OVERVIEW OF ANATOMY AND PHYSIOLOGY OF THE HEART

• Heart: - A muscular organ, which pumps blood through the blood vessels of the circulatory system.
• Blood vessels – Pathway that permits blood to flow from heart to cells and back to the heart. It includes Arteries, arterioles, capillaries, venules, veins.

Anatomy of the Heart

• Relatively small in size and conical organ.
• it weighs about 250 to 350 grams
• Located left of the body midline posterior to the sternum in the middle mediastinum.
• Rotated such that its right side (right atrium and ventricle) is located more anteriorly, while its left side (left atrium and ventricle) is located more posteriorly.
• The posterosuperior surface of the heart, formed primarily by the left atrium, is called the base.
• The pulmonary veins that enter the left atrium border this base.
• The inferior, conical end is called the apex.
• It projects slightly anteroinferiorly toward the left side of the body.

Pericardium

• Fibrous, serous sac
  • Contains the heart
  • In the mediastinum
  • Held in place by connective tissue
    • The external wall of the great vessels' superior to the heart
    • diaphragm inferior.
• Restricts heart movements
• Prevents the heart from overfilling with blood.
• Outer portion
  • tough, dense connective tissue
  • called the fibrous pericardium.
  • attached to both the sternum and the diaphragm
• Inner portion
  • thin, double-layered serous membrane
  • called the serous pericardium.
    • parietal layer
    • visceral layer

Heart Wall Structure

• Three distinctive layers:
  • External Epicardium
  • Middle Myocardium
  • Internal Endocardium

Epicardium-

• outermost heart layer
• also known as the visceral layer of serous pericardium.
• Simple squamous epithelium underlined by fat
• As we age, more fat is deposited in the epicardium
  • this layer becomes thicker and more fatty.

Myocardium-

• middle layer of the heart wall
• composed chiefly of cardiac muscle tissue.
• thickest of the three heart wall layers.
• lies deep to the epicardium and superficial to the endocardium

Endocardium-

• covers internal surface of the heart and the external surfaces of the heart valves

External Anatomy of the Heart

Chambers

• four hollow chambers:
  • two smaller atria
  • two larger ventricles.

• Atria
  • thin-walled, located superiorly.
  • anterior part of each atrium is a wrinkled
  • Atria receive blood through both circulatory circuits.
    • right atrium receives blood from the systemic circuit
    • left atrium receives blood from the pulmonary circuit
  • Blood that enters an atrium is passed to the ventricle on the same side of the heart.
• Ventricles
  • the inferior chambers.
  • Two large arteries, the pulmonary trunk and the aorta exit the heart at the basal surface.
• The pulmonary trunk carries blood from the right ventricle into the pulmonary circuit.
• The aorta conducts blood from the left ventricle into the systemic circuit

Internal Anatomy of the Heart

Heart Chambers

• There are four heart chambers:
  • right atrium
  • right ventricle
  • left atrium
  • left ventricle
• Each plays a role in the continuous process of blood circulation.
• Valves permit the passage of blood in one direction and prevent its backflow.

Right Atrium

• Receives venous blood
  • from the systemic circuit
  • from the heart muscle itself.
• Three major vessels empty into the right atrium:
  • superior vena cava (SVC)
    • drains blood from the head, upper limbs, and superior regions of the trunk
  • inferior vena cava (IVC)
    • drains blood from the lower limbs and trunk
  • coronary sinus drains blood from the heart wall
• The interatrial septum forms a wall between the right and left atria.

Right Ventricle

• Receives deoxygenated venous blood from the right atrium.
• An interventricular septum forms a wall between the right and left ventricles.
• Papillary muscles
  • on the internal wall surface
  • cone-shaped, muscular projections
  • anchor chordae tendineae
    • attach to the cusp of the right AV valve and prevent everting and flipping into the atrium when contracting

Left Atrium

• Once gas exchange occurs in the lungs, the oxygenated blood travels through the pulmonary veins to the left atrium.
• Smooth posterior wall of the left atrium contains openings for approximately four pulmonary veins.
  • two left pulmonary veins
  • two right pulmonary veins
• Has pectinate muscles along its anterior wall as well as an auricle.

Left Ventricle

• Largest of the four heart chambers.
• Wall is typically three times thicker than the right ventricular wall.
• Requires thick walls in order to generate enough pressure to force the oxygenated blood from the lungs into the aorta and then through the entire systemic circuit.
  • right ventricle only has to pump blood to the nearby lungs
• Trabeculae carneae in the left ventricle are more prominent.
• Two large papillary muscles attach to the chordae tendineae that help support the left AV valve.
• At the superior end of the ventricular cavity, the aortic semilunar valve marks the end of the left ventricle and the entrance into the aorta.

Heart valves

Right Atrioventricular (AV) Valve

• Separates the right atrium from the right ventricle.
• Also called the tricuspid valve.
  • has three triangular flaps
• Venous blood flows from the right atrium, through the valve into the right ventricle.
• Is forced closed when the right ventricle begins to contract
• the three triangular flaps attach to the chordae tendineae that helps to preventing blood backflow into the right atrium

Left Atrioventricular (AV) Valve

• Separates the left atrium from the left ventricle.
• Also called the bicuspid valve or the mitral valve.
• Left AV valve has chordae tendineae similar to those of the right AV valve.
• Oxygenated blood flows from the left atrium into the left ventricle.
• Is forced closed when the left ventricle begins to contract
  • prevents blood backflow into the left atrium

Semilunar Valves

• Located within the walls of both ventricles
  • immediately before the connection of the ventricle to the pulmonary trunk and aorta.
• Composed of three thin, pocket like semilunar cusps.
• As blood is pumped into the arterial trunks, it pushes against the cusps
  • forcieg the valves open.
• When ventricular contraction ceases
  • blood is prevented from flowing back into the ventricles.
  • causes the cusps to "inflate" and meet at the artery center, effectively blocking blood backflow

Coronary Circulation

• Left and right coronary arteries travel in the coronary sulcus (atrioventricular groove) of the heart to supply the heart wall.
  • the only branches of the ascending aorta
• Located immediately superior to the aortic semilunar valve.
• The right coronary artery typically branches into the
  • marginal artery
    • supplies the right border of the heart
  • posterior interventricular artery
    • supplies both the left and right ventricles
• Left coronary artery typically branches into the anterior interventricular artery.
  • also called the left anterior descending artery
  • supplies the anterior surface of both ventricles and most of the interventricular septum
• Circumflex artery.
  • supplies the left atrium and ventricle
• Arterial pattern can vary greatly among individuals.

Conduction System of the Heart

• The specialized heart cells of the cardiac conduction system generate and coordinate the transmission of electrical impulses to myocardial cells
• The result is sequential atrioventricular contraction which provides for the most effective flow of blood , thereby optimizing cardiac output
• Characteristics of Cardiac Conduction Cells
  • Automaticity: ability to initiate an electrical impulse
  • Excitability: ability to respond to an electrical impulse
  • Conductivity: ability to transmit an electrical impulse from one cell to another
• Four basic components
  • Sinu-atrial node
  • Atrioventricular node
  • Atrioventricular bundle with its right and left bundle branches
  • Subendocardial plexus of conduction cells (the Purkinje fibers)

Sinoatrial (SA) Node

• Heartbeat is initiated by the cardiac muscle fibers of the sinoatrial (SA) node.
  • located in the posterior wall of the right atrium, adjacent to the entrance of the superior vena cava
• Act as the pacemaker.
  • rhythmic center that establishes the pace for cardiac activity
• Initiates impulses 70 - 80 times per minute.

Atrioventricular (AV) Node

• Impulse travels to both atria, stimulating atrial systole.
• And via an internodal conduction pathway through an opening in the fibrous skeleton to the atrioventricular (AV) node.
  • located in the floor of the right atrium between the right AV valve and the coronary sinus

Atrioventricular (AV) Bundle

• Cardiac impulse then travels from the AV node to the atrioventricular (AV) bundle(bundle of His).
  • extends into the interventricular septum and then divides into one right and two left bundle branches.
• Conduct the impulse to conduction fibers called Purkinje fibers in the heart apex.
  • Purkinje fibers are larger than other cardiac muscle fibers.
• Muscle impulse conduction along the Purkinje fibers is extremely rapid.
• The impulse spreads immediately throughout the ventricular myocardium.

Physiology of cardiac conduction

• Cardiac electrical activity is result of the movement of ions (charged particles such as Na, K and Ca) across to cell membrane.
• In the resting state cardiac muscle cells are polarized, that means –
  • electrical difference exists between –
    • the negatively charged inside the cell membrane and
    • the positively charged outside of the cell membrane
• Cardiac Action Potential
  • Depolarization: electrical activation of a cell caused by the influx of sodium into the cell while potassium exits the cell
  • Repolarization: return of the cell to the resting state caused by re-entry of potassium into the cell while sodium exits
• Phase 0: Cellular depolarization is initiated as positive ions influx into the cell. During this phase, the atrial and ventricular myocytes rapidly depolarize as sodium moves into the cells through sodium fast channels.
• Phase 1: Early cellular repolarization begins during this phase as potassium exits the intracellular space.
• Phase 2: This phase is called the plateau phase because the rate of repolarization slows. Calcium ions enter the intracellular space.
• Phase 3: This phase marks the completion of repolarization and return of the cell to its resting state.
• Phase 4: This phase is considered the resting phase before the next depolarization.

• Refractory periods:
  • Effective/Absolute refractory period: phase in which cells are incapable of depolarizing
  • Relative refractory period: phase in which cells require a stronger-than-normal stimulus to depolarize
• Refractoriness protects the heart from sustained contraction (tetany) which would result in sudden cardiac death 
• Change in serum calcium contraction may alter the contraction of the heart muscle fibers and also change in serum potassium concentration is also important because potassium affects the normal electrical voltage of the cell.

Cardiac Cycle

• The inclusive period of time from the start of one heartbeat to the initiation of the next.
• All chambers within the heart experience alternate periods of contraction and relaxation.
• Contraction of a heart chamber is called systole.
  • forces blood into another chamber (from atrium to ventricle)
  • forces blood into a blood vessel (from a ventricle into the attached large artery)
• Relaxation phase of a heart chamber is termed diastole.
  • myocardium of each chamber relaxes between contraction phases and the chamber fills with blood

Great Vessel and Heart Chamber Pressures

Terminology

• Stroke volume: the amount of blood ejected with each heartbeat
• Cardiac output: amount of blood pumped by the ventricle in liters per minute
• Preload: degree of stretch of the cardiac muscle fibers at the end of diastole
• Contractility: ability of the cardiac muscle to shorten in response to an electrical impulse
• Afterload: the resistance to ejection of blood from the ventricle
• Ejection fraction: the percent of end-diastolic volume ejected with each heartbeat
• Control of heart rate
  • Autonomic nervous system and baroreceptors
• Control of strike volume
  • Preload: Frank-Starling law
  • Afterload: affected by systemic vascular resistance and pulmonary vascular resistance
  • Contractility increased by catecholamine's, SNS, some medications and decreased by hypoxemia, acidosis, some medications