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Digestive, Excretory & Urinary Systems DIGESTION & ITS ORGANS Gastrointestinal Tract Is the pathway along which the process of digestion occurs Digestion is the process of breaking down food into molecules that the body can use It begins at the mouth and ends at the anus It is divided into several distinct organs which are directly involved in the digestive process There are additional organs, not part of the system that aid in digestion by providing secretions Digestion Includes the: Mechanical & chemical breakdown of food into nutrients Absorption of nutrients Elimination of waste The Mouth The location of the mechanical phase of digestion Incisors and molars cut & grind up your food The tongue keeps food between your chewing surfaces Chemical digestion also begins here Salivary glands produce saliva: a mixture of water, mucus, & salivary amylase Parts of the Mouth Teeth Tongue Soft Palate Hard Palate Salivary glands Pharynx Salivary Glands 3 types: Parotid Sublingual Submandibular All produce saliva & salivary amylase whose function is to break down starch into maltose Next step of Digestion After food has been thoroughly chewed, moistened, and rolled into a ball: bolus It is then forced into the pharynx by swallowing action The bolus passes into the esophagus and gets pushed down through a series of rhythmic muscular contractions & relaxations: peristalsis Peristalsis Esophagus Is a muscular tube approx. 25 cm long that runs form the back of the mouth to the stomach Esophagus Step 3 of Digestion Takes place in the stomach By now the bolus is now fairly well minced and turned into a pulpy mass as it passes into the stomach The mechanical action of the muscles in the stomach constrict and relax in a continuous motion blending, whipping, and stirring the stomach's contents into chyme Chyme is a pulpy substance that can be handled by the small intestine. Stomach Is involved in both mechanical & chemical digestion Located in the upper left side of the abdominal cavity & just below the diaphragm It is an elastic bag that is J-shaped, when full and lies in folds when empty It looks like a deflated balloon when empty, but when full, it becomes about a foot long and six inches wide It is able to hold about two quarts of food and drink. Stomach Continued The function of the stomach is best described as a food processing unit and a storage cistern Various chemicals in the stomach like the digestive enzymes interact to break down the food Digestive enzymes of the stomach include: pepsin, rennin, & lipase Hydrochloric acid creates suitable environment for the enzymes and assists in the digestion A watery mucus provides a protective lining for the muscular walls of the stomach so it will not be digested by the acid or enzymes Mechanical Digestion In detail As food enters the stomach, the esophageal sphincter closes to prevent food from reentering the esophagus Food remains in the stomach for 3-4 hours Muscle contractions churn the contents, breaking them down and mixing them with gastric fluid Stomach Anatomy The stomach is composed of six tissue layers From the outside in they are: Serosa Muscularis Submucosa Lamina muscularis Mucosa Gastric surface Stomach Anatomy The walls of the stomach have several layers of smooth muscle Circular Longitudinal Diagonal These muscles work together to churn the contents of the stomach Internal Stomach Anatomy The inner lining of the stomach is a thick, wrinkled mucous membrane composed of epithelial cells Inside of this membrane are small openings called gastric pits Gastric pits: are open ends of gastric glands, which release secretions from special cells These secretions make up the acidic digestive fluid Stomach Wall Anatomy 4 Major Secretory Cells 1.Chief cells Produces pepsinogen When pepsinogen enters the low pH environment of the stomach it becomes the digestive enzyme pepsin Pepsin is responsible for digesting proteins 4 Major Secretory Cells 2. Parietal cells Produce HCl (hydrochloric acid) Antacids work by block the receptors which control HCl secretion Also produce intrinsic factor, which binds to and allows B12 absorption in the intestines 4 Major Secretory Cells 3. G-cell secretes gastrin hormone gastrin activates gastric juice secretion & gastric smooth muscle “churning” gastrin also activates the gastroileal reflex which moves chyme from the ileum to the colon in the intestines 4 Major Secretory Cells 4. Mucus cells Produce mucus which protects against acids and digestive enzymes Chemical Digestion Is done by gastric fluid which contains water, mucous, & digestive enzymes Pepsin digests proteins by breaking them into short amino acid chains known as peptides HCl provides the necessary low pH of 1.5 to 2.5 to active the enzymes, dissolve minerals, and kill bacteria Chemical Digestion Peristalsis forces chyme out of the stomach and into the small intestine through the pyloric sphincter Each time the pyloric sphincter opens, approx. 5 to 15 ml of chyme moves into the small intestine Chemical intestine digestion continues in the small Accessory Organs to Digestion The liver, gallbladder, & pancreas are accessory organs to the process of digestion They perform several important functions including the production of secretions They do not come directly in contact with food The Liver Is located to the right of the stomach Not only an organ but the largest gland of the body Its main functions are to filter and process nutrient-rich blood & aid in digestion Additional functions include: Storing glucose as glycogen Regulates lipid metabolism: such as making cholesterol Making proteins Breaking down toxins Secretes & makes bile Bile Is a bitter, alkaline, brownish-yellow or greenish-yellow fluid It aids in the emulsification, digestion, and absorption of fats Made up of bile salts & bilirubin Bilirubin is the main pigment of bile Gallbladder Is a 10-12 cm, dark green saclike organ found underneath the liver Also Its known as the cholecyst functions include: Store & release bile (50 mL) Stimulate the release of cholecystokinin Pancreas Lies beneath the stomach It is an organ & a gland It has several functions It produces several enzymes & secretions Pancreas Functions Acts as in endocrine gland by producing hormones to regulate blood sugar levels, specifically insulin Two roles in the digestive system: Produces sodium bicarbonate to neutralize stomach acid Produces enzymes to break down carbohydrates, lipids, proteins & nucleic acids Pancreas Enzymes Pancreatic amylase -breaks down carbohydrates Pancreatic lipase -breaks down lipids Trypsin-comes from the precursor trypsinogen, and digests proteins Chymotrypsin-comes from the precursor Chymotrypsinogen, and digests proteins Nucleases: digests nucleic acids DNAase RNAase
