Transport in Mammals (As Level Biology)

• Explain why the Mammalian circulatory system is described as a closed double circulation.

  In the mammalian circulatory system, the blood travels through the vessels and never leaves them(closed) and the blood passes through the heart twice, systematic and pulmonary circulation(double)
• What is the name of the tissue that lines the lumen of all blood vessels?

  Endothelium
• Outline the structure and function of capillaries.

  Capillaries are one cell thick and only consist of the endothelium. Their main function is to bring blood as close as possible to all body cells and allow rapid transfer of substances between cells and blood. They are one cell thick with thin walls to allow rapid diffusion. Their size is about 7um in width to allow only one RBC to pass through at a time to expose its full surface area for exchange of materials. They also have gaps between endothelial cells for rapid exchange of materials.
• What are valves?


  Valves are present in veins, these are extensions of endothelium and prevent the backflow of blood.
• Describe the structure of Red Blood Cells.

  They have a biconcave disc shape and a 7um diameter to easily squeeze through capillaries. They do not have a nucleus so there is more space for oxygen storage. Contain haemoglobin which binds reversibly to oxygen. also contain the enzyme carbonic anhydrase.
• Describe how tissue fluid is formed

  At the arterial end of the capillary, the hydrostatic pressure pushes out the plasma into the cells which causes tissue fluid to form, it does not contain plasma proteins but white blood cells and no red blood cells. At the venous end, the hydrostatic pressure is less and osmotic pressure is greater so 90% of the tissue fluid is forced back into the capillary.
• Explain cooperative bonding of haemobglobin

  One Hb molecule has four haem groups, as a molecule of oxygen combines with a haem group, it causes confirmational changes in the shape of the molecule making it easier for the next oxygen to combine with another haem group. This is why the middle part of the oxygen disassociation curve is steep, however, it gets dificult for the fourth oxygen to bind with the haem group due to a shortage of remaining binding sites.
• What is the difference between fetal and adult haemolobin

  Fetal haemoglobin has a gamma chain instead of a beta chain and higher affinity for oxygen compared to adult haemoglobin. it shifts the disassociation curve to the left and higher.
• Describe how co2 is transported as hydrogen carbonate ions.

  RBCs contains the enzyme carbonic anhydrase which catalyses the reaction between co2 and water to produce carbonic acid(h2co3). within RBCs, H2CO3 disassociates into hydrogen and hydrogen carbonate ions. the hydrogen carbonate ions move out the RBCs into the plasma where they combine with sodium to form sodium hydrogen carbonate meanwhile the hydrogen ions combine with haemoglobin to form weak haemoglobinic acid. in order for this to happen, oxygen is released for the RBCs through diffusion.
• what is the chloride shift?

  During the transport of CO2, hydrogen carbonate ions formed by disassociation of carbonic acid leave the RBCs, leaving behind hydrogen ions which lower the pH and can cause the cell to become positively charged. to prevent this electrical imbalance, chloride ions move into the cell via facilitated diffusion.
• Why do ventricles have thick walls compared to atria?

  Blood from ventricles travels a longer distance and is pumped at a higher pressure. left ventricle has the thickest walls as it pumps blood to the entire body and that covers more distance than the right ventricle which pumps blood just to the lungs.
• Cardiac cycle

  • Atrial diastole: atria and ventricles are relaxed blood returns left and right atria via veins. The atria gets filled with blood, the valve is pushed open some blood goes into the ventricle.
  • Atrial systole: muscles in both atria contract pushing blood into the ventricles, pressure is not too high
  • Ventricle systole: ventricles contract, more forceful contraction than atria. AV valve shuts and semilunar vales open because of the pressure of the blood
  • Ventricular diastole:Ventricular muscles relax and pressure falls, semilunar valves close to prevent the backflow of blood
• Heart action

  • The heart is myogenic, it beats without any external stimuli.
  • The sinoatrial node initiates the wave of excitement which passes through the walls of the atria causing them to contract.
  • A band of non-conducting tissues lying between the atrium and ventricle does not allow the wave to spread to the ventricles.
  • WOE can only pass to the ventricles through the atrioventricular node which causes a slight delay to ensure that the ventricles contract.
  • AVN passes the wave to conducting fibres in ventricle walls which causes them to contract
• Formation of lymph

  • some tissue fluid enters the lymph capillaries
  • they have closed ends and large pores to allow large molecules to pass through
  • larger molecules that are not able to pass through the capillary wall enter the lymphatic system as lymph.
  • the liquid moves through the larger vessels of this system by compression caused by body movement
  • the lymph eventually reenters the bloodstream through veins located close to the heart
  • Any plasma proteins that have escaped from the blood reenter the blood