Circulatory system

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
(Tunica Intima) Middle layer
(Tunica Media) Outer layer
(Tunica Adventitia)
Elastic artery Endothelium
Connective tissue
Smooth muscle Smooth muscle
Elastic lamellae Connective tissue Some elastic fibers Thinner than tunica media

Muscular artery
Large Endothelium
Connective tissue
Smooth muscle Smooth muscle
Collagen fiber
Relatively little elastic tissue Connective tissue Some elastic fibers
Thinner than tunica media
Small Endothelium
Connective tissue
Smooth muscle Prominent internal elastic membrane Smooth muscle (8-10 cell layers)
Collagen fibers Connective tissue Some elastic fibers
Thinner than tunica media
Arteriole Endothelium
Connective tissue
Smooth muscle
Internal elastic membrane Smooth muscle (1-2 cell layers) Thin , ill-defined sheath of connective tissue
Capillary Endothelium
None None



















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 Inner layer
(Tunica Intima) Middle layer
(Tunica Media) Outer layer
(Tunica Adventitia)
Post capillary venule Endothelium
None None


Muscular venule Endothelium
Smooth muscle
(1 or 2 layers) Connective tissue
Some elastic fibers
Thicker than tunica media
Small vein
Endothelium
Smooth muscle (2 or 3 layers) Smooth muscle
(2 or 3 layers continuous with tunica intima) Connective tissue Some elastic fibers
Thicker than tunica 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 .