探索世界之生命物语 The Human Body: Life Story 4

Everything that's alive and quite a few things that aren't, gives off heat. That's what you are looking at now. Every part of your body is burning up energy and creating heat as it goes about the business of being you. But the hottest bit of you is up here, your brain is burning up more energy than anything else in your body, almost a fifth of all the calories you eat and drink. And it uses up almost the same amount whether you are concentrating on something really difficult or just wondering whether to put the cat out, because day and night, your brain is challenged with the most difficult task it will ever face, keeping you alive.

Right now for instance, there are quite a few things on your mind, tonight's dinner for example. For starters, your brain is coordinating a major haulage operation.

Even though the route is usually downhill, food doesn't fall from your mouth to your stomach. When your brain tells you to swallow, it triggers waves of muscle contractions. They squeeze things along in your esophagus at around 4 centimeters per second.

The first scheduled stop is the stomach. It's essentially a biological liquidizer. Its lining is covered in delicate folds which allow it to expand with each mouthful. Inside a mixture of enzymes and hydrochloric acid start to digest your food. And very probably your dinner is still in there, it churns away for about 4 hours. Soon it will be released into your small intestine and at a cue from your brain, bile will be added. This will help you break down fats. You will be carrying tonight's dinner for about 24 hours, so I hope you liked it. When it does finally leave you, it will take around 25 grams of dead cells from your gut with it.

But don't worry, you are constantly replacing cells from head to toe.

Your brain is managing the body's workforce of approximately 50, 000 billion cells and most of them don't last more than a few years. Cells are constantly dying and being replaced by new ones. Up to a billion new cells for old every hour. What this really means is that most parts of your body are in fact a good deal younger than you are. But remarkably these cells aren't replaced and they are particularly special. Each one has a natural tendency to twitch.

words and expressions

haulage:The act or process of hauling.
拖拽的行为或过程

muscle contraction:肌肉收缩

esophagus: The muscular, membranous tube for the passage of food from the pharynx to the stomach; the gullet.食道

lining:A covering or coating for an inside surface:衬里,内层

enzyme:Any of numerous proteins or conjugated proteins produced by living organisms and functioning as biochemical catalysts.
酶：由活的有机体产生的各种蛋白质或蛋白质结合体的一种，起生物催化剂的作用

hydrochloric acid :盐酸

churn:搅拌, 搅动

small intestine:小肠

bile:胆汁