Anatomy of the Heart

The heart is a 4-chambered muscular pump made primarily of cardiac muscle tissue. The heart is divided into 4 chambers: 2 upper chambers for receiving blood from the great vessels, known as the right and left atria, and 2 stronger lower chambers, known as the right and left ventricles, which pump blood throughout the body. Blood flows through the heart in 1 direction, moving from the right side of the heart, through the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, and then returning to the left side of the heart, where it is pumped out to the rest of the body. As blood moves through the heart, 4 important valves prevent backflow. The heart muscle itself is supplied by the coronary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries. The heart also has its own conduction system, triggering its own rhythmic contractions.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Table of Contents

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General Structure and Location of the Heart

Overview of the heart structure

The heart is a 4-chambered muscular pump made of cardiac muscle tissue.

  • 4 primary muscular chambers:
    • Right atrium (RA)
    • Right ventricle (RV)
    • Left atrium (LA)
    • Left ventricle (LV)
  • Connections to the great vessels:
    • Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins (bring blood back to the heart):
      • Superior and inferior vena cava (deoxygenated) → RA
      • Pulmonary veins (oxygenated) → LA
    • Arteries (carry blood away):
      • Pulmonary trunk and pulmonary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries (deoxygenated) → from the RV
      • Aorta (oxygenated) → from the LV
  • Valves:
    • Located between different vessels and chambers 
    • Blood flows in 1 direction into, through, and out of the heart; valves prevent backflow.
    • Named valves (in order through which blood passes):
      • Tricuspid valve
      • Pulmonic valve
      • Mitral valve
      • Aortic valve
  • Vasculature:
    • Heart muscle itself is supplied by the coronary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries.
    • Drained by coronary veins
  • Conduction system: 
    • Electrical “wiring” that generates and allows conduction of the electrical signal throughout the heart
    • This electrical signal triggers the heart to contract.
  • Protective covering: surrounded by a tough 2-layered membrane called the pericardium 
General structure and flow of blood through the heart

General structure and flow of blood through the heart

Image by Lecturio.

Size and shape

  • About the size of a fist 
  • Shape: upside-down pyramid or cone
    • Base: superior portion of the heart, made up of the atria
    • Apex: inferior, rounded tip pointing to the left
  • Dimensions of the adult heart:
    • Width (at the base): approximately 9 cm 
    • Length (from base to apex): approximately 13 cm
    • Depth (thickest point): approximately 6 cm
  • Weight: approximately 300 g

Location and orientation

  • Located in the mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels in the thoracic cavity, between the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs
  • Sits in its own space called the pericardial cavity
  • At the level of T5–T8
  • Approximately ⅔ is to the left of the median plane
  • The heart is slightly rotated so that:
    • Right side is more anterior
    • Left side is more posterior
  • Base of the heart: at the level of the 3rd intercostal cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage
  • Apex of the heart:
    • Formed by the inferolateral part of the left ventricle
    • At the level of the left 5th intercostal space 
    • Approximately 7–9 cm to the left of the median plane
  • Primary structures forming the different surfaces of the heart: 
    • Sternocostal surface (anterior): RV 
    • Diaphragmatic surface (inferior): LV and RV
    • Left pulmonary surface (lateral): LV and LA
    • Right pulmonary surface (lateral): RA

Anatomic relationships

  • Superior (to the heart): 
    • Bifurcation of the main pulmonary trunk
    • Superior vena cava
  • Anterior (to the heart): 
    • Sternum
    • Rib cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage
  • Lateral: lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs 
  • Posterior: 
    • Great vessels (aorta, vena cava, pulmonary veins)
    • Primary bronchi
    • Esophagus
    • Vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column (T5–T8)
  • Inferior: diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm

The Pericardium

Like the pleural cavity around the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs and the peritoneal cavity inside the abdomen, the pericardial cavity around the heart is enclosed in a double layer of fibroelastic connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue known as pericardium.

Layers of the pericardium

  • Outer layer:
    • Defines outer layer of pericardial cavity
    • Made up of:
      • Fibrous pericardium: dense fibrous tissue
      • Serous parietal pericardium: thin, smooth, inner serous layer
    • Fibrous and serous pericardial layers are in direct contact with one another
    • Anchors the heart to:
      • Diaphragm below
      • Great vessels above
  • Visceral pericardium:
    • Directly covers the surface of the heart
    • Serous layer of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue made up of:
      • Simple squamous epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium 
      • Areolar tissue
    • Contains adipose tissue Adipose tissue Adipose tissue is a specialized type of connective tissue that has both structural and highly complex metabolic functions, including energy storage, glucose homeostasis, and a multitude of endocrine capabilities. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or "brite" adipose tissue, which is a transitional form. Adipose Tissue that fills in grooves on the heart surface → protects the coronary vessels
    • Also known as the epicardium (outer layer of the heart wall)
    • Continuous with the parietal pericardium at the base of the heart
  • Pericardial sac:
    • The space between the parietal and visceral layers of pericardium
    • Contains about 15–50 mL of pericardial fluid (an ultrafiltrate of plasma)
    • Clinical relevance: cardiac tamponade Cardiac tamponade Pericardial effusion is the accumulation of excess fluid in the pericardial space around the heart. The pericardium does not easily expand; thus, rapid fluid accumulation leads to increased pressure around the heart. The increase in pressure restricts cardiac filling, resulting in decreased cardiac output and cardiac tamponade. Pericardial Effusion and Cardiac Tamponade
      • The fibrous pericardium does not expand much if the pericardial fluid accumulates rapidly.
      • Rapid increases of serous or hemorrhagic fluid (as little as 80 ml) into this space will ↑ the pressure on the heart and restrict blood flow through the heart
      • Slowly progressing effusions can increase to 2 L without symptoms.
Pericardial cavity of the heart

Pericardial cavity of the heart

Image by Lecturio.

Sinuses within the pericardial cavity

There are 2 important sinuses, or spaces, within the pericardial cavity:

  • Oblique pericardial sinus: 
    • A space behind the heart, between the pulmonary veins
    • Created by the:
      • Fibrous/parietal pericardium posteriorly
      • Pulmonary veins laterally
      • Left atrium anteriorly
  • Transverse pericardial sinus:
    • A space above the heart, behind the aorta and pulmonary trunk
    • A place for surgeons to cross-clamp the aorta during cardiac surgery Cardiac surgery Cardiac surgery is the surgical management of cardiac abnormalities and of the great vessels of the thorax. In general terms, surgical intervention of the heart is performed to directly restore adequate pump function, correct inherent structural issues, and reestablish proper blood supply via the coronary circulation. Cardiac Surgery

The Heart Wall

Layers of the heart wall

  • Epicardium: 
    • Outermost layer of the heart
    • Formed by the visceral layer of the pericardium
  • Myocardium: 
    • Middle muscular layer
    • Thickest layer of the heart (by far)
    • Composed of:
      • Helical layers of cardiac muscle
      • Fibrous skeleton made up of collagen and elastic fibers
    • Purpose of fibrous skeleton:
      • Structural support (especially around valves)
      • Gives cardiac muscle cells something to pull against
      • Limits the routes of electrical excitation through the heart (electrically separates atria from ventricles)
  • Endocardium:
    • Innermost layer 
    • Forms a smooth inner lining within heart and over valves
    • Continuous with endothelium of blood vessels
    • Composed of:
      • Endothelium: simple squamous epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium
      • Subendothelial layer: loose areolar connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue below the endothelium (similar to lamina propria)
  • Subendocardial layer: 
    • Loose fibrous connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue
    • Separates the delicate endocardium from the vigorous pumping action of the myocardium

Orientation of the myocardium

  • Complex patterns
  • 1 figure eight around the atria and great vessels
  • Another figure eight around the ventricles
  • Additional superficial layers wrap around the ventricles
  • Leads to effective pumping action
Orientation of the myocardium

Orientation of the myocardium, allowing the heart to pump blood effectively

Image by Lecturio.

Heart Chambers and Valves

Cardiac chambers

The heart has 4 chambers: 2 atria (receiving chambers) and 2 ventricles (pumping chambers).

RA: 

  • Receives deoxygenated blood from the systemic circulation via:
    • Superior vena cava (SVC): drains the upper portion of the body
    • Inferior vena cava (IVC): drains the lower portion of the body
    • Coronary sinus: drains the heart muscle itself
  • After collecting the blood returning from systemic circulation, RA pumps it into the RV.
  • Right atrial appendage (auricle): 
    • Muscular sac overlying the atria, close to the ascending aorta
    • Slightly increases atrial capacity
  • Sinus venarum: 
    • Smooth, thin-walled area on the posterior portion of the RA
    • Entry point of the SVC and IVC into the RA
  • Pectinate muscles: 
    • Rough, muscular ridges of myocardium
    • Located on anterior wall and within (both right and left) auricles
  • Crista terminalis: separation between sinus venarum and pectinate muscles
  • Interatrial septum: muscular wall separating RA from LA
  • Fossa ovalis (or the oval fossa): 
    • Oval depression in interatrial septum
    • Remnant of the foramen ovale in the fetus (an opening in the interatrial septum that allows blood to move straight from the RA to the LA, bypassing the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, which are not used for oxygenation in the fetus)

LA: 

  • Collects oxygenated blood from the:
    • Left and right superior pulmonary veins
    • Left and right inferior pulmonary veins 
  • After collecting blood returning from the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, LA pumps it into the LV.
  • Walls are:
    • Primarily smooth (no pectinate muscles within the main atrial chamber)
    • Slightly thicker than the walls of the RA (due to higher pressures on the left side of the heart)
  • Left atrial appendage (auricle): similar to the right auricle
  • Valve of the foramen ovale: semilunar ridge on the interatrial septum representing the remnants of the valve of the fetal foramen ovale

RV: 

  • Receives deoxygenated blood from RA through the tricuspid valve
  • Pumps blood out through the pulmonic valve to the pulmonary trunk → lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs
  • Walls are thicker than those in the RA, but thinner than in the LV.
  • Trabeculae carneae: internal muscular ridges in the ventricles (similar to the pectinate muscles of the atria)
  • Papillary muscles: muscles arising from the floor of the RV that control closure of the tricuspid valve
  • Chordae tendineae: tendinous cord-like structures that connect the papillary muscles to the tricuspid valve
  • Interventricular septum (IVS):
    • Wall between RV and LV 
    • Has membranous and muscular components

LV: 

  • Receives oxygenated blood from LA through mitral valve
  • Pumps blood through aortic valve into aorta → systemic circulation
  • Has the thickest layer of the myocardium (highest-pressure chamber)
  • Like the RV, the LV also contains:
    • Trabeculae carneae
    • Papillary muscles to control mitral valve
    • Chordae tendineae between papillary muscles and mitral valve

Sulci

  • Visible grooves on the surface of the heart
  • Mark the boundaries of the 4 chambers
  • Contain:
    • Coronary vessels
    • Adipose tissue
  • Atrioventricular (AV) sulcus: 
    • Divides the atria from the ventricles
    • Commonly called the coronary sulcus because the coronary vessels run within it
  • Anterior interventricular sulcus: marks location of IVS on anterior surface of heart
  • Posterior interventricular sulcus: marks location of IVS on posterior surface of heart

Cardiac valves

  • Valves: 
    • Anchored on fibrous rings
    • Prevent retrograde flow
    • Closure produces audible heart sounds Heart sounds Heart sounds are brief, transient sounds produced by valve opening and closure and by movement of blood in the heart. They are divided into systolic and diastolic sounds. In most cases, only the first (S1) and second (S2) heart sounds are heard. These are high-frequency sounds and arise from aortic and pulmonary valve closure (S1), as well as mitral and tricuspid valve closure (S2). Heart Sounds on auscultation of the chest
  • Tricuspid valve: 
    • Between the RA and RV
    • Also called the right AV valve
    • 3 cusps: anterior, posterior, and septal
  • Mitral valve: 
    • Between LA and LV
    • Also called the left AV valve
    • Bicuspid = 2 cusps: anterior and posterior
  • Pulmonary valve: 
    • Between RV and pulmonary trunk
    • Semilunar in shape, with 3 cusps: right, left, and anterior
  • Aortic valve: 
    • Between left ventricle and aorta 
    • Semilunar in shape, with 3 cusps: right, left, and posterior
View of the valves of the heart from an atrial perspective

View of the valves of the heart from an atrial perspective:
Atria removed

Image by BioDigital, edited by Lecturio

Blood Flow through the Heart

Blood flows into, through, and out of the heart by sequentially passing through the following structures (in order):

  1. Deoxygenated blood enters the heart via the SVC/IVC → 
  2. RA → 
  3. Tricuspid valve → 
  4. RV → 
  5. Pulmonary valve → 
  6. Pulmonary trunk → 
  7. Pulmonary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries → 
  8. Lungs (blood is oxygenated) → 
  9. Pulmonary veins → 
  10. LA → 
  11. Mitral valve → 
  12. LV → 
  13. Aortic valve → 
  14. Aorta → 
  15. Systemic arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries (blood is deoxygenated) → veins → 
  16. SVC/IVC
  17. Back to the heart
Circulation of blood through the body

Circulation of blood through the body:
Deoxygenated blood enters the right side of the heart and passes through the pulmonary trunk to the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs, where it is oxygenated. The blood then returns to the left side of the heart via the pulmonary veins, where it is pumped into the aorta and distributed throughout the body. The blood travels through systemic capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries, where it is deoxygenated again, and travels back to the heart via the superior and inferior vena cava.
LA: left atrium
LV: left ventricle
RA: right atrium
RV: right ventricle

Image by Lecturio.

Coronary Circulation

Coronary circulation describes the flow of blood through the vessels supplying the heart muscle itself. There are 2 primary coronary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: the left and the right, and both originate from the aorta, just above the aortic valve.

Left coronary artery (LCA)

  • Runs posterior to the pulmonary trunk → around the left side of the heart under the left auricle in the coronary (AV) sulcus
  • Primary blood supply for:
    • Left side of the heart 
    • IVS
  • Branches into:
    • Anterior interventricular artery: 
      • Commonly referred to as the left anterior descending (LAD) artery
      • Travels down the anterior interventricular sulcus toward the apex
      • Supplies the septum and anterior wall of both ventricles
      • Clinical relevance: artery most frequently occluded, causing MI MI MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction
    • Circumflex artery:
      • Continuous around the left side of the heart in the coronary sulcus
      • Anastomoses with small branches of the right coronary artery
      • Supplies the LA and posterior wall of the LV
    • Multiple other smaller branches come off of each of these
Left coronary artery and its branches

Left coronary artery and its branches

Image by BioDigital, edited by Lecturio

Right coronary artery (RCA)

  • Runs around the right side of the heart, under the right auricle in the coronary sulcus
  • Primary blood supply for:
    • Right atrium
    • Parts of both ventricles
    • Conduction system (clinical relevance: occlusion of the RCA is associated with arrhythmias)
  • Branches into:
    • Posterior interventricular artery: 
      • Commonly referred to as the posterior descending artery ( PDA PDA The ductus arteriosus (DA) allows blood to bypass pulmonary circulation. After birth, the DA remains open for up to 72 hours and then constricts and involutes, becoming the ligamentum arteriosum. Failure of this process to occur results in patent ductus arteriosus (PDA), a condition that causes up to 10% of congenital heart defects. Patent Ductus Arteriosus (PDA))
      • Travels down the posterior interventricular sulcus toward the apex
      • Supplies the posterior wall of both ventricles
      • Occasionally branches off the LCA rather than the RCA
    • Right marginal artery: supplies the lateral aspect of the RA and RV

Coronary dominance

A person’s coronary dominance pattern is determined by the artery that gives off the PDA PDA The ductus arteriosus (DA) allows blood to bypass pulmonary circulation. After birth, the DA remains open for up to 72 hours and then constricts and involutes, becoming the ligamentum arteriosum. Failure of this process to occur results in patent ductus arteriosus (PDA), a condition that causes up to 10% of congenital heart defects. Patent Ductus Arteriosus (PDA).

  • Right-dominant (approximately 75% of people): PDA PDA The ductus arteriosus (DA) allows blood to bypass pulmonary circulation. After birth, the DA remains open for up to 72 hours and then constricts and involutes, becoming the ligamentum arteriosum. Failure of this process to occur results in patent ductus arteriosus (PDA), a condition that causes up to 10% of congenital heart defects. Patent Ductus Arteriosus (PDA) comes from RCA
  • Left-dominant: PDA PDA The ductus arteriosus (DA) allows blood to bypass pulmonary circulation. After birth, the DA remains open for up to 72 hours and then constricts and involutes, becoming the ligamentum arteriosum. Failure of this process to occur results in patent ductus arteriosus (PDA), a condition that causes up to 10% of congenital heart defects. Patent Ductus Arteriosus (PDA) comes from the left circumflex artery (off the LCA)
  • Codominant (least common): an individual has 2 PDAs
    • 1 comes from RCA
    • The other comes from the left circumflex artery

Coronary veins

  • Great cardiac vein: 
    • Drains the anterior side of the heart
    • Travels with the LAD in the anterior IV sulcus
  • Middle cardiac vein:
    • Drains the posterior side of the heart
    • Travels with the PDA PDA The ductus arteriosus (DA) allows blood to bypass pulmonary circulation. After birth, the DA remains open for up to 72 hours and then constricts and involutes, becoming the ligamentum arteriosum. Failure of this process to occur results in patent ductus arteriosus (PDA), a condition that causes up to 10% of congenital heart defects. Patent Ductus Arteriosus (PDA) in the posterior IV sulcus
  • The great and middle cardiac veins drain into the coronary sinus, which:
    • Lies in the left posterior AV sulcus
    • Drains directly into the RA
  • Approximately 20% of the coronary circulation drains directly into the right ventricle via small vessels.
Cardiac vein

Posterior view of the heart showing the coronary sinus and middle cardiac vein

Image by BioDigital, edited by Lecturio
Table: Summary of coronary circulation
Vessel Trajectory Area of supply/drainage
Left coronary artery LAD artery Runs in the anterior IV sulcus toward the apex
  • Most of the LA and LV
  • Anterior portion of the IVS
Left circumflex artery Wraps around toward the posterior aspect of the heart in the coronary (AV) sulcus Posterior aspect of LA and LV
Right coronary artery PDA PDA The ductus arteriosus (DA) allows blood to bypass pulmonary circulation. After birth, the DA remains open for up to 72 hours and then constricts and involutes, becoming the ligamentum arteriosum. Failure of this process to occur results in patent ductus arteriosus (PDA), a condition that causes up to 10% of congenital heart defects. Patent Ductus Arteriosus (PDA) Runs in the posterior interventricular sulcus toward the apex RV and LV and posterior ⅓ of IVS
Right marginal artery Runs over the RV toward the apex RV and apex of the heart
Great cardiac vein With the LAD within the anterior IV sulcus Areas of the heart supplied by the LCA
Middle cardiac vein With the PDA PDA The ductus arteriosus (DA) allows blood to bypass pulmonary circulation. After birth, the DA remains open for up to 72 hours and then constricts and involutes, becoming the ligamentum arteriosum. Failure of this process to occur results in patent ductus arteriosus (PDA), a condition that causes up to 10% of congenital heart defects. Patent Ductus Arteriosus (PDA) within the posterior IV sulcus Posterior wall of LV and RV

Cardiac Conduction System

Overview of the cardiac conduction system

  • The heart generates its own electrical signal. 
  • This signal travels through conduction fibers throughout the heart as an electrical wave.
  • This electrical wave triggers myocardial contraction.
  • Certain anatomic features of this system ensure that the wave triggers contraction in an efficient and coordinated manner.

Anatomy of the conduction system

  • The conduction system is:
    • Myogenic = signal originates from modified myocardial cells within the heart itself (rather than from nerves)
    • Autorhythmic = depolarize spontaneously, at regular time intervals
  • The sinoatrial (SA) node: 
    • The primary pacemaker of the heart 
    • Depolarizes (creating an action potential) 60–80 times per minute → baseline heart rate = 60–80 beats per minute 
    • A patch of modified myocytes located in the RA:
      • Just deep to the epicardium 
      • At the junction of the SVC and RA
    • Supplied by the SA nodal artery, which can arise as a branch off the RCA (60%) or LCA (40%)
  • The AV node:
    • Located in the interatrial septum:
      • In the posteroinferior region, near the opening of the coronary sinus
      • Located in a space called Koch’s triangle
    • Natural pacemaker rate of 40–60 beats per minute (normally overridden by the SA nodal rate) 
    • Acts as an “electrical gateway” to the ventricles: 
      • Slows down the signal → allows time for atrial contraction, before passing the signal on to the ventricles
      • The fibrous skeleton prevents the signal from bypassing the AV node and causing early ventricular depolarization
    • Supplied by the AV nodal artery
  • Ventricular conduction system: 
    • Common AV bundle (of His): pathway within the IV septum by which the signal leaves the AV node
    • Right and left bundle branches: 
      • The AV bundle divides into right and left bundles within the IV septum.
      • Both run toward the apex.
    • Purkinje fibers: 
      • Arise from the bundle branches near the apex
      • Turn upward and spread throughout the ventricular walls
      • Fastest conduction fibers
      • Natural pacemaker rate of 25–40 beats per minute
Cardiac conduction system

Cardiac conduction system:
Starting with the sinoatrial (SA) node and ending in the Purkinje fibers
AV: atrioventricular

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Clinical Relevance

  • Heart sounds: on auscultation, 2 heart sounds Heart sounds Heart sounds are brief, transient sounds produced by valve opening and closure and by movement of blood in the heart. They are divided into systolic and diastolic sounds. In most cases, only the first (S1) and second (S2) heart sounds are heard. These are high-frequency sounds and arise from aortic and pulmonary valve closure (S1), as well as mitral and tricuspid valve closure (S2). Heart Sounds are heard from a normal heart—1st, closing of the AV valves (tricuspid and mitral), followed by closing of the pulmonic and aortic valves. Additional sounds may also be heard, produced by physiologic and/or pathologic conditions. For example, murmurs are generated by turbulent blood flow through the heart.
  • Stable and unstable angina Stable and unstable angina Stable and unstable angina are considered an important symptom of coronary heart disease (CHD) and present with chest pain due to transient myocardial ischemia. These disorders can be a warning sign for the risk of heart attack (MI) in the future. Clinically, stable and unstable angina are differentiated by exacerbating factors, duration of symptoms, and response to rest and medications. Stable and Unstable Angina: precordial chest pain Chest Pain Chest pain is one of the most common and challenging complaints that may present in an inpatient and outpatient setting. The differential diagnosis of chest pain is large and includes cardiac, gastrointestinal, pulmonary, musculoskeletal, and psychiatric etiologies. Chest Pain or pressure, due to reduced blood flow to the cardiac muscle causing transient myocardial ischemia. Angina is most commonly caused by narrowing or occlusion of the coronary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries or one of their main branches. 
  • Myocardial infarction Myocardial infarction MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction: ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation. MI MI MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction is usually due to thrombus formation on a ruptured atherosclerotic plaque in the supplying arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries.
  • Pericarditis Pericarditis Pericarditis is an inflammation of the pericardium, often with fluid accumulation. It can be caused by infection (often viral), myocardial infarction, drugs, malignancies, metabolic disorders, autoimmune disorders, or trauma. Acute, subacute, and chronic forms exist. Pericarditis: inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the pericardium resulting from infection, autoimmune disease, radiation, surgery, or MI MI MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction. Pericarditis Pericarditis Pericarditis is an inflammation of the pericardium, often with fluid accumulation. It can be caused by infection (often viral), myocardial infarction, drugs, malignancies, metabolic disorders, autoimmune disorders, or trauma. Acute, subacute, and chronic forms exist. Pericarditis manifests as fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, pleuritic chest pain Chest Pain Chest pain is one of the most common and challenging complaints that may present in an inpatient and outpatient setting. The differential diagnosis of chest pain is large and includes cardiac, gastrointestinal, pulmonary, musculoskeletal, and psychiatric etiologies. Chest Pain that increases when lying supine, and an audible pericardial rub on auscultation.
  • Pericardial effusion Pericardial effusion Pericardial effusion is the accumulation of excess fluid in the pericardial space around the heart. The pericardium does not easily expand; thus, rapid fluid accumulation leads to increased pressure around the heart. The increase in pressure restricts cardiac filling, resulting in decreased cardiac output and cardiac tamponade. Pericardial Effusion and Cardiac Tamponade and cardiac tamponade Cardiac tamponade Pericardial effusion is the accumulation of excess fluid in the pericardial space around the heart. The pericardium does not easily expand; thus, rapid fluid accumulation leads to increased pressure around the heart. The increase in pressure restricts cardiac filling, resulting in decreased cardiac output and cardiac tamponade. Pericardial Effusion and Cardiac Tamponade: accumulation of excess fluid in the pericardial space around the heart. The pericardium does not easily expand, so rapid accumulation of fluid leads to increased pressure around the heart. This increased pressure restricts cardiac filling, resulting in decreased cardiac output and cardiac tamponade Cardiac tamponade Pericardial effusion is the accumulation of excess fluid in the pericardial space around the heart. The pericardium does not easily expand; thus, rapid fluid accumulation leads to increased pressure around the heart. The increase in pressure restricts cardiac filling, resulting in decreased cardiac output and cardiac tamponade. Pericardial Effusion and Cardiac Tamponade. Signs and symptoms usually occur in the setting of cardiac tamponade Cardiac tamponade Pericardial effusion is the accumulation of excess fluid in the pericardial space around the heart. The pericardium does not easily expand; thus, rapid fluid accumulation leads to increased pressure around the heart. The increase in pressure restricts cardiac filling, resulting in decreased cardiac output and cardiac tamponade. Pericardial Effusion and Cardiac Tamponade and include dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea, hypotension Hypotension Hypotension is defined as low blood pressure, specifically < 90/60 mm Hg, and is most commonly a physiologic response. Hypotension may be mild, serious, or life threatening, depending on the cause. Hypotension, muffled heart sounds Heart sounds Heart sounds are brief, transient sounds produced by valve opening and closure and by movement of blood in the heart. They are divided into systolic and diastolic sounds. In most cases, only the first (S1) and second (S2) heart sounds are heard. These are high-frequency sounds and arise from aortic and pulmonary valve closure (S1), as well as mitral and tricuspid valve closure (S2). Heart Sounds, jugular venous distention, and pulsus paradoxus. 
  • Arrhythmias: abnormal heartbeat—either too fast, too slow, or in an irregular manner. When the heart is irritated or damaged, abnormal electrical cardiac impulses can develop spontaneously in the atria or the ventricle. Some spontaneous contractions are normal, but certain contraction patterns are very dangerous and can result in permanent heart damage, stroke, or death.
  • Congenital heart defects: structural abnormalities of the heart due to abnormal development in utero. Signs and symptoms depend on the specific type of anomalies and range from asymptomatic to life-threatening. Examples include abnormalities associated with the position of the great vessels, including transposition of the great vessels Transposition of the Great Vessels Transposition of the great vessels (TGV) is a cyanotic congenital heart disease characterized by "switching" of the great arteries. There are 2 presentations: the dextro (D)- and levo (L)-looped forms. The L-looped form is rare and congenitally corrected, as the ventricles are also switched. Transposition of the Great Vessels, truncus arteriosus Truncus arteriosus Truncus arteriosus (TA) is a congenital heart defect characterized by the persistence of a common cardiac arterial trunk tract that fails to divide into the pulmonary artery and aorta during embryonic development. Truncus arteriosus is a rare congenital malformation with a high mortality rate within the 1st 5 weeks of life if not managed promptly. Truncus Arteriosus, and tetralogy of Fallot Tetralogy of Fallot Tetralogy of Fallot is the most common cyanotic congenital heart disease. The disease is the confluence of 4 pathologic cardiac features: overriding aorta, ventricular septal defect, right ventricular outflow obstruction, and right ventricular hypertrophy. Tetralogy of Fallot; holes in the heart known as atrial and ventricular septal defects or a patent foramen ovale Patent Foramen Ovale A patent foramen ovale (PFO) is an abnormal communication between the atria that persists after birth. The condition results from incomplete closure of the foramen ovale. Small, isolated, and asymptomatic PFOs are a common incidental finding on echocardiography and require no treatment. Patent Foramen Ovale; and several others.
  • Dilated cardiomyopathy Cardiomyopathy Cardiomyopathy refers to a group of myocardial diseases associated with structural changes of the heart muscles (myocardium) and impaired systolic and/or diastolic function in the absence of other heart disorders (coronary artery disease, hypertension, valvular disease, and congenital heart disease). Overview of Cardiomyopathies, restrictive cardiomyopathy Restrictive Cardiomyopathy Restrictive cardiomyopathy (RCM) is a fairly uncommon condition characterized by progressive stiffening of the cardiac muscle, which causes impaired relaxation and refilling of the heart during diastole, resulting in diastolic dysfunction and eventual heart failure. Restrictive Cardiomyopathy, and hypertrophic cardiomyopathy Hypertrophic Cardiomyopathy Hypertrophic cardiomyopathy (HCM) is the most commonly inherited cardiomyopathy, which is characterized by an asymmetric increase in thickness (hypertrophy) of the left ventricular wall, diastolic dysfunction, and often left ventricular outflow tract obstruction. Hypertrophic Cardiomyopathy: group of myocardial diseases associated with impaired systolic and diastolic function. These disorders may present as shortness of breath, fatigue, syncope Syncope Syncope is a short-term loss of consciousness and loss of postural stability followed by spontaneous return of consciousness to the previous neurologic baseline without the need for resuscitation. The condition is caused by transient interruption of cerebral blood flow that may be benign or related to a underlying life-threatening condition. Syncope, arrhythmias, or heart failure and are classified on the basis of adaptive changes experienced by the myocardium. The main types of cardiomyopathy Cardiomyopathy Cardiomyopathy refers to a group of myocardial diseases associated with structural changes of the heart muscles (myocardium) and impaired systolic and/or diastolic function in the absence of other heart disorders (coronary artery disease, hypertension, valvular disease, and congenital heart disease). Overview of Cardiomyopathies include hypertrophic, dilated, and restrictive.
  • Heart failure (diastolic dysfunction and systolic dysfunction): Cardiac insufficiency is characterized by an inability of the heart to pump the amounts of blood required to meet the body’s needs. Signs and symptoms commonly include shortness of breath that worsens with physical exertion and lying down and lower-extremity edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema.

References

  1. Drake R.L., Vogl, A.W., Mitchell, A.M.W. (2020). Regional Anatomy, Mediastinum. In Drake, R-L., et al. (ed.), Gray’s Anatomy for Students (4th ed., pp. 190–215). Churchill Livingstone/Elsevier
  2. Moore, K. L., et al. (ed). (2014). Thorax. In Moore, K. L., et al. (ed), Clinically Oriented Anatomy (7th ed., pp. 135–159). Lippincott Williams & Wilkins.
  3. John Volpe BS (2021). Anatomy, Thorax, Heart and Pericardial Cavity https://www.statpearls.com/ArticleLibrary/viewarticle/36077
  4. Saladin, K.S., Miller, L. (2004). Anatomy and Physiology (3rd ed., pp. 716–727). Mc Graw-Hill.

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