Giulia Enders

The first surprise is the sophistication of our sphincters. The vast majority of people are familiar only with the outer sphincter: the muscle we can consciously control, opening and closing it at will. There is another, very similar muscle close by—but this is one we can’t control consciously.

These two sphincter muscles have to work as a team. When what’s left of our food reaches the internal sphincter, that muscle’s reflex response is to open. But it does not just open the floodgates and let everything out, leaving the outer sphincter to deal with the deluge. First, it allows a small “taster” through. The space between the internal and external sphincter muscles is home to a large number of sensor cells. They analyze the product delivered to them, test it to find out whether it is solid or gaseous, and send the resulting information up to the brain. This is the moment when the brain realizes, “It’s time to go to the toilet!” Or maybe, “It’s just a bit of wind.” It then does what it is so good at with its conscious awareness: it adapts to the environment we find ourselves in

THE SUBLINGUAL PAPILLAE, those two constant suppliers of saliva, are situated right behind our lower front teeth, which are particularly susceptible to the buildup of tartar. This is because there are substances in our saliva that contain calcium whose sole function it is to make our teeth harder. But if a tooth is constantly bombarded with calcium, it can be a case of too much of a good thing. Tiny molecules floating innocently by are caught up and “fossilized” without so much as a by-your-leave. The problem is not the tartar itself, but the fact that it has such a rough surface, affording a much better foothold for bacteria that cause tooth decay and gum disease than smooth, clean tooth enamel.

But what are fossilizing, calcium-containing substances doing in our saliva? Saliva is basically filtered blood. The salivary glands sieve the blood, keeping back the red blood cells, which are needed in our arteries, not in our mouth. But calcium, hormones, and some products of our immune system enter the saliva from the blood. That explains why each person’s saliva is slightly different. In fact, saliva analysis can be used to test for diseases of the immune system or for certain hormones

When we are asleep, we produce very little saliva. That’s good news for those who tend to drool into their pillow. If they produced the full daytime quota of 2 to 3 US pints (1 to 1.5 liters) during the night, too, the results would not be particularly pleasant. The fact that we produce so little saliva at night explains why many people have bad breath or a sore throat in the morning

One condition often overlooked by general practitioners and family doctors is Roemheld syndrome, when so much gas collects in the stomach that it presses up against the heart and the nerves in the gut. Sufferers can display a range of different symptoms, including dizziness and discomfort. In more severe cases, Roemheld syndrome can cause anxiety or difficulty in breathing, and may also lead to severe chest pain that feels like a heart attack. Doctors often write off undiagnosed Roemheld sufferers as overanxious malingerers whose symptoms are all in their mind

Now let us turn to the stomach’s strange shape. One side is much longer than the other and so the entire organ has to bend double. That creates large folds inside it. The stomach could be called the Quasimodo of the digestive organs. But its misshapen appearance has a deeper meaning. When we take a drink of water, the liquid can flow straight down the shorter, right-hand side of the stomach to end up at the entrance to the small intestine. Food, on the other hand, plops against the larger side of the stomach. Our digestive pouch cunningly separates the substances it still needs to work on to break them down, from the fluids that it can wave straight on through to the next digestive station. So our stomach is not simply lopsided; rather, it has two sides with different specializations. One side copes better with fluids, the other with solids. Two stomachs for the price of one, so to speak.

As with the small intestine, all the treasures absorbed by the large intestine are transported first to the liver for checking, before entering the main blood system.

Even though we know intellectually that too much snacking is bad for us, we can’t really blame our instincts for encouraging us to grab every opportunity for a treat. When we eat too much sugar, our body simply stores it away for leaner times. Quite practical, really. One way the body does this is by relinking the molecules to form long, complex chains of a substance called glycogen, which is then stored in the liver. Another strategy is to convert the excess sugar into fat and store it in fatty tissue. Sugar is the only substance our body can turn into fat with little effort.

Our small intestine also knows the special value of fat. Unlike other nutrients, it cannot be absorbed straight into the blood from the gut. Fat is not soluble in water—it would immediately clog the tiny blood capillaries in the villi of the gut and float on top of the blood in larger vessels, like the oil on spaghetti water. So fat must be absorbed via a different route: the lymphatic system

However, maintaining a healthy meat-free diet that does not lead to nutritional deficiencies is more difficult than most people think. Plants construct different proteins than animals, and they often use so little of a given amino acid that the proteins they produce are known as incomplete. When our body tries to use these to make the amino acids it needs, it can continue to build the chain only until one of the amino acids runs out