Just the thought or smell of food is enough to get digestive juices flowing in the mouth and stomach. As soon as food enters the body, it’s mashed to a pulp and attacked by chemicals that begin to break it down.
Most of the nutrients in food are long, chainlike molecules that are too big to dissolve in water and be absorbed by the blood. The process of digestion turns these giant molecules into tiny units the body can absorb. The first stage of the process is physical: we tear, crush, and mash food as we chew it with our teeth. This physical action helps digestive juices penetrate the food to break it down chemically.
Unlike cats and dogs, which can wolf down large chunks of food, human beings have to chew food before swallowing it. Inside the mouth, food is mashed by the teeth and mixed with a watery fluid called saliva (spit). Saliva moistens food to make it slippery and easier to swallow. It also contains chemicals called digestive enzymes, which break large food molecules into fragments.
The enzyme amylase breaks down starch molecules from foods like bread and rice, turning them into sugar. The enzyme lipase breaks down fat molecules.
The flavor of food comes largely from our sense of smell. Aromas enter the nose from the back of the mouth.
After being chewed and mixed with saliva, food is molded by the tongue into a soft, squishy mass to make it easy to swallow.
This agile and powerful, muscular organ maneuvers food with amazing precision, placing food particles between the teeth to be crushed. It also mixes food with saliva, molds it into a lump, and pushes it into the throat. The tongue also tastes everything it touches.
Food is chopped, torn, crushed, and ground into smaller pieces by the teeth.
Six large salivary glands and around 1,000 tiny ones produce a total of about 2 pints (1 liter) of saliva every day.
Food doesn’t simply fall to the stomach when you swallow it. Instead, it’s pushed through a tube called the esophagus by muscle action. The wall of the esophagus contracts behind the food to squeeze it along. A wave of contraction shoots all the way down the esophagus, pushing food to the stomach in 7-8 seconds.
Our teeth are the first line of attack in the digestive process, chomping and grinding food into smaller pieces. We have two sets of teeth during our lifetime: 20 milk teeth that last 6-10 years, followed by 32 permanent teeth.
There are several different types of teeth. The front teeth (incisors) have thin edges that make them good for snipping and biting into things. The rear teeth (molars and premolars) are broader, with bumpy tops suited to grinding and crushing food. Canines are pointed teeth used for piercing and gripping. Human canines are small, but other mammals have long, sharp canines called fangs.
Inside a tooth
Teeth are built to withstand a lifetime of wear and are coated in the hardest substance found in the human body: enamel. Beneath this is a bonelike tissue called dentine, which in turn surrounds a soft center that is highly sensitive to pain.
Like a food processor, the stomach churns and mixes food until it turns into a thick liquid. Glands in the stomach wall secrete acid and enzymes that work together to break down protein molecules in food. The stomach’s wall can’t absorb nutrients from food, but it can absorb water and medicines such as aspirin.