Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Chapter 8
Chapter 9
Chapter 10
Chapter 11
Chapter 12
Chapter 13
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As a preclinical medical student I remember being told by a tutor that the body was 90% water. I also remember not being too sure about how I was supposed to react to this information: by despair, that I was just a bag of dirty water? By amazement, that just 10 percent of the molecules in my body could turn me into a human being? It was only after I had seen patients in the hospital that I understood what he was trying to say. People with gut diseases unable to retain intestinal fluid who were being kept alive by having fluids dripped into their veins, others with kidney diseases who spent their days passing huge quantities of urine, casualties who had lost a large amount of blood being revived by a transfusion. I realised that keeping the body’s fluid balance in shape was a major responsibility, and that when it went wrong, everything else began to fail…….
You can easily test how efficient your body is about controlling its water and salt content by taking a walk in a desert. After a few hours, slogging through the sand and the heat, you begin to be very thirsty. As you plough on, your thirst increases, becoming ever more imperative, until it begins to occupy your every thought. You think you’ll go crazy unless you get a drink. And you notice that you don’t pass much urine, and what you do pass seems very concentrated. Your sweat runs into your eyes, making them smart, and into your mouth, tasting salty. Finally, you reach the oasis, and a cool drink. It’s one of the most wonderful experiences of your life. At the subsequent meal, you notice that your food seems to need a lot of salt to make it taste good. Put a
little
of this food – after you’ve added the right amount of salt
– into a container, and try it again a few days later, when
you’re back in the normal world. It tastes awful. Much too
salty……..As well as water, it’s lack of salt we need to worry about – or rather, what our body is set up to detect most readily, since we, like most other mammals, live in a world in which salt is quite hard to obtain. The scarcity and value of salt was recognised by primitive human tribes, who sometimes used it for bartering. Salt was even thought a suitable present for kings in earlier times. One of the triumphs of the human brain (the cortex) is to have worked out how to supply salt easily and cheaply, so that our problem nowadays is not too little salt, but too much. Low body salt induces animals and people to search for salt (as we saw after our walk in the desert). Unlike thirst, salt appetite develops much more slowly, over hours or even days, so the mechanism is different…
Cholera has been a scourge throughout most of human history. In 19th century England, epidemics of cholera were quite usual. In 1854 there was an outbreak in London. Within a few days, hundreds of people living in the Soho region of the city had died. Thousands fled their homes……. Cholera is caused by a bacterium transmitted by contaminated food or water. Without treatment, about 50% of those with severe cholera die. But it’s not the bacterium that kills them. It’s the profuse, painless, watery diarrhoea. Within a few days they are dead. Why? Because they have lost too much water and salt - not only sodium chloride, but potassium as well, and their heart fails…..
Sources of water have been prized since before recorded history began, and, like all valuable resources, they have been fought over. They still are. Disputes about ownership of rivers are still rife. But once again, the human cerebral cortex has helped the limbic system out. We are so used to turning on a tap at any time of the day or in any season that we forget what a remarkable – and recent – luxury this is, though one still lacking in many parts of the world. The invention of reliable, clean and cheap water supplies is one of the human brain’s greatest and most beneficial achievements. First came the concept: the idea that reliable water was important. Then followed the invention of pumps and pipes to deliver it and filters to keep it clean. Every other species, including our near relatives the great apes, has to depend on natural, unreliable and rather scarce sources of water.
Scientific and literary quotes.