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“I have 2 ears, 2 eyes, 2 lungs and 2 kidneys. Why don’t I have 2 hearts?”

Food for Thought


“I have 2 ears, 2 eyes, 2 lungs and 2 kidneys. Why don’t I have 2 hearts?”


There are some thoughts that come into mind when trying to answer this question. And as it’s such a broad question, you can really answer it in a multitude of different ways. Here are two ways you may answer a question like this:


1. Why do I have 2 of some organs, yet 1 of others?

This is an interesting point to make as it is a question that some of us may have had as a ‘shower thought’. And with most things in biology, the answer is usually simple: evolutionary benefit. As you’re aware, there are selection pressures from the environment on species to be able to adapt and survive amongst a population. So, the reason why I must have 2 of some organs and not others, is because it must confer a survival advantage. But what advantage does it give?


Let’s take the lung for example: the lungs are split into two as the bronchi bifurcate from the trachea. What are the benefits of this? A clear benefit, and a benefit for all organs that come in multiples, is that if one fails the other is able to compensate. You may know that removal of one lung is possible, but not all that common. But losing or damaging just one lung is probably not only the reason why we have 2, rather it’s probably a bonus.


It’s more likely the case that the lungs need to be of a certain size in order to meet the oxygen requirements of the body, but why don’t we just have a single lung that fills the entire thorax? Well, there’s a pesky organ in the middle that kind of stops that: the heart. If you decide to fill the whole thorax with the lung, the central thorax may not have enough space to house the heart, and so the heart may not be able to expand properly. That’s also not to mention the descending aorta and oesophagus which traverse between the lungs to get down to the abdomen. So having two lungs separated by a space is likely more efficient for the body than just having one giant lung.


2. What would happen if I did have 2 hearts?

You can actually argue that we do have 2 hearts. After all, in a healthy human, the right and left side of the heart are separate and should not be able to communicate with one another without first passing through the lungs or the body. This is why we call it a ‘double circulatory system’ since blood has to pass through the heart twice in one ‘circuit’.


However, it is ultimately ONE organ, since the two sides don’t beat independently they’re not really ‘two hearts’.


So why is that? Well, physiologically speaking you should know that the ‘pacemaker’ is typically in the Sino-Atrial node, which goes down electrical pathways to lead to the two sides of the heart beating in unison.


Why is this necessary? As the organs are placed in series, if one side of the heart beats out of sync with the other, you will imagine that there can be ‘traffic’ somewhere else in the body - this is exactly what happens in arrhythmias. If the heart cannot synchronize its beats, it cannot efficiently pump blood around the body.


And so, how does this translate if we were to have 2 hearts? Put it simply: it would be hard. It is difficult enough for the two sides of the heart to beat in sync in one organ, consider this if they were separate! You would need nerves, perhaps of equal length, to transfer a signal from a separate third source. Or have the source in one heart, but be able to delay the depolarisation in that heart whilst the signal travels to the other heart. Ultimately this is inefficient, and this is likely why the ‘two hearts’ are really just one organ.


You may also think about some organisms which have multiple hearts, such as octopuses. Their hearts also beat in-sync but we are not 100% sure how this is happening - this is definitely something to think about if you like zoology.


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