The heart is a muscular organ located in the middle mediastinum that pumps blood through the circulatory system. The heart is surrounded by the pericardium and is divided into four chambers: two atria and two ventricles. The right atrium and ventricle are often referred to as the right heart while the left atrium and ventricle are often referred to as the left heart. The atria and ventricles are separated by the atrioventricular valves, while the ventricles and the arterial outflow tracts of the heart (namely the pulmonary trunk and the aorta) are separated by the semilunar valves. The heart wall consists of the endocardium (innermost), the myocardium, and the epicardium. The conduction system of the heart is composed of specialized nodes and pacemaker cells that initiate and coordinate the contraction of the heart.
The right heart receives deoxygenated blood from the systemic circulation and pumps it through the pulmonary circulation, where it becomes oxygenated. The left heart then receives the oxygenated blood from the pulmonary circulation and pumps it through the blood vessels of the systemic circulation. The coronary arteries, namely the right coronary artery and the left coronary artery, arise in the root of the aorta and supply the myocardium and endocardium. The heart develops embryologically from the heart tube, which undergoes looping and septation to separate it into the four chambers.
- Two ventricles and two atria, which connect the with the
- Four valves, which ensure that blood flow occurs in only one direction
- Roughly the size of a fist
- Weighs approx. 300–500 g
- Surrounded by fluid-filled sac) (a fibroserous,
Location: in the between the lungs
- Anterior to the heart: sternum and rib cartilage
- Posterior to the heart
- The upper part of the heart is at the level of the third costal cartilage.
- The site of attachment for the venae cavae, aorta, and pulmonary trunk
- The lower part of the heart (cardiac apex) lies left of the sternum at the level between the fourth and fifth ribs.
- Pumps blood through the body via the
The left atrium is the posteriormost part of the heart, located directly in front of the esophagus. It can be visualized using TEE. The right ventricle is the anteriormost part of the heart and is at greatest risk of injury following chest trauma.
The cardiac apex beat can typically be palpated to the left of the sternum, medial to the midclavicular line at the 4th–5th intercostal space. In patients with dextrocardia, the orientation of the heart is inverse so the apex is located to the right of the mediastinum rather than the left.
- Receives deoxygenated blood via the superior vena cava (SVC), inferior vena cava (IVC), and the coronary veins
- Pumps deoxygenated blood into the right ventricle
- Right auricle (right atrial appendage)
- Receives oxygenated blood from the four pulmonary veins
- Pumps oxygenated blood into the left ventricle to enter the systemic circulation
- Left auricle (left atrial appendage)
- Right ventricle
- Receives oxygenated blood from the left atrium
- Pumps oxygenated blood into the aorta as part of the systemic circulation
- Frontal view
- Lateral view
The cardiac borders form the cardiac silhouette on chest x-ray!
- Two types of cardiac valves that differ in location and morphology
- Cardiac skeleton
- Closure of heart valves produces (See in for details.)
This mnemonic provides the order in which blood flows through the heart valves: Try PULling My AORTA (Tricuspidal, Pulmonary, Mitral, Aortic)!
- Structure: leaflets supported by subvalvular apparatus
- Chordae tendineae: fibrous cords that support the AV valves and connect them to the papillary muscles
Papillary muscles (two in the left ventricle; three in the right ventricle)
- Derive from the myocardium
- Extend from the anterior and posterior ventricular walls and the septum
- Have apices that are attached to the chordae tendineae
- Contract during systole and thereby tighten the chordae tendineae: prevent prolapse of valve leaflets and regurgitation into the atria when pressure rises during ventricular contraction
This mnemonic provides the rule of twos and threes for the atrioventricular valves: The tricuspid valve has three leaflets and is located on the right side, as is the three-lobed right lung. The bicuspid (mitral) valve has two leaflets and is located on the left side, as is the two-lobed left lung!
- Structure: : three crescent-shaped cusps without subvalvular apparatus
- The left and right coronary arteries arise from the root of the aorta and supply the heart muscle with arterial blood.
- Dominance of circulation
- Coronary blood flow peaks during early diastole at a point when the pressure differential between the aorta and the ventricle is the greatest (see left ventricular pressure-volume diagram.)
|Left coronary artery (LCA)|| |
|Right coronary artery (RCA)|| || || |
| || |
- Largest vein of the heart, into which all other coronary veins drain
- Lies in the left posterior atrioventricular groove
- Drains into the right atrium between the IVC orifice and the right atrioventricular orifice
- Veins draining into coronary sinus (coronary veins):
|Somatic nervous system|| |
|Sympathetic nervous system|| |
|Parasympathetic nervous system|
- Definition: collection of nodes and specialized conduction cells that initiate and coordinate contraction of the heart muscle.
- Consists of
- Normal course of electrical conduction: SA node (pacemaker) creates an action potential → signal spreads across atria and causes their contraction . → Signal reaches AV node and is slowed down → AV node conducts the signal to bundle of His down the interventricular septum to Purkinje fibers in myocardium → they carry the signal across the ventricles → the ventricles contract
| ||ca. 40–50/min|
|Bundle of His|| || ||ca. 30–40/min|
| || |
See in cardiovascular physiology for more details.
During states of increased heart rate (e.g. during exercise), the duration of diastole decreases so that there is less time for the coronary arteries to fill with blood and supply the heart with oxygen. Patients with narrow coronary arteries, e.g., due to atherosclerosis, will therefore experience chest pain (angina pectoris) during exertion!
Layers of the heart
- The heart wall itself consists of three layers (from inside to outside):
- Pericardium: membrane that directly surrounds the heart
Description: innermost layer of heart tissue, consisting of the following three sublayers:
- Endothelium (innermost): simple squamous epithelium
- A layer of loose connective tissue
- Subendocardium (outermost): loose connective tissue containing
Description: thick myocardial layer composed of the following:
Cardiomyocytes: striated muscle cells containing a single, centrally located nucleus
- Contain many mitochondria, which produce ATP for contraction.
- Myofibrils within cardiomyocytes are organized into (smallest functional contractile unit of cardiac muscle).
Connected by intercalated discs to form long fibers.
- Intercalated discs connect individual cardiomyocytes into a functional syncytium and force transmission during muscle contraction.
- They contain; adherent junctions (transmit mechanical stimuli) and gap junctions (transmit electrical stimuli)
- They appear as slightly darker-staining lines between cardiac muscle cells under light microscopy and electron microscopy.
- See also in the learning card on .
- Atrial cardiomyocytes release atrial natriuretic peptide (ANP) when stretched (i.e., at higher BPs) → ↑ water and sodium excretion by the kidneys → ↓ BP
- Fibroblasts (these become myofibroblasts after injury)
- Extracellular matrix: collagen, elastin, and glycosaminoglycans
- Cardiomyocytes: striated muscle cells containing a single, centrally located nucleus
- Fibroserous sac enclosing the heart
- Defines the pericardial cavity
Serous pericardium (innermost)
- Visceral layer of serous pericardium (epicardium)
- Parietal layer of serous pericardium
- Fibrous pericardium (outermost)
- Serous pericardium (innermost)
- Pericardial cavity: space between the visceral and parietal layers of the serous pericardium that contains serous, pericardial fluid
- Innervated by the phrenic nerve
The capacity of the pericardial cavity is limited by the stiff, fibrous pericardium. If fluid abnormally accumulates in the pericardial space (pericardial effusion), intrapericardial pressure increases and may impair cardiac function → pericardial tamponade.
Microscopy of the heart valves
- Composed of connective tissue and endocardium
- Mostly vessel-free, with nutrition derived from the surrounding blood (This makes valvular involvement in endocarditis difficult to treat because both the cells of the immune system and antibiotics typically reach sites of infection via the circulatory system.)
- Mesodermal origin
Steps of heart development
- Two single endocardial tubes merge to form the heart tube.
- The straight heart tube transforms into an S-shaped heart loop.
- Endocardial cushions grow towards each other and fuse to separate the atria and ventricles.
- Two atria and two ventricles form and one common outflow tract divides into an aortic trunk and a pulmonary trunk.
- Valves form from the endocardial cushion.
- Fetal circulation is covered in the corresponding section of prenatal and postnatal physiology.
- Postnatal derivatives of fetal vascular structures are covered in the section on in the prenatal and postnatal physiology learning card.
|Embryonic structures||Details||Give rise to|
Two protuberances located on the dorsal and ventral inner surfaces of the primitive heart tube
Atrial septum, interventricular septum, and heart valves
A single cavity separated from the ventricular cavity by the endocardial cushion
|Trabeculated portions of the atria|
A single cavity separated from the atrial cavity by the endocardial cushion
|Trabeculated portions of ventricles|
Forms from a pouch in the dorsal wall of the primitive left atrium
|Smooth portion of the left atrium|
Cavity at the caudal end of the embryonic heart tube in which the veins from the embryonic circulatory arcs unite
|Bulbus cordis||Left and right ventricular outflow tracts|
|Truncus arteriosus|| |
A single arterial trunk that originates from both ventricles of the embryonic heart
|Ascending aorta and pulmonary trunk|
Posterior, subcardinal, and supracardinal veins
|Inferior vena cava|
|Superior vena cava|
See and for associated heart defects.
Week 4 of gestation: looping of the primary heart tube establishes left-right polarity
The heart begins to beat spontaneously by week 4 of gestation and is detectable via transvaginal ultrasound by week 6 of gestation.
- Development of the septum primum and foramen primum
- Narrowing of the foramen primum occurs as the septum primum grows towards the endocardial cushion.
- Development of the foramen secundum
- Development of the septum secundum
- Development of the foramen ovale cordis
Formation of the atrial septum (no further interatrial communication)
- The septum secundum fuses with the remnant of the septum primum to form the atrial septum.
- occurs shortly after birth, when the left atrial pressure increases (due to loss of low resistance placental circulation) and the right atrial pressure decreases (due to increased pulmonary circulation upon lung inflation).
If the septum primum and septum secundum fail to fuse after birth, a patent foramen ovale (PFO) remains. Later in life, a PFO can result in a paradoxical embolus, in which a venous thrombus may travel via the PFO from the venous to the arterial circulation and cause end-organ infarction (e.g., stroke).
- A caudally located muscular interventricular septum forms with an interventricular foramen between the two ventricles.
- The cranially developing aorticopulmonary septum rotates and caudally fuses with the muscular interventricular septum → this fusion forms the membranous interventricular septum and closes the interventricular foramen (heart).
- Endocardial cushions grow to further separate the ventricles and the atria.
Development of the outflow tract
- Development of the aorticopulmonary septum (AP septum)
- Division of the ventricular outflow tract
Associated conotruncal abnormalities:
- Failure to spiral: transposition of the great vessels
- Malaligned AP septum: tetralogy of Fallot
- Partial AP septum development: persistent truncus arteriosus
|Development of semilunar valves|| |
|Development of atrioventricular valves|| |
Defective development of the heart valves can result in
- Stenotic valves
- Regurgitant valves
- Displaced valves (e.g., Ebstein anomaly)
- Atretic valves (e.g., Tricuspid atresia)
- Minor abnormalities (e.g., Bicuspid aortic valve)