Circulatory system
the essential materials are conveyed to the cells by :
1-closed system of tubes
2-action of muscular pump the heart
3- capillaries where the exchange of the oxygen , carbon dioxide and
nutrients occurs these varied structure make up the circulatory system in
the high animals .the circulatory system include 2 major sub division :
1-cardiovascular system (CV) that composed of heart ,arteries , veins and
capillaries .
2-lymph vascular system composed of lymph and lymphatic capillaries .
The circulatory system transport vital life supporting substances such as
nutrients ,O2 and removes detrimental life by products of metabolism
such as CO2 and urea .
The circulating blood has 2 routes :
1- blood from the body enters the right side of heart and is pumped
into the lungs (pulmonary circulation).
2- 2-oxygenated blood from the lungs enters the left side of the heart
and distributed to the body (systemic circulation)
Cardiovascular system
All elements in cardiovascular system are tubes even the heart is a tube
with 4 segments 2 atria and 2 ventricles , the wall of the system vary in
thickness depending on the pressure exert on them by circulatory system .
All components of the system are lined with layer of endothelial cells .
General features of Arteries &Veins :
They are composed three layers called tunics they are from the lumen
outward :
1-Tunica intima : The innermost layer that includes the endothelial lining
(a single layer of simple squamous epithelial tissue ),resting on a basal
lamina and the underlying connective tissue .
2-Tunica media : The muscular middle layer (consists primarily of
circumferentially arranged layers of smooth muscle cells ).
3- Tunica adventitia : The outermost connective tissue layer.
The heart
In the early embryo the heart is straight tube inside the primitive
pericardial cavity during the development the tubular heart grows faster
than the walls of the cavity , such differential growth forces the tube to
bend and fold on its self to form as S-U shaped loop.
The end to the loop come to lie together and become the great vessels of
the heart the rest of the tube undergoes a series of dilations and fusion
and results in the formation of the 2 small atrial chambers and 2 larger
ventricular cavities .
The heart acting as a pump supplies the principal force for the circulation
of the blood the elasticity of the large arteries dampens the force of the
heart beat and causes the blood to flow rather evenly and continuously
instead of moving in spurts the smaller distributing arteries carry the
blood to all organs and tissues of the body where they terminate in
capillaries beds , here is the semi permeable endothelium lining the
capillaries allows the exchange of O2,CO2, hormones and nutrients and
metabolic wastes.
The blood return to the heart via the thinner walled veins, which function
under much lower pressure.
Histological features
1-Epicardium:
The 3 layered heart is suspended in the pericardial cavity by its great
vessels as they enter to exit from the cavity the internal lining of this fibro
elastic sac is layer of flattened mesothelial cells(pericardium) the outer
surface of the heart as the visceral pericardium at the site of the exit or
entrance of the vessels , this layer of the cells is the outer most layer of
the epicardium underlining the cells is thin layer of connective tissues
contain elastic fibers,
A deeper looser connective tissues called subepicardial layer is variable
thickness depending on the a mount of adipose tissue present
2-Myocardium:
The middle layer consist of contractile cardiac muscle fibers and non
contractile muscles fibers called purkinji fibers .
The thickness of myocardium varies depending on the pressure within the
varies cavities it is thinnest in the low pressure atria and thickest in the
high pressure in ventricles,
3-Endocardium:
The endocardium the thinnest layer of the heart has 3 components.
A-endothelium resting on basal lamina and layer of loose collagenous
Fiber
B-elastic fibers and few smooth muscle
C-subendocardial zone of loose connective tissues
Endocardium varies in thickness , the thickest in the left atrium and the
thinnest in the left ventricle.
Arteries :
The layers of the heart continuous with the walls of the arteries , inner
most is the intima is composed of single layer endothelial cells resting on
connective tissue ,the middle layer media is composed of muscular layer
(circular of smooth muscle)and the outer layer adventitia is composed of
connective tissue , arteries decreased in size and increase in number as
they distally from the heart they are usually classified according to size
to:
Arteries :-Are classified into three types on the basis of size and the
characteristics of the tunica media.
Artery
Vessel
Inner layer
Middle layer
Outer layer
(Tunica Intima)
(Tunica Media)
(Tunica Adventitia)
Elastic
Endothelium
Smooth muscle
Connective
tissue
artery
Connective tissue
Elastic lamellae
Some
fibers
Smooth muscle
elastic
Thinner than tunica
media
Muscular
Endothelium
Smooth muscle
Connective
tissue
artery
Connective tissue
Collagen fiber
Some elastic fibers
Large
Smooth muscle
Relatively
little Thinner than tunica
elastic tissue
Small
media
Endothelium
Smooth muscle (8- Connective
tissue
Connective tissue
10 cell layers)
Some elastic fibers
Smooth
muscle
Collagen fibers
Thinner than tunica
Prominent
internal
media
elastic membrane
Arteriole
Endothelium
Smooth muscle (1-2 Thin
Connective tissue
cell layers)
Smooth muscle
Internal
sheath of connective
tissue
elastic
membrane
Capillary
Endothelium
None
, ill-defined
None
T. S. of different blood vessel
Veins :- Generally, the tunica adventitia is the thickest layer of the wall ,
and are classified into three types on the basis of size and the
characteristics of the tunica media :
Veins
Vessel
Post
capillary
venule
Muscular
Inner layer
(Tunica Intima)
Endothelium
Middle layer
(Tunica Media)
None
Outer layer
(Tunica Adventitia)
None
Endothelium
Smooth muscle
Connective tissue
(1 or 2 layers)
Some elastic fibers
venule
Thicker than tunica
media
Small vein
Endothelium
Smooth muscle
Connective tissue
Smooth muscle (2
(2 or 3 layers
Some elastic fibers
or 3 layers)
continuous with
Thicker than tunica
tunica intima)
media
Medium vein
Endothelium
Connective tissue
Smooth muscle
Internal elastic
membrane in some
cases
Smooth muscle
Collagen fibers
Connective tissue
Some elastic fibers
Much thicker than
tunica media
Large vein
Endothelium
Connective tissue
Smooth muscle
Smooth muscle (2
– 15 layers)
Cardiac muscle
near heart
Collagen fibers
Connective tissue
Some elastic fibers
Much thicker than
tunica media
Capillaries :- Smallest diameter blood vessels , are consist of single layer
of endothelial cells and their basal lamina .