The heart is a muscular organ that contracts in a rhythmic way, pumping the blood through the circulatory system. The heart is composed by three layers:
The most internal: endocardium
The middle layer: myocardium
The most external layer: pericardium
Sketch of the heart by Leonardo Da Vinci
The fibrous central region called fibrous skeleton, composed by fibrils, supports the valves and it’s the location where cardiac muscular cells are inserted. The endocardium is, as endothelium, a squamous keratinized epithelial layer that lays on a thin layer of connective tissue with elastic fibres and collagen and a few smooth muscular cells making possible the communication between myocardium and a subendothelial layer made of conjunctive tissue with veins, nerves and ramifications of the heart's conductive system. The myocardium is the thickest level of the heart and it’s formed by cardiac muscular cell layers. Most of those layers are part of the cardiac fibrous skeleton.
The heart's layers
Externally, the heart is covered by a squamous epithelial layer (mesothelium) that has right beside it a thin layer of dense conjunctive tissue making this structure the epicardium. Right below, it has a structure made with loose conjunctive tissue, veins, nerves, and adipose tissue called pericardium, a serous membrane. Between parietal leaflet and visceral leaflet, there is a small amount of liquid that makes easier the heart movements.
The cardiac skeleton is composed by dense conjunctive tissue with large collagen fibres oriented in several directions and in some specific regions we can find fibrocartilage nodes. It has four principal components:
Annuli fibrosi - Ring around openings for attachment of valves
Membranous septum - Upper end of interventricular septum
Fibrous septum - Fibrous tissue in muscular part of interventricular septum
The cardiac valves are a frame-like structure made of dense conjunctive tissue with collagen and elastic fibres protected on both sides by an endothelial layer. The valves bases are supported by fibrotic rings of the cardiac skeleton.
The heart has a very singular system to create its own electrical and rhythmical impulse. It’s a group of two nodes.
Sinoatrial node: in the right atrium, this nodes is an amount of cardiac muscular cells that are very unique – they are fusiform, smaller than the other atriums’ cardiac muscular cells and they have a smaller amount of microfibrils
Atrioventricular node: it’s similar to the sinoatrial node but the cells in this structure branch and make cytoplasmic projections in several directions originating a net.
Atrioventricular bundle: formed by cells that are similar to the atrioventricular node but the distant you get from the beginning of the bundle, the bigger the cells get. They also get a new shape. These cells are called Purkinje cells and they are uni or binucleated cells with a large amount of mitochondria and glycogen. The microfibers are rare and located in the periphery of the cytoplasm. At a certain point the atrioventricular bundles subdivide and get into the ventricle becoming intramyocardial bundles, making the stimulus to reach into deeper areas of the ventricle.