1B

• Small organisms don't need a transport system because:

  • diffusion distance is very short
  • the surface area : volume ratio is high
  • metabolic demands are low
• Transport systems in larger organisms are needed to move nutrients, chemicals, and waste, as well as substances made internally, like hormones.
• mass transport system - an arrangement of structures by which substances are transported in the flow of a fluid with a mechanism for moving it around the body
• Features of mass transport systems:

  • exchange surfaces
  • system of vessels
  • system to ensure movement in correct direction
  • system to move materials fast enough
  • suitable transport medium
  • adaptable rate of transport
• Insects have open circulation systems, so the blood circulates in large open spaces.
• Larger animals, including mammals, have closed circulatory systems where the blood is contained in vessels, and moves continuously.
• Advantages of double circulation system:

  • oxygenated and deoxygenated blood do not mix
  • tissues receive as much oxygen as possible
  • oxygenated blood is delivered quickly at high pressure
  • deoxygenated blood in lungs are at low pressure so no damage
• single circulation system - a circulation in which the heart pumps the blood to the gas exchange organs and the blood then travels around the body before returning to the heart
• double circulation system - a circulation that involves 2 separate circuits, where the heart pumps oxygenated blood around the body, then it returns to the heart (systemic) to be pumped into the lungs to be oxidised (pulmonary), before it is pumped back around the body
• systemic circulation - carries oxygenated blood from heart around body, then back to the heart when it is deoxygenated
• pulmonary circulation - carries deoxygenated blood to the lungs from the heart, and oxygenated blood back to the heart
• Advantages of closed system:

  • pressure can be increased for increased flow
  • flow can be directed to specific organs
• Blood plasma is the fluid of the mass transport system. It makes up 50% of blood volume. It helps to maintain a constant body temperature by moving heat around the body. It can also act as a buffer to regulate pH changes.
• Blood plasma contents:

  • digested food products (small intestine to liver, liver to body for use or storage)
  • nutrient molecules (storage areas to cells)
  • excretory products (cells to excretion organs e.g. lungs, kidneys)
  • hormones (source to body)
• Erythrocytes (red blood cells) contain haemoglobin that carry oxygen. They are made in the bone marrow. They live for around 120 days, and there are approximately 5 million per mm^3 of blood.
• Erythrocytes shape:

  • biconcave disc for large sa:v ratio, for rapid diffusion
  • no nucleus for more space for haemoglobin
  • 250 - 300 million haemoglobin molecules
• Leucocytes (white blood cells) are much bigger than erythrocytes, but can change their shape to fit through small vessels. There are around 4000 - 11000 per mm^3 of blood. They are made in the bone marrow, but sometimes mature in the thymus gland.
• Leucocytes function is to defend the body from infection, and play a role in the inflammatory response of the body.
• Leucocytes shape:

  • nucleus
  • colourless cytoplasm
• Erythrocytes function is to transport oxygen around the body, using haemoglobin molecules.
• Platelets are tiny fragments of megakaryocytes, found in the bone marrow. They are used in blood clotting. There are around 150 000 - 400 000 platelets per mm^3 of blood.
• Haemoglobin is a globular protein, with 4 peptide chains, that each have an iron-containing prosthetic group that can carry 4 molecules of oxygen, to form oxyhaemoglobin.
• The first oxygen to bind to haemoglobin changes its shape, called conformational change, making it easier for the next molecules to bind. There is a steep concentration gradient between the plasma and erythrocyte cytoplasm. The strong affinity of haemoglobin for oxygen means that a small change in the proportion of oxygen can have a large effect on saturation of blood with oxygen.
• Waste CO2 diffuses out of cells into the blood. It then dissolves in the blood to form carbonic acid, which separates into H+ and HCO3- ions, in a reversible reaction. Carbonic anhydrase is an enzyme that controls the rate of reaction that produces carbonic acid. In high CO2 concentrations, more carbonic acid is produced but in low CO2 concentrations, more CO2 is formed.
• Transport of CO2:

  • 5% in plasma
  • 10-20% in haemoglobin (carbaminohaemoglobin)
  • 75% in cytoplasm of red blood cells as HCO3-
• The Bohr effect is the changes in the oxygen dissociation curve of haemoglobin that occurs due to a rise in CO2 and a reduction of the affinity of haemoglobin for oxygen. Haemoglobin releases oxygen in active tissue with CO2 levels.
• Fetal haemoglobin is in the blood of a foetus, and it has a higher affinity for oxygen than the mother so that it can remove oxygen from the maternal blood, even when the proportion of oxygen is low.
• Blood clotting occurs to prevent severe blood loss and to stop pathogens from entering open wounds.
• Platelet involvement in the formation of blood clots:

  When platelets make contact with the components of the tissue from a fissure, they break open and release serotonin and thromboplastin. Serotonin causes the smooth muscle of the blood vessel to contract, which narrows the lumen, reducing flow. Thromboplastin is an enzyme that starts a sequence of chemical changes that clot the blood.
• blood clotting process:

  1. damaged tissues release platelets which release thromboplastin
  2. thromboplastin catalyses prothrombin (large soluble protein in plasma) into thrombin (soluble enzyme), with the correct concentration of Ca2+
  3. thrombin catalyses fibrinogen (soluble protein in plasma) into fibrin (insoluble substance)
  4. fibrin forms a mesh of fibres to cover the wound
  5. platelets and red blood cells get stuck in the mesh, forming a clot
  6. protein in the structure of the platelets contract, making the clot tighter to form a scab that protects the skin as it heals.
• A clot in the wrong place can cause a heart attack or a stroke.
• The circulatory system is made up of different types of blood vessels. The arteries carry blood away from the heart, the veins carry blood towards the heart. The capillaries are small vessels that spread throughout the body.
• Almost all arteries carry oxygenated blood, apart from the pulmonary artery and the umbilical artery (foetus to placenta).
• The lumen of arteries gets narrower as the arteries move away from the heart. The heart pushes blood at a high pressure into the arteries, so the elastic fibres expand to accommodate the volume of blood. Between beats, the fibres return to their original size, pushing the blood in a continuous flow. The smooth muscles contract or relax to regulate blood flow.
• The capillaries link the arterioles and venules, which are the smallest branches furthest from the heart. Capillaries branch between the cells so that substances can diffuse quickly. Since the lumen is small, blood flows slowly allowing more diffusion to occur. Capillary walls are 1 cell thick for faster diffusion.
• Almost all veins carry deoxygenated blood, apart from the pulmonary vein and umbilical vein (placenta to foetus) The blood pressure in veins is low, so it needs muscle pressure and one-way valves to reach the heart.
• Larger veins are between muscle blocks, so when muscles contract from physical activity, the veins are squeezed which pushes blood to the heart.
• There are one-way valves called semilunar valves in the veins, which force the blood to flow in one direction. If blood flows backwards, the valves close, blocking the flow.
• The heart has 2 pumps working in time together. The right side pumps blood from the body to the lungs, the left side pumps blood from the lungs to the body. The two sides are divided by the septum.
• The heart is made of cardiac muscle, which can contract without resting. Cardiac muscle contains myoglobin, which is a respiratory pigment with a higher affinity for oxygen than haemoglobin, so it stores oxygen for respiration.