The Circulatory System
The circulatory system comprises both the blood and
lymphatic vascular systems. The blood vascular system is
composed of the following structures:
1- The heart, an organ whose function is to pump the blood.
2- The arteries, a series of efferent vessels that become
smaller as they branch, and whose function is to carry the
blood, with nutrients and oxygen, to the tissues.
3- The capillaries, the smallest blood vessels, constituting a
complex network of thin tubules that anastomose profusely
and through whose walls the interchange between blood
and tissues takes place.
4- The veins, which result from the convergence of the
capillaries into a system of channels. These channels
become larger as they approach the heart, toward which
they convey the blood to be pumped again.
The internal surface of all components of the blood and
lymphatic systems is lined by a single layer of a squamous
epithelium, called endothelium.
Blood vessels are usually composed of the following
layers, or tunics:
1- TUNICA INTIMA : consists of one layer of endothelial cells
supported by a subendothelial layer of loose connective tissue
containing smooth muscle cells. In arteries, is separated from
the tunica by an internal elastic lamina.
Tunica
Intima
a- Endothelium
b- Subendothelial layer.
c- Internal elastic lamina.
2- TUNICA MEDIA :Layers of smooth muscle cells with
amounts of elastic fibers, retiantar and collagen fibers. In
arteries, the media has a thinner external elastic lamina, which
separates it from the tunica adventitia.
3- TUNICA ADVENTITIA : consist of collagen and elastic
fibers.
The arteries are classified according to their increasing
size into :
1- Arterioles and small artenies.
2- Medium (muscular) artenies.
3- Large (elastic) artenies.
Venous blood are usually grouped into three classes:
1- Venules.
2- Small to medium – sizes venis.
3- Large venis.
Tunica
Intima
a- Endothelium.
b- Subendothelial.
c- Internal elastic lamina
Media
External elastic lamina
Adventitia
Tunica
Intima
a- Endothelium.
b- Subendothelial.
c- Internal elastic lamina
Media
External elastic lamina
Adventitia
Arterioles and small arteries
Medium arteries
Present
Very thin
Absent
1-2 layers of smooth muscle
cells.
Absent
Present
Thick
Present
Up to 40 layers
Very connective tissue
Thin
tissue
Venules
Present
Very thin
Absent
Thin layer absent
Thick
Fiber blast, elastic, collagen
fibers
Large arteries
Present
Thick
Present
Elastic fiber forming 40-70
of elastic lamina with
smooth muscle cells and
reticular fibers present.
connective Relatively under developed.
Small to medium
Large veins
Present
thin
absent
Small bundles of
smooth
muscle,
reticular elastic,
absent
Collagenous tissue
Present
thin
Present
Much thinner, smooth
muscle cells and connective
tissue absent.
Thickest
Contains
longitudinal
bundles of smooth muscle.
Capillaries :
The smallest blood vessels in the body. They are
composed of a single layer of endothelial cells surrounding by
connective tissue. There are three types of capillaries in the
body.
1- The continuous: Are characterized by the absence of
fenestrace in their wall they are found in all types of muscle
tissue, connective tissue exocnine glands and nervous tissue.
2- The fenestrated are characterized by the presence several
openings in the endothelium membrane. This type found in the
endocrine organs, small intestine and the glomeruli of kidney.
3. sinusoids are found in the liver, spleen and bone marrow. The
sinusoids have a tortuous path and greatly enlarged diameter,
though the organ.
Vasa Vasorum :
The walls of large vessels are too thick to receive
nourishment by direct diffusion from the lumen. As a result, the
walls of these vessels have their own small blood vessels called
the vasa vasorum (vessels of the vessel).
Reference :
1- Junquera, L. C. and Carnero, J. (2005). Basic histology.
11th ed.