Anatomy of the Heart

• The heart is a four-chambered pump that provides much of the force required for blood flow.

Anatomy of the Heart

• The heart is located within the mediastinum of the thoracic cavity, specifically within the pericardial cavity.

Pericardium and Layers of the Heart Wall

• The heart is surrounded by the pericardium, a serous membrane that protects it and reduces friction during beats.
• The layers of the heart wall, from superficial to deep, are:
  • Fibrous pericardium: anchors the heart to the diaphragm.
  • Parietal layer of serous pericardium
  • Pericardial cavity
  • Visceral layer of serous pericardium (epicardium)
  • Myocardium: the thickest layer, containing cardiac muscle.
  • Endocardium: a layer of simple squamous epithelium.

Chambers of the Heart

• The two superior chambers are the atria, which receive blood returning to the heart.
• The two inferior chambers are the ventricles, which pump blood out of the heart.
• Each heart has a left and right atrium, plus a left and right ventricle.

Surface Features of the Heart

• Base: the broad, posterior margin of the heart where several great vessels are located.
• Apex: the narrow, inferior aspect of the heart.
• Sulci: grooves on the surface that mark the separations between the four chambers.
  • Coronary sulcus: marks the separation between the atria and the ventricles below.
  • Anterior interventricular sulcus: a groove on the front marking the separation between the left and right ventricles.
  • Posterior interventricular sulcus: a groove on the back marking the separation between the left and right ventricles.
  • The sulci contain different blood vessels.

Atrial Features

• Auricles are ear-like flaps on top of the atria that allow them to receive additional amounts of blood.
• Interatrial septum: the wall between the two atria.
  • It contains the fossa ovalis, a remnant of the foramen ovale from fetal life.
  • The foramen ovale allowed blood to move directly from the right atrium to the left during fetal life, bypassing the lungs.
• Pectinate muscle: the name for the pattern of muscle seen within the walls of the atria.

Ventricular Features

• Interventricular septum: a partition between the ventricles that becomes more muscular inferiorly.
• Trabeculae carneae: the name for the pattern of muscle within the ventricular walls, organized into large bundles.
• Papillary muscle: larger bundles of muscle protruding from the ventricular wall, attached to chordae tendinae.
• Chordae tendinae: tendon-like chords anchoring the cusps of the atrioventricular (AV) valves in place when they close.

Great Vessels of the Heart

• The great vessels directly allow blood to enter or exit the heart.
• The superior vena cava, inferior vena cava, and coronary sinus all allow blood to enter the right atrium.
• The pulmonary trunk emerges from the right ventricle and quickly divides into pulmonary arteries.
• The pulmonary veins return blood to the heart from the lungs.
• The aorta allows blood to leave the left ventricle.

Myocardial Thickness

• A chamber of the heart will have muscle depending on the amount of work it undertakes.
• The atria have the thinnest walls/myocardium, while the ventricles have more muscle.
• The left ventricle has the thickest wall/myocardium, followed by the right ventricle, then the atria.

Heart Valves and Circulation of Blood

• Blood circulation should proceed in one direction, entering the heart on either side into the atrium, then moving into the ventricle, and exiting into the great vessel.
• Backflow can lead to congestive heart failure if it becomes progressively worse.
• Valves are located at 4 specific openings to prevent backflow.
• Atrioventricular (AV) valves are located between the atria and ventricles.
  • They are made of cusps of connective tissue.
  • The left AV valve is the bicuspid (mitral) valve with two cusps.
  • The right AV valve is the tricuspid valve with three cusps.
  • Valves open and close in response to pressure changes.
• Blood moves from areas of higher to lower pressure.
• As blood flows from the atria to the ventricle, it pushes open the valve.
• As the ventricles contract, their pressure rises, and blood attempts to move back toward the atria.
• The cusps of the AV valves fill with blood, blocking the opening and preventing backflow.
• Papillary muscles pull on the chordae tendinae to hold the cusps in place.
• Semilunar (SL) valves are found at the exit points of the heart and are named for the great vessel they allow blood to enter.
  • The pulmonary valve is on the right side, and the aortic valve is on the left side.
  • They also open and close in response to pressure changes.
  • SL valves do not have chordae tendinae or papillary muscles.

Systemic and Pulmonary Circulations

• Two major circuits exist: systemic and pulmonary.
• Systemic circuits send oxygenated blood from the left ventricle to all organs and return deoxygenated blood to the right atrium.
• Pulmonary circuit sends deoxygenated blood from the right ventricle to the lungs for reoxygenation and returns oxygenated blood to the left atrium.

Path of Blood Flow Through the Heart

• The superior vena cava, inferior vena cava, and coronary sinus return deoxygenated blood to the right atrium.
• From the right atrium, deoxygenated blood passes through the tricuspid valve and enters the right ventricle.
• The right ventricle ejects deoxygenated blood over the pulmonary valve into the pulmonary trunk, which divides into pulmonary arteries carrying deoxygenated blood to the lungs for reoxygenation.
• The pulmonary veins return oxygenated blood from the lungs to the left atrium.
• From the left atrium, oxygenated blood passes over the bicuspid valve into the left ventricle.
• The left ventricle ejects oxygenated blood over the aortic valve into the aorta.

Coronary Circulation

• The left and right coronary arteries branch directly off the aorta.
• Branches of the left coronary artery:
  • Anterior interventricular branch (left anterior descending): supplies blood to both ventricles.
  • Circumflex branch: "bends" in the coronary sulcus.
• Branches of the right coronary artery:
  • Posterior interventricular branch: lies within the posterior interventricular sulcus.
  • Marginal branch: runs along the right margin of the heart.
• Major venous drainage of the heart is the coronary sinus.
• Other major veins include
  • Great cardiac vein: in the anterior interventricular sulcus, drains the ventricles primarily.
  • Middle cardiac vein: in the posterior interventricular sulcus.
  • Anterior cardiac veins: seen on top of right ventricle.
  • Small cardiac veins: in coronary sulcus.
• Myocardial ischemia: partial obstruction of a coronary artery leading to hypoxic heart muscle.
  • Cells become starved for oxygen, and lactic acid causes pain (angina pectoris).
  • Muscle tissue is starved for oxygen but does not die.
  • Aspirin, nitroglycerin can be helpful.
  • May lead to angiogram and subsequent angioplasty or CABG if needed.
• Myocardial infarction (heart attack): complete obstruction of a coronary artery leading to anoxic heart muscle.
  • Also associated with angina pectoris.
  • Cardiac muscle will die when an infarct occurs.

Cardiac Muscle Tissue and the Cardiac Conduction System

• Cardiac muscle tissue is found in the walls of the heart.
  • Has a "branching" shape.
  • Is under involuntary control.
  • Is striated.
  • Fibers are connected by intercalated discs.
  • Has larger and more numerous mitochondria.
• The heart is autorhythmic, setting its own rhythm.
• Specialized cardiac muscle cells generate and conduct AP's within the heart; known as the cardiac conduction system.
  • Sinoatrial (SA) node: located in the wall of the right atrium; cells here spontaneously depolarize; known as the "pacemaker".
  • Atrioventricular (AV) node: located at the bottom of the interatrial septum; known as the "gatekeeper"; slows down the movement of a cardiac AP.
  • Atrioventricular (AV) Bundle (Bundle of His): located in the interventricular septum.
  • Right and left bundle branches: located in the interventricular septum, move AP toward apex of heart.
  • Purkinje fibers: located in outer (lateral) walls of ventricles.

Cardiac Action Potential and Contraction

• Depolarization: contractile cardiac fibers have stable resting membrane potential (-90mV).
  • Voltage-gated fast Na+ channels open, and sodium enters cell.
  • The inside of the cell becomes positively charged.
• Plateau: a period of sustained depolarization.
  • Voltage-gated slow Ca++ channels cause this.
  • Calcium flows into cell leading to contraction
  • The depolarization will be sustained due to voltage-gated K+ channels.
• Repolarization: must return fiber to resting membrane potential.
  • Additional voltage-gated K+ channels open, allowing for greater outflow of potassium. -The voltage-gated Ca++ channels of the sarcoplasmic reticulum close
  • The membrane potential becomes negative again, returning to -90mV.
• Refractory period: the muscle cannot contract again.
  • It is longer than the contraction, preventing tetany in cardiac muscle.
• ATP production in cardiac muscle primarily relies on aerobic respiration.
  • Oxidizes glucose and fatty acids and creatinine is used to produce ATP.
  • If cardiac muscle dies, creatinine kinase (CK) may leak into the blood.
• Electrocardiogram (ECG/EKG): a picture of the electrical activity of the heart.
  • Deflections indicate electrical activity
  • Depolarization of cardiac muscle leads to systole (contraction) of the muscle.
  • Repolarization of cardiac muscle leads to diastole (relaxation) of the muscle.
  • At time zero, the SA node will depolarize. As depolarization spreads throughout the atria, the P wave will be produced
    • P wave = atrial depolarization -The QRS complex will be seen after the cardiac AP reaches the Bundle of His, bundle branches and Purkinje fibers.
    • QRS complex = ventricular depolarization
  • Following full systole, the ventricles will repolarize. Ventricular repolarization leads to the T wave on the EKG
    • T wave = ventricular repolarization
  • P-Q interval time from atrial excitation (SA node depolarizing) until the cardiac AP reaches ventricles
  • Q-T interval time from beginning of ventricular depolarization until the end of ventricular repolarization -Enlarged P waves = atrial enlargement -Enlarged Q waves = ventricular enlargement -Prolonged P-Q interval = AV node (heart) block -S-T segment elevation = myocardial infarction

Cardiac Cycle

• Atrial contraction (0.1 sec) -P wave will be produced on the EKG as the atria depolarize -Increases atrial pressure and causes more blood to flow into the ventricles
  • The total volume of blood in each ventricle at the end of this phase is the End Diastolic Volume (EDV)
    • The amount of blood in a ventricle at the end of ventricular diastole or the end of atrial systole
  • The QRS complex will begin on the EKG as the ventricles start to depolarize
• Isovolumetric contraction (0.05 sec)
  • QRS complex will be seen on the EKG due to ventricular depolarization
  • Depolarization of the ventricles leads to ventricular systole beginning -The cusps of the AV valves will close and produces the S1 heart sound -This is called isovolumetric because ALL 4 valves of the heart are closed and there will be no change in the volume of blood
• Ventricular ejection (0.25 sec) -S-T segment seen on EKG as ventricles are in full systole and T wave begins as ventricles begin to repolarize -The pressure in the ventricles will exceed the pressure in the aorta and pulmonary trunk -The stroke volume is the volume of blood ejected from a ventricle with each beat (about 70mL)
  • The percentage of blood ejected from each ventricle is called the ejection fraction (At rest it is about 55%)
  • The volume of blood that remains in the heart is called end systolic volume (ESV)(60mL)
    • ESV is the amount of blood in a ventricle at the end of ventricular systole
• Isovolumetric relaxation (0.05 sec)
  • T wave will be seen on EKG -The pressure within the aorta and pulmonary trunk will be higher than the pressure within the ventricles
  • The cusps of the SL valves fill with blood and close and produces the S2 heart sound -This is an isovolumetric phase
• Ventricular filling (0.35 sec) -EKG should be at baseline as no electrical activity is occurring in any chambers
  • Blood will open the AV valves and begin to flow from atria to ventricle
  • The ventricles will fill primarily due to ventricular relaxation
• Heart Sounds
  • S1 – closure of AV valves
  • S2 closure of SL valves
  • S3 - turbulence during rapid ventricular filling (newborns) -S4 – turbulence during atrial systole (due to noncompliant LV, eg. Left ventricular hypertrophy)

Cardiac Output (CO)

• Cardiac output is the amount of blood ejected from the left ventricle (or right ventricle) into the aorta (or pulmonary trunk) per minute -CO = SV x HR : SV is stroke volume, HR is heart rate
• In an adult, the average cardiac output is roughly equal to that individual's entire blood volume
• Cardiac reserve is the difference between a person's maximum CO and the CO at rest (usually 4–5 times the resting CO).
• Change what is needed to be changed depending on SV or HR
• Regulation of Stroke Volume -Prelaod -It increases preload, stroke volume and CO -Venous return to the atria increases -The Frank-Starling Law will establish the relationship and place limit to it -Contractility -Contractility will increase preload, stroke volume and CO - The use of positive inotropic agents such as T3/T4, Epi, NEpi will increase release of calcium from sarcoplasmic reticulum -The use of negative inotropic agents, such as high levels of potassium and acid (acidosis) inhibit release of calcium from sarcoplasmic reticulum will decrease preload, stroke volume and CO -Afterload -This is the force that the heart must overcome to open the SL valves -For the right side, its the pressure in the pulmonary trunk and for the left it is the pressure in the aorta -There is an inverse relationship between afterload and stroke volume
• Regulation of Heart Rate -Nervous system regulation -Responsibility lies on autonomic nervous system (ANS) -Information is supplied by various receptors (inputs) such as higher cortical areas, Proprioceptors, Baroreceptors, and Chemoreceptors -Information is sent to CV center (integrating center) located in medulla oblongata of brain stem -If heart rate needs to be increased, the cardiac accelerator nerves send impulses to SA node causing heart to beat faster -If heart rate needs to slow down, the vagus nerves send impulses that slow down the rate of the SA node -Endocrine Regulation - Hormales such as T3/T4 and Epi/NEpi will increase heart rate -The positive inotropic agents wil increase heart rate -Chemical Regulation -Na+ and K+ at high levels will slow heart rate - Ca++ levels lead to a faster heart rate -Outside factors -Fitness level – slower resting HR in people who are more active due to conditioning -Age - newborns have resting HR above 100 bpm; will decline as they age -Gender - females usually slightly higher HR than males -Body temperature – higher body temp, higher HR. lower body temp, lower HR.