Mass transport - the circulatory system

• function of the circulatory system: multicellular organisms like mammals have a low surface area to volume ratio so need a specialised mass transport system to carry raw materials from specialised exchange organs to their body cells
• structure of circulatory system: made up of the heart and blood vessels

  • heart pumps blood through blood vessels (arteries, arterioles, veins ad capillaries) to reach diff parts of the body
• pulmonary artery - carries blood from the heart to the lungs
• pulmonary vein - caries blood from the lungs to the heart
• aorta - carries blood from the heart to the body
• vena cava - carries blood from the body to the heart
• renal artery - carries blood from the body to the kidneys
• renal vein - carries blood from the kidneys to the vena cava
• blood transports respiratory gases, products of digestion, metabolic wastes and hormones around the body
• there are 2 circuits

  • one takes blood from the heart to the lungs then back to the heart
  • the other loop takes blood around the rest of the body
  • so the blood has to go through the heart twice to complete one full circuit
• the heart has its own blood supply - the left and right coronary arteries
• vena cave = deoxygenated blood to the heart
• aorta = oxygenated blood to the body
• pulmonary artery = deoxygenated blood to the lungs
• blood always flows from a higher pressure to a lower pressure

  vena cava is the final blood vessel that takes blood back to the heart from the body - lowest pressure
• arteries:

  • carry blood from the heart to the rest of the body
  • thick and muscular walls with elastic tissue to stretch and recoil as the heart beats - helps to maintain high pressure
  • inner lining (endothelium) is folded allowing artery to stretch - also helps to maintain high pressure
  • all arteries carry oxygenated blood except pulmonary artery - deoxygenated blood to lungs
• Arterioles:

  • arteries divide into smaller vessels called arterioles
  • form a network throughout the body
  • blood directed to different areas of demand in body by muscles inside arterioles
  • contract to restrict the blood flow
  • relax to allow full blood flow
• Veins:

  • take blood back to the heart under low pressure
  • wider lumen than equivalent arteries
  • very little elastic tissue or muscle inside
  • valves - prevent backflow
  • blood flow through veins is helped by contraction of body muscles surrounding them
  • all carry deoxygenated blood (oxygen has been used up by body cells) except for pulmonary veins - carry oxygenated blood to hear from lungs
• Capillaries:

  • arterioles branch into capillaries - smallest of the blood vessels
  • substances (e.g. glucose and oxygen) exchanged between cells and capillaries - adapted for efficient diffusion
  • always found very near cells in exchange tissues (e.g. alveoli in lungs) - shorter diffusion path
  • walls are only 1 cell thick - also shortens diffusion pathway
  • large number of capillaries to increase surface area for exchange
  • networks of capillaries in tissue = capillary beds
• Tissue fluid:

  • the fluid that surrounds cells in tissues
• Tissue fluid:

  • made from small molecules that leave the blood plasma e.g. oxygen, water and nutrients
  • doesn't contain any red blood cells or big proteins like blood does bc too large to be pushed out through capillary walls
• cells take in oxygen and nutrients from the tissue fluid and release metabolic waste into it
• in a capillary bed substances move out of the capillaries into the tissue fluid by pressure filtration
• • at the start of the capillary bed, nearest the arteries the hydrostatic (liquid) pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid.
  • This difference in hydrostatic pressure mans an overall outward pressure forces fluid out of the capillaries and into the spaces around the cells forming tissue fluid
  • as fluid leaves the hydrostatic pressure reduces in the capillaries - so the hydrostatic pressure is much lower at the venule end of the capillary bed (nearest the veins)
• • due to the fluid loss and an increasing conc. of plasma proteins (don't leave he capillaries), the water potential at the venule end of the capillary bed is lower than the water potential in the tissue fluid
  • this means that some water re-enters the capillaries from the tissue fluid at the venule end by osmosis
  • any excess tissue fluid is drained into the lymphatic system (network of tubes that acts a bit like a drain) - transports this excess fluid from the tissues and passes it back into the circulatory system
• blood plasma is the liquid that carried everything in the blood
• pressure filtration - filtration happening under pressure - describes the process by which small molecules are filtered out of the capillaries under hydrostatic pressure forming tissue fluid
• pressure is the highest at the start of a capillary bed nearest the arterioles - this is caused by the left ventricle contracting and sending the blood out of the heart, though the arteries and arterioles at high pressure
• high blood pressure means a high hydrostatic pressure in the capillaries which can lead to an accumulation of tissue fluid in the tissues