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I noticed what seems to be a flaw in my Kepler Bb humanoids' 2 circulatory systems. That would be bleeding. Sure, the 2 hearts get more oxygen so an MI is less likely and if it does happen, the other healthy heart compensates so they are only considered to have heart disease if both hearts are damaged in some way.

But there is a big problem here with having the arteries right next to each other. If you have a pair of arteries that are the same size and right next to each other, the force from blood pressure on the walls will change. The force from the right side of the left artery will cause the right side of the right artery to have more force. Likewise, the force from the left side of the right artery will cause the left side of the left artery to have more force. Maybe not double but definitely more. Also, the right side of the left artery and the left side of the right artery will have less force. So overall, the force is going to be going outwards. This will cause asymmetric vasodilation in almost all the arteries(I say almost because the aorta is elastic enough that this added force won't affect it much).

Vasodilation when not because of heat or infection is abnormal due to lowered blood pressure so the hearts will beat faster and harder to raise the blood pressure back up. This increase in heart rate and stroke volume is a problem because this will lead to more vasodilation and it is a vicious cycle. Eventually the arteries will tear. Once you get a tear in an artery, it can either continue down the artery until it reaches a branching point, tear back into the artery, or tear through all layers of the artery and cause internal bleeding.

This internal bleeding due to physiology is very bad news. It means that the humanoid will bleed to death no matter what is done to stop the bleeding initially. This is not a problem with the coronary arteries because 1 set goes from the top of the heart down and the other goes from the bottom of the heart up. This isn't a problem with the veins because of physiologically low blood pressure in the veins. And cappilary bleeding stops very fast and is nothing to worry about. So it is just the arterial bleeding that is worrisome.

So clearly something is needed to stop this vicious cycle. But moving the arteries so that they are further apart makes no sense(more space taken up by arteries than there needs to be) And having 1 heart only supply blood to 1 side of the body makes no sense either(this gets rid of the compensation benefit). So what else could stop this vicious cycle leading to bleeding?

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    $\begingroup$ It seems that you are referring to some Kepler Bb humanoids - if you're referring to some work someone else has done, or maybe other questions, etc. please include link, it makes a question like this less daunting $\endgroup$ – dot_Sp0T Oct 1 '17 at 18:41
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    $\begingroup$ "so they are only considered to have heart disease if both hearts are damaged in some way." While having only one malfunctioning heart is less likely to be fatal, I seriously doubt any member of your race would consider one malfunctioning heart to be "not heart disease." This is like saying that humans don't consider it kidney disease or lung problems as long as one is working, which is of course absurd. Additionally, if I recall correctly, while we can survive with only one of each, the extra strain is not exactly healthy for the remaining one, and our bodies don't function quite as well. $\endgroup$ – jpmc26 Oct 2 '17 at 5:08
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    $\begingroup$ "Eventually the arteries will tear." Well eventually everything in biology fails, we call such tears aneurysms, internal bleeding, strokes, bruises from impact and other forms of "ooh, that's not good." Biological systems under stress get reinforced; it is why exercise builds bone and muscle. Vessel stretching through pulse and muscular movement is constantly causing micro-cracks in the vessel walls; in healthy organisms these are constantly repaired so they do not grow into leaks. Eventually vessels fail as a cascade of other key system failures, typically 80 years or so. $\endgroup$ – Amadeus Oct 2 '17 at 10:58
  • $\begingroup$ Does someone need a doctor? $\endgroup$ – Shirkam Oct 3 '17 at 9:25
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There are a few easy solutions to the problem you have mentioned.

1- Move the arteries apart

Your assumption that But moving the arteries so that they are further apart makes no sense(more space taken up by arteries than there needs to be) is incorrect, because the arteries would be taking the same amount of space regardless of where they are placed. For example, if you have two ropes, they will take up the same space, no matter where you place them. Same is true for the blood vessels.

For a start, you could have heart-1 arteries present at the right side of all bones and heart-2 arteries at the left side. This will make sure that there is no vasolidation at all, since the pressure ways would not be travelling through the bones.

2- Make the hearts at perfect out-sync

Two hearts having systole and diastole in sync would be no better than one heart of double size, since this only increases blood pressure and nothing else. Instead, keeping the hearts in perfect out-sync (one heart has peak systole when the other has peak diastole) would ensure a constant inflow of blood to all organs and limbs at all times. Additionally, this will also be better for the twin-artery system, since they would not be beating in rhythm, and the problems of vasolidation would be greatly reduced.

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    $\begingroup$ Turn it from a 2 stroke engine to a 4 stroke engine :) $\endgroup$ – ivanivan Oct 1 '17 at 23:41
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    $\begingroup$ Two hearts in perfect out-sync are essentially one heart with 4 chambers. Which has some precedent. $\endgroup$ – Willk Oct 2 '17 at 0:28
  • $\begingroup$ @Will, a 4-chamber heart is in-sync. The inner wall is not strong enough to allow the chambers to contract separately. $\endgroup$ – Jan Hudec Oct 2 '17 at 6:24
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As I am not familiar with "Kepler Bb humanoids", I apologize if my answer makes no sense.

In general, having multiple hearts does not imply that they need to be identical in purpose (see redundant). One heart could be the "basal" heart providing general baseline blood circulation. For the most part, its arteries contain the arteries of the "work" heart, which is smaller but can provide significantly higher pressure. Think of the work system to be a series of smaller vessels entirely* contained in the basal system vessels. (*) will clarify this point below.

Under resting condition, the work system passively follow the basal system. Under stress condition, the work system can increase the blood pressure. Perhaps it can also increase the beating rate to twice that of the basal system.

The two systems run alongside the bones for the deep circulation. Where the basal system bifurcates to reach capillary coverage, then the work system separates and its vessels leave the basal vessels. At the capillary level the two systems have different destinations and coverage. For instance, the digestive trait may receive 90% of blood from the basal vessels, while the skeletal muscles may be covered by the work system for 50% of their volume.

The pressure problem that you mentioned should not be an issue because: 1) the pressure differential should be equal across the section of the arteries, 2) the basal arteries are larger in section and can be designed to be large enough not to be significantly affected by the pressure of the inner work artery, 3) at the bifurcations, we are already at the limit of capillary circulation where local pressure is nearly equal for both systems.

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