Blood vessels form a closed system of tubes that carries blood away from the heart, transport it to the various tissues of the body and then returns it to the heart. Blood vessels help in delivering oxygen, nutrients, and hormones to the tissues and carry away carbon dioxide and wastes from them.

• There are five types of blood vessels involved in all these above described processes- arteries, arterioles, capillaries, venules and veins.
• Arteries are large blood vessels that carry blood away from the heart.
• Arterioles are smallest division of arteries that help in carrying blood to the various tissues of the body.
• Capillaries are tiny / smallest blood vessels (5-10 μm in diameter) that branch from arterioles when arterioles enter a tissue. Capillaries allow exchange of substances between blood and tissue due to its thin wall. They form “U-turn” shaped structure to allow flow of blood coming from arteries to venules for return after exchange.
• Venules are small veins that forms when group of capillaries reunite in tissues.
• Veins are large blood vessels that forms when group of venules reunite. Veins carry blood form the tissues back to heart.

Basic structure of a blood vessel consists of 3 layers in its wall: Tunica interna (innermost), Tunica Media (middle layer), and Tunica externa (outermost).

TUNICA INTERNA or TUNICA INTIMA- it forms inner lining of blood vessel. It remains in direct contact with blood. It forms lumen (interior opening) of vessel. It also has 3 layers in it- innermost endothelium (simple squamous epithelium), middle layer is basement membrane which provides physical support and outer is internal elastic lamina which has swiss cheese like appearance and help in diffusion of materials.

TUNICA MEDIA- it is the middle layer of blood vessels and mainly comprised of smooth muscle cells. This layer shows greatest variation among all type of blood vessels. Smooth muscle cells contraction help in regulating the diameter of blood vessel’s lumen which is important to maintain proper blood pressure and blood flow in different tissues of the body. These muscles circularly encircle the vessel.

TUNICA EXTERNA- it is the outer covering of the blood vessel made of elastic and collagen fibers. It consists of network of nerves and tiny blood vessels to supply blood to the vessel wall. These tiny blood vessels are called as vasa vasorum (vessels to the vessels). Tunica externa also anchor the vessels to the surrounding tissue.

• Arteries have well defined tunica interna, thick tunica media and tunica externa. Thick tunica media help in expanding and stretching of arteries more easily during vasodilation and vasoconstriction.

Basic structure showing Walls of Artery

• While in Veins, tunica interna lacks internal elastic lamina layer. Veins also contain valves (thin folds of tunica interna). These valves are only present in veins of limbs (to prevent backflow of blood). Tunica media is much thinner than arteries and tunica externa is the thickest of the three layers in veins.

Basic structure showing Walls of Vein

• Lumen of veins is much larger than of arteries.
• Veins have thin walls than arteries. Arteries walls are stronger than veins.  Also, veins have thin walls in respect to the diameter of their lumen.
• Arteries are designed to withstand high pressure changes while veins are not.
• Average blood pressure in veins is lower than in arteries.
• If we cut a vein, blood flow through it is slow and even, while in case of artery blood flow is rapid with a spurt.
• Veins are much numerous in body than arteries.
• Veins vary in size from small to medium to large (0.5mm to 3cm) while arteries are not.
• Capillaries lack both tunica media and tunica externa. They only have endothelium and basement membrane.
• Elastic arteries are the largest arteries in the body, e.g. aorta and pulmonary trunk.