Title: InkBiology / Animal transport and cardiovascular biology
On every note of this deck, unless a note says otherwise:
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Front: CHECK YOURSELF
The wall of the left ventricle is about three times thicker than the wall of the right. Explain why, and suggest what would go wrong if the right ventricle wall were as thick as the left.
Back: Both ventricles push out the same volume of blood per beat, roughly 70 cm³, but they push it to very different places. The right ventricle sends blood to the lungs, which are a few centimetres away and offer little resistance, so about 24 mmHg is enough. The left ventricle sends blood through the aorta to the whole body, against far greater resistance and over a far greater distance, and needs about 120 mmHg.
A thicker wall contains more cardiac muscle, so more force is generated when it contracts, so the pressure developed inside the chamber is greater. Structure follows the pressure required.
If the right ventricle generated 120 mmHg, that pressure would arrive at the pulmonary capillaries, whose walls are one cell thick. The high hydrostatic pressure would force far more fluid out of them than could be drawn back or drained away, and fluid would collect in the alveoli, which is pulmonary oedema. Gas exchange would be impaired by the extra diffusion distance, and the person would become breathless.
The general point is worth keeping: a pump is matched to its circuit, and a mammal has two circuits precisely so that the gas exchange surface never sees systemic pressure.

Front: DEFINE
Define: Tissue fluid.
Back: The fluid surrounding cells, formed by ultrafiltration from capillaries; it is very poor in plasma protein, though not literally protein-free, and has no red blood cells and no platelets.

Front: DEFINE
Define: Atrioventricular node.
Back: The only electrical connection between atria and ventricles; it delays the impulse so the atria empty first.

Front: CHECK YOURSELF
Explain why a capillary wall being one cell thick is useful, and why the same feature makes it essential that arterioles reduce blood pressure before blood arrives.
Back: A wall one endothelial cell thick, roughly 0.6 μm, gives a very short diffusion pathway. Oxygen, glucose and other small solutes cross quickly, and the rate of diffusion is inversely proportional to the distance travelled, so a thin wall means fast exchange. The gaps between the endothelial cells also allow the bulk flow of fluid out of the capillary that forms tissue fluid.
The cost of that thinness is fragility, and low resistance to being stretched. Blood leaving the left ventricle at 120 mmHg in pulses would rupture such a wall, or at the very least drive far more fluid out of it than could ever be drawn back, producing oedema.
Arterioles are narrow and heavily muscled, and resistance to flow rises steeply as radius falls. Millions of them in parallel drop the mean pressure from around 90 mmHg to around 35 mmHg and damp out the difference between systolic and diastolic entirely, so what reaches the capillary bed is a slow, steady, low-pressure flow.
The two features are a matched pair: the capillary can only afford to be that thin because the arteriole upstream has already taken the pressure out, and the arteriole's expensive muscle is only worth having because the capillary downstream is that thin.

Front: DEFINE
Define: Partial pressure of oxygen.
Back: The contribution oxygen makes to the total pressure of a gas mixture, measured in kPa; a measure of how much oxygen is present.

Front: DEFINE
Define: Haemoglobinic acid.
Back: Haemoglobin that has taken up hydrogen ions, buffering the contents of the red blood cell.

Front: DEFINE
Define: Bohr shift.
Back: The rightward shift of the oxygen dissociation curve caused by an increased partial pressure of carbon dioxide and the fall in pH that goes with it, so that oxygen dissociates more readily at any given partial pressure.

Front: DEFINE
Define: Metabolic rate.
Back: The rate at which an organism uses energy, and so the rate at which it must be supplied with oxygen and relieved of carbon dioxide.

Front: DEFINE
Define: Percentage saturation.
Back: The proportion of the oxygen-binding sites on haemoglobin that are actually occupied.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Arteries carry oxygenated blood and veins carry deoxygenated blood.
Back: Artery means carrying blood away from the heart, which is a statement about direction and not about cargo. The pulmonary artery carries deoxygenated blood and the pulmonary vein oxygenated blood, and the umbilical vessels break the pattern the same way round.

Front: CHECK YOURSELF
During a sprint, blood leaving a leg muscle is far less saturated with oxygen than blood leaving the same muscle at rest, yet blood leaving the lungs is almost as saturated as ever. Explain both observations.
Back: A sprinting muscle respires far faster, so the partial pressure of oxygen within it falls, perhaps from around 4 kPa at rest to 2 kPa or lower. On the dissociation curve that region is steep, so a small fall in partial pressure produces a large fall in saturation, and a great deal of oxygen is unloaded.
The muscle is also producing far more carbon dioxide. That carbon dioxide forms carbonic acid, which releases hydrogen ions and lowers the pH of the blood. Hydrogen ions binding to haemoglobin change its shape so that it holds oxygen less tightly, and the dissociation curve shifts to the right. At the same partial pressure the blood is now less saturated still, so even more oxygen is released. The two effects work in the same direction and both are driven by the muscle's own activity.
At the lungs the partial pressure of oxygen is around 12 to 13 kPa, which lies on the flat plateau of the curve. Even the Bohr-shifted curve is above 90 per cent saturated by that partial pressure, so loading is barely affected. Carbon dioxide is also being lost to the alveoli, which raises the pH again and moves the curve back towards its normal position.
So the shift is asymmetric in its effects, and deliberately so: it acts where the curve is steep and hardly acts where the curve is flat.

Front: DEFINE
Define: Tendinous cords.
Back: Inelastic strands running from the atrioventricular valve flaps to papillary muscles projecting from the ventricle wall. The muscles tense as the ventricle contracts, so ventricular pressure cannot turn the flaps inside out; the pressure does the opening and shutting, the cords only stop the flaps going too far.

Front: DEFINE
Define: Chloride shift.
Back: The movement of chloride ions into a red blood cell as hydrogencarbonate ions move out, maintaining electrical neutrality.

Front: DEFINE
Define: Purkyne fibres.
Back: Conducting fibres running from the bundle of His up through the ventricle walls, so contraction begins at the apex.

Front: DEFINE
Define: Sinoatrial node.
Back: The patch of tissue in the right atrium wall that sets the rhythm of the heart.

Front: DEFINE
Define: Lymph node.
Back: A swelling in a lymph vessel where lymphocytes collect and pathogens in the lymph are removed.

Front: DEFINE
Define: Semilunar valves.
Back: The valves at the base of the aorta and the pulmonary artery, preventing backflow into the ventricles.

Front: DEFINE
Define: Carbonic anhydrase.
Back: The enzyme in red blood cells catalysing the reversible reaction between carbon dioxide and water to form carbonic acid.

Front: DEFINE
Define: Hydrostatic pressure.
Back: The physical pressure the blood exerts against the capillary wall, pushing fluid out through the gaps between endothelial cells.

Front: DEFINE
Define: Oncotic pressure.
Back: The inward pull created by plasma proteins, which stay in the capillary and lower the water potential of the blood relative to the tissue fluid. Also called colloid osmotic pressure.

Front: DEFINE
Define: Mass transport system.
Back: A system in which substances are moved in bulk, in a fluid driven by a pump, over distances too great for diffusion to serve.

Front: DEFINE
Define: Coronary arteries.
Back: The vessels branching from the base of the aorta that supply the heart muscle itself with oxygenated blood.

Front: DEFINE
Define: Conjugated protein.
Back: A protein with a non-protein prosthetic group permanently associated with it.

Front: DEFINE
Define: Loading.
Back: Association of oxygen with haemoglobin, at the gas exchange surface.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Valves open and close because muscle pulls them open and shut.
Back: Nothing pulls a valve: there is no muscle attached to the free edge of one. A valve moves when the fluid on one side pushes harder than the fluid on the other, so every valve movement in the cycle follows a pressure difference.

Front: SPOT THE ERROR
Say what is wrong with this claim:
The Bohr shift means that in an exercising person, haemoglobin picks up less oxygen at the lungs.
Back: The Bohr shift changes unloading, not loading. At the roughly 12 kPa found in the lungs the shifted curve has already flattened and saturation is still above 90 per cent. It is at the steep part of the curve that the shift releases much more oxygen.

Front: DEFINE
Define: Myogenic.
Back: Contracting from within the muscle itself, without needing a nerve impulse to initiate the beat.

Front: DEFINE
Define: Prosthetic group.
Back: The non-protein component; in haemoglobin, the haem group with its iron(II) ion.

Front: DEFINE
Define: Atrioventricular valves.
Back: The valves between each atrium and its ventricle; tricuspid on the right, bicuspid or mitral on the left.

Front: DEFINE
Define: Unloading.
Back: Dissociation of oxygen from haemoglobin, at respiring tissue.

Front: DEFINE
Define: Surface area to volume ratio.
Back: The exchange surface available per unit of volume; it falls as an object gets larger.

Front: DEFINE
Define: Lymph.
Back: Tissue fluid that has drained into the lymphatic system; similar to tissue fluid but containing lymphocytes and more fatty acids after a meal.