(i) smooth muscle and elastin fibres and a



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(i) Tunica externa (outermost) – Formed of connective tissue containing collagen and elastic fibres, lymph vessels and nerve fibres. (ii) Tunica media (middle) – Formed of elastic connective tissue and smooth muscle fibre. (iii) Tunica interna (innermost) – Made up of two parts: elastic membrane and endothelium.

Types of blood vessels –

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(i) Artery:

They have thick wall containing many smooth muscle and elastin fibres and a small lumen. It distributes blood after it has been pumped by the heart. Blood flow is pulsile through arteries and the elastic recoil of their wall when the heart is in diastole helps maintain blood pressure. Arteries divide to form finer branches, the arterioles as they reach the Tunica destination. The smallest arterioles do not have an elastica interna, and their media may contain only scattered modified smooth muscle cells called pericytes. Arterioles can control blood flow to the capillary beds they supply. Blood flow changes from pulsile to smooth in the arterioles. The arterioles finally break up into blood capillaries.

Arteries are more susceptible to injury and diseases. E.g.

Atherosclerosis.

(ii) Veins:

A vein starts by the union of venous capillaries leading to the formation of venules which then join and forms a vein. Venules drain blood from capillaries into veins. Veins have thin walled and a large lumen which means there is less resistance to blood flow than in capillaries and therefore the velocity of blood in veins increases. Veins contain pocket valves which prevent the backflow of blood (which is a risk in veins because the pressure of blood is very low here).

(iii) Capillaries (discovered by Marcello Malpigi, 1661):

They are made up of tunica interna (single layer of endothelium).

They are most effective blood vessels because exchange of substances takes place through them. It contains mixed, oxygenated and deoxygenated blood. It transports blood from the arteries to the veins. Blood flow is at its slowest through the capillaries because they are the narrowest vessels which mean they have the greatest friction which restricts blood flow. This slow velocity aids the exchange of materials.

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