Heart Histology General Information

Despite popular opinion, the heart is not just made of contractile cardiac muscle tissue but contains a fibrous skeleton and nervous tissue.  Disappointingly, it is not shaped like the traditional heart that adorns Cupid’s underwear.  However, the myocardium, made of the contractile cardiac muscle tissue is engorged with blood from coronary vessels and takes up a red color with the traditional H&E stain. 

The heart wall is composed of a few layers of tissue.  The endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces f, lining the open chambers of the heart, atria and ventricles, is simple squamous epithelium.  It is essentially just a continuation of the endothelium or the collective simple squamous epithelium that lines all blood vessels.  These flattened cellsThe basic structural and functional units of life. reduce the friction created when blood flows over these surfaces.

Superficial to the endothelium is the myocardium or the layers of contractile cardiac muscle.  The myocardium is arranged in spiral bundles surrounding the atria and ventricles.  These spiral bundles surround the ventricles differently.  The left ventricle is completely circular with a very thick myocardium.  The right ventricle is more of a semi-circle and has a thinner myocardium.  These differences account for the different circuits that these ventricles serve.  Blood leaving the left ventricle is bound for the systemic circuitThe part of the circulatory system that carries oxygenated blood from the heart to the body and retu, which is large and requires a large amount of pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange..  In factA statement based on direct observation that is repeatedly confirmed., in order for blood to leave the left ventricle, that pressure needs to be at least 120 mmHg to overcome the afterload or the pushback from the aorta.  Blood leaving the right ventricle is bound for the pulmonary circuitThe circulation of blood between the heart and the lungs, where blood is oxygenated., a much smaller and closer circuit.  Because the volume of blood leaving the right and left ventricles has to be equivalent, the right ventricle pumps the same volume of blood with a lower pressure as a compromise. 

The pericardiumThe membrane surrounding the heart. is a fluid-filled sac surrounding the heart.  It is a serous membrane, similar to the pleuraThe double-layered membrane surrounding the lungs and lining the thoracic cavity. of the lungs and the peritoneumThe membrane lining the abdominal cavity and organs. of the abdominal and pelvic cavities.  The visceral layer of the pericardium hugs the heart like a stocking.  Because of this very close association, the visceral pericardium is also called the epicardium.  Either way, it’s a layer of tissue between the myocardium and the pericardial cavity filled with fluid.  The parietal pericardium has only one name and is the superficialNear the surface of the body. layer of the heart that weeps serous fluid into the pericardial cavity.  Excess fluid in this area prohibits the heart from making physical movements associated with contraction.  This is called tamponade and is easily remedied by a really big needle into the chest to remove excess fluid.

The fibrous skeleton of the heart is analogous to the bones to which skeletal muscle is attached.  When skeletal muscle contracts, it provides tension to pull on a bone, creating movementA fundamental property of life involving motion of the body or its parts..  When cardiac muscle contracts, it pulls on the fibrous skeleton to produce a change in the shape of the ventricles and atria.  The fibrous skeleton consists of many collagenA structural protein in the dermis that provides strength and elasticity. bundles, making it fall in the spectrum between dense regular and dense irregular connective tissue.  These collagen bundles extend to create the valves of the heart, which are also lined with the endocardium to reduce friction.