The Digestive System Basic process of digestion 1. Ingestion: animal eats food. 2. Digestion: animal body breaks food down. Mechanical digestion: chewing (mastication). Chemical digestion: enzymes and acids break down food. 3. Absorption: small food molecules move from digestive system to cells of the animal. 4. Elimination: material that can t be digested passes out of the animal s digestive tract. Mouth and Teeth Many vertebrates (but not all!) have teeth used for chewing or mastication. Birds Lack teeth. Break up food in a two-chambered stomach. Gizzard muscular chamber that uses ingested pebbles to pulverize food. These pebbles are called gastroliths. 6 1
Carnivores pointed teeth that lack flat grinding surfaces. Herbivores large flat teeth suited for grinding cellulose cell walls of plant tissues. Humans (omnivores) have carnivore-like teeth in the front and herbivore-like teeth in the back. 8 2
Mouth and Teeth Inside the mouth, the tongue mixes food with saliva. Moistens and lubricates the food. Contains salivary amylase, which initiates the breakdown of starch. Salivation is controlled by the nervous system. Tasting, smelling, and even thinking or talking about food stimulate increased salivation. 14 Mouth and Teeth Mouth and Teeth: Anatomy and Swallowing Swallowing Starts as voluntary action. Continued under involuntary control. When food is ready to be swallowed, the tongue moves it to the back of the mouth. Soft palate seals off nasal cavity. Elevation of the larynx (voice box) pushes the glottis against the epiglottis. Keeps food out of respiratory tract. 15 16 Vertebrate Digestive Systems Gastrointestinal tract is layered. Mucosa innermost Epithelium that lines the interior, or lumen, of the tract Submucosa Connective tissue Muscularis Circular and longitudinal smooth muscle layers Serosa outermost Epithelium covering external surface of tract 17 3
The Esophagus Esophagus: muscular tube connecting the mouth to the stomach. Actively moves a bolus through peristalsis. Bolus: lump of food. Peristalsis: squeezing of food through tube (esophagus and intestines). Successive one-directional waves of contraction. Sphincter opens to allow food to enter stomach. 19 20 Convoluted surface allows for expansion. Contains gastric juice, which is an acidic secretion. Layers of muscle allow for contraction to move and turn food (muscularis). Pyloric sphinctercontrols food entrance into the intestines (duodenum). 21 Pepsinogen is the inactive form of a proteindigesting enzyme. Once it mixes with HCl, it activates and becomes pepsin(protein-digesting enzyme). The mixture of partially digested food and gastric juice that leaves the stomach for the intestines is called: chyme. 23 4
Mucosa has gastric glands that contain three kinds of secretory cells. Mucus-secreting cells: secretes mucous. Parietal cells: secrete hydrochloric acid (HCl). Chief cells: secrete pepsinogen. Why doesn t the gastric acid digest the stomach? Mucous produced by the mucous-secreting cells, lines the stomach and protects it from erosion. Forms a gel called mucin. Ulcers form when H. pyloribacteria infect the stomach, causing holes in the mucin. Moving into the intestines Anything absorbed through the stomach? A little. Water Alcohol Aspirin Chymeleaves the stomach through the pyloric sphincterand enters the duodenum(beginning of the small intestine). The Small Intestine About 15 feet long in living person. 20 feet after death when muscles have relaxed. Small in diametercompared to shorter large intestine. Consists of duodenum, jejunum, and ileum. Receives: Chyme from stomach (partially digested food). Digestive enzymes and bicarbonate from pancreas. Bile from liver and gallbladder. 30 5
Accessory organs: Liver & Gallbladder Liver secretes bile into the small intestine. Bilesalts emulsify (suspend in water) fats, helping them mix into the liquid in the intestine. Easier to digest them this way. Gallbladder stores and concentrates bile. Epithelial wall is covered with villi. Villi are covered by microvilli. Greatly increase surface area. Microvilli participate in digestion and absorption. Enzymes essential to this process. Many adults lack the enzyme lactase. Have lactose intolerance. 31 Accessory organ: Pancreas Pancreasreleases pancreatic juiceinto the small intestine. This juice is full of enzymes. Lipase: breaks apart fat molecules; hydrolyzed into fatty acids and monoglycerides. Pancreatic amylase: breaks long carbohydrates into disaccharides (short chain sugar). Then they break into monosaccharides (single sugars). Trypsin and chymotrypsin: break apart peptide fragments (protein). Aminopeptidases: finish breaking down peptides into individual amino acids. After several hours: food now broken down into smallest components, ready for absorption. Absorption Sugars and amino acids pass directly into cells by active transport or facilitated diffusion. Pass from cells into blood capillaries. Then into the liver which detects abnormalities (like too much glucose). Adjustments can be made here. Absorption Emulsified fat (fatty acids and monoglycerides) diffuses across the membrane because they are nonpolar. Once in epithelial cells, transformed into triglycerides. Then combine with proteins to form chylomicrons. Absorbed into lymphatic capillaries. 6
The Large Intestine (colon) Much shorter than small intestine, but has larger diameter. Small intestine empties directly into the large intestine. No digestion occurs. Many types of bacteria live in the large intestine. Bacteria break down fiber. Bacterial fermentation produces gas. Function: to reabsorb water, remaining electrolytes (sodium & potassium ions), and vitamin K. Too much water absorption = constipation. Too little water absorption = diarrhea. Prepare waste (fecal matter) for expulsion. 39 40 Why you must wash your hands Vitamin K is produced in the large intestine with the help of a bacteria: E. coli Very helpful in the large intestine, but extremely harmful if ingested. Causes vomiting, diarrhea, and if unchecked can lead to sepsis (bacteria in your blood stream traveling all over your body causing a multitude of infections). E. coli bacteria excreted in feces. What is poop? Poop = fecal matter (excrement). Refuse material of digestion. Anything that couldn t be digested. Excess bacteria (the main cause of odor). Mucous and dead cells. Sometimes parasites. 7