Blood Vessels

Order of structure:
- Lumen
-Endothelium
-Elastic tissue
-Smooth Muscle
-Collagen fibres

1- Lumen
- Small - maintain high pressure

2- Walls are thick
-contain collagen fibres -fibrous protein - withstand high pressure and give strength

3- Walls contain elastic tissue
-  Allow artery to stretch and recoil 

4- Wall contains smooth muscle
- Contract and constrict artery - constriction narrows lumen

5- Endothelium folded
-Unfolds when artery stretches  

Same structure as arteries.

• Thinner layers of collagen fibres, smooth muscle and elastic tissue
• - because blood carried at low pressure, walls do not have to withstand high pressure.
• Deoxygenated blood is carried back to the heart via the veins.

1-Lumen
- large - to ease flow of blood

2- Walls
-thinner layers of collagen, smooth muscle and elastic tissue

3- Do not need to stretch and recoil

4- Main feature
- Contains valves
- Valves help flow of blood back to  the heart
- Valves prevent back flow 

Only have  endothelium and lumen 

Endothelium - 1 cell thick
- allows erythrocytes to be squeezed up against the wall of the capillary
- reducing diffusion distance  

Lumen
-very narrow
- erythrocytes to be squeezed as they pass along capillary

Open circulatory systems - in insects
-bathe in blood - blood does not remain in vessels

1- Because blood does not have to travel far

2- Insects do not rely on blood to transport oxygen and carbon dioxide

3- Insects use a seperate system for this ^

4- Open circulatory systems are not suited to large animals
  - Because blood travels at low pressure
  - Not sufficient levels of oxygen and nutrients will get to tissues

Closed circulatory systems - large animals 
-blood remains in vessels
-tissue fluid -bathes tissues and cells

1- Enables heart to pump blood at higher pressure
  - therefore, blood flows more quickly

2- It can also deliver oxygen and nutrients more quickly and remove waste products quickly.