Exchange and Transport in Animals

• all mammals and humans have a double circulatory system.

  • this is where blood passes through the heart twice so ensures the blood moves to the body tissues rapidly and under high pressure
• large organisms can't rely on diffusion (like single celled organisms) for the exchange of materials/nutrients.

  • when molecules are carried in a transport medium (for example, blood) this is called mass transport
• open circulatory system - blood flows through the body in a single circuit, with no return to the heart. blood isn't contained in blood vessels. this usually happens in insects.

  for example haemolymph (carries sugars and nutrients) is pumped out of the insects heart and directly to the body cavity
• closed circulatory system - blood is always contained in blood vessels as it travels to and from the heart in veins and arteries.
• 1. blood leaves the heart in arteries
  2. before the blood reaches the organs it passes through blood vessels called arterioles
  3. the oxygenated blood then reaches the organ ands and oxygen passes through the capillaries into the tissues
  4. capillaries are the sight of gas exchange where oxygen diffuses from the blood into the body tissues. carbon dioxide also diffuses from the body tissues into the blood
  5. the deoxygenated blood then passes through venules and into veins which take it back to the heart to start the cycle again
• in veins the blood is under low pressure. all veins connect to the vena cava which returns the deoxygenated blood back to the heart
• oxygenated blood travels from the heart to the body from the aorta. the aorta divides into multiple different arteries which carry oxygenated blood to different organs
• arteries carry blood AWAY from the heart

  • they are under high pressure and when the heart beats, a surge of higher pressure is pushed down the arteries (this is called a pulse)
• the blood is always moving forwards in arteries. they have a thick muscular wall to withstand the high pressure
• arteries

  collagen rich outer layer - strengthens the artery

  smooth muscle - contracts and narrows the artery as it controls how much blood flows to an organ

  elastic fibres - protein (elastin) which can stretch so as blood moves through the artery it will stretch and then recoil, which helps the blood pass through smoothly

  lumen - large lumen, high pressure of blood. layer of endothelial cells which presents a smooth surface, reducing the friction
• arteries carry the blood to the arterioles which then carries the blood to the capillaries in each organ.

  • in the capillary molecules move from the blood to the body cells (for example oxygen and glucose)
  • other molecules move from the body cells to the blood in the capillaries (for example carbon dioxide and urea)
• arterioles

  blood pressure is lower and the effect of the pulse is weaker

  collagen and elastic fibres - much thinner as doesn't have to withstand such a high pressure

  smooth muscle - much thicker because it has to contract to reduce the blood flow
• there are extensive networks of capillaries in every organ and tissue. these capillaries are very branched so provides maximum surface area for molecules to be exchanged

  capillary bed - where substances are exchanged between the blood and body cells
• • red blood cells move through the capillary in single file, very slow movement, so increases the time available for molecules to diffuse from the blood into the body tissues
  • red blood cells are close to the capillary walls so creates a short diffusion distance between the blood and body tissues. this increases the rate of diffusion between molecule.
  • red blood cells are similar diameter to the capillary so reduces the diffusion distance
• veins have thinner walls than arteries because they don't have to withstand a high pressure

  they have a large lumen so they can carry a greater volume of blood back to heart
• smooth muscle - thinner in veins as blood is at a lower pressure so doesn't need to control the amount of blood flowing as it is slower moving

  elastic fibres - thin as there is no elastic recoil
• venules connect to veins, these veins carry deoxygenated blood to the vena cava where it passes to the heart
• veins have a lining of endothelial cells which creates a smooth surface and reduces the friction between the blood and the walls of the vein
• valves ensure that there is no back flow as blood can be moving against gravity (e.g. in the legs and arms)

  if the blood moves forward the valves stay open but if the blood starts to move back then they will close to prevent backflow
• veins are found lying between skeletal muscles, such as large muscles in the legs.

  • when the muscles contract they squeeze the veins so it forces the blood along
• blood is made up of 2 main components: plasma and blood cells
• tissue fluid - fluid that is passed out of the capillaries and it bathes the cells.

  tissue fluid leaves the blood at the arteriole end and transfers molecules such as glucose and oxygen to the tissue cells

  waste molecules from the tissue cells (carbon dioxide) pass into the tissue fluid and return back into the blood at the venule end
• tissue fluid is forced out of the blood at the arteriole end of the capillary due to the hydrostatic pressure. this is because the blood still has a relatively high pressure as it has just come from an artery
• plasma proteins stay in the capillary so lowers the water potential meaning that more water will move back into the capillary via osmosis (oncotic pressure)
• some tissue fluid drains into a series of blind ended vessels called lymph capillaries which connect to larger lymph vessels forming the lymphatic system.
• tissue fluid goes back into the capillaries by osmosis. the tissue fluid is less negative and inside the capillary is more negative due to the plasma proteins so water moves into the capillary by osmosis down a water potential gradient
• haemoglobin is made up from 4 polypeptide chains, each chain is bound to a prosthetic group called haem. so there are 4 haem groups in each haemoglobin molecule.

  • 1 molecule of haemoglobin can combine with 4 molecules of oxygen.
  • oxyhaemoglobin - reversible reaction and can release the oxygen when required.
• affinity - how strongly oxygen is bound to haemoglobin
• at low pressures of oxygen, haemoglobin has a low affinity

  • once one oxygen molecule is bound the affinity of haemoglobin increases, so it becomes easier to bind further oxygen molecules
• takes relatively large partial pressure of oxygen for the first molecule to bind

  after the first oxygen molecule binds the quaternary structure changes so increases the affinity of the haem groups for oxygen

  second and third haem groups only require a small increase in partial pressure

  the 4th haem group requires relatively high partial pressure because 3/4 haem groups have already been filled so the chances of an oxygen molecule colliding with the 4th haem group is relatively low
• at a certain point one oxygen molecule unloads from the haemoglobin molecule and this changes the structure of the haemoglobin and the effect of this is to decrease the oxygen affinity for the remaining haem groups.

  if its more active tissue more oxygen molecules will unload, the partial pressure is lower

  for the final oxygen molecule to unload the partial pressure has to be very low and only takes place in very active tissue
• the Bohr effect - shifts the oxygen dissociation curve to the right. there is an increase in carbon dioxide so causes the oxygen affinity of haemoglobin to decrease
• low affinity - unloads oxygen easily

  high affinity - loads oxygen easily
• partial pressure of carbon dioxide will be high in active tissue undergoing aerobic respiration so haemoglobin has a lower oxygen affinity so it is more likely to unload oxygen molecules in respiring tissues
• if blood passes through very active tissue the partial pressure of oxygen will fall. because haemoglobin has a reduced affinity for oxygen it is easier for more oxygen molecules to unload
• fetal haemoglobin

  in the placenta the fetal blood and maternal blood pass closely to each other but DO NOT mix
• fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin

  • shifts the oxygen dissociation curve to the left
  • higher affinity for oxygen so increases the oxygen transfer across the placenta
• • the heart is formed from cardiac muscle
  • 2 separate sides (right, deoxygenated blood. left, oxygenated blood)
  • 4 chambers (2 atria, 2 ventricles)
• atria and ventricles are separated by the atrioventricular valves

  • left - bicuspid
  • right - tricuspid
• the right and left sides of the heart are completely separate. they are separated by the septum which prevents blood passing between the two sides of the heart