Whether we eat an apple drink some milk, or swallow a pill, our bodies respond in the same general Way. The digestive system breaks down what we have ingested, converts much of it to chemicals the body can use, and excretes the rest. The chemicals the body uses are absorbed into the bloodstream, which transports them to all of our body cells. Chemical nutrients in the foods we eat provide energy to fuel our activity, body growth, and repair.
Food’s Journey through Digestive Organs
Think of the digestive system as a long hose about 20 feet long with stations along the way. The journey of food through this hose begins in the mouth and ends at the rectum. These digestive organs and the major organs in between are shown in the diagram.
How does this system break down food? One way is mechanical: for example, we grind food up when we chew it. Another way is chemical: by the action of enzymes, substances that act as catalysts in speeding up chemical reactions in cells. How do enzymes work? You can see the effect of an enzyme by doing the following experiment. Place a bit of liver in some hydrogen peroxide and watch what happens: An enzyme in liver called catalase causes the peroxide to decompose, frothing as oxygen is given off as a gas. This is the same reaction you see when you use peroxide to disinfect a wound.
In most cases, the names for enzymes end in the letters -ase, and the remainder of each name reflects the substance on which it acts. The following list gives some examples:
• Carbohydrase acts on carbohydrates.
• Lactase acts on lactose (milk).
• Phosphatase acts on phosphate compounds.
• Sucrase acts on sucrose (sugar).
As food is broken down into smaller and smaller units in the digestive tract, water molecules become attached to these units. When food is in the mouth, there is more digestive action going on than just chewing. Saliva moistens food and contains an enzyme that starts the process of breaking down starches. The salivary glands release saliva in response to commands from the brainstem, which responds primarily to sensory information from taste buds. Simply seeing, smelling, or even thinking about food can produce neural impulses that cause the mouth to water.
The journey of food advances to the esophagus, a tube that is normally flattened when food is not passing through it. The esophagus pushes the food down to the stomach by wavelike muscle contractions called peristalsis. By the time food enters the esophagus, the stomach has already begun digestive activities by releasing small amounts of gastric juice even before food reaches it. Tasting, smelling, seeing, or thinking about food can initiate this process. Once food reaches the stomach, this organ amasses large amounts of gastric juices, including hydrochloric acid and pepsin, an enzyme that breaks down proteins. The stomach also produces a sticky mucus substance to protect its lining from the highly acidic gastric juices.
The muscular stomach walls produce a churning motion that we are generally not aware of which mixes the food particles with the gastric juices. This mixing continues for 3 or 4 hours, producing a semi liquid mixture. Peristalsis in the stomach then moves this mixture on, a little at a time, to the initial section of the small intestine called the duodenum.
Important digestive processes occur in the small intestine. First, the highly acidic food mixture becomes chemically alkaline as a result of substances added from the pancreas, gallbladder, and wall of the small intestine. This is important because the linings of the small intestine and remainder of the digestive tract are not protected from high acidity, as the stomach is. Second, enzymes secreted by the pancreas into the duodenum break down carbohydrates, proteins, and fats further. Third, absorption increases. Because the stomach lining can absorb only a few substances, such as alcohol and aspirin, most materials we ingest are absorbed into the bloodstream through the lining of the small intestine. If alcohol is consumed along with fatty foods, very little alcohol is absorbed until it reaches the small intestine. By the time food is ready to be absorbed through the intestine wall, nutrients have been broken down into molecules carbohydrates are broken down into simple sugars, fats into glycerol and fatty acids, and proteins into amino acids.