All the same at Heart

Two weeks ago now, I was talking to someone about the sort of work I do (when I’m playing at being a scientist) and she mentioned that she knew someone else who studied rat hearts. She said she thought this was very odd, as surely there can’t be much in common between the heart of a rat and a human heart, so what could there be to learn? I was quite pleased with the response I was able to give so I wanted to put it up here too. 

Not so different really. Sarah Palin helps to demonstrate that we're not actually so different from any other animals. (Images from Therealbs2002 and feastoffun.com)

It’s quite a common misconception that the biology of a human is very different from the biology of any other animals. What can we ever hope to learn from them that would be any good to us? The classic example of this comes from Sarah Palin – that fount of well-informed scientific knowledge – who famously commented that ‘[Tax] dollars go to projects that have little or nothing to do with the public good — things like fruit fly research in Paris, France. I kid you not.’ I was also reminded of this by the recent story about scientists making a ‘jellyfish’ out of rat heart cells. This sounds like a bit of fun but may actually come to revolutionise heart transplant procedures by allowing us to grow operational heart muscle from just a few cells. More on that later.

There is very little about the way that humans work that cannot be compared to some extent with the ways that other animals work, or even other living things. Even our brains – which we like to think of as uniquely sophisticated – are not so different from those of mice, rats, frogs, fish or dinosaurs (as far as we can tell). We can even learn things about the way our minds work by studying the humble fruit fly.

Nerve cells in the nerve cord of a fruit fly. A very small and simple equivalent of our spinal cord. (Image by James Chell & Andrea Brand here)

According to evolutionary theory all living things are related to each other, closely related things are more similar and so it is easier to study them and learn things which can be used in people but many simple aspects of life are pretty constant in all organisms. When scientists want to study a complicated system in humans they often start with a similar but simpler equivalent because it’s much easier for them to understand. They can then use what they learn to understand more complicated creatures.

Rats, relatively speaking, are very closely related to us, we have a lot of things in common and the biology of a rat is almost as complicated as our own. Humans and rats, as well as dogs, monkeys, elephants, sheep, platypi, whales, lions and many others are mammals. We all have fur or hair, are warm blooded and females feed their young on milk. These are all characteristics that define mammals but there are many more things that all mammals have in common when you compare them to other living things. Mammals are all vertebrates, animals with backbones, along with amphibians (frogs), reptiles (lizards, crocodiles), birds and fish. Vertebrates are all animals along with invertebrates like worms, insects, arachnids, snails and sea sponges. It goes on from there with connections between us and all other living things.

This tree diagram shows how we think animals (that's us, top right) are related to other living things.

At first glance, the body of an insect is very different to that of a human. For a start, the skeleton is on the outside, there are too many legs and most of the key organs have a very different organisation. Insects don’t even have blood (they have a fluid called haemolymph which fills the body and surrounds all of the organs), or a circulatory system, but despite this they do have a heart, of sorts. The insect heart is a simple tube of muscles that lies along the back of the animal. It draws in heamolymph at one end and pushes it out at the front through a very simple equivalent of the aorta (the main blood vessel in humans). As in humans, the action of the heart ensures that nutrients are constantly delivered throughout the body. Haemolymph is like blood, but not as efficient or complex as our blood, it also doesn’t really carry oxygen and doesn’t have any cells in it. In insects oxygen is delivered to most parts of the body directly be a complex system of airways, there are no lungs. (The video below shows the heartbeat of a larval fruitfly, you can see the haemolymph moving through the heart from left to right. The two large tube structures on either side of the heart are the main airways that supply oxygen to the body).

Fish are a fun vertebrate when it comes to the heart, they’re more like us and the fish heart has been well studied as another simplified equivalent of our own. Our heart has four chambers, the two on the right are smaller and weaker, they pump blood to the lungs where it can pick up oxygen to deliver to the rest of the body. Blood then returns to the stronger left-side of the heart to be pumped to other parts of the body

 In fish the blood goes to the heart once, and the heart only has two chambers, not four. Blood from the heart passes the gills (the fish equivalent of lungs) where it picks up oxygen but then it continues straight away to the rest of the body. The first chamber of the fish heart is called the atrium, it gathers blood under low pressure and delivers it into the second chamber, the ventricle. The ventricles of a heart are usually much larger and stronger than the atria, they must generate the force needed to move blood all around the body. The human heart is made up of two atria and two ventricles, one on each side.

The circulation of a fish as a simplified diagram. The heart is pictured in the centre with the two chambers labelled a (atrium) and v (ventricle) the third section is called the conum (m), it is very elastic but is not part of the heart because it does not pump blood. Blood leaves the heart on the right and passes through the blood vessels of the gills (far right). It then continues directly to the rest of the body (shown as the network of vessels on the left, before returning to the heart. Vessels carrying blood low in oxygen are in white with black outline and those carrying oxygen rich blood are in black (Image here).

The hearts of amphibians and reptiles are more complex still, having three or four chambers depending on which species you look at, they represent some interesting evolutionary stages between the simple single circulation of fish and the double circulation of mammals and birds. Three chambered hearts generally have two atria (one collecting blood from the body and the other collecting from the lungs) which deliver into one ventricle. This is less efficient than our heart as some blood that has already been to the lungs may end up going back again and some blood that needs to go to the lungs to collect oxygen will not get there. In some species, the ventricle is subdivided which helps to prevent this. Over millions of years this subdivision may come to fully divide the ventricle into two. This is probably what happened at some point in our past.

This model repesents the heart of a blue whale (a mammal, the same as us) and that is an actual person in the top right, just to give you an idea of scale. The heart of a whale weighs about 1300lb (590kg), about the weight of nine average adult humans! That's about 2000 times the size of your own heart!

Once you start looking at mammals, hearts are pretty much all the same, a complete four chambers with one side delivering to the lungs and the other to the body. The only thing that really changes in the size.

So you can see, depending on your interests, you can study similar things in very different organisms. It’s always important to test the things you learn in simple animals in animals that are more similar to us and even then you must be careful when applying the same ideas to people. There might always be some small, unnoticed, subtle difference that means something unexpected can happen, but in general, studying animals can be an excellent way to learn about ourselves.

And the rat-jellyfish? Well, if you cut a piece of muscle from a normal rat heart and gave it all the food and oxygen it needs to stay alive then it would move and contract just like a swimming jellyfish. The experiment isn’t a mad attempt at creating franken-jellies but a way of growing a new piece of heart that, if it works with human cells – which it should, but might not, we’re not exactly the same as rats after all – could be used to repair the damage done by heart attacks.