Heart: The heart is composed of 3 layers or tunics;
(a) Endocardium
(b) Myocardium
(c) Epicardium.
The endocardium is a continuation of tunica intima constisting of 3 parts:
(a) endothelium resting on a basal lamina and associated thin layer of collagen fibers.
(b) A deeper dense layer of connective tissues containing elastic fibers and a few smooth muscles cells.
(c) The subendocardial zone of loose connective tissues that contains small blood vessels and nerves. In the ventricle, the subendocardial zone may contain scattered purkinje fibers (Singh, 2005).
Myocardium: Consist of 2 components;
(a) Fascicles of cardiac muscles cells that connects to the fibrous connective tissue skeleton of the heart.
(b) Non-contractile, modified muscle cells that form the impulse (action potential) generating and conducting system of the hear. This system is composed of the atrioventricular and Sino atrial nodes, and the purkerije fibers. The cells of this system are muscle cells that are specialized for impulse (action potential) generation and impulse conduction. These cells generate and conduct action potential that synchronize the heart beat. The purkenje fibers are scattered along the innermost portion of the myocardium next to the endocarium (Singh, 2005).
Epicardium: Is the epithelium and connective tissues covering the heart. It is a squamous to cuboidal epithelial external linning, a continuation of the epithelium that lines the periciardial cavity. Below this, is a layer of connective tissues with high concentration of elastic fibers between the elastic layer and myocardium is the subepicardial layer that consist of loose connective tissues, nerves, veins and adipose tissue (Gartner et al, 2007 and Singh, 2005).
ARTERIES
The histological structures of an artery varies considerably with its diameter. However, all arteries have some features in common which are as follows;
(1) TUNICA INTIMA: A layer of simple squamous epithelium called endothelium. The endothelial layer is encircled by a subendothelial layer of loose connective tissue containing a few smooth muscle cells. The intima often appears wrinkled in sections due to contraction of the smooth muscle cells (Gartner et al, 2007)
(2) TUNICA MEDIA: This layer encircles the tunical intima. It consist of circumferential smooth muscle with extracellular matrix secreted by cells between themselves that consist of collagen fibers and elastin as well as various proteoglycans. In most arteries, a layer of elastin called internal elastic lamina separates the intima and media (Gartner et al, 2007).
(3) TUNICA ADVENTITIA: Is a connective tissues layer containing many collagen and elastic fiber. This layer gradually becomes continuous with the connective tissues of the organ, the vessel is in (Gartner et al, 2007)
ARTERIOLES: are small arteries of 0.04-0.5 mm in diameter. The tunical intima has a very thin subendothelial layer of loose connective tissue. Its tunica media is muscular and composed of 1-3 layers of smooth muscle. The adventitia is fairly prominent and contains thin collagen and elastic fibers (Gartner et al; 2007).
CAPILLARIES
Is the smallest segment of blood vascular system usually 7-9um in diameter, but may be as small as 5um and as large as 12um in diameter. The wall consists of a simple squamous epithelium called endothelium. This simple squamous epithelium is surrounded by a basal lamina. Cells called pericytes may be sporadically found between the basal lamina and the endothelial cells. The cells are able to contract and thus constrict capillaries (Gartner et al, 2007).
The walls of certain types of capillaries may have pores (fenestrae) penetrating the squamous endothelial cells. Capillaries are of 2 types;
1) Fenestrated (perforated capillaries): Pores penetrate the endotherlial cells. They are found in tissues where rapid exchange of substance occurs (Kidney or villi of the intestine).
There are of 2 types,
1) Fenestrated capillaries with diaphragm.
2) Fenestrated capillaries without diaphragm (Singh, 2002).
2) Non-fenestrated capillaries: Are those capillaries whose endothelial lining are continuous and do not have holes or poles on their walls. They present functional complexes that allow a slight degree of permeability (Singh, 2002)
VENULES
Is the smallest veins of about 0.2-1mm in diameter. The intima lacks a subendothelia layer of connective tissues. It has a thin media consisting of a few smooth muscle fiber. As the size of the venules increases, the amount of muscle increases. A well developed adventitia rich in collagen fibers is present (Singh, 2002 and Gartner et al, 2007).
VEINS
The basic structure of veins is similar to that of arteries. The tunica intima, media and adventitia can be distinguished especially in large veins. The walls of a vein are distinctly thinner than that of an artery having the same sized lumen. The tunica media contains a much larger quantity of collagen fibers. Veins are easily compressed and thus collapse easily after death. The adventitia of veins is thicker than the media (especially in large veins). In some large veins (e.g. inferior vena cava) the adventitia contains a considerable amount of elastic and muscles fibers which run in a predominantly longitudinal direction. These fibers facilitates elongation and shortening of the vena cava with respiration (Singh, 2002).
BLOOD
Is a red fluid that circulates in the body’s veins, arteries and capillaries. It is pumped by the heart through the arteries to the lungs and all other tissues and then returned to the heart in veins (Illustrated medical dictionary, 2008).Blood forms about 79% of the body weight consisting of plasma, corpuscles and platelets. Erythrocytes (R.B.C) transport oxygen and carbon(iv)oxide. Leucocyte (W.B.C) produced in the red bone marrows (Myeloid tissue) and lymphocytes fight infections and thrombocytes (Platelets) are essential to blood clothing at the site of injury (Mackenzie, 2011).