anatomy & physiology of the vascular system

Anatomy and Physiology of the Vascular System:

Arterial walls are thicker than veins.
Thickness of the arterial walls gradually diminishes as it becomes smaller.
Walls of the blood vessel consist of three basic structures, intima, media, and adventitia, which vary with the types of the vessels.
Blood vascular tree is a circuit that conducts blood from the heart through large- diameter, low-resistance conducting vessels to small arteries and arterioles, which lower blood pressure and protect the capillaries.
Capillaries are thin-walled and allow the exchange of nutrients and waste products between tissue and blood, a process that requires a very large area.
The circuit back to the heart is completed by the veins, which are disrensible and provide a volume biffer that acts as a capacitance for the vascular circuit.

Arteries are of three types:

1. Larger or elastic arteries (aorta and its main branches) : 
Intima is composed of lining endothelial cells, separated from the media by internal elastic lamina.
Media is composed of smooth muscle cells.
Outer limit of the media is separated is separated from adventitia by external elastic lamina. Small arterioles (vasa vasorum) pass through the adventitia into the outer one-half to two-thirds of the media to perfuse the vessel wall.
Atherosclerosis is the disease largely of elastic and muscular arteries which is associated with functional and structural changes in the muscular arteries and arterioles.

2. Muscular arteries, are branches of elastic arteries (e.g. coronary or renal arteries) regulate the blood pressure by vasodilatation or vasoconstriction under the control of the autonomic nervous system.

3. Arterioles regulate the blood flow into capillary beds by the smooth muscle of media, thus controls the systemic arterial blood pressure.

Capillaries:Capillaries are small vessels having a diameter of a red blood cell, lined by one-cell thick endothelium, a thin basement membrane, and slow blood flow.

These are ideally suited to the rapid exchange of diffusible materials between blood and extravascular tissue.

Veins:
Veins are thin-walled vessels with poorly defined internal elastic lamina and media. Hence, they often show abnormal, irregular dilation, and easy penetration by cancer cells and inflammation.

Lymphatics:
Lymphatics are identified in tissue sections as collapsed, endothelium-lined channels devoid of blood cells.