1. The overall function of the human digestive system is to provide materials to be used by the individual cells of the body. (para 6-1)
2. The radiant energy of the Sun is stored by plants as the chemical bonds of glucose molecules. The process for doing this is called photosynthesis.
Ultimately, humans consume either the green plants themselves or the animals that ate the green plants.
Through the processes of digestion, the glucose is released. It is then delivered to the cells of the body by the circulatory system.
Within the cells of the body, the energy is released from the glucose molecules by the process known as metabolic oxidation. The released energy is then used to produce the compound ATP. Metabolic oxidation and the production of ATP occur within the mitochondria. The energy for any of the life processes is obtained directly from ATP. (para 6-2)
3. The three categories of foodstuffs are carbohydrates, lipids, and proteins. Other necessary items include water, minerals, and vitamins. (para 6-3b, c)
4. The following processes are involved in the human digestive system:
Ingestion of foods.
Elimination of unused materials. (para 6-3d)
5. When an individual needs food, he experiences the sensation of hunger. This is controlled by the hypothalamus area of the brain, which receives information from various parts of the body for this purpose.
The process of food selection is related both to previous learning and to internal chemical requirements.
As food enters the oral cavity, bite-size chunks of food are cut off by the upper and lower incisors. These chunks are about the right size for the mouth to handle. (para 6-4)
6. There are two key facts about digestion:
a. First, digestion is a chemical process. Food is broken down into its constituent parts through the process of hydrolysis.
b. Second, this chemical process takes place only at wet surfaces of the food. (para 6-5)
7. Food processes are broken down into smaller and smaller pieces through the process of mastication, or chewing. This greatly increases the total surface area of the food. The grinding and crushing are accomplished by the premolar and molar teeth. Keeping the food between the surfaces of the grinding teeth are the tongue and the cheeks. (para 6-6)
8. The fluids secreted into the oral cavity by the salivary glands and the buccal glands are collectively known as saliva. These fluids serve to wet the surface areas of the food particles. Saliva also dissolves some of the molecules of food items. These dissolved molecules are tested by the taste buds. Food particles are held together as a bolus by the mucus, which also makes the bolus somewhat slippery. (para 6-7)
9. The bolus is moved posteriorly out of the mouth into the pharynx and then down through the esophagus to the stomach. Both the upper and lower air passageway must be protected as the bolus passes through the pharynx. (para 6-8)
As the bolus approaches the pharynx, the upper air passageway is closed by the soft palate, which also tenses to resist the pressure from the bolus. (para 6-9)
11. Wavelike contractions of the three pharyngeal constrictor muscles force the bolus into the beginning of the esophagus.
As the larynx is raised along with the tongue and the hyoid bone, its epiglottis turns down over the opening of the larynx. Thus, food is prevented from entering the lower air passageway. (para 6-10)
12. The esophagus is a tube with muscular walls. It extends from the pharynx above, through the neck and thorax, to the stomach in the abdomen. Wavelike contractions, called peristalsis, move the bolus through the esophagus to the stomach. (para 6-11)
13. Because of the stomach's capacity, the individual can engage in activities other than eating. In addition, certain digestive processes are initiated in the stomach. (para 6-12)
14. One way the stomach is adapted as a storage area is that its wall is quite stretchable. Its lining has folds called rugae.
Another adaptation is that, at each end, there is a valve or similar structure to keep contents from leaving. The "gastroesophageal valve" has not been demonstrated anatomically. At the other end of the stomach is the well-developed pyloric valve. (para 6-13)
15. The mucosal lining of the stomach contains a number of gastric glands. The mixture produced by the stomach, called chyme, is quite acid.
The three layers of muscles help to ensure that the contents of the stomach are thoroughly mixed. (para 6-14)
16. Digestion occurs through the actions of chemicals called enzymes. The end products (molecules or particles) are small enough to be absorbed through the walls of the small intestines. (para 6-15)
Digestive enzymes are present to maintain the appropriate rates of reaction. A catalyst is a substance that improves the rate of reaction without being consumed itself. Digestive enzymes are catalysts. Without digestive enzymes, digestion would be too slow to provide materials needed by the body. (para 6-16)
17. The majority of digestion in humans takes place in the small intestines. Draped over these is a flap called the greater omentum. This flap has fat for insulation and many blood vessels for heat. Thus, the greater omentum may be compared to an "electric blanket" for the small intestines. (para 6-16c)
18. The saliva contains enzymes that initiate the digestion of complex carbohydrates.
In the small intestines, there are digestive enzymes for carbohydrates, lipids, and proteins. These enzymes are found in the fluids produced by the pancreas and glands in the mucosa of the small intestines. Moreover, the liver produces a fluid called bile, which is stored in the gallbladder for release into the small intestines; this fluid helps in the digestion of lipids. (para 6-17)
19. The absorptive area of the walls of the small intestines is increased by permanent circular folds (plicae circulares) and by fingerlike processes called villi. (para 6-19a)
21. The blood capillaries absorbing substances from the digestive tract join to form the hepatic portal venous system. A venous portal system begins in capillaries, which join to form veins, which in turn end in another group of capillaries. The hepatic portal vein carries blood from the absorptive area of the digestive system to the liver.
22. In the liver, excess materials are removed and stored. For example, some glucose is stored as glycogen. Toxic materials are degraded. Microorganisms are removed. The "treated" blood is then routed from the liver to the heart and then throughout the body. (para 6-21)
23. Lipid materials are stored as fat throughout the body so that they will be available when needed for energy. (para 6-22a)
24. The lumen of the digestive system connects directly with the surrounding environment. For this reason, special protective mechanisms are associated with the human digestive system. Such mechanisms belong to the reticuloendothelial system.
A primary component of the reticuloendothelial system are the lymphoid tissues. An important type of cell found within these tissues is the lymphocyte. These cells signal other types of white blood cells to phagocytize foreign materials. These tissues are more important in the child.
The aggregate of lymphoid tissue at the beginning of the pharynx are called tonsils.
Peyer's patches might be considered the "tonsils" of the small intestine.
Lining the sinusoids of the liver and removing harmful substances from the blood are Kupffer's cells. These cells are also considered to be part of the reticuloendothelial system. (paras 6-23 thru 6-29)
25. During nursing, the initial secretion of the mammary glands is called colostrum. Although this secretion lacks nutrients, it is loaded with antibodies. These provide the infant with its primary protection for the first 6 months of life.
Later, if the infant has an upper respiratory infection, the mammary gland will produce the appropriate antibodies. This is due to a reflux of fluid into the milk ducts of the mammary gland as the infant sucks. (para 6-30)
26. Required in very small quantities from outside the body are substances called vitamins. These can be considered in two major categories--water-soluble and fat-soluble.
Water-soluble vitamins include the B-complex vitamins, vitamin C, and others. There is a daily requirement for water-soluble vitamins because they are continuously excreted with the urine.
On the other hand, fat-soluble vitamins accumulate in the fat of the body. (paras 6-31 thru 6-33)
27. An important nondigestible food material is cellulose, commonly referred to as "fiber" or "bulk." Other undigested materials may be due to the consumption of great quantities of food. Undigested food materials enter the large intestines through the ileocecal valve. (para 6-34)
28. A major function of the large intestines is to salvage water. To facilitate movement, mucus is added to the contents of the large intestines. Microorganisms in the large intestines manufacture vitamin K. Some microorganisms can act upon certain foods to produce gases. Feces is stored in the rectum and the lower portion of the sigmoid colon. Defecation is accomplished by relaxation of the anal sphincter muscles. (paras 6-35 thru 6-37)
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