Anatomy, Thorax, Heart


Article Author:
Ibraheem Rehman


Article Editor:
Afzal Rehman


Editors In Chief:
Juan Batlle
Jitendra Sisodia
Jeffrey Miller


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Frank Smeeks
Kristina Soman-Faulkner
Trevor Nezwek
Radia Jamil
Patrick Le
Sobhan Daneshfar
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Pritesh Sheth
Hassam Zulfiqar
Navid Mahabadi
Steve Bhimji
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Mark Pellegrini
James Hughes
Beata Beatty
Nazia Sadiq
Hajira Basit
Phillip Hynes
Tehmina Warsi


Updated:
2/10/2019 8:13:30 AM

Introduction

The heart is a muscular organ situated in the center of the chest behind the sternum. It consists of four chambers: the two upper chambers are called the right and left atria, and the two lower chambers are called the right and left ventricles. The right atrium and ventricle together are often called the right heart, and the left atrium and left ventricle together functionally form the left heart.[1][2][3][4]

Structure and Function

The heart consists of four chambers organized into two pumps (right and left) for the purpose of providing blood flow to the systemic and pulmonary circulations. The right atrium receives deoxygenated blood from the entire body except for the lungs (the systemic circulation) via the superior and inferior vena cavae. Also, deoxygenated blood from the heart muscle itself drains into the right atrium via the coronary sinus. The right atrium, therefore, acts as a reservoir to collect deoxygenated blood. From here, blood flows through the tricuspid valve to fill the right ventricle which is the main pumping chamber of the right heart. The right ventricle pumps blood through the right ventricular outflow tract, across the pulmonic valve, and into the pulmonary artery that distributes it to the lungs for oxygenation. In the lungs, the blood oxygenates as it passes through the capillaries where it is close enough to the oxygen in the alveoli of the lungs. This oxygenated blood is collected by the four pulmonary veins, two from each lung. All four of these veins open into the left atrium that acts as a collection chamber for oxygenated blood. Just like the right atrium, the left atrium passes the blood onto its ventricle both by passive flow and active pumping. Oxygenated blood thus fills the left ventricle, passing through the mitral valve. The left ventricle, which is the main pumping chamber of the left heart, then pumps, sending freshly oxygenated blood to the systemic circulation through the aortic valve. The cycle is then repeated all over again in the next heartbeat. 

All four valves of the heart mentioned above have a singular purpose: allowing forward flow of blood but preventing backward flow.

Conduction System

Pumping of the heart and timing of contraction of various chambers is regulated by an electrical conduction system.  Heart muscle contracts in response to the electrical stimulus received. The sinus node which is the main pacemaker of the heart is situated at the junction of the superior vena cava and the right atrium. It rhythmically generates an electrical discharge about 70 times a minute. This electrical signal is carried to the left atrium via the Bachmann’s bundle. Conduction occurs through the right atrial muscle to the atrioventricular node (AV node), located in the triangle of Koch, a small triangular area formed by tricuspid valve, tendon of Todaro, and lip of the coronary sinus ostium. The AV node receives the electrical signal and conducts it to the bundle of His with some delay. This delay allows the emptying of the atria into the ventricles before the ventricles contract in response to the electrical signal. The bundle of His divides into the right and left bundles that successively branch into thousands of small branches called Purkinje fibers. The His-Purkinje tree serves to rapidly conduct the electrical signal to all parts of both ventricles to produce a near simultaneous contraction of all parts both ventricles, producing a uniform and coordinated squeeze.[5][6][7][8]

Embryology

The heart develops from two endocardial tubes that merge, loop, and septate to form the heart. During the intrauterine stage, the septum between the two atrial is open, and a ductus connects the pulmonary artery to the aorta effectively bypassing the pulmonary circulation because the lungs are not functional. Rapidly after birth, these two connections close, establishing separate pulmonary and system circulations.

Blood Supply and Lymphatics

The heart is supplied by two coronary arteries: the left main coronary artery and the right coronary artery. The left main coronary artery carries 80% of the flow to the heart muscle. It is a short artery that divides into two branches, (1) the left anterior descending artery that supplies anterior two-thirds of the inter-ventricular septum and adjoining part of the left ventricular anterior wall, and (2) the circumflex coronary artery that supplies blood to the lateral and posterior portions of the left ventricle.

The right coronary artery and its branches supply the right ventricle, right atrium, and the inferior wall of the left ventricle.

Coronary arteries and veins course over the surface of the heart. Most coronary veins coalesce into the coronary sinus that runs in the left posterior atrioventricular groove and opens into the right atrium. Other small veins, called thebesian veins, open directly into all four chambers of the heart.

Small lymphatic vessels form a dense network beneath the epicardium and endocardium of the ventricles and open into a lymphatic duct in the atrioventricular groove. However detailed lymphatic anatomy of the human heart has not been worked out.

Nerves

The sinus node and the AV node are both supplied by sympathetic nerve fibers from the sympathetic ganglia and parasympathetic fibers through the vagus nerve and parasympathetic ganglia behind the heart.

Muscles

The heart is a muscular organ. It has no bones. Sheets of muscle fibers are arranged over a fibrous skeleton to give the heart chambers their shapes. However, the atrial muscle is completely separated from the ventricular muscle by a fibrous atrioventricular scaffolding such that no electrical conduction can occur between the two, except through the AV node. 

Physiologic Variants

The general structure of the heart is quite uniform in healthy individuals. However, some variations do occur. The heart is arranged more horizontally in the chest in short and obese individuals, while it is more vertical in tall and thin people. An athlete’s heart may be physically larger. Coronary arteries show variations in branching patterns and relative sizes.

Surgical Considerations

Cardiac valves can become fibrosed and calcific with age or disease, producing clinically significant stenosis requiring surgical or trans-catheter replacement. Similarly, valves may become incompetent allowing backward flow called regurgitation, also necessitating replacement or repair.[9]

Coronary arteries can become clogged with thrombus or atherosclerotic plaque, causing reduced blood supplies to cardiac muscle. This may result in angina or myocardial infarction and often requires revascularization.

Clinical Significance

The heart is a vital organ.  If the heart stops, cessation of blood flow and oxygen supply will occur, leading to irreversible brain damage within 4 to 5 minutes.  Cessation or impairment of cardiac function may occur due to lack to the blood supply to cardiac muscle (coronary artery disease), stenosis or regurgitation in cardiac valves (valvular heart disease), intrinsic weakness of heart muscle (cardiomyopathy), or ineffective cardiac rhythms.

Other Issues

In six per 1000 live births, congenital cardiac malformations occur. Ventricular septal defects (VSD), atrial septal defects and tetralogy of Fallot among the commonest.  Tetralogy of Fallot consists of a combination of VSD of the membranous portion of the interventricular septum, stenosis of the orifice of the pulmonary artery, the aortic orifice overriding the VSD and hypertrophy of the right ventricle.  This requires surgical correction, usually at an early age.


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Anatomy, Thorax, Heart - Questions

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What makes up the left lateral border and apex of the cardiac silhouette on a chest radiograph?



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Which of the following makes up the left lateral border and apex of the cardiac silhouette?



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The great cardiac vein empties into which structure?



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A 68-year-old female presents for a general check-up. Her blood pressure is 162/94 mmHg. On her first visit to you 3 months ago her blood pressure was 156/92 mmHg. The patient is nonadherent with her medication. On physical examination, the apex of her heart is shifted to the left, and a third heart sound is heard. These findings are due to left ventricular hypertrophy from chronic hypertension. Normally, the apex of the heart is located in which intercostal space?



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The middle cardiac vein which ascends in the posterior interventricular groove is accompanied by which artery?



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Which of the following forms the largest part of the sternocostal surface of the heart?



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The circumflex branch of the left coronary artery supplies which of the following areas of the heart?



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Into which of part of the heart does the anterior cardiac vein drain?



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Which of the following is the artery that supplies the left ventricle?



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Which of the following forms a bridge between the interventricular septum and the base of the anterior papillary muscle?



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The sinoatrial node is in the wall of which heart chamber?



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Which of the following prevents the cusps from being everted into the atrium during ventricular contraction?



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The artery which arises as a branch of the right coronary artery opposite the origin of the posterior interventricular branch supplies which of the following structures?



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What artery supplies the left ventricle wall?



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The middle cardiac vein that ascends in the posterior interventricular groove is accompanied by which artery?



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The left atrium and ventricle receive blood from which of the following arteries?



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Where in the heart is the sinoatrial node located?



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The artery that arises as a branch of right coronary artery opposite the origin of the posterior interventricular branch supplies which of the following structures?



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Blood enters the left atrium of the heart via which of the following?

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Blood pumped from the right ventricle initially flows into which of the following?



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What orthogonal plane is closest to the vertical long axis view of the heart?



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Which is a correct description of cardiothoracic anatomy?



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Which of the following is the correct order in which a first pass agent will reach various anatomic structures?



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A 64-year-old retired carpenter is suddenly seized with excruciating chest pains, following climbing two flights of stairs. A paramedic is called who administers morphine for the pain and recommends hospitalization. The patient refuses and insists on remaining at home. Several hours later a second "attack" occurs, and another paramedic is called. During the examination, the patient has a cardiac arrest. The paramedic attempts closed cardiac massage, adrenaline, and mouth-to-mouth breathing. Eventually, the patient is pronounced dead and an autopsy is performed. The patient's heart weighs 540 grams. What is the normal weight of a male heart?



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The apex of the heart is situated:



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What the term for the outermost layer of the heart?



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The pulmonary veins connect to the:



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You are asked to listen for a murmur at the apex of the heart. On the surface of the chest, where will you place your stethoscope?



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Which of the following is not a boundary of the triangle of Koch?



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What structure of the heart forms the anteromedial border of the left ventricular outflow tract?



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Anatomy, Thorax, Heart - References

References

Saxton A,Manna B, Anatomy, Thorax, Heart Right Coronary Arteries 2018 Jan;     [PubMed]
Carter YM,Mahajan K, Anatomy, Thorax, Heart Anomalous Left Coronary Artery 2018 Jan;     [PubMed]
Hussain A,Burns B, Anatomy, Thorax, Wall 2018 Jan;     [PubMed]
Saxton A,Bordoni B, Anatomy, Thorax, Cardiac Muscle 2018 Jan;     [PubMed]
Tucker WD,Burns B, Anatomy, Thorax, Heart Pulmonary Arteries 2018 Jan;     [PubMed]
Ogobuiro I,Tuma F, Anatomy, Thorax, Heart Coronary Arteries 2018 Jan;     [PubMed]
Capobianco SM,Sicari V, Anatomy, Thorax, Subclavian Veins 2018 Jan;     [PubMed]
Oliver KA,Ashurst JV, Anatomy, Thorax, Phrenic Nerves 2018 Jan;     [PubMed]
Mori S,Tretter JT,Spicer DE,Bolender D,Anderson RH, What is the real cardiac anatomy? Clinical anatomy (New York, N.Y.). 2019 Jan 24;     [PubMed]

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