heart's Conduction System

• A wave of electrical energy is required to stimulate heart muscle and induce contraction, similar to other muscles within the body.
• Action potential is generated by tissues within the heart and spreads over structures that form the heart's conduction system.
  • Nodes (tissue masses) and specialized fibers (branch through myocardium).
• Sinoatrial (SA) node in the upper wall of the right atrium initiates heartbeats.
  • Generates action potential at regular intervals.
  • The Pacemaker is the SA Node which sets the rate of contractions
• Atrioventricular (AV) node is located in the lower interatrial septum.
• Atrioventricular (AV) bundle of his at the top of the interventricular septum.
  • Branches extend to all parts of the ventricular walls.
  • Fibers travel down both sides of the interventricular septum in groups called the right and left bundle branches.
  • Purkinje fibers travel in a branching network throughout the myocardium of the ventricles.
  • Intercalated disks allow the rapid flow of impulses throughout the heart muscle.
• Impulses travel:
  • 1 - SA node is generated
  • 2 - myocardium contracts.
  • AV node is stimulated via internodal pathways.
  • 3 - The relatively slower Conduction rate allows atria to contract, completing filling of ventricles before ventricles contract.
  • 4 - The excitation wave travels rapidly through the AV bundle > ventricular walls contract.
• Sinus rhythm is normal and originates in the SA node
• Safety: If the SA node is not performing correctly, a region of the node will generate its own heartbeat, but at a slower rate

Control of Heart Rate

• The heart's essential beat originates within the heart, itself. Influenced by the nervous system, hormones, and the internal environment.

• ANS is an important role in modifying the heart rate to match the body's increasing/decreasing needs

  • System is sympathetic: to increase in response to increased activity
    • boost cardiac output 4-5 times the resting value.
    • Rate is increased via stimulating the SA and AV nodes and by strengthening contractions by influenceing the fibers directly to increase cardiac output.

• Stimulation is parasympathetic: To decrease the heart rate to restore homeostasis, the Vagus Nerve signals to reduce as it acts on the SA and AV node.

• Hormones: Epinephrine and Thyroxine

• Ions: K+, NA+, Ca2+

• Drugs

• Exercise that is regularly undertaken strengthens/increase the volume of blood ejected with each beat, reducing the bodies circulatory needs while at rest/lower heart rate.

  • Trained Athletes usually have a low resting heart rate. The following variations in heart Rate are more common:

• Bradycardia: Relatively slows the heart-rate.

• Tachycardia Heart rate of +100 beats

• Sinus Arrythmia Regular variation can be caused by changes in normal breaths.

• Premature beat / Extrasystole: When a heartbeat does not follow the cycle.

Normal and Abnormal Heart Sounds

• These normal heart sounds can be described as "lub" and "dub"
  • S1 (Lub) The ventricular systole
  • S2 (Dub) Ventricular relaxation
  • If a sound is ABNORMAL it can be from faulty valve actions.
  • The sound is abnormal it can be from the narrowing, or stenosis, of valve openings due to many conditions
  • The Organic Murmer can be a result of heart/vessel structure change
  • Functional murmers are sound the can differentiated.