Transport in mammals

• Pressure does the work everywhere in this topic: it forces tissue fluid out, it opens and shuts every valve, and the partial pressures of oxygen and carbon dioxide decide where haemoglobin loads and unloads.

• 8.1.1 I can state — state that the mammalian circulatory system is a closed double circulation consisting of a heart, blood and blood vessels including arteries, arterioles, capillaries, venules and veins
• 8.1.2 I can describe — describe the functions of the main blood vessels of the pulmonary and systemic circulations, limited to pulmonary artery, pulmonary vein, aorta and vena cava
• 8.1.3 I can recognise — recognise arteries, veins and capillaries from microscope slides, photomicrographs and electron micrographs and make plan diagrams showing the structure of arteries and veins in transverse section (TS) and longitudinal section (LS)
• 8.1.4 I can explain — explain how the structure of muscular arteries, elastic arteries, veins and capillaries are each related to their functions
• 8.1.5 I can recognise — recognise and draw red blood cells, monocytes, neutrophils and lymphocytes from microscope slides, photomicrographs and electron micrographs
• 8.1.6 I can state — state that water is the main component of blood and tissue fluid and relate the properties of water to its role in transport in mammals, limited to solvent action and high specific heat capacity
• 8.1.7 I can state — state the functions of tissue fluid and describe the formation of tissue fluid in a capillary network
• 8.2.1 I can describe — describe the role of red blood cells in transporting oxygen and carbon dioxide with reference to the roles of: • haemoglobin • carbonic anhydrase • the formation of haemoglobinic acid • the formation of carbaminohaemoglobin
• 8.2.2 I can describe — describe the chloride shift and explain the importance of the chloride shift
• 8.2.3 I can describe — describe the role of plasma in the transport of carbon dioxide
• 8.2.4 I can describe — describe and explain the oxygen dissociation curve of adult haemoglobin
• 8.2.5 I can explain — explain the importance of the oxygen dissociation curve at partial pressures of oxygen in the lungs and in respiring tissues
• 8.2.6 I can describe — describe the Bohr shift and explain the importance of the Bohr shift
• 8.3.1 I can describe — describe the external and internal structure of the mammalian heart
• 8.3.2 I can explain — explain the differences in the thickness of the walls of the: • atria and ventricles • left ventricle and right ventricle
• 8.3.3 I can describe — describe the cardiac cycle, with reference to the relationship between blood pressure changes during systole and diastole and the opening and closing of valves
• 8.3.4 I can explain — explain the roles of the sinoatrial node, the atrioventricular node and the Purkyne tissue in the cardiac cycle (knowledge of nervous and hormonal control is not expected)

Precise vocabulary is part of the biology. Water moves down a water potential gradient; an active site is complementary to its substrate; enzymes are denatured, not killed; ATP releases energy when it is hydrolysed, and respiration never produces energy. Give a calculated answer to the same number of significant figures as the least precise data, or one more, with its unit. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as variables, method, recording, graphs and evaluation rather than as theory.

• “Arteries carry oxygenated blood and veins carry deoxygenated blood.” Correct An artery carries blood away from the heart, not necessarily oxygenated blood; a vein carries blood towards it. The pulmonary artery carries deoxygenated blood and the pulmonary vein oxygenated blood.
• “Tissue fluid forms because proteins leak out of the capillaries.” Correct The plasma proteins stay in. Hydrostatic pressure forces water and small solutes out at the arterial end; the proteins left behind give the plasma a lower water potential, so water returns by osmosis at the venous end.
• “Carbon dioxide binds to the haem group.” Correct Carbaminohaemoglobin forms when CO2 binds to the amine groups of the globin chains. The haem groups are where oxygen binds.
• “In the chloride shift, chloride ions move out of the red blood cell.” Correct In respiring tissue Cl− moves into the red cell as HCO3− moves out. It reverses only in the lungs.
• “A high partial pressure of carbon dioxide moves the curve to the left.” Correct The Bohr shift is to the right: at the same pO2 haemoglobin is less saturated, so more oxygen is released.
• “The left ventricle wall is thicker because it pumps more blood.” Correct Both ventricles pump the same volume each beat. The left generates a much higher pressure, because it pumps blood round the whole body rather than only to the lungs.
• “The heart muscle opens and closes the valves as it contracts.” Correct Heart valves are passive. Each opens when the pressure behind it is higher than the pressure in front, and closes when the pressure in front is higher.
• “The SAN sends the impulse straight down into the ventricles.” Correct Non-conducting tissue between the atria and ventricles blocks that route. The wave reaches the ventricles only through the AVN, after a delay, and the Purkyne tissue carries it to the apex first.
• “Arteries carry oxygenated blood and veins deoxygenated blood.” Repair Arteries carry blood away from the heart; the pulmonary artery carries deoxygenated blood.
• “Elastic arteries contract to push the blood along.” Repair Elastic fibres stretch and recoil; they do not contract. It is the smooth muscle of muscular arteries that contracts, and it changes the lumen diameter rather than pumping.
• “A lymphocyte has a lobed nucleus.” Repair The neutrophil has a lobed nucleus; a lymphocyte's round nucleus almost fills the cell.
• “The plan diagram shows the smooth muscle cells in the tunica media.” Repair A plan diagram shows the layers in proportion and no cells.
• “Tissue fluid forms because proteins leave the capillary.” Repair Proteins stay in the plasma; hydrostatic pressure forces water and small solutes out.
• “Water returns at the venous end because the pressure pushes it in.” Repair It returns by osmosis, because the plasma proteins give the plasma a lower water potential than the tissue fluid, and the hydrostatic pressure there is now lower than that effect.
• “Carbon dioxide binds to the haem group.” Repair It binds to the amine groups of the globin chains (carbaminohaemoglobin).
• “Carbonic anhydrase is in the plasma.” Repair Carbonic anhydrase is inside red blood cells; the plasma has none, which is why most HCO3− is made in the red cells.
• “The chloride shift moves chloride out of the red cell.” Repair In respiring tissue Cl− moves in as HCO3− moves out.
• “Most carbon dioxide is carried by haemoglobin.” Repair Most is carried as HCO3− in the plasma.
• “The Bohr shift moves the curve to the left in the tissues.” Repair High CO2 moves the curve to the right: lower affinity, more oxygen released.
• “At 13 kPa haemoglobin is on the steep part of the curve, so it loads quickly.” Repair Lung pO2 is on the flat part; that is why loading is almost complete even if pO2 falls a little. The steep part is at tissue pO2.
• “The left ventricle is thicker because it pumps more blood.” Repair Both ventricles pump the same volume; the left pumps at higher pressure round the whole body.
• “The semilunar valves close when the ventricles contract.” Repair When the ventricles contract, the atrioventricular valves close and then the semilunar valves open. The semilunar valves close when ventricular pressure falls below arterial pressure, as the ventricles relax.
• “The SAN sends the impulse straight to the ventricles.” Repair Non-conducting tissue blocks it; the AVN delays it and the Purkyne tissue carries it to the apex.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• Draw what is there, in proportion. Three faults break the plan-diagram rules: drawing individual cells, shading the layers, and making the artery's media as thin as the vein's. Measure the thicknesses against the lumen with your eyepiece graticule and keep the ratios.
• Explain means feature + function + reason. “Veins have valves” is a feature; “veins have valves, which close if blood starts to flow backwards, so blood returns only towards the heart” is an explanation.
• Describe with numbers, explain with shape change. When a question says describe the curve, quote readings from it; when it says explain, the key idea is that binding one oxygen molecule changes the shape of the whole haemoglobin molecule.
• The rule for every valve: valves are opened and closed only by differences in blood pressure. A valve opens when the pressure behind it is higher than the pressure in front of it, and closes when the pressure in front is higher.
• Interleave with the chapters that use this one. Topic 9 (gas exchange) uses the capillary and the partial pressures of oxygen: re-answer “why does haemoglobin load almost fully in the lungs?”. Topic 11 (immunity) uses the white cells: re-answer “how do you recognise a neutrophil, a lymphocyte and a monocyte?”. Topic 14 (homeostasis) uses blood and tissue fluid: re-answer “how does tissue fluid form and return?”. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 8 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A multiple-choice item (Paper 1) can turn on one direction or one name: which way the chloride ion moves, which way the Bohr shift moves the curve, which vessel carries deoxygenated blood away from the heart, which cell has the lobed nucleus. A structured question (Paper 2) asks you to describe a sequence — tissue fluid formation, carbon dioxide transport, the conducting system — and to explain it: a wall thickness, the curve's shape, a valve event.
• The data skills this topic lends itself to: the dissociation curve (read a saturation, calculate the oxygen released from a supplied oxygen capacity), a graph of pressures in the atrium, ventricle and aorta (find each valve event and the heart rate), a photomicrograph or electron micrograph of a vessel or a blood cell to identify and measure, and a heart diagram to label. The numerical skills are recall: heart rate = 60 ÷ cycle time, magnification, graticule calibration, percentage change.
• Paper 3 may give you a prepared slide of an artery and a vein to draw as plan diagrams, or a blood smear to draw cells from and measure with a calibrated graticule. For an investigation, the closest context is the heart rate of Daphnia: temperature (or a drug concentration) as the independent variable, beats counted over a timed period as the dependent variable, animal size and time to equilibrate standardised, and a miscounted fast beat as the main error.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.