part 1

• The double circulatory system is made up of the pulmonary and systemic circuits.

  The low pressure in the pulmonary circulation pushes blood slowly to lungs allowing more time for gas exchange. The high pressure in the systemic circulation ensures blood is pumped to all the other body organs and allows tissue fluid to form in each organ.
• The arteries and veins have the same basic structure. Their walls have three layers and this is why they are considered to be organs (capillaries contain only one type of cell and hence are regarded as a tissue):
• arteries and veins have A thin inner layer known as TUNICA INTIMA. This is composed of SQUAMOUS ENDOTHELIUM that is smooth, and enables the blood to flow through the vessels with minimum friction.
• arteries and veins have A middle layer known as TUNICA MEDIA. This is composed of SMOOTH MUSCLE and ELASTIC FIBRES.
• arteries and veins have A tough outer layer known as TUNICA EXTERNA. This is composed of FIBROUS TISSUE that helps to protect the vessel from the pressure exerted by other organs rubbing against it.
• arteries have More elastic tissue than veins since they have to cope with blood under high pressure. The elastic tissue allows artery wall to stretch and then recoil to maintain blood flow
• Arteries are more muscular than veins since they control (smooth muscle brings about vasodilation and vasoconstriction) the amount of blood being distributed to an organ.
• Arteries have a small lumen since blood in arteries must be maintained at high pressure
• Arteries have a folded endothelium which allows expansion during pulses
• veins Have large lumen because as blood is at a low pressure it needs a large volume (to reduce friction) into which it can move
• veins Possess semi lunar valves which ensure that low pressure

  movement of blood is one way
• Atherosclerosis is a disease in which the artery wall thickens as a result of a build up of

  fatty materials.