Digestion

1.         Nutrients are chemicals the body needs to function properly.

            a.         We get the nutrients we need from the food we eat.

            b.         The food is not in a form that can be used.

            c.         The large molecules must be broken down into small molecules that cells can use.

            d.         This process is called digestion.

2.         Digestion starts even before you put food in your mouth.

            a.         When you are hungry or smell food, you start to produce saliva.

3.         Digestion involves both physical and chemical changes.

            a.         A physical change is a change in the properties of a substance.

            b.         A chemical change results in new substances.

            c.         All food contains vitamins, minerals, proteins, lipids, and/or carbohydrates.

                        i.         Vitamins and minerals can be absorbed directly into the body from the digestive system.

                        ii.        Proteins, lipids, and carbohydrates must be digested before being absorbed.

Stages of Digestion

There are two stages in digestion:

1.         Mechanical digestion is the physical breakdown of food.

            a.         It breaks food into smaller pieces but does not change the substances in food.

            b.         Food must be broken into smaller pieces so it can be swallowed.

            c.         Most mechanical digestion is done by chewing.

2.         Chemical digestion is the breakdown of large food molecules into smaller molecules.

            a.         Chemical digestion is performed mostly by special proteins called enzymes.

            b.         Enzymes perform and speed up chemical reactions in the body.

            c.         Each enzyme can break down only one type of food molecule.

                        i.         For example,

                                    (1)       Pasta is mostly starch. Starch is made of glucose, a simple sugar.

                                    (2)       Glucose is the main source of energy for cells but they cannot absorb starch.

                                    (3)       Chemical digestion must break the starch into glucose molecules.

            d.         In general,

                        i.         Carbohydrates are changed to simple sugars.

                        ii.        Proteins are changed to amino acids.

                        iii.       Fats are changed to fatty acids.

Organs of the Digestive System

1.         The mouth is the entrance to the digestive system.

            a.         Teeth are used to grind food into smaller pieces. This is the beginning of mechanical digestion.

            b.         The tongue moves food around in the mouth and pushes it where it can be chewed.

            c.         Saliva is mixed with food to soften and moisten it. This makes it easier to swallow.

            d.         Saliva also contains an enzyme that starts to chemically digest starch.

2.         The esophagus is the passageway from the mouth to the stomach.

            a.         The walls of the esophagus produce mucus which lubricates the food so it moves more easily.

            b.         Muscles lining the esophagus contract to move the food along. These contractions are called peristalsis.

            c.         When you swallow, the epiglottis covers the windpipe so that food does not enter the lungs

3.         The stomach is a large bag-like organ that begins the breakdown of protein in food.

            a.         The stomach has muscles that cause it to churn the contents, contributing to mechanical digestion.

            b.         This churning usually starts automatically at meal times. This is what is happening when you hear your stomach “growling.”

            c.         The churning also mixes the food with gastric juice.

                        i.         Gastric juice is a mixture of digestive juice made by the stomach. It includes:

                                    (1)       mucus - protects the lining of the stomach from being digested

                                    (2)       pepsin - is an enzyme that digests proteins

                                    (3)       hydrochloric acid - a strong acid that is needed for pepsin to work properly. It also helps break down food and kills bacteria in the stomach

            d.         By the time food leaves the stomach it is a thick liquid called chyme

            e.         Chyme is released from the stomach into the small intestine.

4.         The small intestine is a narrow, coiled tube about 6.5 m long and 2.5 cm wide.

            a.         The walls are muscular and these muscles move food through the intestine by peristalsis.

            b.         Most chemical digestion occurs in the small intestine.

            c.         Food entering the small intestine is mixed with several digestive juices.

            d.         These are produced by the small intestine, the pancreas and the liver.

            e.         Once food is digested, it is absorbed into the bloodstream in the small intestine.

            f.         Once in the bloodstream, nutrients can be taken to all parts of the body.

            g.         The absorption takes place through the walls of the small intestine.

            h.         The inner wall is folded and has many little projections called villi that create a huge surface area so nutrients can be absorbed quickly.

5.         The pancreas is a small organ located below the stomach.

            a.         When food enters the small intestine, the pancreas releases digestive juices that digest starch, proteins, and fats.

            b.         It also releases a chemical that neutralizes the acid from the stomach so the intestine is not damaged.

6.         The liver is the largest internal organ.

            a.         The liver produces bile which acts like a detergent to break apart large clusters of fat into tiny droplets.

            b.         These droplets make it easier for enzymes to digest them.

            c.         Bile is made in the liver but is stored in the gallbladder until it is needed.

            d.         The gallbladder is a small pouch under the liver.

            e.         When food enters the small intestine, bile moves from the gallbladder to the small intestine.

7.         The large intestine is the last part of the digestive system, about 2 m long and 7 cm wide.

            a.         The undigested food that enters the large intestine contains lots of water and minerals.

            b.         The water and minerals are reabsorbed by the large intestine so that they are not lost.

            c.         Feces is the term for what remains after the water has been reabsorbed.

            d.         Feces move into the rectum until it is full and the waste is eliminated from the body.

Problems of the Digestive System

1.         Appendicitis

            a.         The appendix is a small pouch near where the large and small intestines join.

            b.         If food becomes trapped in the appendix it can become infected with bacteria. If not treated, the appendix can rupture which is usually fatal.

            c.         This infection is called appendicitis.

2.         Indigestion

            a.         This is usually caused by poor eating habits - eating too much, too little or too fast.

            b.         Sometimes called “heartburn,” it results when acidic juices from the stomach enter the esophagus and causes a burning sensation.

3.         Ulcers

            a.         Ulcers are caused when stomach acid digests the lining of the stomach, causing a hole.

            b.         They are caused when excess stomach acid destroys the mucus that protects the stomach lining.

            c.         Most ulcers are caused by bacteria in the stomach but can also be caused by stress.

4.         Diarrhea

            a.         When the large intestine fails to absorb water from waste, the feces remain very watery.

            b.         It can be caused by stress as well as certain viruses and bacteria.

            c.         When suffering from diarrhea, it is important to drink plenty of fluids to avoid dehydration.