blood vessels

• arteries 


  carry blood form the heart to the rest of the body
• arteries

  • walls are thick and muscular and have elastic tissue to stretch and recoil as the heart beats, which helps maintain the high pressure.
  • The endothelium is folded, allowing the artery to expand - this also helps it to maintain the high pressure.
• All arteries carry oxygenated blood except for the pulmonary arteries, which take deoxygenated blood to the lungs.
• Arteries branch into arterioles, which are much smaller than arteries.
• arterioles have a layer of smooth muscle, but they have less elastic tissue. The smooth muscle allows them to expand or contract, thus controlling the amount of blood flowing to tissues.
• Arterioles branch into capillaries, which are the smallest of the blood vessels.
• Substances like glucose and oxygen are exchanged between cells and capillaries, so they're adapted for efficient diffusion, e.g. their walls are only one cell thick.
• Capillaries connect to venules, which have very thin walls that can contain some muscle cells. Venues join together to form veins.
• Veins take blood back to the heart under low pressure.
• Veins have a wider lumen than equivalent arteries, with very little elastic or muscle tissue. Veins contain valves to stop the blood flowing backwards. Blood flow through the veins is helped by contraction of the body muscles surrounding them.
• All veins carry deoxygenated blood except for the pulmonary veins, which carry oxygenated blood to the heart from the lungs.
• artery structure

  A) elastic tissue in walls

  B) thick muscle layer

  C) folded endothelium

  D) lumen

  
• capillary structure

  A) endothelium (one cell thick)

  
• vein structure

  A) large lumen

  B) endothelium

  C) thin muscle wall

  
• tissue fluid


  the fluid that surrounds cells in tissues.
• what is tissue fluid made from

  substances that leave the blood plasma, e.g. oxygen, water and nutrients. doesn't contain red blood cells or big proteins, because they're too large to be pushed out through the capillary walls.
• Cells take in oxygen and nutrients from the tissue fluid, and release metabolic waste into it. In a capillary bed (the network of capillaries in an area of tissue), substances move out of the capillaries, into the tissue fluid, by pressure filtration.
• Hydrostatic pressure


  Pressure exerted by a fluid at rest
• At the start of the capillary bed, nearest the arteries


  The hydrostatic pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid
• Fluid movement in capillaries

  1. Difference in hydrostatic pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid

  2. As fluid leaves, the hydrostatic pressure reduces in the capillaries - so the hydrostatic pressure is much lower at the end of the capillary bed that's nearest to the venules

  3. As water leaves the capillaries, the concentration of plasma proteins in the capillaries increases and the water potential decreases
• Oncotic pressure


  Pressure generated by plasma proteins in the capillaries
• At the venule end of the capillary bed


  There's a high oncotic pressure and a low water potential
• The water potential in the capillaries is lower than the water potential in the tissue fluid


  Some water re-enters the capillaries from the tissue fluid at the venule end by osmosis
• • Not all of the tissue fluid re-enters the capillaries at the vein end of the capillary bed - some excess tissue fluid is left over.
  • This extra fluid eventually gets returned to the blood through the lymphatic system - a kind of drainage system, made up of lymph vessels
• • The smallest lymph vessels are the lymph capillaries.
  • Excess tissue fluid passes into lymph vessels.
  • Once inside, it's called lymph.
  • Valves in the lymph vessels stop the lymph going backwards
  • Lymph gradually moves towards the main lymph vessels in the thorax (chest cavity)
  • Here, it's returned to the blood, near the heart.
• red blood cells are only found in the blood as they're too big to get through capillary walls into tissue fluid
• Most white blood cells are in the lymph system. They only enter tissue fluid when there's an infection.
• platelets are only found in the blood meaning they are only present in the tissue fluid if the capillary are damaged
• proteins are mainly in the blood as most plasma proteins are too big to get through capillary walls
• solutes can move freely between blood, tissue fluid and lymph