VEINS

Annotations:

• capillaries -> venules -> veins

1. bring/ return blood to the heart
2. blood pressure is very low
   1. typically 5mm Hg or even lesser
   2. How does blood move then?
3. three layered wall, just like arteries
   1. except the low blood pressure means less elasticity is required
      1. tunica media is much thinner and has far fewer elastic fibres
4. The blood is pushed by muscle contraction
   1. when you tense muscles, they squueze inwards, temporarily rasing the pressure in veins
5. contain half moon/ semilunar valves
   1. formed from the endothelium
   2. allow blood to move towards the heart but not away from it
      1. blood in the veins is squeezed up through the valves but can not pass down through them
6. used when
   1. blood transfusions
   2. collecting blood sample