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).