Chapter 14 The Digestive System
Children have a special fascination with the workings of the digestive system: They relish crunching a potato chip, delight in making "mustaches" with milk, and giggle when their stomach "growls." As adults, we know that a healthy digestive system is essential for good health, because it converts food into the raw materials that build and fuel our body's cells. Specifically, the digestive system takes in food (ingests it), breaks it down physically and chemically into nutrient molecules (digests it), and absorbs the nutrients into the bloodstream. Then it rids the body of the indigestible remains (defecates).
Part I: Anatomy and Physiology of the Digestive System
Anatomy of the Digestive System
· Name the organs of the alimentary canal and accessory digestive organs and identify each on an appropriate diagram or model.
o See figure 14.1 on page 433
· Identify the overall function of the digestive system as digestion and absorption of foodstuffs, and describe the general activities of each digestive system organ.
o Function of the digestive system is to take in food (ingest it), break it down physically and chemically into nutrient molecules (digest it), to absorb the nutrients into the bloodstream, and to rid the body of the indigestible remains (defecate)
o The alimentary canal (gastrointestinal (GI) tract) is a continuous, coiled, hollow, muscular tube that is open at both ends made up of many parts
o The walls of the GI tract from the esophagus to the large intestine are made up of the same four basic tissue layers, or tunics
§ The mucosa – innermost layer – a moist membrane that lines the cavity, or lumen, of the organ – surface epithelium, plus a small amount of connective tissue (lamina propria) and smooth muscle layer – beyond the esophagus, which has friction-resisting stratified squamous epithelium, the epithelium is mostly simple columnar
§ The submucosa – just beneath the mucosa – soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic vessels
§ The muscularis externa – muscle layers typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells
§ The serosa – outermost layer of the wall – single layer of flat serous fluid-producing cells, the visceral peritoneum (continuous with the slick, slippery parietal peritoneum, which lines the abdominopelvic cavity by way of membrane extension, the mesentery)
o General activities of each digestive system organs
§ Mouth – oral cavity – food is mixed with saliva and masticated (chewed) – chemical and mechanical digestion begins
§ Pharynx – common pathway for food and air and continuous with the esophagus – contractions (peristalsis) propels food down the tube
§ Esophagus – gullet – runs from the pharynx through the diaphragm to the stomach that conducts food from to the stomach via peristalsis
§ Stomach – acts as a temporary storage tank for food as well as a site for food breakdown via chemical (gastric juices) and mechanical processes to produce a heavy cream-like substance called chyme
§ Cardioesophageal sphincter – controls food entering the stomach from the esophagus
§ Fundus – expanded part
§ Body – midportion
§ Pylorus – terminal part
§ Pyloric sphincter – controls the food entering the small intestine
§ Small intestine – the body’s major digestive organ where chemical digestion completes the breakdown of the food and nearly all food absorption occurs via the microvilli
§ Three subdivisions – duodenum, junum, ileum
§ Ends at the ileocecal valve to join the large intestine
§ Large intestine – absorption of water dries the indigestible food residue so it can finally be eliminated from the body as feces through the anus
§ Five subdivisions – cecum, appendix, colon (ascending, transverse, descending), sigmoid rectum, anal canal, which ends at the anus
§ The anal canal has an external voluntary sphincter made of skeletal muscle and an internal involuntary sphincter formed by smooth muscle – both closed except during defecation
§ Goblet cells produce mucus that acts as a lubricant to ease the passage of feces to the end of the digestive tract
§ Bacteria in the large intestine make some vitamins (vitamin K and some B), which are absorbed along with some ions and most of the remaining water
§ Pancreas – produces enzymes that break down all categories of digestible foods and are secreted into the duodenum in an alkaline fluid, which neutralizes the acidic chyme coming in from the stomach
§ Endocrine function is to produce the hormones insulin and glucagon
§ Liver – produces bile which leaves the liver through the common hepatic duct and enters the duodenum through the bile duct
§ Bile is a yellow-to-green, watery solution containing bile salts, bile pigments (bilirubin, a breakdown product of hemoglobin), cholesterol, phospholipids, and electrolytes – bile salts emulsify fats by physically breaking large fat globules into smaller ones providing more surface area for the fat-digesting enzymes to work
§ Gallbladder – stores bile when digestion is not occurring – bile backs up the cystic duct and enters the gallbladder where is it concentrated by the removal of water
· Describe the composition and function(s) of saliva.
o A mixture of mucus and serous fluids
§ Mucus moistens and helps to bind the food together into a mass called a bolus, which makes chewing and swallowing easier
§ The clear serous portion contains salivary amylase, which begins the process of starch digestion in the mouth
§ Lysozyme and antibodies (IgA) inhibit bacteria
o Saliva also functions to dissolve food chemicals so they can be tasted
· Name the deciduous and permanent teeth and describe the basic anatomy of a tooth.
o See figure 14.9 on page 442
o The function of teeth is to masticate, or chew, the food – incisors and canines cut and tear the food while molars grind the food
o Deciduous teeth – baby teeth – milk teeth – erupt around six months and a complete set of 20 teeth is present by age two
o Permanent teeth – second set of teeth set deeper under the deciduous teeth – as they enlarge and develop, the roots of the baby teeth are reabsorbed causing them to loosen and fall out around ages 6 – 12 years with all of the permanent teeth but the third molars (wisdom teeth, which erupt around age 17 - 25) having erupted by the end of adolescence ending in a set of 32 teeth although sometimes the wisdom teeth are absent or fail to erupt
o A tooth consists of two major regions – crown and the root
§ Crown – enamel-covered and exposed above the gingival, or gum
§ Enamel heavily mineralized with calcium salts
§ Dentin – underlies the enamel and forms the bulk of the tooth and surrounds the central pulp cavity, which contains connective tissue, blood vessels, and nerve fibers collectively called the pulp, which supplies nutrients to the tooth tissues and provides for tooth sensations
§ Where the pulp cavity extends into the root, it becomes the root canal, which provides a rout for the blood vessels, nerves, and other pulp structures to enter the pulp cavity of the tooth
§ Root – portion of the tooth embedded in the jawbone and connected to the crown via the neck
§ Outer surface covered by cementum, which attaches the tooth to the periodontal membrane (ligament) that holds the tooth in place in the bony jaw
· Explain how villi aid digestive processes in the small intestine.
o Three structures that increase the absorptive surface in the small intestine are the microvilli, villi, and circular folds that function to increase the surface area for digestion and absorption – decrease in number toward the end of the small intestine while local collections of lymphatic tissue called Peyer’s patches increase and function to prevent bacteria from entering the bloodstream
§ Microvilli – are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance called the brush border
§ Villi – fingerlike projections of the mucosa that give it a velvety appearance and feel – within each is a rich capillary bed and a modified lymphatic capillary called a lacteal
§ The digested foodstuff is absorbed through the mucosa cells into both capillaries and the lacteal
§ Circular folds – plicae circulars – deep folds of both mucosa and submucosa layers
o Absorption – via active or passive transport processes into the mucosal cells
Functions of the Digestive System
· Describe the mechanisms of swallowing, vomiting, and defecation.
o Swallowing – deglutition – involves the coordinated activity of the tongue, soft palate, pharynx, and esophagus and involves two major phases
§ The voluntary Buccal phase – food is voluntarily chewed, mixed with saliva, and formed into a bolus (food mass) then forced into the pharynx by the tongue where it is then processed via involuntary reflex activity
§ The involuntary pharyngeal-esophageal phase – transports food bolus through the pharynx and esophagus under the control of the parasympathetic division of the autonomic nervous system (the vagus nerve)
§ The tongue blocks off the mouth, the soft palate blocks off the nasal passages, the larynx rises so that the epiglottis covers the respiratory tract so the bolus must enter the esophagus and move down via wavelike peristaltic contractions of the muscular walls – first the longitudinal muscles then the circular muscles contract to allow swallowing of the bolus
§ At the end of the esophagus, the food presses against the cardioesophageal sphincter, causing it to open and allow the food into the stomach
§ Gravity plays no part in swallowing – automatic process
o Vomiting – reverse peristalsis occurring in the stomach and possibly the small intestine, accompanied by contractions of the abdominal muscles and the diaphragm, which increases the pressure on the abdominal organs
§ Caused by local irritation of the stomach or disturbance of the equilibrium apparatus of the inner ear that may activate the emetic center in the brain (medulla), which then causes vomiting
o Defecation – the elimination of indigestible substances from the body via the anus in the form of feces
§ The defecation reflex is initiated when feces are forces into the rectum by mass movement (long, slow-moving but powerful contractile waves that move over large areas of the colon that force the contents toward the rectum) and the walls of the rectum are stretched
§ Spinal reflex (sacral region) that causes the walls of the sigmoid colon and the rectum to contract and the anal sphincters to relax
§ Defecation can be delayed temporarily voluntarily – the reflex contractions end and the rectal walls relax until the next mass movement and the process begins again
· Describe how foodstuffs in the digestive tract are mixed and moved along the tract.
o Peristalsis is the major means of propelling food through the digestive tract – involuntary waves of contraction that move along the length of the intestine, followed by waves of relaxation – net effect is to squeeze the food along the tract
o The mixing of food in the mouth by the tongue, churning of food in the stomach, and segmentation in the small intestine are examples of physical processes of mechanical digestion
· Describe the function of local hormones in the digestive process.
o See table 14.2 on page 453
o Gastrin – stimulates release of gastric juice, stimulates mobility of small intestine, relaxes ileocecal valve
o Histamine – activates parietal cells to secrete hydrochloric acid
o Somatostatin – inhibits secretion of gastric juice and pancreatic juice and inhibits emptying of stomach and gallbladder
o Secretin – increases output of pancreatic juice, increases bile output, inhibits gastric mobility and gastric gland secretion
o Cholecystokinin (CCK) – increase output of pancreatic juice, stimulate gallbladder, relaxes sphincter or duodenal papilla to allow bile and pancreatic juice to enter the duodenum
o Gastric inhibitory peptide (GIP) – inhibits gastric mobility and secretion of gastric juice
· List the major enzymes or enzyme groups produced by the digestive organs or accessory glands and name the foodstuffs on which they act.
o Chemical digestion – sequence of steps in which large food molecules are broken down into their building blocks by enzymes – these reactions are called hydrolysis reactions because a water molecule is added to each bond to be broken
o Salivary amylase in the mouth – starch is broken down into maltose – produces by salivary glands, which include the parotid gland, sublingual gland, and submandibular gland
o Pepsin (pepsinogens are activated into pepsin by hydrochloric acid found in gastric juices) in the stomach – breaks down proteins into amino acids – produced by chief cells in stomach lining
o Rennin in the stomach of infants – breaks down milk protein and converts it into sour milk-like substance – not typically found in adults
o Brush border enzymes in the small intestine – break down double sugars and complete protein digestion
o Pancreatic juice entering the small intestine – completes the breakdown of starch (via pancreatic amylase), does half of the protein digestion (via trypsin, chymotrypsin, carboxypeptidase), and fat digestion (via lipases), and digestion of nucleic acids (via nucleases)
§ Pancreatic juices also contain bicarbonate, which makes it very basic causing it to neutralize the acid chyme coming in from the stomach in order to activate the enzymes in the small intestine
§ Release of pancreatic juices is stimulated by the vagus nerve and local hormones produced by mucosa cells – secretin and cholecystokinin (CCK) stimulate the pancreas, liver, and gallbladder
· Name the end products of protein, fat, and carbohydrate digestion.
o See table 14.1 on page 447
§ Carbohydrates – disaccharides and polysaccharides are broken down into monosaccharides (simple sugars such as glucose, fructose, and galactose) – only carbohydrates our system breaks down into simple sugars are sucrose, lactose, maltose, and starch – cannot digest cellulose because we lack the enzymes necessary
§ Proteins – broken down into amino acid building blocks
§ Fats – lipids – broken down into fatty acids and glycerol
o Most substances are absorbed through the intestinal cell plasma membranes by active transport – they enter the capillary bed in the villus to be transported in the blood to the liver via the hepatic portal vein