Bodily Defenses Image Map
Where does the digestive process begin?
Let's follow the journey that pizza experiences once it is eaten!  First, the mouth begins to chew the pizza. Enzymes in the saliva begin to break down the carbohydrates in the crust.

Enzymes break down carbohydrates, proteins, and fats by using water to break the bond between a subunit of these three types of compounds (see the "Hydrolysis" on the right).  For example, a starch molecule is broken down into glucose by the enzyme, amylase.

From the mouth, the saliva and the tongue work together to prepare the pizza to enter the pharynx. The pharynx directs air to the lungs and food to the stomach. Inside the pharynx is the epiglottis, which covers the opening to the trachea and prevents the food from entering the lungs. 

What happens in the stomach?
Once through the pharynx, the food is moved by the actions of smooth muscle in the esophagus to the stomach. The stomach secretes highly acidic gastric juice (pH = 2.5; see discussion of ph in Water module). When you have an upset stomach, it is often because your stomach is not passing the food on into the small intestine, which causes an accumulation of acid in the stomach. The hydrochloric acid (HCl) and enzymes break down the protein components of cheese and pepperoni.  Because the proteins are broken down, the peptide bonds between the amino acids are more accessible to the enzymes (see the unit on proteins).   The gastric juice also continues the digestion of the crust, started by the saliva.  Muscles in the stomach are working to churn the once solid pizza into a thick liquid. See Story Time about how Dr. Beaumont actually observed a lot of this in a living human being!

One cell type makes so much HCl that the pH of the stomach goes as low as 2, which means it is VERY acidic. The rest of the body has a pH of 7.2.  In other words, the fluid in the stomach is 160,000 times more acidic than the fluid in the rest of your body!  Why doesn't the stomach digest itself? The stomach secretes a mucus barrier to protect the stomach lining from the highly acidic contents and from the protein-destroying enzyme. But if this barrier is broken, you get ulcers. See the Hazards section on ulcers.


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