Digestion: the facts

This is the same part of the course as the heart and lungs I've written about before, and seems particularly suited to laying out in bullet points. It's also extremely interesting, finding out about bits of my body that I hadn't really thought about before. I have moved on to the subject of digestion, which has thrown up many nuggets of fascinating information for use in pub conversations.

• The technical term for swallowing is 'deglutition.' Swallowing is the most complex reflex in the body, employs about 25 separate muscles, and once initiated it cannot be stopped.
• The three pairs of salivary glands in the mouth produce saliva of different viscosities. One of the many functions of saliva is to dissolve food so we can taste it.
• You can start or stop chewing voluntarily but it's mostly automatic, a bit like breathing, which must be why it's almost impossible not to chew sweets or gum when they are in your mouth.
  
• The oesophagus doesn't do very much except peristalsis and secreting mucus, but presumably causes much dispute between American and English spelling pedants. Same goes for faeces, caecum and diarrhoea in the digestion department.
• The commonly asked question "Why doesn't the stomach digest itself?" is easily answered when you look at the detail. A protective layer of alkaline mucus is secreted to line the stomach wall, and the digestive enzyme pepsin only works in strong acid. Pepsin is secreted in an inactive form as pepsinogen, and only becomes a protein digester when it meets the strong acid environment of the stomach. The same sort of thing happens in the intestine, with trypsinogen converting to protein-digesting trypsin only in the presence of enzyme enterokinase.
  
• Stomach and intestinal secretions are stimulated by lots of things: sight, smell, taste of food, the action of chewing and swallowing, the distension of the stomach, the presence of protein and fat in the small intestine. The autonomic nervous system, a load of neurotransmitters and a bunch of hormones all get involved.
  
• The liver produces bile the whole time, but when the Sphincter of Oddi at the bottom of the bile duct is closed it can't flow out into the intestine, so it backs up and is stored in the gall bladder. If that has been removed, the bile just backs up into the liver. Not a problem, because bile doesn't contain any digestive enzymes.
  
• The cells lining the small intestine are sloughed off and replaced every three days or so. We even digest and reabsorb anything that's useful from these dead cells.
• There's loads of spare absorption capacity in the small intestine, so you can cope when up to half of it has been removed, except for one section at the end which is the only place where bile salts and vitamin B12 are absorbed.
  
• Most of the time, your large intestine is absorbing water without moving the contents along ('haustral contractions'), but after a meal it carries out a 'mass movement' and shoves the contents a long way forward. This is why some of us always feel the need for a Number Two after breakfast.
• The stomach is passive during vomiting - it is simply squeezed between the muscles of the diaphragm and the abdominal muscles.

I'm not getting on fast enough with revision, and find it pretty hard to focus at the moment. Just one more week of teaching left, and then a week when I hope I'll be scared enough to knuckle down properly.