Biology 39.2 Digestion

1. Digestion Biology 39.2 Digestion

2. Breaking Down Your Food: Imagine you just ate your favorite meal. What happens to your food? Before your body can use the nutrients in food for energy, the large food molecules must be broken down both physically and chemically before cells can use it. The process of breaking down food into molecules the body can use is called digestion. Breaking Down Your Food

3. The digestive system is the body system that is involved in taking in and processing food for use by your body cells. The digestive system takes in food, breaks it down into molecules small enough for the body’s cells to absorb, and gets rid of the undigested molecules and waste from the cells. The digestive system

4. The digestive system is made up of a long, winding tube, the digestive tract, that begins at the mouth and runs through the body to the anus. Food travels more than 26 feet (8 m) through your digestive tract. The digestive tract includes your mouth, pharynx, esophagus, stomach, small intestine, large intestine and rectum. Although the liver and pancreas are not part of the digestive tract, they deliver secretions into the digestive tract through ducts. Digestive system

5. The digestion of food begins as soon as the food is ingested. The teeth rip and chew the food into shreds and the tongue mixes the pieces with a watery solution called saliva. Taste buds in the tongue are sensitive to certain chemicals in the food. Saliva is secreted in the mouth by three pairs of salivary glands. Saliva moistens and lubricates food so that it can be swallowed more easily. Starting Digestion

6. Saliva also contains amylases, enzymes that begin the breakdown of carbohydrates, such as starch, into monosaccharides (single sugars). The mechanical action of chewing and the chemical action of amylase are both part of the digestive process taking place in the mouth. Saliva

7. Notice that the structure of our teeth helps in the breakdown of food. The two front teeth, the incisors, cut food. The cuspids, or canines, shred food. The back teeth, the molars, crush and grind food. Teeth

8. After passing through the region in the back of the throat called the pharynx, the food triggers a swallowing response. The action of swallowing moves the epiglottis (a flap of cartilage) over the opening of the trachea, the tube that leads to the lungs. This action prevents food from entering the trachea and eventually the lungs. Instead, food enters the esophagus taking it to the stomach. Pharynx & epiglottis

9. The esophagus is a long tube that connects the mouth to the stomach. No digestion takes place in the esophagus. It’s role is to act as a kind of chute, moving food down into the stomach. The esophagus is about 25 cm or 10 inches long. The lower two-thirds of the esophagus is wrapped in sheets of smooth muscle. Esophagus

10. Food does not simply fall down into the stomach, it is pushed down by muscle contractions in the esophagus. Successive rhythmic waves of smooth muscle contractions happen in the esophagus, called peristaltic contractions, or peristalsis, move the food toward the stomach. Peristalsis can be thought of as waves moving through the muscle with the area where the wave is passing causing the muscle to narrow. It takes about 10 to 15 seconds for food to pass down the esophagus and into the stomach. esophagus

11. Food exits the esophagus and enters the stomach through a muscular valve called the sphincter. The sphincter prevents acid-soaked food in the stomach from making it’s way back into the esophagus. The stomach is a saclike organ located just beneath the diaphragm. Besides temporarily storing food, the stomach, also mechanically breaks down food and chemically unravels and breaks down proteins. The Stomach

12. When food enters the stomach, gastric juice is secreted by the cells that line the inside of the stomach. Gastric juice is a combination of hydrochloric acid and pepsin. The acid breaks the bonds in proteins and unfolds large protein chains into single strand proteins. Pepsin, a digestive enzyme secreted by the stomach, cuts the single protein strands into smaller chains of amino acids. Pepsin works only in an acidic environment. Stomach

13. The stomach mixes its contents by using peristaltic waves. Swallowed food can spend 2 to 6 hours in the stomach. Your stomach secretes about 2 liters (2.11 quarts) of hydrochloric acid every day, which creates a solution about 3 million times more acidic than your bloodstream. The hormone gastrin regulates the synthesis of hydrochloric acid, thus it is made only when the pH level of the stomach is higher than 1.5. Stomach

14. A coating of mucus protects the lining of the stomach from gastric acid. Bicarbonate in the stomach helps neutralize digestive fluids. Blood circulation in the stomach lining also helps protect stomach tissues. Stomach

15. Food passes from the stomach into the small intestine when a sphincter between the two organs opens. The small intestine is a coiled tubular organ about 20 feet long that is continuous with the stomach and that functions mainly in the digestion and absorption of nutrients. The word small refers to the small diameter of the small intestine when compared to the large intestine, not the length of it. Peristalsis mixes the food, which remains in the small intestine for about 3 to 6 hours. The Small Intestine

16. The first part of the small intestine, the duodenum, receives secretions from the pancreas, liver, and gallbladder. Cells that line the small intestine and the pancreas secrete digestive enzymes involved in completing the digestion of carbohydrates into monosaccharides, proteins into amino acids, and lipids into fatty acids and glycerol. Small Intestine

17. Before fats can be digested by pancreatic enzymes called lipases, the fats must first be treated with bile, a greenish fluid produced by the liver. Bile breaks up fat globules into tiny fat droplets, a process called emulsification. The gall bladder, a green muscular sac attached to the liver, concentrates and stores bile until it is needed in the small intestine. Bile

18. Most absorption (passage of nutrients to the blood or lymph) occurs in the small intestine. The lining of the small intestines covered with fingerlike projections called villi. Villi are too small to see with the naked eye. Villi

19. In turn, the cells covering each villus(singular form) have projections on their outer surface called microvillus. The villi and microvilli greatly increase the area available for absorption of nutrients. Sugars and amino acids enter capillaries in the villi and are carried in the blood to the liver for further metabolism. Villi

20. All components of food that are not for energy production are considered wastes. The wastes move into the large intestine. The large intestine, or colon, is much shorter than the small intestine. The diameter of the large intestine is about three times the diameter of the small intestine. The colon is not coiled like the small intestine. Instead, it is composed of three relatively straight segments. No digestion takes place in the colon. The Large Intestine

21. The volume of material that flows through the digestive system each day is large; about 10 liters of food, drink, and secretions enter the small intestine daily. But the amount of material that leaves the body as waste is small. This is because almost all the fluids and solids (about 90-95 percent) are absorbed during their passage through the small intestine. Most mineral ions and water are absorbed through the wall of the large intestine. The Large Intestine

22. Most of the colon’s contents are dead cells, mucus, digestive secretions, bacteria and yeast. A thriving colony of bacteria live in the human colon. These microbes synthesize many compounds that your body needs and can not get easily from food, including vitamin K and several B vitamins. In addition, bacteria aid in transforming and compacting the undigested materials into the final waste product, feces. Intestinal bacteria

23. The final segment of the large intestine is the rectum. Solids in the colon pass into the rectum as a result of peristalsis in the large intestine. From the rectum, the solid feces are eliminated from the body through the anus. Undigested material passes through the large intestine and is expelled through the anus in 12 to 14 hours. Rectum

24. Balancing water absorption in the intestine is important. Wastes rushed through the large intestine before the remaining water is absorbed result in diarrhea (watery feces). When food remains in the colon for long periods of time, which causes much water to be absorbed, constipation (hard feces) results. Hard feces are more difficult to pass. The Large Intestine

25. The human liver is about the size of a football and weighs more than 3 pounds. It presses upward against the diaphragm and occupies the upper right hand side of the abdominal cavity. The liver plays several roles in digestion. The liver secretes bile, which aids in the emulsification of fats. Bile also promotes the absorption of fatty acids and fat soluble vitamins A, D, E and K. Liver’s role in digestion

26. Bile pigments give bile a yellowish green color. Jaundice, a condition in which the eyes, skin, and urine become abnormally yellow, is a result of increased amounts of bile pigments in the blood. Jaundice often occurs as a result of hepatitis, an inflammation of the liver. Liver’s role in digestion

27. Digested food molecules in the blood stream are transported to the liver. The liver stabilizes blood sugar by converting extra sugar to glycogen for storage. The liver than breaks down the glycogen when it is needed for energy. The liver also modifies amino acids. Fat soluble vitamins and iron are stored in the liver. Liver’s role in digestion

28. The liver monitors the production of cholesterols and detoxifies poisons. If the liver is unable to change a substances harmful form, it stores it. In this way, toxins, including heavy metals and pesticides, accumulate in the liver. The liver can be damaged by a viral infection, chronic drug or alcohol use, or a traumatic injury. As a result of any of these, healthy liver cells are destroyed and replaced by scar tissue. The scarring of the liver is called cirrhosis. Liver’s role in digestion