Introduction to the Heart

Introduction to the Heart

• The heart is essential for continuous blood circulation.
• If blood stops, nutrients and oxygen deplete while waste accumulates.
• The heart beats around 100,000 times daily, averaging 70 beats per minute.
• Approximately 1.5 million gallons of blood are pumped yearly.
• The heart pumps about 2.9 gallons (5 to 30 liters) per minute.

Overview of the Cardiovascular System

• The heart's size approximates an adult's clenched fist.
• It has four chambers: two atria and two ventricles.
• The heart pumps blood through two circuits: pulmonary and systemic.

Blood Vessels

• Arteries transport blood away from the heart.
• Veins transport blood toward the heart.
• Capillaries are vessels that connect arteries and veins.

The Pericardium

• The pericardium surrounds the heart and has two parts: the outer fibrous and the inner serous pericardium.
• The serous pericardium has a visceral layer (epicardium) attached to the heart's surface.
• The serous pericardium also has an outer parietal layer that borders the fibrous pericardium.
• The pericardial cavity lies between the serous layers and contains pericardial fluid.
• The pericardial fluid reduces friction by lubricating the heart's surfaces.

Structure of the Heart Wall

• The heart walls have three layers: epicardium, myocardium, and endocardium.
• The epicardium is the external surface and equals the visceral pericardium.
• The myocardium contains cardiac tissue, including muscle cells, connective tissue, blood vessels, and nerves.
• The endocardium is the internal, endothelial surface of the heart.

Cardiac Muscle Tissue Characteristics

• Cardiac muscle tissue features a striated appearance.
• It relies on aerobic respiration and has numerous mitochondria and myoglobin.
• An extensive circulatory supply supports cardiac muscle tissue.
• Cardiac muscle cells contract involuntarily without CNS input.
• Cardiac muscle cells connect via intercalated discs.

Intercalated Discs

• Cardiac cells use specialized cell-to-cell junctions
• Desmosomes bind together the plasma membranes of adjacent cardiac cells
• Intercalated discs bind adjacent cells' myofibrils.
• Gap junctions connect heart muscle cells, which facilitates direct ion movement.
• Functionally, all cardiac cells contract as one unit forming a functional syncytium.

Cardiac Skeleton

• An elastic sheath wraps each cardiac cell.
• A fibrous sheet wraps each muscle layer.
• Fibrous sheets separate deep and superficial muscle layers.
• These sheets also encircle the base of the pulmonary trunk, ascending aorta, and valves.
• The cardiac skeleton stabilizes cell and valve positions.
• It supports blood vessels and nerves, distributes contraction forces, and prevents overexpansion.
• The cardiac skeleton also provides elasticity for recoil and isolates atrial cells from ventricular cells.

Orientation and Superficial Anatomy

• The heart is situated slightly left of the midsagittal plane inside the mediastinum.
• The base is the superior heart border, and the apex is the inferior portion.
• The right atrium forms the right border; the right ventricle creates the inferior border.
• The anterior surface includes the right atrium, right ventricle, and left ventricle.
• The posterior surface consists of the left atrium and a small part of the right atrium.
• The diaphragmatic surface features the right and left ventricles.
• Sulci (grooves) on the heart's external surface identify the four chambers.
• The interatrial groove separates the left and right atria.
• The coronary sulcus separates the atria from the ventricles.
• The anterior and posterior interventricular sulci separate the left and right ventricles.
• The left and right atria are positioned above the coronary sulcus and have thin walls.
• Both atria contain an expandable auricle.
• The left and right ventricles are positioned below the coronary sulcus, and have thicker walls.
• The wall of the left ventricle is thicker compared to the right.

Internal Anatomy and Organization

• A frontal heart section reveals the interatrial septum divides the left and right atria.
• The interventricular septum divides the left and right ventricles.
• The endocardium forms the atrioventricular valves.
• The atrioventricular valves are between the atria and the ventricles.

Right Atrium

• Oxygen-poor venous blood flows in through the superior vena cava, inferior vena cava, and coronary sinus.
• The posterior side of the right atrium is where the coronary sinus enters.
• The anterior wall and auricle contain pectinate muscles.
• The fossa ovalis, a fetal remnant of the foramen ovale that allowed fetal blood to bypass the lungs, is in the interatrial septum.

Right Ventricle

• Oxygen-poor blood from the right atrium, enters through the right atrioventricular valve (tricuspid).
• Blood exits the right ventricle through the pulmonary semilunar valve.
• The pulmonary trunk is connected to the pulmonary valve, which then branches into the right and left pulmonary arteries.
• Via chordae tendineae, the right AV valve connects to papillary muscles .
• Three cusps or fibrous flaps and three associated papillary muscles are present.
• Each cusp attaches to a papillary muscle via chordae tendineae.
• Papillary muscles and chordae tendineae prevent valve inversion during ventricular contraction.
• The internal surface of the right ventricle is contains of trabeculae carneae (muscular ridges) and the moderator band.
• The moderator band is exclusive to the right ventricle, that extends from the interventricular septum to the ventricular wall, preventing overexpansion.

Left Atrium

• Oxygenated blood enters from the lungs via the right and left pulmonary veins
• the pectinate muscles are restricted to auricle
• Blood passes through the left atrioventricular valve, which is known as bicuspid or mitral valve

Left Ventricle

• Has the thickest wall needed for strong contractions of the entire systematic circuit.
• The right ventricle is has a thinner wall since it only pumps blood via the pulmonary circuit.
• A moderator band is absent.
• Prominent trabeculae carneae is present.
• The left AV valve has chordae tendineae connecting the two cusps and two papillary muscles.
• Blood exits through the aortic semilunar valve, entering the ascending aorta.
• Blood moves towards the aortic arch after which, moves down the descending and into all body parts of the systematic circuit.

Structural Differences Between Ventricles

• Right ventricle: thinner wall, weaker contraction, and a moderator band.
• Left ventricle: thicker wall, stronger contractions (6-7 times more powerful).

Heart Valves

• The heart has four heart valves: two AV valves (tricuspid and bicuspid), and two semilunar heart valves (aortic and pulmonary).

AV Valves Structure

• Each AV valve has a connective tissue ring; that connects to both, the heart tissue, and the fibrous skeleton of the heart.
• It has cusps, chordae tendineae (connecting the cusps to the papillary muscles, and papillary muscles (preventing AV valve inversion).

AV Valve Function During the Cardiac Cycle

• Papillary muscles relax as pressure in the atria causes the AV valves to open and allowing blood to flow from atria to ventricle
• When ventricles contract, pressure closes the AV valves and opens up the semilunar ones.
• Closure of AV valves inhibits regurgitation (backflow) into the atria, moving blood through the semilunar valve.

Coronary Blood Vessels

• Originate at base of ascending aorta and supply cardiac muscle tissue via coronary circulation.
• Major coronary arteries: Right coronary artery (RCA) and Left coronary artery (LCA).
• RCA's branches include: atrial, marginal branches, posterior interventricular, and conducting system branches.
• LCA's branches include: circumflex, left marginal branch, and anterior interventricular branch.
• The RCA passes between the right auricle and pulmonary trunk.

Left Coronary Artery Branches

• The anterior interventricular branch has branches that lead to the posterior interventricular branch.
• The circumflex branch forms the left marginal and the posterior left ventricular branches.
• Cardiac venous blood drains into the right atrium via the coronary veins.
• Main coronary and cardiac veins: great, middle, posterior of left ventricle, small, and anterior.
• The great and middle cardiac veins deliver blood to the coronary sinus.
• The coronary sinus drains directly into the posterior right atrium.
• The posterior vein parallels the left ventricular branch, and the small cardiac vein parallels the right coronary artery.
• The anterior cardiac veins are branches that extend from the right ventricle cardiac cells.

Coordination of Cardiac Contractions

• The cardiac cycle alternates between contraction (systole) and relaxation (diastole) periods.
• Atrial systole: blood passes into ventricles.
• Ventricular systole: blood is ejected into the pulmonary trunk and ascending aorta.
• During Diastole: heart chambers fill with blood.
• Conducting cells coordinate cardiac contractions, those are either nodal or conducting cells.
• Nodal cells: sinoatrial (SA) and atrioventricular (AV) nodes, that establish the rate of contractions as ell membranes automatically depolarize (autorhythmic).
• Conducting cells: distribute the contractile stimulus to the myocardium.

Sinoatrial (SA) Node

• The Sinoatrial (SA) node is situated in the posterior wall of the right atrium near the entrance of the superior vena cava, which is also known as the cardiac pacemaker.
• Pacemaker cells in the SA node generate 80–100 action potentials per minute resulting in Bradycardia (slower heart rate), and Tachycardia (faster heart rate).
• The Atrioventricular node (AV node) sits within the floor of the right atrium, conducting the impulse from the SA node to the AV node via internodal pathways.
• The AV node slows the impulse so it can travel from the AV node, to the AV bundle, then along the interventricular septum, dividing into the right and left bundle branches, which serve the right and left ventricle.
• The bundle branches conduct impulses to the Purkinje fibers: which in turn connect to cardiac muscle cells, causing ventricular contraction.

Autonomic Control of Heart Rate

• Despite the SA node controlling HR, the SA node may be altered.
• Impulses from the autonomic nervous system (ANS) modify pacemaker activity as nerves associated with the ANS innervate: SA node, AV node, cardiac cells, and smooth muscles in cardiac blood vessels.
• Norepinephrine (sympathetic division) raises heart rate/contraction force.
• Acetylcholine (parasympathetic) lowers heart rate/contraction force.
• Cardiac centers in the medulla oblongata can modify HR through cardioacceleratory center (sympathetic neurons, speed up HR).
• In addition, they can can conduct cardioinhibitory centers (parasympathetic neurons, Vagus [N X] is involved which slow down HR).