e. Production and Use of ATP.  The released energy is then used to produce the compound known as ATP (adenosine triphosphate). This metabolic oxidation and the production of ATP occur in the mitochondria. For this reason, the mitochondria are known as the "powerhouses" of the cell. When energy is required for carrying on any of the life processes, it is obtained from the ATP.

ATP ---® ADP + P0₄ + E

YIELDS

ATP ¬ --- ADP + Phosphate Radical + Energy

IS

FORMED

BY

6-3. FOODS

A food is any substance utilized by a living thing for energy (or for growth and repair).

a. There are both plant and animal sources for foods. One can eat grains in the form of bread, and one can eat the meat of an animal that ate such grains.

b. Making up foods are specific substances known as foodstuffs. In general, foodstuffs are in three categories:

(1) Carbohydrates (starches and sugars).

(2) Lipids (fats and oils).

(3) Proteins.

c. In addition, other necessary items are also parts of foods. These include water, minerals, vitamins, and so forth.

d. In the human digestive system, the following processes are involved:

(1) Ingestion (taking in) of foods.

(2) Initial processing.

(3) Storage.

(4) Digestion.

(5) Absorption.

(6) Elimination of unused materials.

a. Hunger.  When an individual needs foods, he experiences a sensation known as hunger. The hypothalamus area of the brain controls the degree of hunger or satiation (feeling of being well fed). To do this, the hypothalamus receives various types of information from throughout the body.

b. Food Selection.  When food is presented, an individual goes through a process of food selection. He or she has a greater appetite for some foods than others. This process is related both to previous learning and to current, internal chemical requirements.

c. Biting.  Together, the upper and lower incisors (anterior teeth) create two cutting surfaces like a pair of scissors. As food items are placed in the opening of the oral cavity, bite-size chunks of food are cut off. These chunks are usually just the right size for the mouth to handle.

6-5. TWO KEY FACTS ABOUT DIGESTION

In general terms, there are two key facts to understand about digestion:

a. First, digestion is a chemical process. Through a process called hydrolysis, food is broken down into its constituent parts.

b. Second, this chemical process takes place only at wet surfaces of the food.

6-6. MASTICATION

During the process known as mastication (chewing), the food particles are gradually broken down into smaller and smaller pieces. At the same time, the total surface area of the food increases greatly.

a. This grinding and crushing of the food particles are accomplished by the posterior teeth, the premolar and molar teeth. For this purpose, these teeth have broad, opposing surfaces.

b. Together, the tongue and cheeks act to keep the food particles between the surfaces of the grinding teeth. This is accomplished as the lower jaw moves up and down.

a. Secreting fluids into the oral cavity are such glandular structures as the salivary glands and the buccal glands. (The buccal glands are serous and mucous glands on the inner surfaces of the cheeks.) These fluids are collectively known as the saliva.

b. Saliva serves to wet the surface areas of the food particles produced by mastication. In addition, saliva also dissolves some of the molecules of the food items.

c. Taste buds sample these dissolved molecules and test the quality of the food being eaten. Taste buds are located on the tongue and the back of the oral cavity.

d. Another component of the saliva is mucus. The mucus tends to hold the food particles together as a bolus. Since the mucus also makes this bolus somewhat slippery, the bolus can slide readily through the initial portion of the digestive tract.

When the food has been adequately broken down (increased surface area), wetted thoroughly, and tested (tasted), it is ready to be swallowed.

a. The bolus is moved posteriorly out of the mouth (oral cavity) into the pharynx and then down through the esophagus to the stomach.

b. The pharynx is common to both the digestive and respiratory systems. Therefore, as the bolus passes through the pharynx, both the upper and lower air passageways must be protected. Otherwise, food particles might enter the passageways.

6-9. MOVEMENT OUT OF THE ORAL CAVITY

a. Initial Movement of the Bolus.  There are intrinsic muscles in the tongue. Through their action, the tongue arches upward and presses against the hard palate, the roof of the mouth. This initiates the posterior movement of the bolus.

b. Action of the Hyoid Complex.  The muscles of the hyoid bone pull the hyoid bone upward and force the tongue upward into the oral cavity. This closes up the front part of the oral cavity and forces the bolus further to the rear.

c. Action of the Soft Palate.  As the bolus approaches the pharynx, the soft palate is raised. Thus, the soft palate serves as a trap door to close the upper air passageway. By tensing to resist the pressure from the bolus of food, the soft palate ensures the continued backward movement of the bolus into the pharynx.

a. Pharyngeal Constrictor Muscles.  The wall of the pharynx contains three pharyngeal constrictor muscles. By wavelike contractions, these muscles force the bolus down into the beginning of the esophagus.

b. Action of the Epiglottis.  As the hyoid bone's muscles raise the tongue up into the oral cavity, they also raise the larynx. The larynx is raised because it is attached to the inferior margin of the hyoid bone. As the larynx is raised, its epiglottis automatically turns down over the opening of the larynx. Thus, food is prevented from entering the lower-air passage-way.

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 (peristalsis) move the bolus through the esophagus to the stomach.

a. The stomach is a saclike enlargement of the digestive tract. By way of the esophagus, the stomach receives the food that has been processed in the oral cavity.

b. The stomach's capacity is great enough to allow the individual to take in enough food material at one time to last for an extended period of time. This allows the individual to engage in activities other than eating.

c. In addition, certain digestive processes are initiated in the stomach.

d. The food is retained in the stomach for varying lengths of time, depending upon the types of food eaten, the condition of the individual, and many other factors.

The stomach is adapted as a storage area in several ways.

a. Its wall is quite stretchable. The mucosal lining of the stomach is thrown up into longitudinal folds called rugae. These rugae flatten out as the stomach capacity increases.

b. At each end of the stomach, there is a structure to keep the contents from leaving the stomach.

(1) At the point where the esophagus enters the stomach, there is a "gastroesophageal valve." This valve appears to be functional, although it has not been demonstrated anatomically.

(2) At the other end of the stomach is the well-developed pyloric valve.

a. Gastric Glands.  The mucosal lining of the stomach contains a number of gastric glands. These gastric glands produce gastric digestive juices for initiating digestion, particularly of proteins. Some of the gastric glands also produce hydrochloric acid. Thus, chyme, the mixture produced by the stomach, is quite acid.

b. Additional Musculature.  A third inner, oblique layer of muscle has been added to the stomach wall. With the three layers of muscles, the contents of the stomach are thoroughly mixed.

The small intestines are the primary area of the body for digestion of foodstuffs. Digestion occurs through the action of enzymes. The results of the digestion are the end-products. These end-products (molecules or particles) are of such size that they can be absorbed through the walls of the small intestines. The end-products are then distributed throughout the body by the body's circulatory systems.

a. Digestion is the chemical process that breaks foodstuffs down into their basic constituents. In general, chemical processes are expected to occur at a rate proportional to the temperature. However, in the human body, the temperature is not high enough for the chemical process of digestion to produce a sufficient quantity of the materials needed.

b. Therefore, digestive enzymes are present to maintain the appropriate rates of reaction. Digestive enzymes are catalysts. A catalyst is a substance that improves the rate of a reaction without being consumed itself. Because of digestive enzymes, digestion proceeds at a pace fast enough to provide the materials needed by the body.

c. The majority of digestion in humans takes place in the small intestines. The small intestines are located in the central part of the abdomen, immediately beneath the abdominal wall. In healthy individuals, a flap called the greater omentum is draped over the small intestines (between them and the anterior abdominal wall). The greater omentum has a great deal of fat for insulation. It is richly supplied with blood vessels for heat. Some might compare the greater omentum to an "electric blanket" for the small intestines.

Table 6-1. Foodstuffs, enzyme classes, and end-products of digestion.

a. The digestive process begins in the oral cavity. The saliva contains enzymes which initiate the digestion of complex carbohydrates.

b. In the stomach, the gastric glands produce enzymes that initiate the digestion of proteins.

c. In the small intestines, there are digestive enzymes for all three classes of foodstuffs--carbohydrates, lipids, and proteins. Enzymes for completing the digestion of these three classes are found in the fluids produced by the pancreas and glands in the mucosa of the small intestines. Moreover, there is a fluid called bile that is produced by the liver and stored in the gallbladder for release into the small intestines. Bile helps in the digestion of lipids.

d. The presence or absence of certain enzymes is genetically determined. Therefore, some individuals may have difficulty digesting certain foods.

The length of the small intestines appears to be just right. The time it takes for material to travel from beginning to end is just about right for the completion of digestion.

The end-products of digestion are absorbed primarily through the walls of the small intestines.

a. Surface Area.  The amount of absorption is proportional to the surface area of the walls which contact the contents. Two anatomical specializations serve to increase this surface area:

(1) There are permanent circular folds (plicae circulares) in the mucosal lining of the small intestines.

(2) The entire inner surface of the mucosa is covered with villi. Villi are minute, fingerlike processes that extend into the lumen (cavity) of the small intestines.

b. Capillaries.  The simple sugars and amino acids are absorbed into the blood capillaries. Most of the fatty acids and glycerol are absorbed into the lymphatic capillaries.

All of the blood capillaries in the absorptive areas of the digestive tract join to form the hepatic portal venous system. A venous portal system is a system that begins in capillaries, which join to form veins, which in turn end in another group of capillaries. The hepatic portal vein carries the blood from the absorptive areas of the digestive system to the liver.

In the liver, a number of actions are performed on the blood. Excess materials are removed and stored. For example, some glucose is stored as glycogen. Toxic materials are degraded, microorganisms are removed, and so forth. The "treated" blood is then routed from the liver to the heart and then throughout the body.

The lipid materials, such as fatty acids and glycerol, are carried to the venous system beyond the liver.

a. Lipid materials are a high-energy item. They are stored as fat throughout the body so that they will be available when needed for energy.

b. Body fat also serves as insulation in the subcutaneous tissues. It gives buoyancy to the body in water.

c. Cholesterol is a very important substance in the body. It participates in the functioning of the liver and in other activities of the body.

d. However, there are certain medical conditions in which physicians prescribe a low-cholesterol and/or low-fat diet.

The human digestive system is essentially a continuous tube. It is open at both ends. Therefore, the lumen (cavity) connects directly with the surrounding environment.

a. Along with the ingested food, almost anything can pass through the mouth into the digestive system. Almost anything does enter the digestive system.

b. The digestive tract is open to the surrounding environment also at the other end, the anus.

As indicated above, a variety of toxic materials and/or microorganisms may be included with the ingested foods. To protect against these undesired materials, special protective mechanisms are associated with the human digestive system. Such protective mechanisms are said to belong to the reticuloendothelial system. This term refers to the association of such mechanisms with a particular layer of epithelial cells.

a. The lymphocyte is an important type of white blood cell that is also found in the interspaces of a tissue called lymphoid (or lymphatic) tissue. Lymphocytes signal other types of white blood cells to phagocytize (engulf) foreign materials found within the body. The lymphoid tissues are particularly important in individuals from birth until about 15 years of age. The mass of lymphoid tissue found in the body of a 12-year-old is about twice the mass found in a full-grown adult. (Between 6 and 15 years of age, the immune system of the blood becomes the primary protector of the body from disease.)

b. The lymphoid tissues are a primary component of the reticuloendothelial system.

Tonsils are aggregates of lymphoid tissue found at the beginning of the pharynx. There are three pairs of tonsils. Together, they form a ring of lymphoid tissue at the beginning of the pharynx. This ring, called Waldeyer's ring, completely surrounds the entrance to the pharynx from both the mouth (digestive entrance) and the nose and nasal chambers (respiratory entrance).

a. In the upper recess of the pharynx is the pair of pharyngeal tonsils (commonly known as the adenoids).

b. On either side, below the soft palate, are the palatine tonsils. These are the tonsils that one sees most frequently in small children.

c. On the back of the root of the tongue are the lingual tonsils.

Lymphoid aggregates of varying size are found in the walls of the small intestines. In the ileum portion, in particular, these aggregates are large enough to be easily observed and are called Peyer's patches. These might be considered "tonsils" of the small intestines.

At the beginning of the large intestine, at the inferior end of the cecum, is a structure known as the vermiform appendix. Since the vermiform appendix is actually a collection of lymphoid tissue, it should be considered the "tonsil" of the large intestine.

As we have seen, blood from the absorptive areas of the gut tract is collected and delivered to the liver by the hepatic venous portal system. As this blood passes through the sinusoids (channels) of the liver, it is acted upon by cells called Kupffer's cells. These cells line the sinusoids. Since Kupffer's cells remove harmful substances from the blood, they are considered part of the reticuloendothelial system.

a. When the newborn baby is nursed by its mother, the initial secretion of the mammary glands is called colostrum. Although this colostrum lacks nutrients, it is loaded with antibodies. These antibodies provide the infant with its primary protection for the first 6 months of life.

b. After a few days, the mammary gland produces the natural food for the human infant. As the infant suckles at the mother's breast, there is a certain amount of reflux (backward flow) into the milk ducts of the mammary gland. Should the infant develop an upper respiratory infection, the organisms causing the infection will be included in this reflux. Generally by the next time the infant suckles, the mammary gland will have produced the appropriate antibodies. These antibodies are delivered to the infant for its protection.

a. There is a group of chemicals that are required in very small quantities from outside the body for the proper functioning of the body. These substances are called vitamins.

b. Vitamins are found in varying amounts in different foods. In fact, many processed foods contain artificial vitamin supplements.

c. Vitamins can be considered in two major categories--water-soluble vitamins and fat-soluble vitamins.

The water-soluble vitamins include vitamin C, B-complex vitamins, and others. There is a daily requirement for water-soluble vitamins. This is because they are excreted continuously with the urine.

a. Vitamin B₁ (Thiamine Hydrochloride).  Vitamin B₁ is present in liver, bananas, lean pork, and whole grain cereals.

b. Vitamin B₂ (Riboflavin).  Riboflavin is found in milk, milk products, leafy green vegetables, fruit, and liver.

c. Vitamin B₆ (Pyridoxine Hydrochloride).  Vitamin B₆ is found in whole grain cereals, yeast, milk, fish, eggs, and liver.

d. Nicotinic Acid (Niacin) and Nicotinamide (Niacinamide).  These are present in meat, liver, milk, peanuts, and whole grain cereals.

e. Vitamin B₁₂.  Vitamin B₁₂ is found in liver, milk, eggs, and cheese.

f. Folic Acid.  Folic acid is found in leafy green vegetables and liver.

g. Vitamin C (Ascorbic Acid).  Sources of vitamin C include citrus fruits, tomatoes, bell peppers, paprika, and all leafy green vegetables.

On the other hand, fat-soluble vitamins can be accumulated in the fat of the body:

a. Vitamin A.  Vitamin A is mainly obtained from yellow-colored vegetables of all sorts (carrots, squash, and so forth.).

b. Vitamin D.  Vitamin D is produced in the skin by the activity of solar radiation. It is also present in fish liver oils, butter, and egg yolk.

c. Vitamin K.  Vitamin K is important in blood clotting. It is actually produced by microorganisms located in the large intestines. This source of vitamin K may be lost during the administration of antibiotics. Vitamin K also occurs in such foods as alfalfa, spinach, cabbage, and egg yolk.

d. Vitamin E.  The function of vitamin E in humans is not known. Research indicates that vitamin E has important functions in various species, but the specific function varies from species to species.

a. Nondigestible Food Materials.  A number of substances within food materials cannot be digested by the human digestive system. One important material in this group is called cellulose. Cellulose is a complex carbohydrate found in plants. Cellulose is commonly referred to as "bulk" or "fiber."

b. Other Undigested Food Materials.  When individuals consume great quantities of foods, a portion of it will not be digested.

c. Passage Out of the Small Intestines.  This undigested material will pass out of the small intestines with the non-digestible materials. The resulting fluid mass enters the large intestines through the ileocecal valve.

a. Consolidation of Contents.  In the large intestines, this fluid mass is gradually consolidated into a semisolid mass called feces. The major function of the large intestines then is salvage. Water is the primary salvage item. In addition to water, some previously unabsorbed endproducts of digestion can be absorbed here. At the same time certain excretions from the body can be deposited in the fecal mass.

b. Mucus.  As the contents increase in solidity, mucus is added to facilitate their movement through the large intestines. (Previously, we have seen the addition of mucus to the bolus in the mouth to facilitate movement.) This mucus is produced by unicellular glands in the mucosal lining of the large intestines. (Because of their microscopic appearance, these unicellular glands are called goblet cells.)

c. Organisms.  Many microorganisms are found within the lumen or cavity of the large intestines. Certain microorganisms are responsible for the production of vitamin K. Depending on the type of food present, some species of microorganisms produce various gases (flatulence). On occasion, pathogenic organisms may be present and cause problems for the individual.

Toward the lower end of the large intestines, the contents (feces) have become relatively consolidated. This consolidated mass is retained (stored) mainly in the rectum and the lower portion of the sigmoid colon.

At the appropriate time, the feces is passed out of the body (defecation). The feces passes through the anal canal and anus. This is accomplished by the relaxation of the anal sphincter muscles.