Re: What if humans could live without a heart like a jellyfish does?

Hello Patricia,

You have asked a series of very interesting questions. I'm not sure that there are hard and fast answers to most of them, but I can give you rather speculative answers (and some scientific evidence) for some of them.

Q: If we could be like a jellyfish without a heart, would we still be able to do the same things we do?
Well, first of all we have to decide what our heart does, then we can consider what we'd be able to do if we didn't have one. We also need to think about how the jellies, which certainly do not have a heart, get along without one. As you probably know, our heart is the pump that circulates blood through our body. The circulatory system, powered by the pumping action of the heart, transports nutrients and wastes through our entire body. In fact, having a complex circulatory system is one of the things that allows some animals to reach very large size. Animals (and single-celled organisms) that lack a circulatory system have to rely on simple diffusion to transport things from one part of the body (or cell) to another. Diffusion works, but is nowhere near as efficient as an active transport system. Putting all this together, I'd have to conclude that if we didn't have a heart, we would NOT be able to do many of the things that we do. For one thing, we wouldn't be as large as we are, and many of the things we do, we do because we are fairly large, as animals go. For example, without a heart we wouldn't be able to move very quickly at all, because our muscles and other tissues wouldn't receive enough oxygen to function well.

Jellies, as you mentioned, do not have a heart. They do, however, have a circulatory system of sorts. As is explained very well on this webpage, jellies have a body cavity that serves as both a disgestive system and a circulatory system. How does this work? Well, most of the animal's body consists of a thick layer of mostly non-living jelly, covered by a thin epithelium. There is only one opening to the digestive system, so undigested food wastes are expelled via the "mouth". The rest of the gut consists of a series of canals that extend throughout the bell and into the tentacles. This combination digestive/circulatory system is called a gastrovascular cavity, or GVC. As the bell of the jelly pulses when the animal swims, water (carrying dissolved oxygen) is sucked into the mouth and ciliary currents continually move water throughout the canals of the GVC. Carbon dioxide wastes are similarly transported and eventually dumped to the outside. The movie on the webpage I cited above does a nice job of demonstrating this.

Q: Would we look different? or still be the same?
Undoubtedly. If we didn't have a heart we'd be a lot smaller (see above), and quite possibly a lot squishier. What allows us to stand upright against the force of gravity is our skeletal system. Our bones are made up of living tissues, fed by our circulatory system. No heart, no bones. No bones, and we'd collapse in a heap of soft tissue. This ties in with the answer to your next question.

Q: Would we still be able to live on land or would we have to live in the water also?
I think that if it WERE possible for us to live without a heart and circulatory system, we'd have to live in the water, unless we were very tiny. We wouldn't be able to support any appreciable weight on land, but the buoyancy of water would help with that. Another benefit of living in water is that oxygen and carbon dioxide are both more soluble in water than in air. This means that it would be easier for our little bodies to get enough oxygen (and get rid of carbon dioxide wastes) via simple diffusion.

Q: Did anyone every ask you this before?
Nope! But it has been fun thinking about it!

Q: Is it impossible for this to happen?
Fortunately, yes. Our bodies are very complex entities, consisting of many organ systems that depend on each other to function properly. The circulatory system is just one of these organ systems. If we didn't have a heart, things would fall apart almost instantly, so it is extremely unlikely that evolution would take the route of "getting rid of" our heart.

I hope these answers help with your science project!

Allison J. Gong
MAD Scientist