- Located in the middle mediastinum
- Has two ventricles and two atria that connect the two major circulatory systems of the body: the pulmonary circulation with the systemic circulation
- Has valves to ensure blood flow only in one direction
- Surrounded by pericardium (fibroserous, fluid-filled sac)
- About the size of a fist and weighs ∼ 300–500 g
The heart has:
- Right atrium
- Left atrium
Two ventricles of the heart
- Right ventricle
- Left ventricle
- Tricuspid valve: consists of three leaflets located between the right atrium and right ventricle
- Bicuspid valve (mitral valve) : consists of two leaflets located between the left atrium and the left ventricle
The tricuspid valve has three leaflets and is located on the right side, as is the three-lobed right lung. The mitral valve has two leaflets and is located on the left side, as is the two-lobed left lung!
- Pulmonary valve: between right ventricle and pulmonary trunk
- Aortic valve: between left ventricle and aorta
- Pulmonary circulation: Deoxygenated blood in the right heart flows into the lungs, where it is oxygenated, and delivered back to the left atrium.
- Systemic circulation: Oxygenated blood flows from the left ventricle into the systemic circulation and, after passing through the capillary bed, flows back in a deoxygenated state to the right atrium of the heart to restart the process.
- Definition: A layer of connective tissue that separates the atria from the ventricles and anchors the valves with fibrous rings (annuli fibrosi cordis) surrounding the atrioventricular and arterial orifices.
Internal structures of the heart
|Ventricle|| || |
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 spread the signal across the ventricles → the ventricles contract
|Name||Anatomic localization||Characteristics||Frequency||Action potential|
| || ||ca. 40–50/min|
| || |
|Purkinje fibers|| |
See in cardiovascular physiology for more details.
- The left and right coronary artery arise from the ascending aorta and supply arterial blood to the heart muscle.
- They fill with blood during diastole because they are compressed during ventricular systole.
- Course: arises from right aortic sinus of the ascending aorta
- Course: arises from left aortic sinus of the ascending aorta and anastomoses with the RCA
Types of circulation
- Right-dominant (∼ 85% of people): posterior descending artery (PDA) arises from RCA
- Left-dominant (∼ 8% of people): PDA arises from left circumflex artery (LCX)
- Codominant (∼ 7% of people): PDA arises from both RCA and LCX
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!
The venous blood from the heart is drained into the right atrium by the coronary sinus.
- Left side drains into great cardiac vein → coronary sinus → right atrium
- Right side drains into middle and small cardiac veins → coronary sinus → right atrium
- Sympathetic fibers
- Parasympathetic fibers (from the vagal nerve)
- Phrenic nerve: sensory innervation of the pericardium
Lymph of the heart drains into anterior mediastinal nodes and tracheobronchial nodes.
Appearance on imaging
- Cardiac silhouette on chest x-ray:
- Heart tissue consists of four layers.
Endocardium: inner layer with three sublayers
- Endothelium (innermost): simple squamous epithelium
- Smooth muscle and connective tissue
- Subendocardium (outermost) - loose connective tissue
Myocardium: thick myocardial layer
Cross-striated muscle cells (cardiomyocytes) with single centrally located nucleus
- Each cell contains myofibrils organized into cardiac muscle). (smallest functional contractile unit of
- Contain many mitochondria to produce ATP for contraction
- Atrial cardiomyocytes release atrial natriuretic peptide (ANP) when stretched to lower BP→ ↑ water and sodium excretion by the kidneys.
Cardiomyocytes are joined together by intercalated discs to form long fibers.
- Intercalated discs connect the single cardiomyocytes to a functional syncytium and force transmission during muscle contraction.
- Contain adherent junctions (transmit mechanical stimuli) and gap junctions (transmit electrical stimuli)
- Appear as dark-staining lines between cardiac muscle cells under light microscopy
- Cardiac Purkinje cells: Specialized/modified cardiomyocytes of the heart conduction system with fewer contractile myofibrils and more mitochondria and glycogen
- Fibroblasts (turn into myofibroblasts after injury)
- Extracellular matrix: collagen, elastin, and glycosaminoglycans
- Cross-striated muscle cells (cardiomyocytes) with single centrally located nucleus
Epicardium: a connective tissue layer where the visceral layer of the serous pericardium comes into contact with the heart
- Contains blood vessels, lymphatics, and adipose tissue
- Pericardium: a fibroserous sac enclosing the heart that consists of a 2-layered inner serous pericardium and an outer fibrous pericardium
- Endocardium: inner layer with three sublayers
Heart valves: composed of connective tissue and endocardium
- Mostly vessel-free and derive nutrition from the surrounding blood.
As the pericardium is rather stiff, the capacity of the pericardial space is limited. An acute pericardial effusion (e.g., following cardiac wall rupture, a complication of myocardial infarction), can lead to pericardial tamponade and a life-threatening reduction in cardiac output. In such cases, quick pericardial fluid drainage (pericardiocentesis is required).
- The mesoderm is the embryonic progenitor of all muscle (cardiac muscle, smooth muscle, and skeletal muscle).
The following steps are important during heart development:
- Development of a heart tube from two single endocardial tubes
- Transformation of the straight heart tube into an S-shaped heart loop
- Separation of the atria and ventricles through endocardial cushions that grow towards each other and fuse
- Formation of two atria, two ventricles, and division of one common outflow tract into an aortic trunk and a pulmonary trunk.
Heart structures and their embryonic origins
Left and right ventricle outflow tract
Trabeculated portion of atria
Trabeculated portion of ventricles
Smooth portion of left atrium
Smooth portion of right atrium
- Week 4 of gestation: looping of the primary heart tube establishes left-right polarity
- Initially, the two atria communicate via the foramen primum.
- The cranially located septum primum extends caudally towards the dorsal endocardial cushion, which narrows the foramen primum.
- As the foramen primum disappears, the two atria communicate via a newly-formed foramen secundum in the septum primum.
- A newly-formed cranially located septum secundum extends caudally towards the dorsal endocardial cushion. During this time, the septum primum expands, which narrows the foramen secundum and leaves the residual foramen ovale.
- The septum primum then develops into the valve of the foramen ovale.
- The septum secundum and remaining septum primum ultimately join to form the atrial septum.
- Foramen ovale closes shortly after birth as the left atrial pressure increases (due to increased pulmonary circulation upon lung inflation).
- 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.
- Endocardial cushions grow to further separate the ventricles and the atria.
Defective development of the heart valves can result in:
- Ebstein anomaly: a tricuspid valve leaflet is caudally displaced
- Tricuspid atresia: the tricuspid valve is hypoplastic or completely absent
- Bicuspid aortic valve: the most common congenital valvular heart defect.
Formation of the outflow tract
- Neural crest and endocardial cells migrate to form truncal ridges and bulbar ridges from the truncus arteriosus and bulbus cordis, respectively.
- Truncal and bulbar ridges spiral and fuse to form the aorticopulmonary septum (AP septum).
- The AP septum develops into the ascending aorta and pulmonary trunk
Associated conotruncal abnormalities:
- Failure to spiral: transposition of the great vessels
- Malaligned AP septum: tetralogy of Fallot
- Partial AP septum development: persistent truncus arteriosus