22.9: Ingestion is the first step in the breakdown of food.

It will take you longer to read about ingestion, the intake of food into your body, than it would to just do it (FIGURE 22-17). Typically lasting less than a minute, ingestion primarily involves four parts of your anatomy: your mouth, teeth, tongue, and esophagus. We’ll look at each in some detail. To start, imagine that you take a big bite of a hamburger, containing bread, lettuce, and the meat.

Putting food in your mouth stimulates your salivary glands. (In fact, just thinking about food can stimulate saliva secretion!) Via tiny ducts throughout your mouth, the salivary glands secrete mucus that lubricates the food to help it pass into your stomach. They also secrete an enzyme called alpha-amylase that initiates the process of digestion. Alpha-amylase breaks the bonds holding together the starch molecule (a highly branched carbohydrate pieced together from hundreds of linked glucose molecules) and releases a bit of glucose that can be used for energy. About 20% of the ingested starch is broken down in the mouth. Protein and fat molecules, on the other hand, are not broken down at all.

Interestingly, across dozens and dozens of cultures, humans have developed common ways of preparing their food, including using heat and marinating food in acidic solutions, such as vinegar or lemon juice. Because harsh conditions such as heat and acid help to disrupt the tissue of food items, they increase the efficiency with which digestive enzymes can make contact with the food and break it down.

You start chewing food in your mouth in order to tear and grind it into little bits. This is a first important step toward completely breaking down and harvesting the energy stored in the chemical bonds of the food. Several different types of teeth have evolved in mammals and other animal species that allow different types of food items to be processed. Incisor and canine teeth, in the front, are used for biting and tearing food. Behind them are premolars and molars, used for grinding and crushing food. Just by looking at the type of teeth an animal has, we can learn a lot about its diet. Herbivores have molars primarily for grinding and crushing the tough cell walls of the plants they eat. Carnivores have sharp and lethal canines and incisors for killing and tearing apart the flesh of other animals.

Birds have no teeth. This explains why they can sometimes be seen eating gravel. Although the gravel has no nutritional value, it collects in the gizzard, one of the two chambers of their stomach, where it helps to grind up food. This is an important digestive step for birds because, lacking teeth, they must swallow their food whole. Consequently, when it first arrives in the stomach, the food hasn’t been ground up at all, which can reduce the efficiency of digestive enzymes.

While you are chewing your food, your tongue assists in the process by forming the food into a ball shape that can be swallowed. The ball of partly crushed, saliva-coated food is pushed to the back of your mouth by your tongue. There, your throat opens to two passageways into your torso, the trachea and the esophagus (FIGURE 22-18). The trachea, also called the windpipe, connects to your lungs, while the esophagus connects to your stomach. Food destined for your stomach is kept from entering your trachea by a fast but critical maneuver in which your voice box moves up (due to muscle contractions), causing a flap of tissue (called the epiglottis) to be pushed over the entrance to the trachea just as you begin to swallow. If the ball of food you swallow is too big, it can get stuck at the beginning of the esophagus, wedging against the flap of tissue in front of the trachea—and this can cause you to choke by blocking all air from getting into your lungs.

Once the ball of chewed food makes its way into the esophagus, waves of smooth muscle contractions, called peristalsis, propel the food down the esophagus and into the stomach, where digestion continues. Because of peristalsis, it’s not necessary for you to be sitting upright or standing when you eat. Even if you are standing on your head, the food is pushed down the esophagus to the stomach.