Biology

Circulatory System and Hypothermia.

In January 1997, while the leaders who ruled the world tried to face the climatic change in Kioto, ordinary people spend their time in the cinemas crying when, in the last scenes of the most expensive film in history, Jack Dawson, one of the main characters, died frozen in the cold waters of the Atlantic Ocean while the most famous ship in history sinked.

Titanic.

The film I am talking about is, obviously, Titanic. In this movie Jack Dawson dies after some undefined time swimming in the extremely cold water, in a heroic way, helping his lover to survive on a table.

But, why did he die? Why are we able to survive in cold water only for a little time? Why is the temperature so relevant to our body? And, above all, what can we do to survive in extremely cold conditions?

First, Jack Dawson died, mainly, because he was a homeotherm organism. Mammals and birds are homeothermic. This means that their corporal temperature (our body temperature, because, in fact, I am a mammal too) is constant and independent of the environmental temperature. This is an advantage in many situations, because our body activity doesn't depends on the external temperature. Due to this, we can live in many different ecosystems, from the white and snowy North Pole to the scorching and sandy Sahara Desert.

Other animals, such us Amphibians, are Poikilotherms. Their body temperature is similar to their surrounding temperature. If the temperature of the environment rises, their corporal temperature rises too and their vital activity increases. On the other hand, when the environmental temperature decreases, their corporal temperature and vital activity decrease too. These animals tolerate big changes in the temperature of their bodies. But they can't live in habitats where the average temperature is very different to their optimal corporal temperature. When the habitat has seasonal changes of temperature, many of these animals hibernate during the coolest periods.

So we can live in many different ecosystems, but we don't tolerate changes in our core temperature. Human beings, for instance, have an average corporal temperature of 36,5°C. No human being would survive if his temperature rose over 42°C or descended under 34°C.

So it's clear that our little hero died because the environmental temperature was very low, this provoked a drop of his corporal temperature, and when the corporal temperature reached tragic low levels he died.

We can ask, however, another question. He was swimming, so he was in liquid water. Liquid water must be over 0°C at atmospheric pressure. And all of us have been playing in snow in winter, while the thermometer was under 0°C. Why did he die?

The answer is easy: water is a better transmitter for heat than air. This is related to its specific heat, the capacity of matter to increase or decrease its temperature. Water transmits heat and cold much better than air. That's the reason why we can't survive for more than few minutes in cold water, whereas we can survive for hours out of the water. As a result, the other main character of the movie, miss Rose, was able to survive as she was not actually in the water.

The transmission of heat via water is also related to the thermic sensation: when the atmosphere has high amounts of water vapour it transmits temperature better than when the humidity is low.

Let's analyse the final question: what can we do, as homeothermic organisms, to survive under extremely cold conditions? The question is not easy and is related to our circulatory system and the way it works.

The main function of the circulatory system is to transport substances from some parts of the body to others: oxygen from the lungs to the cells, carbon dioxide from the cells to the lungs, nutrients from the digestive tract to the cells, hormones from the endocrine glands to the target organs, and so on.

But the blood is, also, the most important system to move heat in our body. After all, it is a liquid, made of water and we have just talked about the properties of water as heat transmitter. The surface of our body tends to lose heat. The most relevant structure in our body to increase our temperature is the muscular system. Muscular contractions release high amounts of heat.

Gebreselassie running in Dubai, by Anto 1210

This is the reason why when we are in movement, our body temperature rises. If we are running, for instance, the heat released by the muscles of our legs increase the temperature of our body. But we have said that our temperature can't be higher than 36,5°C. To avoid problems related to high temperature, our body carries out some actions to improve the release of heat. First, a big caudal of blood is transported to our surface. The blood is a warm liquid, so it tends to lose its heat, that is released to the exterior. Second, our skin is rich in sweat glands that release liquid to the surface. Sweat absorbs heat from our body to evaporate, reducing the temperature of our skin. Summing up, we blush and sweat.

On the other hand, when the environment is very cold, our body carries out actions to preserve heat. The surface caudal of blood is reduced to avoid losing heat. At the same time, the sweat production falls. If the cold is very intense and the core temperature decreases, another factor starts to work: muscular contractions. To increase our temperature, our muscles starts to carry out short involuntary contractions. We start to shiver.

According to all these facts, it seems logical that if we find ourselves in a cold environment, the most effective system to survive is keeping in movement, because muscular contractions will provide the necessary heat to our body. But this conclusion is absolutely wrong.

Because if the environment is very cold and we keep in constant movement our muscles produce high amounts of heat, but high amounts of heat are also released at the same time. And, soon or later, our muscles will fatigue. At that moment we will dead in a few minutes, because our muscles are loosing heat.

Mont Balnc, by Nattfodd

As we saw in Titanic, the main character survived for some time. But when he finally was very tired, he died frozen very quickly. In fact, the film is not very plausible, in real life a human can only survive for twenty minutes in such cold water.

How can be done, then? Simply, keep calm and stay quiet, with no movement (and out of the water, if possible). We said that our surface loses heat, so we ought to minimise our surface. The most efficient geometrical body, with lower surface volume relation, is an sphere. Due to this, we must try to be as spherical as we can. For instance, in the fetal position.

So here we are, situated on the floor, dry an as spherical as possible. To preserve the temperature of our internal (and vital) organs, our body will close the peripheral blood vessels: our arms and legs reduce their blood caudal in order to prevent the release of heat. At the same time our muscles start to make little contractions to produce heat.

This mechanism is not free from problems. After some minutes with low flow of blood, the oxygen in our hands and feet lowers. As a result, the cells will start to die. Some time later, the damage of our limbs could be irreversible.

Can you imagine Jack Dawson surviving after the accident, but sat in a wheelchair after the dramatic amputation of their legs? After all, it's obvious that Hollywood heroes must preserve their beauty more than their core temperature.

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