The Tin Man Lecture

In The Wizard of Oz, the Tin Man is seeking a heart. Lacking a heart is taken to mean that you cannot love. This is poor science of course, our emotions are felt by our brain, and our heart is a pump. Without a heart, our blood would not be pumped around our body, our tissues would be starved of oxygen and food, and we would rapidly die.

In Friday’s lecture on biological physics I covered why we and all animals bigger than roughly a few millimetres need hearts, why we need pumps to pump fluid round our bodies. The reason is some simple physics: We need pumps as without the flow pumps produce, molecules such as oxygen would only diffuse around our bodies. I am about 1.85 m tall. Diffusion is agonisingly slow over distances like that. Oxygen takes decades to diffuse over a metre. This is far far too slow to support life. Hence the need for a pump.

Fortunately for my mother, I was a lot shorter than 1.85 m when I was born. But I was still far too large to do without a internal pump, even as a small cute baby. Our heart is the first of our organs to kick into action, about two months after conception, i.e., seven months before birth. It has to, as even then we are already too big for oxygen to just diffuse to where it needs to go.

This has the remarkable consequence that our heart starts out as a simple periodically contracting tube that develops, without missing a beat, into the four-chamber heart that you probably remember from biology at school, and that is currently at work within your chest.

I find this a bit mind blowing. How does a pump change so radically while continuing to operate? We can’t see this remarkable process in humans* but it is possible to observe this is in other organisms. Liebling et al. show this for zebrafish embryos. Their figure 3 shows the progressive development of the heart as it grows more sophisticated as it develops, and if you look at that paper’s Supporting Information you can see movies of the beating hearts.

* Liebling et al. use optical microscopy to image the beating hearts of zebrafish embryos. Zebrafish eggs are mostly transparent so that works. Human embryos develop inside their mothers of course, and mothers are not transparent, hence the need to study other animals.