Royal Institution Christmas Lectures 2007
Starvation and Thirst
This lecture talked about how the body copes when it is starved of water or starved of food. Imagine you are shipwrecked in the Caribbean. After 3 days in the life-raft without water, you are literally dying of thirst. You could hope it rains and collect some rainwater but, that's not likely. You're going to have to think of something you might drink and there are two things you might consider if you were sufficiently desperate. While we consider this we will look at why water is so important.
The Water of Life
40 litres of Water
The adult body is about 60% water. The blood contains 3½ litres of water, the juice around your cells contains another 10½ litres and that water is very important because it takes nutrition and oxygen to the cells. More important than that, most of the chemical reactions that take place in the body have to happen in water, that means your cells contain another 28 litres of water. A 70kg adult contains 40 litres of water.
Its not just the water that's important, because in that water is salt. Blood contains roughly 9g per litre of salt. If we increase the concentration it starts sucking water out of the blood cells. If we don't drink enough water then the salt concentration rises as we dehydrate. The organs that are responsible for managing the amount of water and salt in the body are the kidneys. The kidneys filter the blood and pass toxins, and the salt and water you don't need, into the urine.
The kidneys are not just simple filters because having filtered out some water and salt, they then take back what is needed to keep you alive. The average adult has to urinate around 600ml a day to regulate salt levels. We're losing water in other ways too. When you breath, on a cold day, you can sea your breathe. That's moisture being lost. Between this, perspiration and the water content of your stools, you lose between 1½ to 2 litres of water every day. On a hot day, you could lose this every 2 hours.
So, what could we drink on our life raft? Yes, we could drink urine as suggested by the audience, but we'll come back to that. Sea water is full of salt. We know that blood contains 9g per litre, Atlantic sea water contains 38g per litre. That's why its undrinkable. Drinking urine is not such a bad idea, but as you get dehydrated your urine will become increasingly concentrated.
We all need water, but we also need energy from food. How much energy does food contain? To demonstrate, Dr Montgomery asks his audience what they think could be blown up if all the energy in one portion of Christmas dinner was released at once.
- A Dustbin
- A Washing Machine
- A Car
it actually contains enough energy to blow up a car.
In November 1992, Dr Mike Stroud and Sir Ranulph Fiennes set out to try and cross Antarctica. They faced temperatures of -45°C and they walker for 1,350 miles. This trip was unsupported so they had to take eveything they would need to survive for 3½ months. They each had to take a sled weighing 220kg. They were exhausted and starving.
Where does energy come from? Well, ultimately, it comes from the Sun. This is converted by plants into energy stored as sugars. Food is crammed full of energy. Butter, weight for weight, has five times the energy of TNT. For that reason, Mike Stroud, when he was in Antarctica, carried most of his calories in the form of butter.
Dry food, like crackers, cannot be chewed without some form of lubricant. The lubricant we use is saliva, and we produce over a pint of it each day. Saliva is mostly water, but contains mucus to help lubricate the food and digestive chemicals to break down starch and fat. It also contains antibiotics as our mouths are cess-pits One millilitre of saliva contains over a billion bacteria.
Food passes from the mouth to the stomach via the oesophagus of gullet. The stomach, apparently, only holds about 35ml, so where does our Christmas dinner go? The stomach stretches. Folds called rugae open out allowing it to expand to contain about a litre. The stomach is not just a passive sac, it helps in the digestive process. It contains hydrochloric acid that kills any bugs you've swallowed and starts digesting the proteins. Food passes from the stomach into the small intestine where bile from the liver continues the digestive process.
Where is the energy used?
- Kidneys, 7W
- Heart, 10W
- Muscle, 18W
- Brain, 20W
- Liver, 30W
The liver is doing a huge amount of metabolic work Its receiving, storing and converting and its breaking down hormones and toxins.
Dr Mike Stroud returns to discuss his energy requirements in Antarctica. He estimates that his body was using as much as 1,000W. He was eating about 5,500 calories a day, but burning 8-12 thousand calories, so he was starving and wasting away.
If your calorie intake is insufficient, the body resorts to other sources. Firstly sugar stored in the muscles is used, when this is depleted body fat is burned. Finally, the body will break down muscle in order to survive.