Cardiovasular system – Introduction ( The heart )

The circulatory system contains a pump (the heart) , a set of connected tubes (blood vessels or vascular system), and a mixture of extracellular fluid and cells (the blood) that fills them. This body-wide transport system is often termed as cardiovascular system.

Anatomy of heart

·        Muscular organ enclosed in pericardium and closely affixed by epicardium.

·        Cells of the walls of the heart is made of cardiac muscle cells called myocardium. Inner surface of chamber covered by thin layer of cells called endothelium.

·        Has four chambers – two atrium and two ventricles. Atroventricular valves are located between the atrium and ventricles. These valves allow blood to flow from atrium to ventricle. Right AV valve is called tricuspid valve. Left AV  valve is called bicuspid valve.

·        To prevent AV valves from being pushed up into atrium , the valves are fastened to muscular projections (papillary muscles) of the ventricular walls by fibrous strands(chordae tendinae).

·        Opening of right ventricle into pulmonary trunk and the left ventricle into the aorta also contains valves, Pulmonary and Aortic valves respectively. (also called semi-lunar valves). These valves allow blood to flow into arteries during ventricular contraction but prevent blood from moving in opposite direction during ventricular relaxation.

·        There are no valves at the entrances of the superior and inferior venae cavae into the right atrium, and of the pulmonary veins into the left atrium.

Cardiac muscle

·        Arranged in layers that are tightly bound together and completely encircle the blood-filled chambers.

·        Cells are striated – result of an arrangement of thick myosin and thin actin filaments

·        Cardiac muscle cells are shorter than skeletal muscle fibers

·        Adjacent cells are joined end to end at structures called intercalated discs. Adjacent to the intercalated discs are gap junctions.

·        1 percent of cardiac cells have specialized feature that are essential for normal heart excitation. These cells constitute a network known as conducting system of the heart and are in contact with cardiac muscle cells via gap junctions.

Innervation

·        The heart receives a rich supply of sympathetic and parasympathetic nerve fibers, the latter contained in the vagus nerves.

Blood supply

·        The arteries supplying the myocardium are the coronary arteries, and the blood flowing through them is termed the coronary blood flow. The coronary arteries exit from the very first part of the aorta and lead to a branching network of small arteries, arterioles, capillaries, venules, and veins similar to those in other organs. Most of the coronary veins drain into a single large vein , the coronary sinus, which empties into right atrium.