A&P Module 3: Digestion

8 regions of the large intestine

(1) Cecum, (2) appendix, (3) ascending colon, (4) transverse colon, (5) descending colon, (6) sigmoid colon, (7) rectum, (8) anal canal

14. Through what sections of the pharynx does air travel? Through what sections of the pharynx does food travel?

14. Air enters the nose, continues through the nasopharynx, oropharynx and the laryngopharynx. Air then enters the trachea on its way to the lungs. Food and liquid enter the mouth, continue through the oropharynx and the laryngopharynx but then continue into the esophagus.

What part of Digestion happens in the small intestine?

47. In the small intestine chemical digestion continues as enzymes from the pancreatic juices break down starches and proteins, and begin digestion of fat. Mechanical digestion continues as bile breaks down fat into smaller pieces. Absorption begins as digested food molecules, water and fat are absorbed into the bloodstream.

62. How is the energy released in the breakdown of glucose converted to a form usable by the cells?

62. The energy released in the breakdown of glucose allows the body to add a phosphate to an adenosine diphosphate (ADP) molecule and make adenosine triphosphate (ATP) which is then used by cells as a source of energy to perform cellular activities.

63. How does the energy formed in fat catabolism compare to that formed in carbohydrate metabolism?

63. Lipid metabolism yields twice the energy formed during glucose metabolism.

64. Name, tell the location of and explain in detail the first stage of lipid breakdown.

64. Lipid metabolism occurs in the liver by a process known as beta-oxidation to form acetic acid which is further catabolized by the TCA cycle to carbon dioxide forming some ATP.

65. Why are proteins not usually used as a body energy source and when are they used?

65. Proteins are the building blocks for the cells and are therefore preserved by the body only being used to make ATP when carbohydrates and fats are unavailable to the body.

66. What is the first step in the oxidation of an amino acid formed from a protein?

66. When proteins must be used to form ATP, the amino group is removed from the protein to form ammonia.

67. Explain in detail how ammonia is removed from the body.

67. Ammonia is combined with carbon dioxide to make urea which is excreted in the urine.

Absorption and the Small Intestine

Absorption of food by the small intestine is increased by three structures in its wall. The surface area of the wall's mucous membrane is greatly increased by circular folds which have small, finger-like projections called villi on which are many smaller projections called microvilli. These structures increase the surface area of the roughly 20-foot length of the small intestine to about 1800 square feet for greatly increased absorption of nutrients. When chyme enters the duodenum, proteins and carbohydrates are only partially digested, and fat digestion still needs to be carried out. Considerably more digestive activity is required before these nutrients can be absorbed through the intestinal wall.

Large Intestine

As food continues to move along the gastrointestinal tract, it comes to the large intestine (see Figure 3.22). This organ extends from the ileocecal valve to the anus. Muscular contractions move the food contents (chyme) through the large intestine (colon) to be excreted as feces. The large intestine has 8 parts: the cecum, the appendix, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, the rectum and the anal canal (see Figure 3.22). The cecum is the entrance to the large intestine through the ileocecal valve. The second part is the appendix which hangs from the cecum and is usually twisted. Appendicitis occurs when bacteria accumulates and causes inflammation of the appendix. The third part is the ascending colon which runs along the right side of the abdomen superiorly. The right colic (hepatic) flexure connects the ascending colon to the transverse colon. The transverse colon runs across the abdomen. The left colic (splenic) flexure connects the transverse colon to the descending colon. The descending colon runs along the left side of the abdomen superiorly to the sigmoid colon. The sigmoid colon travels through the pelvis connecting with the rectum. The rectum connects to the anal canal where feces are expelled through the anus, the end of the gastrointestinal tract.

The Stomach Wall

As with the rest of the digestive tract, a thick layer of mucus protects the wall of the stomach and the first part of the duodenum. In a circumstance where gastric juice does penetrate the mucus, pepsin starts to digest the stomach or duodenal lining and an ulcer results. An ulcer is an open sore in the wall caused by the gradual disintegration of tissues. There are many causes of stomach and duodenal (small intestinal) ulcers. A bacterial infection from Helicobacter pylori (H. pylori) causes duodenal ulcers by eating away at the mucosal lining. Once a hole is formed, gastric juice enters other areas in the abdominal region, causing bleeding or damage to other organs. Ulcers also occur from the overuse of drugs such as NSAIDs (nonsteroidal anti-inflammatory drugs) and aspirin. NSAIDs damage the mucosal lining of the stomach.

Hunger

Basic physiological responses play a major role in hunger and satiety, or the sensation of being full. Hunger is the internal drive to find and eat food and is often experienced as a negative sensation such as churning, growling or a painful sensation in the stomach.

Vitimans

Besides the three nutrients listed above, our bodies also need vitamins and minerals in a much smaller amount. Vitamins are organic compounds that are not consumed in metabolic reaction but help enzymes carry out the metabolic reactions which convert nutrients to materials need by the body. Vitamins can be classified as water-soluble or fat-soluble. Water-soluble vitamins are not able to be stored in the body and are therefore excreted in urine. Fat-soluble vitamins are stored in body fat. Ingestion of excessive amounts of fat-soluble vitamins can be problematic. Vitamins are needed in the diet to maintain growth and good cellular health. No one food includes all the vitamins needed by the body. This is one reason that humans need to eat a balanced diet that consists of a variety of different foods.

Glucose Metabolism

Carbohydrates are the main source of energy for cells with the body digesting carbohydrates into glucose. Glucose metabolism is used by the body primarily during high energy demands. As glucose is one of the most abundant biomolecules, the catabolism of glucose is an important metabolic pathway used by microorganisms for ATP production. The complete catabolism of a single molecule of glucose (C6H12O6) yields up to 38 ATP and involves 3 distinct transitions. Energetically (ATP) speaking, glycolysis is the first step of this process and yields 2 molecules of ATP. Next, by either fermentation or respiration (see below) 2 additional molecules of ATP can be produced. Last, the electron transport chain produces 34 ATP via an oxidative phosphorylation event at the plasma membrane. Let's look at each step in a little more detail:

The role of digestion

Carbohydrates, proteins, and lipids are the three major biochemical categories of food that the body needs in large amounts. Vitamins and minerals are also needed by the body but in much smaller amounts. Carbohydrates are classified as starches and sugars. Milk and fruits such as apples or peaches contain sugar. Grains such as wheat, and vegetables such as potatoes contain starches.

Cellular Respiration

Food contains stored energy. In order for cells to use this energy, the food must be broken down so that its energy can be released. This released energy is in the form of ATP. Cellular respiration is defined as the process by which cells generate ATP. Most organisms carry out aerobic respiration which requires oxygen and a fuel (glucose, for example) and releases carbon dioxide, water and ATP.

Describe how food enters and exits the stomach

Food enters the stomach from the esophagus via the cardioesophageal sphincter and exits the stomach via the pyloric sphincter into the small intestine.

Hormones that cause hunger

Ghrelin, Endorphins, Neuropeptide-Y

Describe glycolysis.

Glycolysis is the breakdown of a single molecule of glucose into tow pyruvate molecules.

Hormones and Satiety

Hormones also influence our feelings of hunger or satiety. The human body produces a variety of hormones that work together to balance our hunger and satiety. Hormones are compounds secreted into the bloodstream by one type of cell that acts to control the function of another type of cell. The list below outlines several types of hormones related to hunger and satiety.

ATP (Adenosine triphosphate)

If a phosphate group is removed from ATP it becomes adenosine diphosphate or ADP. Thus, ATP has energy to donate while ADP can accept energy (phosphate group) to become ATP. By donating available phosphate groups, ATP transfers energy from catabolic (breaking down) reactions to be used for anabolism (building up). This process of donating and/or accepting energy is what fuels a cell to carry out the necessary biochemical reactions for survival.

Summary of ATP

In summary, cells require ATP to survive. Under aerobic conditions (in the presence of oxygen) the complete catabolism of glucose yields 2 ATP from glycolysis, 2 ATP from the TCA cycle and 34 ATP from the electron transport system, for a total of 38 ATP from a single glucose molecule. The summation of these metabolic processes is shown in Figure 3.32 below.

60. Describe the citric acid cycle (TCA cycle).

In the citric acid (TCA) cycle the pyruvic acid product of glycolysis is converted to carbon dioxide forming some additional ATP.

61. Describe the electron transport chain.

In the electron transport chain hydrogen atoms formed during glycolysis and the TCA cycle are converted to water releasing energy which is used to add phosphate to ADP to form ATP.

The roles and the small intestine

In the small intestine digestion continues and absorption begins. Enzymes from the pancreatic juices enter the small intestine to continue digestion of starches and proteins, and begin digestion of fat. Peptides, which result from the first step in protein digestion, are digested by peptidases to amino acids. Maltose, which results from the first step in starch digestion, is digested by maltase to glucose. Other disaccharides, each of which is acted upon by a specific enzyme, are digested in the small intestine. See Figure 3.28 below for an overview of the digestive processes in the small intestine. Pancreatic juice includes three enzymes: amylase, trypsin, and lipase, which digest starch, proteins, and fats respectively. Bicarbonate, which is also contained in the pancreatic juice, neutralizes the acidic chyme. Chyme stimulates the small intestine to secrete two hormones: cholecystokinin and secretin. Cholecystokinin causes the gallbladder to secrete bile. Secretin causes the liver to secrete bile. Both hormones cause the pancreas to secrete its juices. Bile breaks down fat into smaller pieces and is needed for the absorption of fats. See Figure 3.28 below for an overview of the digestive processes carried out by the pancreas and gallbladder. When food reaches the end of the small intestine, digestion is complete and most absorption of nutrients into the bloodstream has happened. Bacteria, undigested food, and water remain by the time chyme reaches the end of the small intestine.

What part of digestion happens in the stomach?

In the stomach mechanical digestion continues as muscles churn and mix the food, breaking it into smaller pieces in the form of chime which is moved to the small intestine by peristalsis. Chemical digestion continues as pepsin digests proteins.

What is the source of the stomach enzyme pepsin and what is its function?

Inactive pepsinogen is secreted by the chief cells and converted into the active enzyme pepsin by HCl. Pepsin then begins the breakdown of proteins.

Hormones that cause Satiety

Leptin, Serotonin, Cholecystokinin (CCK)There are two key hormones in this process: Leptin and Ghrelin. Leptin is produced by adipose tissue (fat cells). The role of leptin is to alert the brain to turn off the hunger center in the brain and activate the satiety center when consuming a meal. Ghrelin is produced by the stomach and has the opposite role. Ghrelin travels to the brain and stimulates the hunger center as it deactivates the satiety center. Ideally, leptin and ghrelin work together to balance hunger and satiety.

Non Glucose Metabolism

Lipid metabolism occurs in the liver by a process known as beta-oxidation. Lipids are used by the body primarily during low energy activities. Lipid metabolism yields twice the amount of ATP than when a carbohydrate or protein is broken down. A fat molecule is broken down in the mitochondria to form acetic acid. Acetic acid is further catabolized by the TCA cycle to carbon dioxide forming some ATP molecules. The electron transport chain (as described above) then converts hydrogen atoms to water. This process releases energy which is used to add phosphate to ADP to form ATP in the same way.

Lipids

Lipids can be divided into three categories: saturated fat, unsaturated fat, and cholesterol. Meat and dairy products such as butter contain saturated fats, while nuts and vegetable oils such as olive oil contain unsaturated fats. Cholesterol is made by our bodies but can also be ingested by eating animal products such as meat (beef), cheese, and eggs (yolk). Proteins (which are made up of amino acids) can be categorized as complete proteins or incomplete proteins. Examples of complete proteins are meat, poultry, eggs, milk or dairy products, and soybeans. Incomplete proteins are food such as grains, legumes, seeds, and nuts. There are nine amino acids called essential amino acids required in an adult human diet (with ten required in an infant's diet). They are called essential amino acids because the body is unable to make these amino acids. Complete proteins contain all nine essential amino acids but incomplete proteins do not.

Minerals

Minerals are inorganic compounds that, like vitamins, are not used as fuel in metabolic reactions but are combined with other nutrients to form necessary body substances. Humans need the following seven minerals in their diets: calcium, chloride, magnesium, phosphorus, potassium, sodium, and sulfur. There are twelve other minerals needed in trace amounts. Foods such as milk, some meats, legumes and vegetables are the best sources of minerals. No one food contains all the minerals that are needed by the human body. Therefore, it is necessary to eat a balanced diet with a variety of foods.

The Roles and the mouth

Now let's look at these four roles throughout the gastrointestinal tract, starting with the mouth. As stated earlier, ingestion occurs when food enters the mouth. The mouth is also where digestion begins, both mechanical and chemical. Chemically, saliva is released by the salivary glands (note the locations of the parotid, sublingual, and submandibular glands in Figure 3.27 below). The enzyme salivary amylase contained in the saliva begins the chemical breakdown of starches. Mechanically, the food is broken down into smaller physical pieces by both the teeth and the tongue. The food is then called a bolus. The bolus is swallowed into the pharynx. Peristalsis (rhythmic muscular contractions) moves the food along the esophagus to the stomach. No ingestion, digestion, absorption, or defecation takes place in the pharynx or esophagus. These structures only transport the food from the mouth to the stomach by peristalsis.

The roles and the stomach

Once in the stomach, mechanical and chemical breakdown of food continues with the secretion of digestive juices. Very little to no nutrient absorption occurs in the stomach. The stomach's main function is for mechanical and chemical breakdown of food. The hormone gastrin stimulates the stomach to secrete gastric juices, which contains: pepsinogens, mucus, and hydrochloric acid. The parietal cells (located in the wall of the stomach body) secrete hydrochloric acid, generating a pH of 1.3-3.5. This very acidic pH kills many of the bacteria ingested along with food. In addition, the low pH stops the activity of salivary amylase. The secretion of hydrochloric acid is essential in the activation of pepsin. Pepsinogen (a pre-enzyme) is secreted by the chief cells in the stomach. Hydrochloric acid converts the inactive pepsinogen (secreted by the chief cells) into the active enzyme pepsin which begins the breakdown of proteins. Mucus protects the stomach lining from the hydrochloric acid and pepsinogens. The muscularis externa churns and mixes the food, breaking it into smaller pieces. The smaller pieces allow for more surface area of the food to be exposed to the enzymes in the stomach to form chyme. Peristalsis moves the food toward the small intestine where the pyloric sphincter controls the slow release of chyme into the duodenum. See Figure 3.28 below for an overview of the digestive processes in the stomach.

Protein Metabolism

Protein metabolism only takes place when carbohydrates and fats are unavailable to the body. Proteins are the building blocks for the cells and are saved as a last resort. When proteins must be used to form ATP, the amino group is removed from the protein to form ammonia. The remainder of the protein molecule enters the TCA cycle, forming some carbon dioxide and ATP. Hydrogen atoms formed during the TCA cycle are converted to water. This releases energy which is used to add phosphate to ADP to form ATP. Ammonia is harmful to the body and must be converted to a different form. Ammonia is combined with carbon dioxide in the liver to make urea. Urea is then excreted from the body in urine.

The pH of stomach is 1.3 - 3.5. What is this due to and what is its source and function?

The 1.3-3.5 pH of stomach contents is due to hydrochloric acid secreted by the parietal cells. The high acidity level kills bacteria and converts the inactive pepsinogen into the active enzyme pepsin.

Anal Canal

The anal canal has an involuntary smooth muscle internal sphincter and a voluntary skeletal muscle external sphincter (Figure 3.23). These sphincters open and close the anus during defecation to discharge solid waste material called feces.

The Mouth

The beginning of the gastrointestinal tract is the mouth (see Figure 3.5). The mouth is made up of the following structures. The lips form the opening; the tongue forms the floor; the cheeks form the walls; the hard and soft palates form the roof, and the uvula forms the posterior border. The uvula is a process that hangs inferiorly from its attachment to the soft palate. The tongue is a muscle. Like all muscles, it attaches to other structures so it can contract for movement. The tongue's attachments are the hyoid bone, the mandible, and the lingual frenulum (see Figure 3.5 and Figure 3.10). The lingual frenulum is the vertical mucous membrane by which the body of the tongue is attached to the floor of the mouth.

The four main roles of the digestive system

The digestive system has four main roles that it performs: ingestion, digestion, absorption, and defecation. Ingestion is when food enters the mouth. Digestion or food breakdown occurs when food is moved, mixed, and exposed to enzymes along the gastrointestinal tract. Digestion can be divided into two parts: mechanical digestion and chemical digestion. Mechanical digestion occurs when food is physically broken down into smaller pieces. This occurs when the teeth chew food, when the stomach churns and mixes food, and as food is moved and squeezed along the gastrointestinal tract. Chemical digestion is the chemical breakdown of food by enzymes into smaller nutrients. Absorption is the process of moving digested food into the bloodstream. Defecation is the excretion of indigestible food from the anus.

How is the energy released from the breakdown of glucose used by the body?

The energy is released during the breakdown of glucose is used by the body to add a phosphate to an adenosine diphosphate (ADP) molecule and make adenosine triphosphate (ATP).

Esophagus

The esophagus extends from the pharynx to the stomach, travelling through an opening in the diaphragm on its way to the stomach (see Figure 3.13). It is a muscular tube, containing four layers of tissue of the same types found in the stomach, small intestine, and large intestine. The first or innermost layer is the mucosa, which secretes mucus into the interior the gastrointestinal tract. The second layer is the submucosa; this layer contains blood and lymph vessels, lymph nodes, nerves, and mucous glands. The third layer is the muscularis externa which is made of two layers of muscle: the outer longitudinal fibers and inner circumferential fibers. Finally, the fourth and outermost layer is the serosa which is made up of cells that make serous fluid.

Small Intestine

The human small intestine is a coiled tube approximately 20 feet long that runs from the pyloric sphincter to the ileocecal valve (controls the entrance to the large intestine). The small intestine is the longest part of the gastrointestinal tract. It has three sections. The first section is the duodenum, which is the shortest section of the small intestine (see Figure 3.18). The small intestine produces enzymes itself, but more importantly the duodenum is where enzymes from the pancreas and bile from the liver enter the small intestine. The jejunum (see Figure 3.19) is the second section and the ileum (see Figure 3.20) is the third section. The ileum is slightly longer than the jejunum.

Hypothalamus

The hypothalamus (see Figure 3.29) is a region of the brain that plays a role in hunger, satiety, and the feeling of "being full". The amount of nutrients in the blood begin to decline if a person has not eaten in a long period of time. This drop stimulates the feeding center in the hypothalamus and signals the body to feel "hungry" and desire to eat. When the nutrient level of the blood begins to rise, the satiety center is activated and the desire to continue to eat declines. The hypothalamus is constantly processing signals from a variety of nerves throughout the body which influence food intake. If a portion of the hypothalamus is damaged or destroyed, from certain chemicals, trauma, surgery or cancers, the regulation of hunger and satiety can be lost. In such an instance, the body no longer receives signals to stop eating and weight gain is almost certain.

Large Intestines Major Function

The large intestine's major function is to propel wastes from the body (see Figure 3.24). The large intestine also reabsorbs some water and electrolytes. About 1.5 liters of water enter the digestive tract daily because of eating and drinking and an additional 8.5 liters enter the digestive tract each day carrying the various substances secreted by the digestive glands. About 95% of this water is absorbed by the small intestine, and much of the remaining portion is absorbed into cells in the wall of the colon. When materials pass through the colon too quickly, excess water is not reabsorbed, and diarrhea will result. Extreme or prolonged diarrhea can lead to serious dehydration and electrolyte loss. Vitamin K and certain B vitamins, all produced by intestinal bacteria, are also absorbed by the cells in the wall of the colon.

Rectum

The last 20 cm of the large intestine is the rectum, which terminates in an external opening, the anus. Digestive wastes (feces) eventually leave the body through the rectum and anus. Feces are about 75% water and 25% solid matter. Almost one-third of this solid matter is made up of intestinal bacteria. The remainder is undigested plant material, fats, waste products (such as bile pigments), inorganic material, mucus, and dead cells from the intestinal lining.

The Liver and its functions

The liver has numerous functions, including three that are important in digestion (see Figure 3.26). First, it produces bile. Bile is stored in the gallbladder before entering the small intestine, where it emulsifies fats. Bile is sent to the duodenum by way of the common bile duct (see Figure 3.26). Bile looks green because it contains pigments that are products of hemoglobin breakdown. Bile also contains bile salts, which are emulsifying agents that break up fat into fat droplets so that they mix with water. Emulsified fat is more easily acted on by enzymes. Second, the liver stores glucose as glycogen. The liver breaks down glycogen to glucose between meals to maintain a constant glucose concentration (homeostasis) in the blood. Third, the liver produces urea from amino groups that are removed from excess amino acids.

Stomach

The next step in the digestive process continues the bolus into the stomach (see Figure 3.14). Muscles contract to move the bolus through the pharynx to the esophagus and into the stomach by peristalsis. The stomach is a backward C shaped organ that is located along the left side of the abdomen below the diaphragm, as shown in Figure 3.14. Food enters the stomach from the esophagus via the cardiac sphincter, also known as the lower esophageal sphincter (LES) or the gastroesophageal sphincter. Food exits the stomach via the pyloric sphincter into the small intestine. The stomach has two curvatures: the concave lesser curvature which extends inward on the shorter side of the stomach and the convex greater curvature which extends outward on the longer side of the stomach. The stomach is covered by peritoneum (a serous membrane), called the lesser omentum and the greater omentum. The lesser omentum attaches the lesser curvature of the stomach to the liver. The greater omentum (Figure 3.15) hangs down almost like an apron. It attaches the greater curvature of the stomach to the posterior wall of the abdomen, covering the transverse colon and the anterior surface of the small intestines.

The Pharynx

The pharynx connects the mouth to the esophagus and is also known as the throat (see Figure 3.12). It is made up of three parts: the nasopharynx (where air enters the nose or nasal cavity), the oropharynx, and the laryngopharynx. The pharynx has two types of skeletal muscles: a circumferential outer layer and an inner longitudinal layer. These two layers of muscles contract alternatively causing peristalsis which squeezes food into the esophagus. Air enters the nose, continues through the nasopharynx, oropharynx and the laryngopharynx but then enters the trachea on its way to the lungs. Food and liquid enter the mouth, continue through the oropharynx and the laryngopharynx but then continue into the esophagus. Food is usually prevented from entering the larynx by the epiglottis, a flap of elastic cartilage which covers the larynx when swallowing food or liquid. However, if food or liquid enters the larynx, coughing clears the larynx. Coughing is the body's way of preventing aspiration of food or liquid into the lungs. Aspiration of food or liquid into the lungs can cause aspiration pneumonia.

58. Describe cellular respiration.

The process in which cells use to make energy. Glucose combines with oxygen to form carbon dioxide, water, and ATP

Leptin and Ghrelin

The role of leptin is to alert the brain to turn off the hunger center in the brain and activate the satiety center when consuming a meal. Ghrelin is produced by the stomach and has the opposite role. Ghrelin travels to the brain and stimulates the hunger center as it deactivates the satiety center. Ideally, leptin and ghrelin work together to balance hunger and satiety

The Small Intestine is specialized in apsorption

The small intestine is specialized for absorption. Molecules are absorbed by the huge number of villi (singular, villus) that line the intestinal wall (see Figure 3.21). Each villus contains blood vessels and a lymphatic vessel called a lacteal. Sugars and amino acids enter villi cells and then are absorbed through the capillary beds present inside the villi. The nutrients are carried into the hepatic portal circulation (of the liver) before entering general blood circulation. Molecules that are too large to enter blood capillaries, such as glycerol and fatty acids, are transported into lacteals. For example, fatty acids enter villi cells and are reassembled into fat molecules (triglycerides). The newly packaged fat molecule moves into the lacteals, eventually connecting to general blood circulation at the thoracic duct. Absorption continues along the small intestine until almost all products of digestion have been complete. The epithelial cells of the villi produce intestinal enzymes, which remain attached to the plasma membrane of microvilli. These enzymes complete the digestion of peptides and sugars.

Stomach Contents

The stomach contents are called chyme, which is a thick, soupy consistency. At the base of the stomach is a narrow opening controlled by a valve called the pyloric sphincter (Figure 3.16). Relaxation of the sphincter causes a small quantity of chyme to pass through the opening into the duodenum, the first part of the small intestine. When chyme enters the duodenum, it sets off a reflex that causes the muscles of the sphincter to contract and close the opening temporarily. Then the sphincter relaxes again and allows more chyme to enter. The slow process allows chyme to enter the small intestine for thorough digestion.

The Stomach Has two Purposes

The stomach has two purposes. First it is a temporary storage area for food. Usually, the stomach stores up to two liters of partially digested food. The stomach wall contains rugae. Rugae are folds within the stomach wall which allows the stomach to expand when it is full (see Figure 3.17). This storage system enables humans to periodically eat relatively large meals and spend the rest of their time at other activities. But the stomach is much more than a mere storage organ. The muscular walls of the stomach contract vigorously and mix food with juices that are secreted whenever food enters the stomach, allowing for the second purpose: mechanical food breakdown. The stomach has a unique third layer in the muscularis externa, an oblique layer (see Figure 3.16 and Figure 3.17). The extra muscular layer allows the stomach to mix food more efficiently with the gastric juices. Breaking food down into smaller pieces is necessary for digestion and later absorption of nutrients.

where is the stomach relative to the abdomen and diaphragm

The stomach is backward C shaped and along the left side of the abdomen and below the diaphragm

Four Parts of the Stomach

The stomach is made up of four parts (see Figure 3.16). The first is the cardia where the food enters from the esophagus. The second part is the fundus which is dome-shaped top of the stomach which is just under the diaphragm. The third part is the body which is the wider midportion located between the fundus and fourth part which is the funnel-shaped pyloric region.

The Tongue

The tongue (see Figure 3.10) grips the food and constantly repositions it between the teeth, forming it into a compact mass called a bolus. The tongue pushes the bolus back to the pharynx to begin the process of swallowing. It contains papillae, which are raised bumps on the tongue (see Figure 3.11). There are three types of papillae which help to grip food on the tongue: circumvallate, fungiform and filiform. Circumvallate and fungiform papillae both contain taste buds. Taste buds are chemical receptors that are stimulated by the chemical composition of food. They are found primarily on the tongue as well as on the surface of the mouth and the wall of the pharynx.

The Teeth

There are a set of thirty-two teeth in the human mouth (see Figure 3.6 and Figure 3.7). The upper arch of teeth is located within the maxilla. The movable, lower mandible contains the lower arch of teeth. There are four different types of teeth to accommodate both a vegetable and meat diet. One-half of each jaw has two chisel-shaped incisors for biting; one pointed canine for tearing; two increasingly flat pre-molars for grinding; and three flat molars for crushing. Each tooth is made of three main parts: the crown, the neck and the root (see Figure 3.8). The root is the largest portion of each tooth and is located within the alveolar process of the jaw bone (see Figure 3.9). The neck is located within the upper portion of the gums (gingiva). The crown is the visible portion of the tooth and lies above the gum line. At the center of each tooth is a pulp cavity which contains nerves and blood vessels. Nervous innervation allows for sensations of heat, cold and pain. Dentin is a bone-like substance composing most of each tooth. Enamel is located on the surface of the crown. Enamel is very hard connective tissue which protects the rest of the tooth.

The Stomach and Satiety

There are other factors that determine what is eaten and the quantity of food consumed. The GI tract itself contributes to the feeling of satiety. The stomach plays a major role in satiety and weight regulation. As the stomach expands from food intake and the nutrients consumed are absorbed, receptors within the stomach wall send signals to the brain and the desire to eat declines. This is an example of the nerve network and the GI tract communicating with the brain that the body no longer desires food.

Accessory Organs

Three important accessory glands, the liver, the pancreas, and the gallbladder (see Figure 3.26), send secretions to the duodenum (the first section of the small intestine). The pancreas lies deep in the abdominal wall. The pancreas sends pancreatic juice into the duodenum by way of the pancreatic duct. Pancreatic juice contains sodium bicarbonate, which neutralizes the chyme and makes the pH of the small intestine slightly basic. Pancreatic juice also contains digestive enzymes that act on every major component of food. Pancreatic amylase digests starch to maltose; trypsin and other enzymes digest protein to peptides; and lipase digests fat droplets to glycerol and fatty acids. The enzymes travel by way of the pancreatic duct to the hepatopancreatic ampulla into the duodenum of the small intestine (see Figure 3.26).

The Roles and the Large Intestine

What remains of the food enters the large intestine by the ileocecal valve. In the large intestine, the undigested food is compacted in order to be prepared for defecation. Water is absorbed to form feces. Vitamins and ions are absorbed as well. Bacteria metabolize the remaining nutrients, producing vitamins K and B in the process. Feces travel to the rectum where it is defecated via the anus.

Two Main Groups of the Digestive System

alimentary canal (Gastrointestinal tract GI) and accessory digestive organs

The Electron Transport System

also referred to as the electron transport chain, is a continuation of cellular respiration and can proceed either aerobically (with oxygen) or anaerobically (without oxygen). However, anaerobic respiration is less efficient and yields fewer ATP molecules than aerobic respiration. As electrons are transferred from NADH/FADH2 to terminal electron acceptors (O2; aerobic respiration), energy is released and captured by electron acceptor proteins located in the inner membrane of mitochondria. Electrons are then passed down a chain of electron acceptors (thus the name) causing protons (H+; positive charge) to be pumped out of the membrane. This causes a strong differential across the mitochondrial membrane, which forms the proton motive force. The proton motive force drives H+ back through the ATP synthase complex, also located in the membrane, resulting in the production of up to 34 molecules of ATP.

Glycolysis

begins with the breakdown of a single molecule of glucose into two pyruvate molecules. See Figure 3.31, below, for a basic diagram of the process of glycolysis:

Almentary Canal (GI Tract)

is a continuous muscular digestive tube beginning in the oral cavity. It is open at both ends, winding its way through the body cavity. It is composed of the mouth (oral cavity), pharynx (throat), esophagus, stomach, small intestine, large intestine, and anus.

Metabolism

is a set of chemical reactions that are essential for the body to sustain life.

Cellular Respiration

is an aerobic process (oxygen must be present) used by cells to produce energy. Respiration takes place inside and around the double membrane-enclosed organelle known as the mitochondria. Fermentation, on the other hand, takes place in the absence of oxygen. Since most of the potential energy from glucose is still locked in the form of pyruvate following the initial stage of glycolysis, respiration is the process of unlocking that energy. The central pathway of respiration is called the tricarboxylic acid (TCA) cycle, also known as the Kreb's Cycle or the citric acid cycle. In the TCA cycle, the pyruvic acid (product of glycolysis) is converted to carbon dioxide forming some additional ATP. At its conclusion, the TCA cycle produces 2 ATP in total (one for each pyruvate processed) and an abundance of electron carriers, known as NADH and FADH2. The production of these electron carriers is the primary function of the TCA cycle, as the transfer of these electrons will fuel the generation of ATP via the electron transport system.

Catabolism

is the set of reactions in which larger molecules are broken down to be used as energy sources for the body.

Anabolism

is the set of reactions in which smaller molecules are combined to make larger, more complex molecules.

Accessory Digestive Organs

teeth, tongue, gall bladder, salivary glands, liver and pancreas

Mastication

the process of chewing NEXT The food is mixed and moistened with saliva from the salivary glands. The salivary glands contain enzymes (amylase, specifically) that begin the process of chemical digestion by breaking down starches.