chapter 17

1. Chapter 17 � Digestive System

3. Digestive System The alimentary canal is a muscular tube made up of 4 layers: The Mucosa is formed from primarily of epithelium that is invested with goblet cells. This inner lining of this layer has many tiny folds on it in order to increase the overall absorptive surface area. This is the layer of the tract that actually absorbs and secretes (absorbs food, secretes digestive enzymes). The Submucosa in made up primarily of loose connective tissue, blood vessels, glands, and nerves.

4. Digestive System Muscular Layer: this layer is made up of 2 layers of smooth muscle, and provides movement for the tube. The inner layer of muscle is circular while the outer layer is longitudinal. The Serosa is the outer most layer of the tube and is primarily composed of serous epithelium and some connective tissue. The serous layer secretes serous fluid.

5. Digestive System Segmentation is a movement of the tube which helps to mix food with the digestive secretions. Peristalsis is a contraction of the tube which causes food to move forward thru the tube. It begins when food expands the tube, and as the contractions occur the segment ahead of the tube relaxes to allow advancement of the food bolus. The tube has both sympathetic and parasympathetic innervation. An increase in parasympathetic nerve flow is usually responsible for increasing peristalsis and digestive secretions.

6. Digestive System Organs of the Digestive System The Mouth is the first item in line in the alimentary canal. Mechanical digestion begins here with mastication (chewing). The Tongue is an accessory organ of digestion. It functions to position food in the mouth. It is held to the floor of the mouth by the lingual frenulum. The top of it is covered with structures called papillae which help to handle food and provide the sense of taste. The tongue also helps in the formation of speech.

7. Digestive System The Palate forms the roof of the oral cavity. It consists of a hard (bony) anterior portion and a soft posterior portion. A cone shaped projection called the uvula hangs from the posterior most aspect of the palate. The Teeth are the hardest structures in the body. They aid digestion by way of mastication. There are two sets of teeth in humans, deciduous and permanent. Children will typically have 20 teeth that are gradually replaced by the adult teeth. Adults typically have 32 teeth.

9. Digestive System Teeth are arranged in this order: Central Incisor Lateral Incisor Cuspid Bicuspid (Pre-molar) Molar Each type of tooth is designed to do a specific job. Incisors snip food, Cuspids and Bicuspids grip and tear, and Molars grind. Teeth break food into smaller pieces. This increases the overall surface area of the food allowing the digestive enzymes to act more efficiently on the food for the process of chemical digestion.

10. Digestive System Each tooth is made of a few different layers. Enamel covers the top of the tooth . Below that is the dentin. Each tooth has a cavity called the pulp cavity or root canal. This cavity is home to a connective tissue pulp as well as blood vessels and nerves. Below the Gingiva (gums) the tooth is surrounded by a substance called cementum and held in place by a Peridontal Ligament.

11. Digestive System The Salivary Glands secrete the fluid Saliva which helps to moisten food and begin the chemical break down of some nutrients. Saliva helps to cleanse the mouth and teeth, and helps to enhance taste . The pH of saliva is between 6.5 and 7.5. Saliva also contains a digestive enzyme called Salivary Amylase, which is responsible for beginning the break down of starchy foods. The major salivary glands are the Submandibular, the Sublingual, and the Parotid.

12. Digestive System The Pharynx is divided into 3 separate regions: Nasopharynx Oropharynx Laryngopharynx The pharynx is connects the mouth to the Esophagus. Neither of these organs digest food, but are important passage ways to the Stomach.

13. Digestive System The swallowing mechanism (Deglutition) is divided into a voluntary phase and an involuntary phase. The first phase is voluntary and occurs when food is chewed and mixed with saliva. The involuntary phase occurs when the tongue pushes food to the back of the pharynx and the swallowing reflex is triggered. During this time the Epiglottis closes the trachea and allows food to enter the esophagus. The swallowing reflex momentarily inhibits breathing. When food enters the esophagus it is moved down the tube toward the stomach by the action of Peristalsis.

14. Digestive System The Esophagus is a straight, collapsible tube that leads from the pharynx thru the thoracic cavity to the stomach. It penetrates the diaphragm thru an opening called the Esophageal Hiatus. The esophagus terminates at the Cardiac Sphincter just above the stomach.

15. Digestive System The Stomach is a curved pouch-like organ that is located in the upper left quadrant of the abdomen. The inner lining is marked by thick folds called rugae. The stomach secretes Gastric Juice that has a very low pH and contains an enzyme called Pepsinogen that initiates the digestion of proteins. Where most of the organs of digestion have two layers of smooth muscle to help with peristalsis, the stomach has three layers. This helps the stomach to churn and mix food in the process of mechanical digestion.

16. Digestive System

17. Digestive System There are three types of secretory cells associated with gastric glands in the stomach. Mucous cells secrete mucous Chief cells secrete digestive enzymes Parietal cells produce hydrochloric acid Pepsinogen is an inactive form of the enzyme pepsin. Pepsinogen is activated into Pepsin by HCl secreted by the parietal cells. It begins the break down of most dietary protein. Mucous stops the action of the both HCl and Pepsin and thus keeps the stomach from auto-digesting itself.

18. Digestive System The stomach is divided into 4 regions. The cardiac region is located lust inside the cardiac sphincter. The fundus is the superior most portion located above the cardiac sphincter. The body makes up the bulk of the organ in the middle. The pyloric region is the inferior most area, just above the Pyloric Sphincter. The pyloric sphincter retains food in the stomach until it is released into the Small Intestines.

19. Digestive System

20. Digestive System Intrinsic Factor is another component of gastric juice. It is critical for the absorption of Vitamin B12. Please note that very little food is actually absorbed in the stomach. The stomach will absorb some water and many electrolytes, but most other nutrients are absorbed else where. Vomiting is a reflex where the gears of the digestive system work in reverse motion. This reflex ultimately results in the emptying of the contents of the stomach.

21. Digestive System The Pancreas is located primarily in the upper left quadrant of the abdomen, and has both endocrine and exocrine functions. In addition to insulin and glucagon secretion, it secretes many digestive enzymes. These secretions join bile secreted by the liver and enter the small intestines thru the Ampulla of Vater (Hepatopancreatic Ampulla). The pancreas has a head, a body and a tail.

22. Digestive System

23. Digestive System Pancreatic Juice contains enzymes that digest carbs, fats, proteins and nucleic acids. Pancreatic Amylase finishes the break down of carbohydrates into disaccharides. Pancreatic Lipase breaks down triglyceride molecules into fatty acids and monoglycerides. The Proteases are Trypsin, Chymotrypsin, and Carboxypeptidase. These enzymes reduce proteins to amino acid chains. Nucleases are enzymes that break down nucleic acids into nucleotides. Pancreatic juice also contains a high bicarbonate concentration. This gives it an alkaline pH which is favorable for the actions of the above enzymes, as well as neutralizing most of the acid in from the stomach.

24. Digestive System The Liver is the largest of all internal organs. It is located mostly in the upper right quadrant of the abdomen. The liver is enclosed by a fibrous capsule and divided into a right and left lobe. Two minor lobes also exist. They are the caudate lobe and the quadrate lobe. Blood digestive tract, carried in the hepatic portal vein brings newly absorbed nutrients into the sinusoids of the liver. Here blood is cleansed of impurities and microbes by Kupffer Cells (phagocytes).

25. Digestive System

26. Digestive System The liver performs several important functions. It plays a key role in carbohydrate metabolism by helping to maintain blood glucose levels. It aids in lipid and protein metabolism. It forms urea by deaminating amino acids. It synthesizes clotting factors for the blood and helps the spleen in breaking down damaged and old RBC�s. It also synthesizes Bile.

27. Digestive System Bile is a yellow-green liquid that is continuously secreted by the liver. It is then stored in the Gall Bladder until needed. The Gall Bladder is found on the inferior side of the liver and stores bile until it is called for by the body. It concentrates the bile by dehydration and keeps it in this form until release. Under certain conditions this liquid can form a crystal and an accumulation of these crystals is referred to as a gall stone. This process is referred to as Choleolithiasis. Choleolithiasis can lead to Choleocystitis, which can lead to a medical emergency.

28. Digestive System Bile salts aid digestive enzymes by the emulsification of fats. In this process fat droplets are sequestered from each other, increasing the overall surface area and maximizing enzymatic action of the lipases.

29. Digestive System The Small Intestine is a tubular organ that extends from the pyloric sphincter to the Large Intestines. This organ is divided into three parts: The Duodenum is the proximal most portion and receives secretions from both the pancreas and the liver. The Jejunum is usually larger in diameter than the ileum and more vascular The Ileum is the distal portion of this organ and is usually less active than the jejunum. The Mesentery are a double fold of peritoneum that suspend most of the small intestine within the abdominal cavity. The mesentery supports blood vessels and nerves that supply the intestinal wall.

30. Digestive System The interior lining of the intestinal wall are covered by numerous Villi. These structures project into the lumen of the intestine and greatly absorb the absorptive capacity of this organ. Each villus is made up of a layer of epithelium and a core of connective tissue containing blood capillaries, nerve fibers, and a lymphatic Lacteal. At their free surface the epithelial cells have many fine extensions called Microvilli that form what is known as the �Brush Boarder� that further enhances absorption. The capillaries and lacteals carry absorbed nutrients into general circulation.

31. Digestive System The small intestines also have an exocrine function. Mucous is secreted in varying amounts. Also, the following enzymes are secreted: Peptidases which break down protein. Sucrase, maltase, and lactase which split disaccharides into monosaccharides. Intestinal lipase which breaks down fats.

32. Digestive System The small intestine is the main organ of absorption. It is incredibly efficient in this process. Carbs, once broken down into monosaccharides, are absorbed into a villus and enter the blood capillaries. Proteins are broken down into smaller amino acid chains and absorbed by a villus into the blood capillaries. Most fats are absorbed by lymphatic lacteals and ultimately make their way to the cysterni chyli, then on to general circulation. Electrolytes are moved by active transport, and water is generally moved by osmosis.

33. Digestive System Generally, very little in the way of actual nutrients reaches the end of the digestive tract. If the small intestine begins to push chyme thru too fast, nutrients will not be as well absorbed. This includes water, and the result is a watery stool called Diarrhea. Prolonged diarrhea can cause dehydration and electrolyte imbalance. The small intestine ends where it joins the large intestine at the Ileocecal Valve (another sphincter muscle).

34. Digestive System The Large Intestine is so named because of it�s diameter. This organ absorbs water, electrolytes, and remnants of the digestive secretions remaining in the left over waste product that was once food. It also forms and stores Feces. The colon has a few parts: Ascending portion The Cecum is the pouch like end of the ascending portion The Veriform Appendix is the terminal portion of the cecum Transverse portion Descending portion The Sigmoid Colon is the final portion of the descending portion

35. Digestive System The Anal Canal is made up two sets of sphincter muscles. The internal anal sphincter is made of smooth muscle and is under voluntary control. The external anal sphincter is made up of skeletal muscle and is under voluntary control. The large intestine has little or no digestive function, other than to eliminate an unneeded waste product. It�s epithelial lining has many goblet cells in it and thus mucus is the only significant secretion of this organ.

36. Digestive System Many bacteria inhibit the large intestine and make up the Intestinal Flora. Many of these bacteria synthesize vitamins such as K and B12 which are then absorbed by the mucosa. Movement thru the large intestine are caused by peristalsis, but these movements are usually less prevalent here than in the small intestine. Feces are composed of materials that were not digested (such as cellulose), or not absorbed, some water and electrolytes, mucus, and bacteria.