f human cells could use the food eaten in that form, there would be no need for a digestive system. Â The digestive system changes the food into a form that can be used by the cells and that enables the nutrients to get into the blood so they can be transported throughout the body
There are two kinds of digestion: Â mechanical and chemical. Â Mechanical digestion involves physically breaking the food into smaller pieces. Â Mechanical digestion begins in the mouth as the food is chewed. Â Chemical digestion involves breaking down the food into simpler nutrients that can be used by the cells. Chemical digestion begins in the mouth when food mixes with saliva. Â Saliva contains an enzyme (amylase) that begins the breakdown of carbohydrates. Â (An enzyme is a protein that can catalyze certain biochemical reactions).
The Digestive Process
Mouth
Food is chewed into smaller pieces.  Adults have 32 specialized teeth—teeth that can grind, chew, and tear different kinds of food. The tongue is an organ consisting of skeletal muscles (voluntary muscles) that move the food around the mouth to allow for efficient mechanical digestion. Salivary glands beneath and in back of the tongue secrete the saliva that allows for easier swallowing of food and the beginning of chemical digestion.
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Pharnyx
Swallowing forces the chewed food through a tubular entrance (pharynx) to the esophagus (food tube). Â As food is swallowed a flap-like valve, the epiglottis, closes over the trachea (windpipe) to prevent food entering the windpipe and causing choking.
The stomach starts chemical digestion of protein. Â Secretions from the stomach lining consist of about two liters of hydrochloric acid (HCl), pepsin, and other fluids that make up gastric fluids each day. The fluid is extremely acidic and it helps kill bacteria and other pathogens that may have been ingested.
The thick mucus also produced by the stomach lining usually keeps the acids from damaging the lining. If not enough mucus is produced or if too much acid is produced, peptic ulcers form. Â Heredity, stress, smoking, and excessive alcohol intake can make the ulcers worse. Â The condition can worsen and bleeding ulcers can result.
Food stays in the stomach for approximately 3-4 hours and moves through another sphincter muscle to pass into the small intestine.
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Small intestine
Nearly 7 meters in length, the small intestine is folded and curled around a small area in the abdominal cavity. The inside surfaces of the intestine are covered with projections called villi. Â These finger-like structures are covered in smaller projections called microvilli and work to absorb food molecules that have been broken down by the processes of chemical digestion.
The small intestine has three distinct parts: Â the duodenum, the jejunum, and the ileum. Â Each day, about 9 liters of fluid enters the duodenum. Â Most chemical digestion takes place in the duodenum by chemicals secreted by the liver, pancreas and small intestine. The other two sections of the small intestine, the jejunum and the ileum, absorb food molecules by way of the villi directly into the blood stream.
Large intestine
The large intestine receives the material “left-over” from chemical digestion that is basically nutrient free. Only water, cellulose, and undigestible materials are left. The main job of the large intestine is to remove water from the undigested material. Water is quickly removed from the material through villi and returns to the blood stream. Â
Liver
The liver is a large organ located just above the stomach. Â The liver produces bile which helps digest lipids. Â Bile is stored in the gallbladder and flows from the gallbladder to the duodenum where it helps digest fats.
The picture at the left shows a human liver.
Gall bladder
The gall bladder is a small, greenish organ located just under the liver. Â It stores bile produced by the liver until it is secreted directly into the first section of the small intestine.
Pancreas
The pancreas has three important functions that help the digestive system change food into a form that can be used by the cells.
1) Â Â It produces enzymes which help break down proteins, lipids, and carbohydrates.
2) Â Â It produces the hormone, insulin, which helps regulate blood glucose levels.
3) Â Â It produces sodium bicarbonate which helps to neutralize stomach acids.
Checking for Understanding
1. Â Â What is the function of the digestive system?
To change ingested food into a form that can be used by the cells, to get the nutrients into the blood stream, to reabsorb water, and to eliminate wastes.
2. Â Â What is the function of the following parts/organs of the digestive system?
a.   mouth—houses teeth, salivary glands and tongue for mechanical and chemical digestion
b.   teeth—chews, grinds, and tears food into smaller pieces
c.   esophagus—food tube; leads from the pharynx to the stomach
d.   stomach—
churns food to mix with gastric enzymes and acid to continue mechanical and chemical diges
