L2 - blood vessels

• Lumen: centre hollow cavity of blood vessel through which blood flows
• Endothelium: thin inner layer for diffusion (capillaries), very smooth to prevent friction
• Elastic layer (containing elastin fibres): helps to maintain blood pressure by stretching and springing back
• Smooth muscle layer: can contract and relax to control volume of blood passing through them
• Tough outer layer made of collagen fibres: resists pressure changes from outside and within
• Arteries:

  -carry blood away from the heart

  -blood at high pressure so wall thick to withstand pressure

  -lumen relatively small to maintain pressure

  -have endothelial layer, elastic layer, smooth muscle layer, outer collagen, and elastic layer

  -arteries near the heart have more elastic tissue and arteries further from the heart have more smooth muscle
• Arterioles:

  -small blood vessels that distribute blood from an artery to a capillary

  -contain smooth muscle to allow the flow rate to the capillary bed to be controlled

  -contracting the muscle reduces the flow rate leading to vasoconstriction which allows more blood to flow to other areas which are more demanding of oxygen
• Arteries and arterioles, elastic layer:

  -elastic fibres allow arteries to withstand the force of blood pumped out of the heart and stretch (to the limits of the collagen)

  -then recoil to original length

  -this helps to even out pulse surges in arteries

  -arteries have more need for this as closer to the heart, so have more elastin

  -amount of elastin reduces the further from the heart the artery is
• Arteries and arterioles, smooth muscle layer:

  -allows arteries to constrict or dilate to control blood flow to individual organs (vasoconstriction and vasodilation)

  -is need more the further away from the heart you are
• Arteries and arterioles, collagen layer:

  -more structural support needed closer to the heart so aorta has a larger collagen layer
• Arteries down a microscope:

  -tunica intima

  -tunica media

  -tunica externa

  
• Tunica intima:

  -inner layer of elastic tissue which allows the wall to stretch and recoil to help maintain blood pressure
• Tunica media:

  -middle layer of smooth muscle which allows vasoconstriction and vasodilation
• Tunica externa:

  -outer layer of collagen and elastic tissue which provides strength to withstand the high pressure, and recoil to maintain pressure
• Veins:

  -carry blood back to the heart

  -blood at low pressure so walls are thinner

  -large lumen to ease the flow of blood

  -thinner elastic, smooth muscle and collagen layers than arteries

  -valves to help blood flow back to heart and not flow in the opposite direction (back flow)

  -thin walls mean surrounding skeletal muscle applies pressure when it contracts helping the blood flow in the correct direction
• Veins have thinner elastic, smooth muscle and collagen layers than arteries as they do not need to stretch and recoil and are not actively constricted to reduce blood flow
• Venules:

  -take blood from the capillary bed and lead it to the veins

  -have a thin elastic, smooth muscle and collagen layer
• Capillaries:

  -very thin walls (layer of flattened endothelial cells to reduce diffusion distance)

  -walls one cells thick to allow exchange of materials between blood and tissue fluid

  -walls are leaky to allow plasma and dissolved substances to leave the blood in the formation of tissue fluid

  -lumen same diameter as red blood cell to reduce diffusion distance

  -resistance increases reducing the rate of flow allowing time for diffusion to occur
• Precapillary sphincter is used to narrow or widen the arterioles by contracting or relaxing

  
• Moving blood around the body:

  -blood moves through the arteries, arterioles no capillaries due to the pressure created by the heart pumping

  -when your muscles contract, the valves open and blood moves through the veins. When your muscles relax, the valves close, keeping blood flowing in one direction through the veins
• Aneurysm:

  -a bulge or a weakness in a blood vessel (most commonly the aorta)

  -high blood pressure has been the best predictor in recent years

  -scientist have recently discovered that an increase in collagen in the aorta could also lead to an aneurysm

  -the usual ratio of collagen to elastin in the aorta is 1.85 to 1, but during a large aneurysm it’s been found to be 7.91 to 1
• Elastin fibres stretch and recoil to maintain blood pressure
• Smooth muscle fibres contract and relax to control volume of blood passing through them