Small & medium size veins: T. intima: formed of endothelial cells & thin subendothelial layer. T. media: formed of small bundles of smooth muscles with reticular & elastic fibers. T. adventitia: well developed & formed mainly of collagen fibers.
Large veins: (vena Cava): T. intima: well developed with discontinuous internal elastic lamina. T. media: thin & formed of few layers of smooth muscle cells with abundant connective tissues. T. adventitia: the thickest & best developed layer contain longitudinal bundles of smooth muscles.
Tunics of the heart:
Endocardium: lines the chambers of the heart, it vary in thickness in different areas. It consist of single layer of squamous endothelial cells rest on a thin subendothelial layer of loose connective tissues. Between the endocardium and myocardium is a layer of connective tissue called subendocardial layer that contain veins, nerves & some purkinje fibers which are part of the impulse conducting system of the heart.Myocardium: is the thickest layer consist of cardiac muscles those muscles are arranged in layers that surround the heart chambers in a complex spiral. The muscle cells of the heart are grouped into two groups: contractile cells & the impulse generating & conducting cells responsible for the electrical signal that initiate the heartbeat.
Epicardium: the heart is covered externally by simple squamous epithelium supported by a layer of connective tissue called subepicardial layer which contain veins, nerves & nerve ganglia of the heart. the adipose tissues that surround the heart will accumulate in this layer.
Fibrous skeleton of the heart: Is a dense connective tissue with collagen fibers which will give support to the heart valves & also give origin & insertion to the cardiac muscles. Valves of the heart: Cardiac valves consist of a central core of dense connective tissue lined on both sides with endothelial cells & the bases of the heart valves attached to the fibrous skeleton of the heart.
Conducting system of the heart:
The impulse generating and conducting system of the heart consist of several structures that make it possible for the atria and ventricles to beat in succession.The Sinoatrial node: located close to the entrance of the superior vena cava into the right atrium. The cells of the SA node are modified cardiac muscle cells smaller then atrial muscle cells, those nodal cells are arranged around a large nodal artery. The impulse will be carried from the SA node to the AV node through tracts of specialized cells.
Atrioventricular node: mass of specialized cardiac muscle cells lies in the septal wall of the right atrium, the cells of AV node is similar to those of the SA node.
The small fibers that emerge from the AV node will be regularly arranged together & eventually become a distinct bundle of parallel fibers that will conduct the impulse from the AV node to the ventricle & this bundle is called Bundle of His. Those fibers are called Purkinje fibers which will make contact with the cardiac muscles to stimulate their contraction & they have a diameter which is considerably greater then that of the ordinary cardiac muscle cells.