Title: InkBiology / Exchange surfaces and gas exchange
On every note of this deck, unless a note says otherwise:
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Front: DEFINE
Define: Ventilation rate.
Back: The number of breaths taken per minute, about 12 to 15 at rest. Also called breathing rate.

Front: SPOT THE ERROR
Say what is wrong with this claim:
A large animal needs lungs because it has more cells to supply than a small one.
Back: The problem is the ratio, not the total. Double an animal's length and its volume rises eightfold while its surface rises fourfold, so every gram of tissue has half the surface serving it. Demand outgrows surface, not supply.

Front: DEFINE
Define: Diffusion.
Back: The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion. It is passive and requires no metabolic energy.

Front: DEFINE
Define: Vital capacity.
Back: The largest volume that can be breathed out after the deepest possible breath in; roughly 4.6 dm³ in a young adult man and 3.1 dm³ in a young adult woman.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Breathing in works by the lungs expanding and pulling air into themselves.
Back: The lungs cannot ventilate themselves; their airway smooth muscle adjusts width, not airflow. The diaphragm and intercostal muscles enlarge the thoracic cavity, the lungs follow it, the pressure inside falls, and only then does air move in. Muscles, volume, pressure, air, in that order.

Front: DEFINE
Define: Residual volume.
Back: The volume left in the lungs after the hardest possible breath out, about 1.2 dm³. It cannot be measured with a spirometer, because it never leaves the lungs.

Front: DEFINE
Define: Smooth muscle.
Back: Involuntary muscle in the airway wall; its contraction narrows the lumen, and it is the tissue that contracts inappropriately during an asthma attack.

Front: DEFINE
Define: Squamous epithelium.
Back: An epithelium of very flattened cells, giving a wall a fraction of a micrometre thick; the lining of an alveolus.

Front: DEFINE
Define: Exchange surface.
Back: A surface across which substances move between an organism and its environment, or between two compartments within it.

Front: DEFINE
Define: Gill filament.
Back: One of the many thin projections from a gill arch, sometimes called a primary lamella.

Front: DEFINE
Define: Goblet cell.
Back: A cell in the airway epithelium that secretes mucus, which traps dust, pollen and bacteria.

Front: DEFINE
Define: Cartilage.
Back: A firm, flexible supporting tissue. In the trachea it forms incomplete rings, and in the bronchi irregular blocks, holding the airway open against the pressure drop of inspiration.

Front: DEFINE
Define: Diffusion distance.
Back: The thickness of the barrier a substance has to cross, sometimes called the length of the diffusion pathway.

Front: DEFINE
Define: Elastic fibres.
Back: Fibres that stretch as the lungs inflate and recoil as the muscles relax, providing most of the force for quiet expiration.

Front: CHECK YOURSELF
Explain how the structure of an alveolus and the blood supply around it together maximise the rate of oxygen uptake. Refer to all three terms of Fick's law in your answer.
Back: Surface area. There are about 300 million alveoli, giving a combined area of roughly 70 m². Area is on the top of the fraction, so a larger area means a proportionally faster rate.
Thickness. The alveolar wall is a single layer of flattened squamous epithelium and the capillary wall is a single layer of endothelium, sharing one basement membrane, so the whole barrier is under a micrometre. Thickness divides, so a short pathway means a fast rate. The capillaries are narrow enough to force red cells against the wall, which shortens the path further.
Difference in concentration. Ventilation keeps replacing the alveolar air, holding the alveolar oxygen partial pressure near 13.3 kPa, while blood flow keeps removing oxygenated blood and bringing in blood at around 5.3 kPa. Neither side is allowed to equilibrate, so the gradient is maintained rather than used up.
The point that ties them together is that all three are maintained at once. Any one of them can fail on its own: scarring that thickens the wall, emphysema that destroys area, a blocked bronchiole that stops ventilation. Each of those reduces the rate by itself, which is why lung diseases with quite different causes produce the same breathlessness.

Front: DEFINE
Define: Operculum.
Back: The bony flap covering the gills of a bony fish, acting as a valve and part of the pump that maintains the flow of water.

Front: DEFINE
Define: Ciliated epithelium.
Back: An epithelium whose cells carry cilia that beat in a co-ordinated wave, sweeping mucus and trapped particles up towards the throat.

Front: CHECK YOURSELF
Water flows over a gill lamella at a rate of 100 arbitrary units of oxygen per minute and leaves carrying 20. Blood arrives carrying 10 and leaves carrying 90. Show that these figures are consistent, state what would happen if the blood flow were reversed, and explain the difference in terms of the concentration gradient.
Back: Check the balance first. The water loses 100 − 20 = 80 units and the blood gains 90 − 10 = 80 units. Whatever leaves one fluid enters the other, so the figures are consistent.
The blood leaves carrying 90 while the water leaves carrying 20, which looks impossible until you notice which end each fluid leaves from. Blood at 90 is leaving at the end where water arrives at 100, so it is beside water richer than itself right up to the moment it goes.
Reverse the blood flow and the two now travel the same way. They start far apart, 100 against 10, and converge as oxygen crosses. Once they reach the same value there is no gradient left and diffusion stops, so both leave at about 55 units and the rest of the lamella does nothing.
The reason is the gradient. In counter-current flow the water alongside any point of the lamella is always richer in oxygen than the blood at that point, so oxygen diffuses across the full length of the surface. In parallel flow the gradient exists only over the first part of the lamella and the remaining surface is wasted, which is why the counter-current arrangement extracts around 80% of the oxygen and the parallel one could not exceed half.

Front: CHECK YOURSELF
A student claims that a large organism could avoid needing a gas exchange system by simply having a very large surface area. Evaluate this claim, using the ratio and the behaviour of diffusion over distance.
Back: The claim is not wrong so much as incomplete. A large surface area on its own is no use; what matters is the surface area relative to the volume it has to supply, because demand rises with volume.
An organism can raise its ratio by changing shape, and flatworms and tapeworms do exactly that. So the strategy is genuinely available, and some large organisms use it.
It runs out for a second reason, though, which is distance. Even with plenty of surface, any cell buried deep inside is a long way from it, and diffusion time rises with the square of distance: a centimetre takes hours. So a body that stays thin can manage without an exchange system, and a body that is thick cannot, however large its surface is.
A complete answer therefore separates the two limits: the ratio limits how much can cross, and the diffusion distance limits how quickly it reaches the middle. Large active animals fail both tests, which is why they have a specialised exchange surface and a transport system to finish the delivery.

Front: DEFINE
Define: Tidal volume.
Back: The volume of air moved in or out in one normal breath at rest, about 0.5 dm³ in an adult.

Front: DEFINE
Define: Lamella.
Back: One of the thin plates set at right angles along a gill filament, one cell thick and richly supplied with blood; the actual exchange surface.

Front: DEFINE
Define: Counter-current flow.
Back: An arrangement in which two fluids move past each other in opposite directions, so a concentration gradient is maintained along the whole length of the exchange surface.

Front: SPOT THE ERROR
Say what is wrong with this claim:
A fish's gills work well because water flows over a huge surface area, so plenty of oxygen has the chance to diffuse across.
Back: Area alone would not save the fish. Run water and blood the same way and they equalise part-way along the lamella, after which the remaining surface does nothing. Counter-current flow keeps the water richer in oxygen than the blood beside it the whole way.

Front: DEFINE
Define: Concentration gradient.
Back: The difference in concentration between two points, divided by the distance between them.