Aliens are cool, aren’t they? Absolutely, really cool; so why should we care if they are in any way scientifically justified? How about monsters; how do they work?
Nerds are very keen on physics and maths, but biology seems to take more of a back seat. Is there any reason why the monsters or aliens in your story should not work within the realms of your carefully constructed universe? Absolutely not, if you respect (or acknowledge and then ignore) some basic rules of nature.
Why should you bother? Is any of this relevant to the story? Well, it often is. How many times in Buffy, did the Slayers defeat a monster by studying ancient books to learn about a specific weakness? In a more recent example (and trying to avoid spoilers here), the plot of Stranger Things pivoted on knowledge of the lifecycle of the ‘bad guys.’ How your monsters or aliens work is often critical for the story.
Both aliens and monsters generally tend to fall under the heading of “living things.” There may be some argument about ‘undead’ creatures such as vampires and zombies, but I think in general that’s a good way to consider the question. This structure encompases humanoids, plants and animals right down to bacteria.
You may remember from school biology that living things are defined as creatures which
- Respire (breathe)
- Metabolise (eat and drink)
- Respond to stimuli
- Adapt to environment
Respiration, or gas exchange takes place in a variety of different ways. Its function is to get life giving oxygen into the body and remove the waste gas, carbon monoxide, from the body. There are different mechanisms for this to occur, depending on the creature involved. Oxygen is used to “burn” with other chemicals derived from food, releasing energy.
In humans and other mammals, the diaphragm (a very thin muscle between the chest and the abdomen) drops and the intercostal muscles (between the ribs) contract to expand the chest. This creates a vacuum and air rushes into the lungs. The oxygen in this air diffuses into the blood as carbon dioxide waste diffuses in the opposite direction. Breath is expelled as the intercostal muscles relax, dropping the chest and the diaphragm moves upwards, reducing pressure in the chest cavity.
There is a nice video of how this all happens here.
The lungs are made of branching tubes, becoming smaller and smaller, until they reach tiny air spaces, which the equally tiny blood vessels are wrapped around, allowing gas exchange. They have a huge surface area to make this more efficient. The surface area of a pair of human lungs, is approximately the same as one half of a tennis court!
The circulatory system which includes blood vessels and the heart, acts to move oxygenated blood around the body and take carbon dioxide back towards the lungs.
This is a very effective system in an Earth like atmosphere, where the oxygen level in the atmosphere is around 20%. At 15% oxygen, cognitive function of organisms is severely impaired and at about 8% oxygen, life is unsustainable, with death in less than 10 minutes.
In fish, water washing over the gills performs the same function, but this is why most fish continually swim, or at least rest in fast flowing areas of water. Gills also have a large surface area for gas exchange, with water entering through the fish’s mouth and leaving through the gill flaps. As this flow and gas exchange is passive, there is no reason that this mechanism could not work in an atmosphere with a high air flow, such as a planet with constant wind, or even with gills which were outside the body, towed about like a barrage balloon.
Interesting, throw-away fact for you; an octopus beats time lords by having 3 hearts, 2 of which pump blood to their gills. Mammals use haemoglobin (a red pigment) as our oxygen carrying compound, making our blood scarlett when oxygenate. In an octopus it is blue haemocyanin, so they are really “blue blooded.”
Insects have a different mechanism, using spiracles which are blind ending tubes along the abdomen, through which oxygen diffuses in and carbon dioxide diffuses out. They then have a primitive circulatory system to pump this around the body. Using spiracles affects the size it is possible for a creature to achieve. Above a certain length of spiracle tube, the diffusion of gas becomes ineffective and air sits in a dead-space, with no movement. For this reason, it is impossible on earth for insects to become very large. However, with different atmospheric pressures, gases could diffuse further and larger insects would be possible.
Other organs within the circulatory system also have variable functions. In humans, the spleen is an important part of the immune system. Individuals who have had their spleen removed will become immunocompromised. In dogs and horses it also stores red blood cells which can be squeezed out into the circulation when an extra boost is needed. This is seen, for example is racehorses or performance dogs, where the packed cell volume or percentage of red cells in the blood can increase markedly on exercise.
Does respiration have to involve oxygen? No; although reacting with oxygen is a very efficient way to create energy for the body, which is what respiration does. Some bacteria already respire using hydrogen gas, but as this is relatively inefficient, it would be hard to sustain a large creature using this mechanism.
Other bacteria respire in the absence of oxygen (anaerobically) and derive energy from alternative ways of oxidizing glucose. They used nitrates or suphates for this reaction, but even though they are not using air, these compounds release oxygen during the reaction.
So it would be possible to have the chemical reactions in the body sustained in an atmosphere lacking in oxygen gas, but on a cellular level, creatures are most efficient when oxygen is used.
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This is the first of a series of blog posts, which have been derived from a lecture I gave at Right on Paper, a one day conference of academic talks on various subjects, for speculative fiction writers, held in London on 3rd February 2018.
This talk, was in turn sparked by a conversation in the bar at Nine Worlds, a science fiction and fantasy convention, where I announced that there was a really interesting talk which could be given on Xenobiology and decided I’d better put my money where my mouth was.
I also blog on veterinary matters here.