Heart Structure and Blood Flow

Overview

• Historically, the heart was considered the chamber for the soul and the site of emotion
• The mechanical function of the heart is to consume oxygen/nutrients and release carbon dioxide/waste
• A circulatory system is required in multicellular organisms to efficiently deliver oxygen and nutrients, while removing waste
• All cells are in close proximity to a capillary network to facilitate the exchange
• The heart's structure and function ensure a one-way flow of blood

Blood Flow

• Blood flows in a closed continuous loop through two circuits
• The pulmonary circuit supplies vessels to the alveoli (lungs)
• In the pulmonary circuit, blood absorbs O2 and eliminates CO2
• The right chambers of the heart drive the pulmonary circuit
• The systemic circuit includes all other tissues of the body
• In the systemic circuit oxygen/nutrients are delivered, and carbon dioxide/waste is picked-up
• The left chambers of the heart drive the systemic circuit

Location

• The heart is located in the mediastinum within the thoracic cavity
• It's positioned anterior to the vertebral column, posterior to the sternum, and medial to the lungs
• The superior mediastinum houses great vessels like the aorta, pulmonary vessels, and superior vena cava
• The apex of the heart points anteroinferiorly
• The heart is anchored superiorly and to the left on the posterior wall, allowing greater movement during contraction

Auscultation Points

• Auscultation points on the chest are where valve function can be assessed due to vessel reverberations after valve closure
• The aortic valve (A) is best heard at the right second intercostal space at the upper right sternal border
• The pulmonary valve (P) is best heard at the left second intercostal space at the upper left sternal border
• The tricuspid valve (T) is best heard at the left fourth/fifth intercostal spaces at the lower left sternal border
• The mitral valve (M) is best heard at the left fifth intercostal space at the left midclavicular line
• Erb's point (E; A+P) is best heard at the left third intercostal space at the left sternal border

Layers of the Heart

• The heart is surrounded by three layers of connective tissue
• The fibrous pericardium is the outermost layer, made of dense irregular connective tissue
• It separates the mediastinum from the pleural cavities and prevents overfilling of the heart
• The serous pericardium is the inner layer, made of serous connective tissue
• The serous pericardium folds onto itself, creating two layers: the parietal and visceral pericardium
• The parietal pericardium is in contact with the fibrous pericardium
• The visceral pericardium, also called the epicardium, is in contact with the heart
• The space between serous layers contains a small volume of serous fluid, reducing friction during contraction

Heart Wall Tissues

• The epicardium is also known as the visceral pericardium
• It has a smooth surface that limits friction
• The myocardium is the muscular layer of the heart
• It contains interconnected cardiac muscle fibers ensuring a coordinated contraction pattern for effective blood pump
• The myocardium requires the most energy
• The endocardium is made of simple squamous epithelium, similar to blood vessels
• Endocarditis is the inflammation of the endocardium, particularly around the valves
• It correlates with IV drug use and tongue piercing, and may result in permanent scarring or death
• The fibrous skeleton is a connective tissue framework that provides support for valves and electrical insulation between atria and ventricles

Heart Chambers

• The heart is divided into right and left sides
• The right side receives blood from the systemic circulation and sends it to the pulmonary circulation
• The left side receives blood from the pulmonary circulation and sends it to the systemic circulation
• Arteries carry blood away from the heart, while veins carry blood toward the heart

Atria and Ventricles

• Each side of the heart contains an atrium and a ventricle
• The atria are smaller and less muscular chambers that serves as a "primer"
• Atria contracts to maximally fill the ventricle (final 20%)
• The atria are separated by the interatrial septum
• The ventricles are larger, more muscular chambers
• Ventricles contracts to drive blood back into the systemic and pulmonary circuits
• The left ventricle has a much larger wall than the right ventricle
• This is due to greater resistance in the systemic circuit
• Ventricles are separated by the interventricular septum

Right Atrium

• The right atrium receives blood from the entire systemic circulation for passage into the right ventricle through three vessels
• The superior vena cava drains blood from the head, thoracic cage, and upper appendages
• The inferior vena cava drains blood from the abdomen, pelvis, and lower appendages
• The coronary sinus drains blood from the coronary (heart) circulation
• The auricle expands to accommodate venous return as blood fills the right atrium
• Contraction occurs along the anterior wall of the right atrium, which contains pectinate muscle
• The crista terminalis is the margin between the muscular anterior wall and smooth posterior wall
• The right atrium is separated from the left atrium by the interatrial septum
• The fossa ovalis is a depression within the septum, representing the embryological remnant of the foramen ovale

Right Ventricle

• The right ventricle contracts to deliver blood to the pulmonary circulation
• The dense muscular wall is made up of trabeculae carnae, continuous with three prominent muscular projections called papillary muscles
• Papillary muscles attach to valvular cusps of the tricuspid valve through chordae tendinae
• Chordae tendinae reinforces the closed valve during ventricular contraction
• During ventricular contraction, blood is ejected into the pulmonary trunk through the semilunar pulmonary valve
• Semilunar pulmonary valve prevents backflow of blood during ventricular relaxation
• The right ventricle is separated from the left ventricle by the thick interventricular septum

Left Atrium and Ventricle

• The left atrium receives blood from pulmonary circulation for passage into the left ventricle
• Blood enters through two pairs of pulmonary veins
• The wall lining is generally smooth, except for some pectinate muscle in the left auricle
• The left ventricle contracts to deliver blood to systemic circulation
• It is similar in structure to the right ventricle, but with much thicker walls and a rounded appearance
• It has two papillary muscles instead of three, which attach to the bicuspid (mitral) valve
• During contraction it ejects blood into the aorta through the semilunar aortic valve

Heart Valves

• A set of four valves ensure one-way blood flow through the circuit
• Cuspid valves separate atria from ventricles and are made of connective tissue flaps
• The tricuspid valve has three cusps on the right side
• The bicuspid (mitral) valve has two cusps on the left side
• Cusps are secured to the ventricular chamber wall through chordae tendinae, which are anchored to papillary muscles
• Cusps act as a parachute, containing blood during ventricular contraction
• Stretch/weakness in chordae tendinae/papillary muscles can result in valve prolapse (heart murmur), common on the left side (mitral valve prolapse)
• Semilunar valves separate ventricles from the great vessels (aorta/pulmonary trunk)
• Each semilunar valve is composed of three "pocket cusps"

Semilunar Valves

• Semilunar valves flatten against the wall as ventricles contract, allowing blood to flow into great vessels
• They fill with blood and bulge into the lumen as ventricles relax, causing blood to start back flow

Conduction System

• The heart exhibits autorhythmicity, where cells spontaneously depolarize without external influence
• The sinoatrial (SA) node (pacemaker) is the region of the heart where cells depolarize most rapidly
• Functional syncytium results in a coordinated spread of depolarization/contraction throughout the atria
• The fibrous skeleton ensures the spread of depolarization does not continue inferiorly to ventricles
• The atrioventricular (AV) node is a small node of cells embedded in the fibrous skeleton
• Depolarization spreads through the atria, triggering the AV node
• Depolarization travels down the interventricular septum through tracts (bundles of His)
• The tracts terminate in the ventricular myocardium around the apex of the heart (Purkinje fibers)
• Depolarization spreads from inferior to superior, allowing coordinated ventricular contraction

Cardiac Cycle

• The cardiac cycle is a repeating, predictable cycle of blood circulation through the heart and body
• The right side of the heart pumps deoxygenated blood to the lungs
• The left side pumps oxygenated blood to the body
• Blood moves from areas of high pressure to low pressure, maintained by contraction/relaxation
• There are 1-way valves involved
• The cycle can be broken down into ventricular filling, atrial contraction, isovolumetric ventricular contraction, ventricular ejection, and isovolumetric ventricular relaxation

Ventricular Filling

• Ventricular filling occurs during diastole (heart relaxation)
• Pressure in veins and atria is less than the pressure in the ventricles
• Blood flows freely from atria into ventricles
• Approximately 80% of ventricular filling occurs during this phase

Atrial Contraction and Isovolumetric Ventricular Contraction

• Atrial muscle contracts, squeezing the remaining 20% of blood volume into the ventricles
• Ventricles begin to contract as atria relax during isovolumetric ventricular contraction
• AV-valves snap shut, producing the "lub" sound
• Semilunar valves remain closed because the pressure in the aorta and pulmonary trunk is still higher
• Interventricular pressure rises

Ventricular Ejection

• Ventricular pressure exceeds the pressure in the aorta and pulmonary trunk
• Semilunar valves are forced open
• Blood is ejected into systemic/pulmonary systems

Isovolumetric Ventricular Relaxation, Return to Ventricular Filling

• As ventricles start to relax, pressure falls below the pressure in the aorta and pulmonary trunk
• Semilunar valves snap shut, producing the "dub" sound
• AV-valves remain closed because pressure in the ventricles remains higher than pressure in the atria
• Cycle repeats allowing return to ventricular filling

Fetal Circulation

• Fetal circulation modifies blood flow, due to the physiology of fetal development
• Fetal lungs are not functional and are filled with amniotic fluid
• Oxygen is supplied by the placenta
• Umbilical arteries travel to the placenta from arteries in the pelvis
• Oxygenated blood returns through the umbilical vein, which fuses with the inferior vena cava
• Oxygenated blood mixes with deoxygenated blood

Fetal Bypasses

• There are two bypasses in place during fetal circulation
• The foramen ovale is a hole in the interatrial septum that allows blood to bypass ventricles
• Tends to favors passage of oxygenated blood from the IVC, which as a greater oxygen concentration for arteries to brain
• The foramen ovale fuses at birth to form the fossa ovale
• Incomplete fusion results in a “hole in heart”
• Results in mixing of oxygenated, deoxygenated blood puts greater stress on the heart; requires surgical repair
• The ductus arteriosus is a channel between the pulmonary trunk and aorta
• Allows blood from the right ventricle to avoid pulmonary circulation
• less oxygen-rich blood flows to other areas of the body
• Fuses at birth to form the ligamentum arteriosum

Coronary Circulation

• The heart requires its own blood supply of oxygen nutrients
• Oxygen is acquired through the right and left coronary arteries, which are the first branches off the aorta
• The flow is unique because it is located just above the aortic valve
• Receive little blood during ventricular contraction (blocked by semilunar valve flaps)
• As ventricles relax, blood is pushed against the valve and flows into coronary arteries
• Two holes can be thought of at the bottom of a drinking glass

Right and Left Coronary Arteries

• The right coronary artery supplies the right chambers and the posterior surface Marginal artery lies between the anterior/posterior surface of the right ventricle
• The posterior interventricular artery wraps around the back and runs along the interventricular septum
• The left coronary artery supplies the left chambers and the anterior surface
• The anterior interventicular artery runs down the anterior surface along the interventricular septum
• The left circumflex artery wraps around the left atrioventricular septum, supplying the posterior surface of the left ventricle
• Blood collects in veins, merging to form great and middle cardiac veins, flowing through coronary sinus

Veins

• All blood collects in the coronary sinus along the posterior atrioventricular septum before returning to the right atrium

Myocardial Infarction

• The endothelium of blood vessels is constantly subjected to microtrauma due to blood pressure fluctuations (systole vs. diastole)
• Fats and cholesterol tend to congeal at microtears, resulting in fatty streaks following healing
• Over time, streaks increase in size, forming plaques
• If vessel occlusion is greater than 70%, deficient blood flow (ischemia) results
• Coronary vessels are particularly vulnerable due to small size and close proximity to the heart

Ischemia

• Ischemia results in a characteristic pain called angina, an early sign of coronary artery disease (CAD)
• With greater or complete blocks in the coronary vessels, total ischemia can results in death of the heart wall, resulting in myocardial infarction (MI; i.e., heart attack)
• The extent of damage depends on the size and region of the wall affected
• Mild heart attacks affect a small region of the wall and cause minimal loss of function

Severe Heart Attacks

• Even mild heart attacks adds a greater workload placed on the surviving heart muscle, which may lead to further disease and a greater risk for more MIs
• Severe heart attacks affect larger regions of the wall; surviving tissue is unable to compensate for the loss
• Ineffective heartbeat results in heart failure or death