The heart (Latin: cor) is a thick, muscular organ with four cavitated parts. The heart is a central organ and pump of the cardiovascular system. It maintains the unidirectional flow of blood through the circulatory system. The heart is a hollow organ located in the middle mediastinum. The heart is surrounded by a serous sac called the pericardium. The great vessels originating from the heart provide blood flow throughout the whole body. It is a vital organ meaning a person can not live without it.
External heart anatomy
The heart has a cone or pyramid shape with the base projected upward, backward, and right, while the apex locates forward, downward, and to the left.
Approximately two-thirds of the heart is located on the left, while one-third on the right side from the midline of the body.
The apex of the heart is typically located deep to the left 5th intercostal space.
The apex of the heart is formed by the left ventricle.
The longitudinal axis of the heart goes from the base to the apex. It goes from the top to the bottom, from right to left, and from back to the front.
The heart is slightly rotated around the longitudinal axis to the left. The right side of the heart is located closer to the ventral surface of the body, while the left side- closer to the dorsal surface of the body.
The heart weighs around 200 to 400 grams (7 to 14 oz). Each day it beats about 100,000 times and pumps approximately 6-7,5 liters (1,6-2 gallons) of blood. The average heart rate is 60 to 90 heartbeats per minute.
The heart has four borders and five external surfaces formed by different internal parts.
The external surfaces of the heart contain sulci.
Borders of the heart
The heart has four borders separating the surfaces.
The superior border is formed by the right and left atrium, auricles, and the great vessels.
The right border of the heart is formed by the right atrium extending between the superior and inferior vena cava, while the left by the left ventricle and auricle.
The inferior border is formed by the left and right ventricles.
Position of the heart
The superior border of the heart goes horizontally at the level of the 3rd rib cartilage, and the right one goes archwise down from the 3rd rib cartilage to the 5th rib cartilage. This border is located approximately 1,5 centimeters from the sternum on the right side.
The inferior border goes obliquely down from the cartilage of the 5th rib on the right side to the apex on the left side.
The apex of the heart is projected in the fifth intercostal space. It is approximately 1,5 centimeters medially from the left midclavicular line. The left border goes obliquely upwards from the apex to the site where the cartilage of the third rib joins the bony part of the rib.
The heart has five surfaces:
- diaphragmatic surface (inferiorly),
- sternocostal surface (anteriorly),
- right and left pulmonary surfaces (medially and laterally),
- base of the heart (posteriorly).
The heart in its anatomical position rests on the diaphragmatic surface, which is a flat surface. It is faced inferiorly and lies on the diaphragm. This surface is formed by the right ventricle and a small part of the left ventricle separated by the posterior interventricular groove.
The sternocostal (anterior) surface of the heart is convex. It is directed anteriorly and slightly to the left against the sternum and inner surfaces of the ribs.
It consists mainly of the right ventricle, a part of the right atrium, and part of the left ventricle.
On the sternocostal surface above the coronary sulcus are located two auricles- right and left. The right one is larger than the left one.
The right and left pulmonary surfaces are broad, rounded, convex, and face the lungs.
The left pulmonary surface is formed by the left ventricle and a part of the left atrium, while the right pulmonary surface consists of the right atrium.
Base of the heart
The base of the heart is quadrilateral and oriented posteriorly.
It consists of the left atrium, a small portion of the right atrium, and the proximal parts of the great veins (superior and inferior venae cavae and the pulmonary veins).
The base of the heart is fixed posteriorly to the pericardial wall, opposite the bodies of the 4th/5th to 8th thoracic vertebrae (5th/6th to 9th when standing).
The internal partitions forming four chambers produce grooves on the external surface called external sulci, including the coronary sulcus and the anterior and posterior interventricular sulci.
All of these external sulci are continuous with each other inferiorly, just to the right of the apex.
The area on the lower backside of the heart, where meet the coronary sulcus and the posterior interventricular sulcus, is called the cardiac crux or crux of the heart.
The coronary sulcus is also known as the atrioventricular groove. It is a groove that circles the heart, marking the separation between the atria and the ventricles.
Anterior and posterior interventricular sulci
The anterior and posterior interventricular sulci go in a vertical direction and mark the separation of both ventricles.
The anterior interventricular sulcus is situated on the anterior surface (sternocostal surface). It contains the anterior interventricular artery and the great cardiac vein.
The posterior interventricular sulcus is located on the diaphragmatic surface. It contains the posterior interventricular artery and the middle cardiac vein.
Anatomical relations of the heart
Anteriorly to the heart is located the sternum and cartilages of the ribs, left lung.
Posteriorly are situated organs of the posterior mediastinum. They are the esophagus, descending thoracic aorta, azygos, hemiazygos vein and thoracic duct, other blood vessels, nerves, and lymph nodes.
Below the heart is located the central tendon of the diaphragm.
But above the heart is located the large blood vessels of the heart and bifurcation of the main pulmonary trunk.
The sides of the heart are related to the mediastinal part of the parietal pleura and lungs.
Internal heart anatomy
Internally, the heart is divided into four chambers: right and left atria and right and left ventricles.
The chambers are separated by septa containing subdivisions.
The interatrial subdivision is the upper part, located between atria.
The interventricular subdivision is the lower portion located between the ventricles. But the atrioventricular part is located between the atria and ventricles.
The heart consists of two pumps, each formed by an atrium and a ventricle separated by a valve.
The right pump receives deoxygenated blood and pumps it into the lungs, and the left pump receives oxygenated blood from the lungs and sends it to the body.
The atria have relatively thin walls receiving blood, while the ventricles with reasonably thick walls pump blood out of the heart.
The right atrium is situated in the superior right corner of the heart above the right ventricle.
The systemic circulation ends in the right atrium. The deoxygenated blood enters it through the inferior and superior vena cava and the coronary sinus.
Walls of the right atrium
The right atrium has a cuboid shape. Therefore it has six walls:
- and medial.
Between the medial and the posterior wall below the fossa ovalis is located a small opening called the opening of the coronary sinus. Through the opening, the coronary sinus opens to the right atrium. The coronary sinus collects venous blood from the heart muscle.
It is protected by a semicircular fold of the lining membrane, called the valve of the coronary sinus (also known as the Thebesian valve).
Superior and inferior walls
The superior vena cava opens to the superior wall with an opening of superior vena cava.
On the inferior wall is located the right atrioventricular orifice. It is an oval opening between the right atrium and the right ventricle.
Anterior and posterior walls
On the anterior wall is placed the right auricle. The primary function of the right auricle is to increase the volume of the right atrium.
The muscular layer of the heart creates parallel ridges in the anterior wall and the auricle. They are called the pectineal muscles.
On the posterior wall is located the opening of the inferior vena cava. Inferior vena cava opens to it.
Along the inferior margin of the opening extends a semilunar valve called the valve of inferior vena cava. The valve functions until the baby is born. It directs blood from the right atrium through the foramen ovale to the left atrium.
The enlarged posterior part of the right atrium receiving blood directly from both inferior and superior venae cavae is called the sinus of venae cavae.
Lateral and medial walls
On the lateral wall are located the pectineal muscles.
The medial wall is formed by the interatrial septum.
In the interatrial septum lies an oval fibrous depression called the fossa ovalis covering the foramen ovale during fetal development. Around the fossa ovalis is an oval margin named the annulus ovalis or limbus of the fossa ovalis.
Small openings for the smallest cardiac veins are also located on the medial wall of the right atrium.
The right ventricle is one of the heart chambers located in the inferior right portion of the heart under the right atrium and opposite to the left ventricle.
The right ventricle contains deoxygenated blood.
The primary function of the right ventricle is to pump blood up through the pulmonary valve and trunk into the lungs, thus providing pulmonary circulation.
The right ventricle has a pyramidal shape with the base directed upwards and the apex - downwards.
The medial wall is formed by the interventricular septum.
The inferior wall lies close to the central tendon of the diaphragm. The anterior wall of the right ventricle is directed towards the inner surfaces of the sternum and ribs.
The base of the right ventricle contains the right atrioventricular orifice and the opening of the pulmonary trunk.
The part of the right ventricle that leads to the opening of the pulmonary trunk is called the infundibulum (or conus arteriosus).
It is a conical extension formed from the upper and left angles of the right ventricle.
The inner surface of the right ventricle is smooth only in the infundibulum. Elsewhere the muscular layer of the heart creates papillary muscles and irregular muscular columns called trabeculae carneae.
Right atrioventricular orifice and tricuspid valve
Venous blood from the right atrium enters the right ventricle through the right atrioventricular orifice.
Around the right atrioventricular orifice is the tricuspid valve.
The tricuspid valve has three leaflets - the anterior, posterior, and septal leaflets.
Around the orifice is a fibrous ring called the fibrous annulus.
The right atrioventricular valve opens during atrial systole. It allows deoxygenated blood from the right atrium to flow into the right ventricle. The valve closes during the ventricular systole.
The primary function of the tricuspid valve is to prevent the backflow of the blood from the ventricles into the atria.
There are three papillary muscles in the right ventricle. Leaflets are connected with chordae tendineae.
The chordae tendineae arise from one leaflet and insert into two adjacent papillary muscles.
The chordae tendineae prevent the valve leaflets from prolapsing into the right atrium.
The papillary muscles fix the tricuspid valve, but trabeculae carneae prevents blood swirling.
Opening of the pulmonary trunk and valve
The exit opening from the right ventricle is the opening of the pulmonary trunk. Deoxygenated blood enters the pulmonary trunk through this opening.
The trunk divides into two pulmonary arteries- right and left pulmonary arteries.
The opening is located behind the left sternocostal joint of the third rib.
Around the opening of the pulmonary trunk is the pulmonary valve, which is a semilunar valve.
It has three semilunar cusps: anterior left and right.
It is attached to a fibrous annulus located around the opening.
During ventricular systole, the pressure in the ventricle increases as the walls shrink. Valves are pressed against the inner surface of the pulmonary trunk, and the blood flows through it.
As the pressure in the right ventricle decreases, the valve closes.
The left atrium is located in the superior left corner, above the left ventricle, and opposite to the right atrium.
The pulmonary circulation ends in the left atrium, as oxygenated blood from the lungs enter it through the pulmonary veins.
The left atrium has a cuboid shape. It is thicker but smaller in volume than the right atrium.
The medial wall of the left atrium is formed by the interatrial septum, which separates the left and right atrium.
On the anterior surface of the left atrium lies the left auricle, a flap of the heart wall.
The left auricle has an irregular shape with many tiny ridges created by the pectinate muscles. The primary function of the left auricle is to increase the volume of the left atrium.
Elsewhere, the inner surface of the left atrium is smooth.
Openings of the left atrium
There are five openings found in the walls of the left atrium.
These include four openings for the pulmonary veins and one for the left atrioventricular orifice.
- The four openings for the pulmonary veins are located on the superior and posterior walls of the left atrium. The pulmonary veins carry oxygenated blood from the lungs to the left atrium.
- The left atrioventricular orifice is found on the inferior wall of the left atrium. It carries oxygenated blood to the left ventricle.
The left ventricle is one of the heart chambers located in the lower-left portion of the heart below the left atrium, opposite to the right ventricle.
The primary function of the left ventricle is to pump blood into the aorta, providing systemic circulation.
The left ventricle has a cone shape with a base directed upward, while the apex is directed inferiorly.
There are two openings found on the base of the left ventricle:
- the left atrioventricular orifice,
- and the aortic orifice.
There are fibrous rings and valves around both openings.
The anterosuperior portion of the left ventricle below the aortic orifice is called the aortic vestibule.
The inner surface of the heart is smooth at the aortic vestibule. Elsewhere the inner surface is covered by papillary muscles and trabeculae carneae.
Left atrioventricular orifice and mitral valve
Around the left atrioventricular orifice is the left atrioventricular valve (also called the mitral valve, bicuspid valve).
The mitral valve has two cusps - the anterior and posterior. Between both cusps are smaller ones called the commissural cusps.
The opening of the mitral valve is surrounded by a fibrous ring called the mitral annulus.
The chordae tendineae are inelastic tendons attached to the cusps and the papillary muscles within the left ventricle.
The chordae tendineae prevent the valve leaflets from prolapsing into the left atrium.
The left atrioventricular valve opens during atrial systole, allowing arterial blood from the left atrium to enter the left ventricle.
The valve closes during the ventricular systole.
Aortic orifice and valve
The aortic orifice is guarded by the semilunar aortic valve.
The aortic valve is made of three cusps- left, right and posterior.
The aortic orifice is surrounded by a fibrous ring called the annulus fibrosus, which fixes the aortic valve.
The aortic orifice is located in the midline behind the sternum at the 3rd intercostal level.
Heart sound locations and heart auscultation
Every valve can be auscultated at a specific place on the anterior wall of the chest.
Right atrioventricular or tricuspid valve can be auscultated in the fifth intercostal space on the right side of the sternum or the xiphoid process of the sternum.
The sound of the left atrioventricular or mitral valve can also be heard in the fifth intercostal space, approximately 1,5 centimeters medial from the left midclavicular line.
The aortic valve can be found in the second intercostal space on the right side of the sternum.
The pulmonary valve can also be heard in the second intercostal space but on the left side of the sternum.
Pulmonary and systemic circulation
The systemic circulation starts in the left ventricle containing oxygenated blood.
Through the aorta and its branches, blood spreads through the body and reaches the organs. Tissue receives the oxygen, and the oxygenated blood becomes deoxygenated.
It is being accomplished by the arterioles, pre-capillaries, capillaries, post-capillaries, and finally by the venules going back to the heart.
Deoxygenated blood reaches the heart through the superior and inferior venae cavae and coronary sinus, flowing into the right atrium, where ends the systemic circulation.
The pulmonary circulation starts with the right ventricle containing deoxygenated blood.
Blood reaches the lungs through the pulmonary trunk and right and left pulmonary arteries.
Arteries form a network of capillaries in the lungs and in the alveolar walls where happens the gas exchange. Deoxygenated blood becomes oxygenated and reaches the left atrium of the heart through the pulmonary veins.
The pulmonary circulation ends in the left atria.
Layers of the heart
The wall of the heart is formed by three layers:
- endocardium - the inner layer,
- myocardium - the middle layer, and
- epicardium - the outer layer.
The heart with all three layers is covered by the pericardium.
The endocardium is the inner layer, and it lines all the internal surfaces of the heart cavities. It covers the pectinate and papillary muscles, trabeculae carneae, valves, and chordae tendineae.
The structure of the endocardium is similar throughout the heart. However, in the left atrium and ventricle, the endocardium is thicker than in the right atrium and ventricle. And the atrial endocardium is more prominent than the ventricular endocardium. Part of the endocardium covering the chordae tendinae is very thin.
All of the heart valves are derived from the endocardium:
- the valve of the inferior vena cava and coronary sinus in the right atrium,
- right atrioventricular (three cusps) and pulmonary valves in the right ventricle,
- left atrioventricular (two cusps) and aortic valves in the left ventricle.
At the openings of the arteries and veins, the endocardium passes into the walls of blood vessels.
The myocardium is a thick muscular layer between the endocardium and epicardium.
It is formed by cardiac muscle fibers, also known as cardiomyocytes.
The primary function of the myocardium is to contract and pump blood out of the heart to provide organs with blood.
Cardiac muscles are characterized by rapid contractions that can not be controlled.
The myocardial layer is different in the atria and ventricles.
Myocardium in atria
In the atria, the myocardium has two layers:
- an outer circular layer that is common in both atria,
- an inner longitudinal layer that is separate in each atrium.
The longitudinal layer is formed by pectinate muscles.
Myocardium in ventricles
In the ventricles, the myocardium is formed by three layers:
- outer and inner layers are longitudinal and are common in both ventricles,
- a middle layer that is circular and separate in each ventricle.
In the ventricles, the muscles of the inner layer form the papillary muscles and the trabeculae carneae.
The outer longitudinal layer at the apex of the heart creates the vortex of the heart, known as the swirling arrangement of cardiac muscular fibers at the apex of the heart.
At the mentioned site, the outer longitudinal layer transitions into the inner longitudinal layer.
It happens vice versa as the inner longitudinal layer transitions into the outer longitudinal layer.
The epicardium is also called the visceral or inner layer of the serous pericardium. It is the outer serous layer of the heart.
It is a thin, transparent layer composed of loose connective tissue, elastic fibers, and adipose tissue.
The epicardium covers not only the heart but also the beginnings of the large vessels: the ascending aorta, the pulmonary trunk, the superior vena cava, the inferior vena cava, as well as the pulmonary veins before they return to the heart.
From these blood vessels, the epicardium further continues as the parietal layer of the serous pericardium.
The epicardium protects the inner heart layers. It participates in the production of the pericardial fluid.
The heart is surrounded by a closed fibrous sac called the pericardium. The pericardium separates the heart from the surrounding organs.
The pericardium contains two layers- an outer or fibrous and an inner or serous layer.
The outer or the fibrous layer forms a closed sac around the heart and separates the heart from the surrounding organs. It fuses with the diaphragm and pleura.
The anterior surface of the pericardium connects with the inner surface of the sternum and ribs.
The inferior surface of the pericardium fuses with the central tendon of the diaphragm and the anterior part of the muscular part of the diaphragm.
The lateral surfaces fuse with the mediastinal pleura.
The serous layer has two lamina - the parietal and the visceral lamina.
The visceral lamina of the serous pericardium is also known as the epicardium. It covers the outer surface of the myocardium, and at the large blood vessels of the heart, it continues as the parietal lamina.
The parietal lamina covers the heart and fuses together with the fibrous pericardium.
The space between the parietal and visceral laminae of the serous pericardium is called the pericardial cavity.
It contains a small amount of serous fluid that decreases the surface tension. It also lubricates surfaces, allowing free movement of the heart during contractions.
The pericardial cavity contains few recesses and two sinuses- transverse pericardial sinus and oblique pericardial sinus.
The heart works as a pump.
The main function is to pump the blood to the systemic and pulmonary circulations providing the deoxygenated and oxygenated blood exchange. Therefore the heart provides the tissue with the oxygen.
The heart is the main organ in the circulatory system, and its rhythmic contractions keep people alive.
- The heart generates blood pressure.
- The heart routes the blood as it separates the pulmonary and systemic circulations.
- It ensures one-way blood flow because of the valves.
- The heart regulates the blood supply as it changes contraction rate and force, responding to metabolic needs.
- It provides the cardiac cycle.
The rhythmic action of the heart and, therefore, the blood flow to the systemic and pulmonary circulations are provided by the sequential contractions of the atria and ventricles. It is called the cardiac cycle. The cycle starts with one heartbeat and ends with another.
Each cycle can be divided into two main phases- diastolic and systolic phases.
During the diastolic phase, the heart chamber is relaxed, and it fills with blood.
During the systolic phase, the heart chambers are in a contracted state and pump the blood further. Each atrium and ventricles go through both mentioned phases.
When atria are in the systolic phase, the ventricles are in the diastolic phase and opposite.
There are several subphases of the cardiac cycle:
- atrial diastole,
- atrial systole,
- ventricular diastole,
- ventricular systole.
During this phase, the atria are relaxed, and the blood passively fills the right atrium via the superior and inferior vena cava and the left atrium through the pulmonary veins. During this phase, the atrioventricular valves are closed.
When the cavity is filled with blood, the pressure in the atria is more significant than it is in the ventricles, that is why the mitral and tricuspid valves open, and the blood flows into the ventricles.
During the atrial systole, contract both atria and the blood through the tricuspid and mitral valves flow into the ventricles.
In this phase, both ventricles are relaxed and are in a diastolic state. Tricuspid and mitral valves between the atria and ventricles are open.
During this phase, the atria are in a contracted state, and ventricles fill up with the blood.
During this phase, both ventricles are in a contracted state.
The blood through the semilunar valves flows into the blood vessels providing systemic and pulmonary circulations (aorta and pulmonary arteries). In this phase, the ventricles empty.
Tricuspid and mitral valves during this phase are closed, preventing the blood backflow into the atria.
In this phase, the walls of the atria are relaxed and are in a diastolic state.
Arterial blood supply
The arterial blood supply to the heart and heart muscles is provided by the left and right coronary arteries and their branches.
Right coronary artery
The right coronary artery has the following branches supplying the heart- sinoatrial nodal branch, right marginal branch, atrioventricular nodal branch, posterior interventricular branch.
All mentioned branches except the right marginal artery supply the right atrium and the right ventricle.
The right marginal artery supplies the right ventricle and the apex of the heart.
The posterior interventricular artery also supplies the interventricular septum.
To sum up, the right coronary artery with its branches supply the right atrium and interatrial septum, sinoatrial and atrioventricular nodes, all posterior and partially anterior wall of the right ventricle, posterior third of the interventricular septum, all papillary muscles in the right ventricle, posterior papillary muscle in the left ventricle, a small part of the posterior wall of the left ventricle.
Left coronary artery
The left coronary artery has the following branches supplying the heart- left circumflex branch, anterior interventricular branch, left marginal artery.
To sum up, the left coronary artery perfuses the left atrium and the left ventricle (except a small portion of the posterior wall), part of the anterior wall of the right ventricle, two anterior thirds of the interventricular septum, atrioventricular bundle, and anterior papillary muscles in the left ventricle.
The coronary sinus provides the venous drainage of the heart with its tributaries. The coronary sinus drains into the right atrium.
The main tributary of the coronary sinus is the great cardiac vein arising at the apex of the heart.
Besides the great cardiac vein, the middle and small cardiac veins also drain into the coronary sinus.
The left marginal vein and left posterior ventricular veins also provide the drainage of the heart.
The heart drains via subendocardial, myocardial nodes and then via subepicardial plexus forming the right and left cardiac collecting trunks.
The left trunk drains into the tracheobronchial, and the right trunk drains into the brachiocephalic lymph nodes.
The heart is innervated by sympathetic and parasympathetic fibers from the autonomic nervous system.
Nerves form the cardiac plexus, which contains fibers from both parts of the autonomic nervous system.
Parasympathetic efferent and afferent innervation are provided by the vagus nerve (CN X).
These nerves reduce the heart rate and the force of contraction of the heart and provide the vasoconstriction of the coronary arteries.
The vagus nerve contains several cardiac branches such as thoracic cardiac, inferior and superior cervical cardiac branches.
The sympathetic efferent innervation is provided by the sympathetic trunk and cardiac nerves of the lower cervical and upper thoracic ganglia. These nerves opposite the parasympathetic branches increase heart rate and the force of contraction of the myocardium.
The sympathetic afferent innervation is also provided by the lower cervical and upper thoracic ganglia.
Sympathetic afferent fibers provide the pain sensation of the heart.