Heart Anatomy: Circuits and Coverings

Heart Anatomy: Pulmonary and Systemic Circuits

• The heart is a transport system with two side-by-side pumps.
• The right side receives oxygen-poor blood and pumps it to the lungs via the pulmonary circuit to get rid of CO2 and pick up O2.
• The left side receives oxygenated blood from the lungs and pumps it to the body tissues via the systemic circuit.

Size, Location, and Orientation

• The adult heart is approximately the size of a fist.
• The adult heart weighs less than 1 pound.
• Positioned in the mediastinum between the second rib and fifth intercostal space on the superior surface of the diaphragm.
• Two-thirds of the heart is located to the left of the midsternal line.
• The heart sits anterior to the vertebral column and posterior to the sternum.

Coverings of the Heart

• The pericardium is a double-walled sac that surrounds the heart.
• The superficial fibrous pericardium protects, anchors the heart, and prevents overfilling.
• The serous pericardium is a deep, two-layered structure.
• The parietal layer lines the internal surface of the fibrous pericardium.
• The visceral layer (epicardium) is on the external surface of the heart.
• The pericardial cavity, filled with fluid, separates the two layers and reduces friction.

Layers of the Heart Wall

• The heart wall consists of three layers: epicardium, myocardium, and endocardium.
• The epicardium is the visceral layer of the serous pericardium.
• The myocardium is made of circular or spiral bundles of contractile cardiac muscle cells.
• The endocardium is the innermost layer, continuous with the endothelial lining of blood vessels, and covers the heart chambers and valves.

Chambers and Associated Great Vessels

• The internal heart features include four chambers: two superior atria and two inferior ventricles.
• The interatrial septum separates the atria.
• The interventricular septum separates the ventricles.
• The fossa ovalis is a remnant of the foramen ovale from the fetal heart.

Heart Valves

• Heart valves ensure unidirectional blood flow.
• They open and close in response to pressure changes.
• The two major types of valves atrioventricular and semilunar.
• Atrioventricular valves are located between the atria and ventricles.
• Semilunar valves are located between the ventricles and major arteries.

Pathway of Blood Through the Heart

• Right side: Superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus to right atrium to tricuspid valve to right ventricle to the pulmonary semilunar valve to pulmonary trunk to pulmonary arteries to lungs.
• Left side: From the lungs, four pulmonary veins to left atrium to mitral valve to left ventricle to aortic semilunar valve to aorta to systemic circulation.
• Equal volumes of blood are pumped to the pulmonary and systemic circuits.
• The pulmonary circuit is short and low-pressure, while the systemic circuit is long and high-friction.

Ventricle Anatomy

• The anatomy of ventricles reflects differences in circuits.
• The left ventricle walls are 3x thicker than the right ventricle.
• The left ventricle pumps with greater pressure.

Coronary Circulation

• The coronary circulation provides the functional blood supply to the heart muscle itself.
• It is the shortest circulation in the body and is delivered when the heart is relaxed.
• The left ventricle specifically receives most of the coronary blood supply.
• Both the left and right coronary arteries arise from the base of the aorta and supply arterial blood to the heart, encircling it in the coronary sulcus.
• Arteries contain many anastomoses (junctions) that provide additional routes for blood delivery but cannot compensate for coronary artery occlusion.

Coronary Arteries

• The left coronary artery supplies the interventricular septum, anterior ventricular walls, left atrium, and posterior wall of the left ventricle, with two branches: the anterior interventricular artery and the circumflex artery.
• The right coronary artery supplies the right atrium and most of the right ventricle, with two branches: the right marginal artery and the posterior interventricular artery.

Coronary Veins

• Cardiac veins collect blood from capillary beds.
• The coronary sinus empties into the right atrium and is formed by merging cardiac veins.

Clinical - Homeostatic Imbalance

• Myocardial infarction is a heart attack resulting from prolonged coronary blockage.
• Areas of cell death are repaired with noncontractile scar tissue.

Microscopic Anatomy

• Intercalated discs connect cardiac cells and contain desmosomes to hold cells together during contraction and gap junctions.
• Gap junctions allow ions to pass from cell to cell, electrically coupling adjacent cells which allows the heart to function as a functional syncytium to act as a coordinated unit.

Electrical Events

• The heart depolarizes and contracts without nervous system stimulation, with rhythm altered by the autonomic nervous system.
• Heartbeat coordination relies on gap junctions and the intrinsic cardiac conduction system.
• The intrinsic cardiac conduction system is a network of noncontractile cells that initiate and distribute impulses to coordinate heart depolarization and contraction.

Modifying Heart Rhythm Innervation

• Heartbeat is modified by the ANS via cardiac centers in the medulla oblongata.
• Cardioacceleratory center sends signals through sympathetic trunk to increase rate and force, stimulating SA and AV nodes, heart muscle, and coronary arteries.
• Cardioinhibitory center uses parasympathetic signals via the vagus nerve to decrease rate, inhibiting SA and AV nodes.

Mechanical Events

• Systole is the period of heart contraction.
• Diastole is the period of heart relaxation.
• The cardiac cycle is blood flow through the heart during one complete heartbeat: atrial systole and diastole, followed by ventricular systole and diastole, representing pressure and blood volume changes with mechanical events following electrical events seen on the ECG.

Heart Sounds

• Two heart sounds (lub-dup) are associated with the closing of heart valves.
• The first sound is from the closing of AV valves at the beginning of ventricular systole.
• the second sound is from the closing of SL valves at the beginning of ventricular diastole.
• the pause between lub-dups signify heart relaxation.
• Mitral valve closes slightly before tricuspid, and aortic closes slightly before pulmonary valve, allowing auscultation of each valve when a stethoscope its placed in four different regions.
• Heart murmurs are abnormal heart sounds which indicate valve problems.

Types of Valve Problems

• Incompetent valve fails to close completely, allowing backflow of blood, causing a swishing sound as blood regurgitates backward from ventricle into atria
• Stenotic valve fails to open completely, restricting blood flow through a valve, causing a high-pitched sound as clotted blood is forced through a narrow valve.

Regulation of Pumping: Cardiac Output

• Cardiac output is the amount of blood pumped out by each ventricle in 1 minute.
• Cardiac output equals heart rate (HR) times stroke volume (SV).
• Stroke volume is the volume of blood pumped out by one ventricle in one beat.
• At rest, cardiac output average 5.25 L/min.

Regulation of Stroke Volume

• Stroke volume is mathematically equal to EDV (end diastolic volume) - ESV(end systolic volume).
• Normal SV is 70 ml/beat.
• Three things that affect SV: Preload, Contractility and Afterload

Preload: Stretching Heart Muscle

• Preload is degree to which cardiac muscle cells are stretched just before they contract.
• The most important factor in preload stretching of cardiac muscle is venous return , amount of blood returning to heart.
• Slow heartbeat and exercise increase venous return.
• Contractility correlates with contractile strength at given muscle length.

Afterload

• Afterload is the back pressure exerted by arterial blood and is the pressure that ventricles must overcome to eject blood.
• Back pressure from arterial blood pushing on SL valves is major pressure.
• Aortic pressure is around 80 mm Hg, and pulmonary trunk pressure is around 10 mm Hg.
• Hypertension increases afterload, resulting in increased ESV(Edn Systolic Volume) and reduced SV(stroke volume).

Regulation of Heart Rate

• If SV decreases, CO can be maintained by increasing HR.
• Heart rate is regulated by the autonomic nervous system, chemicals, and other factors.

Regulation of Heart Rate: Autonomic Nervous System

• The sympathetic nervous system can be activated by emotional or physical stressors releasing Norepinephrine which binds causing: increased pacemaker activity and HR and increased contractility.
• Parasympathetic nervous system opposes sympathetic effects by acetylcholine hyperpolarizing pacemaker cells by opening K+ channels, which slows HR and has little to no effect on contractility, causing heart at rest to exhibits vagal tone.
• If the parasympathetic is dominant influence on heart rate, it decreases rate about 25 beats/min. Cutting the vagal nerve leads to HR of ~100

Regulation of Heart Rate: Chemicals and Hormones

• Epinephrine from adrenal medulla increases heart rate and contractility.
• Thyroxine increases heart rate and enhances effects of norepinephrine and epinephrine.
• Intra- and extracellular ion concentrations must be maintained for normal heart function.
• Age, gender, exercise, and body temperature also influence heart rate.

Heart rate abnormalities

• Tachycardia is an abnormally fast heart rate > 100 beats/min, which if persistent, may lead to fibrillation.
• Bradycardia is a heart rate slower than 60 beats/min and may result in grossly inadequate blood circulation in nonathletes, but may be desirable result of endurance training.

Congestive Heart Failure

• Congestive heart failure (CHF) is a progressive condition with a low cardiac output that is so low that blood circulation is inadequate to meet body requirements, reflecting weakened myocardium, often caused by: coronary atherosclerosis, persistent high blood pressure >90mmHg, chronic increased end systolic volume or Multiple myocardial infarcts,
• Either side of heart can be affected:
• Left-sided failure results in pulmonary congestion- Blood backs up in lungs
• Right-sided failure results in peripheral congestion causing blood pools in body organs, causing edema.
• Failure of either side ultimately weakens other side