9. Veins:
Will return blood to the heart aided by the action of smooth muscle in their walls & specialized valves.
As with arteries the veins are classified into
venules,
Small veins
medium veins
large size veins.

10. Venules:
they share in the process of exchange between blood & tissues together with the capillaries
they are similar to capillaries in structure so they have a very thin wall but with more pericytes in their wall.
T. intima: consist of endothelial cells with very thin subendothelial layer.
T. media: in small venules contain only contractile pericytes and they are called Post-capillary venules or called Pericytic venules.

11. In larger venules the pericytes will be gradually replaced by smooth muscle cells forming layer of 1-2 smooth muscle thickness.Those muscular venules drains into small veins.
T. adventitia: become identifiable in the larger venules however it is non existent in the postcapillary ones

12. Small & medium size veins:
T. intima: formed of endothelial cells & thin subendothelial layer.
T. media: formed of small bundles of smooth muscles with reticular & elastic fibers.
T. adventitia: well developed & formed mainly of collagen fibers.

13. Large veins: (vena Cava):
T. intima: well developed with discontinuous internal elastic lamina.
T. media: thin & formed of few layers of smooth muscle cells with abundant connective tissues.
T. adventitia: the thickest & best developed layer contain longitudinal bundles of smooth muscles.

14. Large veins in the arms & legs carrying blood against gravity are equipped with valves to prevent blood flowing back.
These valves are thin flaps of intima that project into the lumen lined on both sides by endothelial cells, ther free edges of the valve pointing toward the heart so they prevent backflow of blood.
These valves mainly found in medium and large size veins. Incompetence of those valves in the veins of the legs causes varicose veins.

15. Lymphatic vascular system:
A system of endothelial lined, thin walled channels that collect fluid from tissue spaces & return it to the blood.
Lymphatic channels are blind ended & identical in structure to blood capillaries.
In some areas the lymphatic capillaries have a fenestrated endothelium which permit the entry of large molecules such as proteins to the vessel.
The larger lymphatic vessel have a structure similar to that of veins except they have a thinner wall & lack a clear cut separation between the intima, media & adventitia. They also have numerous internal valves & like the large arteries & veins large lymphatic vessels contain vasa vasorum.

16. Heart:
is a muscular organ that contract rhythmically pumping the blood through the circulatory system it is also responsible for the production of the hormone Atrial natriuretic hormone. its wall consist of:
three tunics
the fibrous central region of the heart (fibrous skeleton)
valves of the heart
conducting system of the heart

18. Tunics of the heart:
Endocardium: lines the chambers of the heart, it vary in thickness in different areas.
It consist of single layer of squamous endothelial cells rest on a thin subendothelial layer of loose connective tissues.
Between the endocardium and myocardium is a layer of connective tissue called subendocardial layer that contain veins, nerves & some purkinje fibers which are part of the impulse conducting system of the heart.

19. Myocardium:
is the thickest layer consist of cardiac muscles
those muscles are arranged in layers that surround the heart chambers in a complex spiral.
The muscle cells of the heart are grouped into two groups:
contractile cells & the impulse generating & conducting cells responsible for the electrical signal that initiate the heartbeat.

20. Epicardium:
the heart is covered externally by simple squamous epithelium supported by a layer of connective tissue called subepicardial layer which contain veins, nerves & nerve ganglia of the heart. the adipose tissues that surround the heart will accumulate in this layer.

21. Fibrous skeleton of the heart:
Is a dense connective tissue with collagen fibers which will give support to the heart valves & also give origin & insertion to the cardiac muscles.
Valves of the heart:
Cardiac valves consist of a central core of dense connective tissue lined on both sides with endothelial cells & the bases of the heart valves attached to the fibrous skeleton of the heart.

22. Conducting system of the heart:
The impulse generating and conducting system of the heart consist of several structures that make it possible for the atria and ventricles to beat in succession.

23. The Sinoatrial node:
located close to the entrance of the superior vena cava into the right atrium.
The cells of the SA node are modified cardiac muscle cells smaller then atrial muscle cells,
those nodal cells are arranged around a large nodal artery.
The impulse will be carried from the SA node to the AV node through tracts of specialized cells.

24. Atrioventricular node:
mass of specialized cardiac muscle cells lies in the septal wall of the right atrium, the cells of AV node is similar to those of the SA node.

25. The small fibers that emerge from the AV node will be regularly arranged together & eventually become a distinct bundle of parallel fibers that will conduct the impulse from the AV node to the ventricle & this bundle is called Bundle of His.
Those fibers are called Purkinje fibers which will make contact with the cardiac muscles to stimulate their contraction & they have a diameter which is considerably greater then that of the ordinary cardiac muscle cells.