Anatomy and Location of the Heart

• Cardiology is the study of the cardiovascular system
• The apex of the heart is the bottom point, formed by the ventricles
• The base of the heart is formed by the atria, which is the broad superior surface
• Ventricles are muscular chambers which eject blood
• Atria are collecting chambers that contract to fill the ventricles
• The heart beats about 100,000 times every day, or 35 million times a year
• The left side of the heart pumps to the systemic circuit
• The right side of the heart pumps to the pulmonary circuit

Location & Heart Anatomy

• The heart is about 12cm long and 9cm wide at its broadest (5” x 3.5”)
• It weighs about 250g (8 oz) in females
• It weighs about 300g (10 oz) in males
• The heart rests on the diaphragm in the mediastinum between the lungs
• It is located near the midline of the thoracic cavity; two-thirds are to the left of the midline
• The base of the heart is directed posteriorly and to the right
• The apex is the hearts most inferior point

Pericardium

• This is a fibrous layer made of dense, irregular connective tissue
• The fibrous pericardium functions to protect and anchor the heart
• The serous pericardium has two layers, the parietal pericardium and the visceral pericardium, with the pericardial cavity in between filled with pericardial fluid
• The parietal pericardium is fused to the fibrous pericardium
• The visceral pericardium is also called the epicardium

Heart Wall

• The epicardium is the visceral layer of the serous pericardium
• The myocardium is composed of cardiac muscle that is involuntary and contains branched cells
• The cardiac muscle is connected through intercalated discs, gap junctions, and desmosomes
• The endocardium is continuous throughout the cardiovascular system

Right Atrium (RA)

• The right atrium receives deoxygenated blood from:
  • The superior vena cava
  • The inferior vena cava
  • The coronary sinus
• The posterior wall is smooth, while the anterior wall is rough with pectinate muscles
• The pectinate muscles increase surface area/power of contraction; extends into the auricle to receive blood until contraction
• The right atrium is divided from the left atrium by the interatrial septum
• The oval depression in the septum called the fossa ovalis is a remnant of the foramen ovale
• Blood leaves the right atrium through the tricuspid valve

Right Ventricle (RV)

• The right ventricle receives blood from the right atrium
• It forms most of the anterior surface of the heart
• It contains trabeaculae carneae made of raised bundles of cardiac muscle
• Cusps of the tricuspid valve are connected to chordae tendineae
• Chordae tendineae are connected to cone-shaped trabeaculae carneae, also called papillary muscles, that tense the chordae tendineae
• It is divided from the left ventricle by the interventricular septum
• Deoxygenated blood is ejected to the pulmonary valve and pulmonary trunk to reach the lungs for gas exchange

Left Atrium (LA)

• Receives oxygenated blood from the lungs through four pulmonary veins
• Structurally similar to the right atrium
• Blood passes from the left atrium to the left ventricle through the bicuspid (mitral) valve

Left Ventricle (LV)

• Receives oxygenated blood from the left atrium via the bicuspid valve
• Has internal structures, including trabeaculae carneae, chordae tendineae, papillary muscles, and the interventricular septum similar to the right ventricle
• Oxygenated blood is ejected into the aorta
  • Some aortic blood travels to coronary arteries
  • The remainder passes to the aortic arch
  • During fetal life, blood passes from the pulmonary trunk to the aorta (bypassing the lungs) through the ductus arteriosus, which closes shortly after birth

Myocardium

• Atrial walls are thinnest
• The right ventricle is thinner than the left ventricle because it pumps blood a shorter distance
• The left ventricle walls are thickest
• Both the right and left ventricles pump the same volume of blood with each beat

Heart Valves

• Heart valves ensure one-way blood flow
• Atrioventricular Valves:
  • Valves located between the atria & the ventricles
  • The right side valve is known as the tricuspid valve
  • The left side valve is known as the bicuspid or mitral valve
  • Chordae tendineae connect the valves to papillary muscles
• Semilunar Valves exist at the beginning of the arteries that leave the heart
  • These valves have 3 cusps
  • Pulmonary Semilunar Valve moves deoxygenated blood to the lungs
  • Aortic Semilunar Valve moves oxygenated blood into the aorta which circulates throughout the body

Systemic and Pulmonary Circulation

• Systemic Circulation:
  • The left side of the heart
  • Receives oxygenated blood from the lungs
  • Pumps oxygenated blood to the aorta & body tissues
• Pulmonary Circulation:
  • The right side of the heart
  • Receives deoxygenated blood from the body tissues (veins)
  • Pumps deoxygenated blood to the pulmonary trunk & lungs

Coronary Circulation

• Supplies blood to the heart tissues
• Arteries arise from the base of the aorta within a groove
• Left coronary artery:
  • Runs toward the left side of the heart
  • Divides into the anterior interventricular artery
  • Supplies blood to the interventricular septum and anterior walls of both ventricles
• Right coronary artery:
  • Runs toward the right side of the heart
  • Divides into the marginal artery and posterior interventricular artery
• The marginal artery serves the lateral myocardium of the right side, the second mentioned artery serves the heart apex and posterior ventricular walls

Coronary Veins

• Veins also known as the coronary sinus
• Great cardiac vein (anterior)
• Middle cardiac vein (posterior)
• Small cardiac vein
• Anterior cardiac veins

Cardiac Muscle

• Cardiac muscle cells are short, fat, branched, and interconnected
• Cardiac cells are connected at intercalated discs, which have desmosomes and gap junctions
• Cardiac muscles operate as a functional syncytium, i.e co-ordinated contraction

Difference Between Cardiac and Skeletal Muscle

• Some cardiac muscle cells are self-excitable
• The heart has two kinds of myocytes: pacemaker cells (that spontaneously depolarize) and contractile cells
  • Wave of depolarization spreads from cell to cell because of gap junctions to spontaneously depolarize the wave
• The heart contracts as a unit
• An influx of Ca++ from extracellular fluid triggers Ca++ release from the SR
• Tetanic contractions cannot occur in cardiac muscle, due to the very long absolute refractory period because of the long plateau phase (figure 18.15)
• The heart goes through aerobic metabolism

Action Potentials by Pacemaker Cells

• Figure 18.12
• Pacemaker cells have an unstable resting membrane potential and can depolarize spontaneously
• Pacemaker potential (Prepotential – Figure in book, found in SA and AV nodes): Na+ makes inside less negative, drifting toward threshold
• Depolarization: Calcium channels open at the threshold
• Repolarization: K+ channels open

Action Potential in Contractile Cells

• Depolarization: Na+
• Plateau phase: Ca++ (leads to loge absolute refractory period)
• Repolarization: K+ efflux

Anatomy of Cardiac Conduction System

• Cardiac excitation begins at the SA node (100/min)
• Excitation arrives at the AV node, located in the interatrial septum, and is slowed down (75/min)
• Action potential flows to the AV bundle
• Excitation then enters the right and left bundle branches traveling upward
• Final action potential arrives at the Purkinje fibers, contracting the ventricular myocardium from the apex up ejecting blood through semilunar valves

Conduction System

• Sinoatrial (SA) node:
• Spreads to both atria at 90 - 100 action potentials per minute
• Atrioventricular (AV) node:
• 40 -50 action potentials per minute
• Atrioventricular (AV) bundle (bundle of His):
• 20-40 action potentials per minute
• Right & left bundle branches conduct in the interventricular septum
• Purkinje fibers conduct using conduction myofibers.

Electrocardiography

• A recording of AP transmission through the cardiac conduction system
• Electrodes are placed on the body surface, including on the arms and legs, and six positions on the chest
• A graph of a series of up and down waves is produced during each heartbeat
• An electrocardiograph instrument produces 12 different tracings

ECG Waves

• P wave:
  • Atrial depolarization
• QRS complex:
  • Ventricular depolarization
  • Onset of ventricular contraction
• T wave:
  • Ventricular repolarization
  • Just before ventricles start to relax
• Atrial repolarization is usually not visible because it is masked by the larger QRS complex
• Systole is contraction
• Diastole is relaxation

Cardiac Cycle

• All events associated with one heartbeat occur during the cardiac cycle
• Two atria contract (atrial systole) while two ventricles relax (ventricular diastole)
• Two ventricles contract (ventricular systole) while two atria relax (atrial diastole)

Pressure & Volume Changes

• The resting heart rate is about 75 beats/min, so each beat occurs in approximately 0.8 seconds
• Relaxation period is 0.4 seconds, greatest variation in timing
• Atria contract for 0.1 seconds
• Atria relax & ventricles contract for 0.3 seconds

Atrial Systole

1. SA node depolarization
2. Atrial systole forces blood through AV valves into ventricles
3. Ventricles fill with an EDV measuring approximately 120mL just prior to ventricle contraction

Ventricular Systole

4. Ventricles contract as atria relax
5. Blood is pushed against AV valves forcing them shut; all valves shut for an instant
6. Pressure in ventricles exceeds pressure in arteries, both SL valves open and blood is ejected from ventricles
7. Resting body volume of blood ejected is about 70mL (just over half of EDV); ESV is about 50mL

Relaxation Period

8. Ventricular repolarization; shown as a T wave in an ECG
9. Ventricular diastole

• Ventricles relax
• Chamber pressure drops
• Blood flows from pulmonary trunk and aorta back toward ventricles when SL valves close
• Isovolmetric relaxation occurs, all four valves closed

11. Ventricular pressure is less than atrial pressure, where AV valves open and ventricle fill

• Occurs without atrial systole

12. Another cardiac cycle begins at atrial depolarization (P wave)

Heart Sounds

• Heart sounds may be evaluated through auscultation, or by listening to heart sounds
• Sounds caused by heart valves closing
• Four sounds, but only two (S1 and S2) are loud enough to hear by stethoscope
• S1 = lubb = long, booming sound of AV valves closing
• S2 = dubb = short, sharp sound of SL valves closing
• S3 blood turbulence during ventricular filling
• S4 blood turbulence during atrial systole

Cardiac Output

• Cardiac output equals stroke volume times heart rate
  • CO = SV x HR
• The difference between resting and maximal cardiac output is called cardiac reserve
• Stroke volume equals the amount of blood in ventricle during diastole (EDV) minus the amount of blood in ventricle after it has contracted (ESV)
  • SV = EDV - ESV

Regulation of Stroke Volume

1. Preload

• The degree of stretch of ventricles before contracting
• Frank-Starling law of the heart states that the greater the stretch (within limits), the greater the contraction
• Stretch is due to blood in the ventricles at the end of diastole (EDV)

2. Contractility

• Forcefulness of contraction of individual fibers
• Increased contractility (positive inotropic), direct consequence of greater Ca2+ influx, like glucagon, thyroxine, epinephrine
• Decreased contractility (negative inotropic), like acidosis, rising extracellular potassium, calcium channel blockers

3. Afterload

• Pressure that must be exceeded before ejection begins
• Pressure at semilunar valves of large arteries

Control of Heart Rate

• The cardiovascular center of the medulla oblongata controls heart rate
• Sensory inputs are:
  • Movement as monitored by proprioceptors increase input to cardiovascular center
  • Chemical changes in the blood monitored by chemoreceptors
  • Blood pressure changes, monitored by baroreceptors
• Sympathetic Effect:
  • Cardiac accelerator nerves that release norepinephrine (NOR) and bind to beta 1 receptors
  • Increases spontaneous firing of SA & AV nodes
  • Increases Ca++ to contractile fibers
• Parasympathetic Effect:
  • Using the vagus nerve
  • Release of acetylcholine causes hyperpolarization (open K+ channels)
  • Slows spontaneous depolarization of intrinsic fibers
  • PNS activation may be persistent in some grief and depression conditions

Chemical Regulation of Heart Rate

1. Hormonal effects -Epinephrine (EPI) & norepinephrine (NOR), and thyroid hormones all increase heart activity
2. Cations

• Sodium (Na+)
• High levels block Ca2+ inflow
• Potassium (K+)
  • High levels block action potential generation
  • Calcium (Ca2+)
  • High blood levels increase heart rate and activity
  • Low levels depress heart activity

Other Factors Influencing Heart Rate

• Age
• Gender
• Physical fitness
• Bradycardia may be exhibited, strong effective slow beats under 60bpm
• Body temperature
• Increased temperature increases rate
• Decreased temperature decreases rate