blood vessels

• Circulation : the movement of blood through the heart and blood vessels
• arteries carries blood away from the heart
• artery walls contain elastic fibres and smooth muscle
• elastic fibres in arteries walls allow for them to expand to accommodate for the extra blood. they also recoil when ventricle relax, keeping blood moving and maintaining pressure
• Vasoconstriction : muscle contract to reduce diameter of artery, reducing blood flow to organ
• Vasodilation : muscle relax to increase diameter of artery, increasing blood flow to an organ
• smooth muscle in artery walls do not contract and relax to move blood
• Arterioles supply blood to capillaries and are made of smooth muscle
• Vasodilators are substance that produces a local widening, or dilation, of arterioles. they include carbon dioxide, lactic acid which are wastes from cellular respiration
• Heat energy from cellular respiration tends to increase blood temp which contributes to an increase in heart rate
• capillaries are the links between arteries and veins
• capillaries are microscopic blood vessels that forms a network to carry the blood close to nearly every cell. they allow the cells to get their requirements for cellular respiration from the blood and for them to pass their wastes into the blood.
• capillary walls are one cell thick allowing substances to pass easily between blood and cells
• veins carry blood towards the heart
• veins do not have muscular walls meaning they cannot change diameter. the pressure in veins are constant meaning they do not have to be elastic
• Blood pressure is relatively low because is loses pressure from flowing through capillaries, therefore the walls are thinner than arteries, and many have valves to prevent blood flowing backwards 
• capillaries join into venules, which join to larger veins, which all culminate into the inferior / superior vena cava and pulmonary veins
• the vena cava brings blood from body to right atrium, with the superior bringing blood from above heart and inferior brings blood from below heart
• the pulmonary veins bring blood from lungs to left atrium, there are 4 veins, 2 from each lung