AQA AS UNIT 2 BIOLOGY

• Tissue Fluid is a watery liquid that contains amino acids, fatty acids salts and oxygen. 
• Blood is pumped by the heart and passed through the vessels into the capallaries causing a pressure called hydrostatic pressure.
• The hydrostatic pressure acts on the inside of the capillaries forcing the tissue fluid out.
• However it is opposed by two forces : 

     - Osmotic forces as the blood has a lower water potential than the water potential of the tissue cells

          -The hydrostatic pressure of the the tissue fluid outside of the capillaries

• But the hydrostatic pressure of the tissue fluid inside the capillaries is much greater than the two opposing forces
• So tissue fluid moves out of the capillaries 

Tissue Fluid can return to the blood by two ways: 

-The Lymphatic system 

-back into the capillaries at the venule end

• Tissue Fluid moves back into the capillaries as the hydrostatic pressure of the tissue fluid outside of the capillaries is now greater than the hydrostatic pressure of the tissue fluid inside the capillaries. 
• Also the osmotic forces are greater than the hydrostatic pressure of tissue fluids from inside of the capillaries
• Therefore water and tissue fluid move back into the capillaries 
• Lymphatic system returns the rest of the tissue fluid as not all of it can be carried by the capillaries it is not moved by the heart but by the hydrostatic pressure of the tissue fluid 

Arteries- 

• Thick Muscle- can be constricted and dilated to control the blood volume
• Thick Elastic Layer- Helps to maintain a high blood pressure as it stretches and recoils 
• Overall wall thickness is large- make sure that walls do not burst under high pressure

Aterioles - 

• Thicker muscle than arteries- constricts lumen and restrict the flow 
• Elastic Layer is thinner- Blood is at a lower pressure 

Veins-

• Muscle layer thin- Doesn't need flow to be restricted
• Elastic Layer thin- Too low pressure for recoil action 
• Overall wall thickness is small- pressure too low for vessel to burst
• valves - prevent back flow

Capillaries- 

• walls one cell thick- short diffusion pathway 
• narrow diameter- so cells closer to capillaries
• Lumen narrows - red blood cells squeezed flat so closer to cells
• Spaces in Lining- allow white blood cells escape and deal with infections