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Digestive System Introduction Organs General Structure Structures and functions Digestion Absorption Digestive system Nutrients required by body cells have to be in simple and soluble form. Ingested food is in complex form which has to be broken into simpler form. The conversion of complex form of food into simple soluble form brought by digestive system. Different organs that makes up digestive system are involved in digestion. Main Organs (in order) Mouth and Oral Cavity Pharynx Esophagus Stomach Small intestine Large intestine Accessory Organs Teeth and tongue Salivary glands Liver Gallbladder Pancreas General Structure Walls of the digestive tract consist of four layers throughout its length. From lumen to outward: Mucosa/mucous membrane – epithelial layer coated with mucous Submucosa – loose connective tissue: contain blood vessels, lymphoid, nerves and glands Muscularis/musculosa–smooth muscle that produces wave-like contractions (peristalsis) to move food, consists of two layers, outer-longitudinal, inner-circular Serosa/adventitia – connective tissue; outermost covering. Baisc structure of Alimentary canal Structures and functions Oral cavity Hollow chamber with a roof, floor and walls. Food enters through the mouth. Hard palate – bony structure in the anterior or front portion of the mouth. Soft Palate – located above the posterior or rear portion of the mouth; muscle tissue. Uvula – hangs down from the soft palate; helps prevent food or liquid from entering the nasal cavities above the mouth. Tongue – skeletal muscle covered with mucous membrane. Involved in mixing, swallowing and tasting. Structures and functions Teeth – 4 major types Incisors – cutting function during mastication. Canines (cuspids) – pierce or tear food. Premolars (bicuspids) – large flat surfaces with 2 or 3 grinding or crushing cusps on their surface. Grind food into a bolus so it can be swallowed. Molars (tricuspids) – same as premolars. Deciduous teeth = baby teeth, 20 in number; adults have 32 teeth Structures and functions Teeth cont’d Parts of a tooth Crown – portion that is exposed and visible in the mouth; covered with enamel. Neck – narrow portion surrounded by the pink gingiva or gum tissue. Root – fits into the socket (joint) of the upper or lower jaw. Structures and functions Structures and functions Salivary Glands(3 pairs) Parotids – largest, lie just below and in front of each ear at the angle of the jaw (mumps). Submandibular – open into the mouth on either side of the lingual frenulum. Sublingual – open into the floor of the mouth. Saliva – contains mucus and a digestive enzyme called salivary amylase which begins the process of chemical digestion of carbohydrates. Location of salivary glands Structures and functions Esophagus Muscular, mucus lined tube that connects the pharynx with the stomach (about 25 cm or 10 inches long). Peristalsis moves food from the mouth to the stomach. Structures and functions Stomach Lies in the upper part of the abdominal cavity just under the diaphragm. Serves as a pouch that food enters after it has been chewed, swallowed and passed through the esophagus. At the junction of the esophagus and the stomach is the cardiac sphincter (muscle) that keeps food from reentering the esophagus Divided into three regions: Fundus – enlarged portion to the left of and below the cardiac sphincter. Body – central part of the stomach Pylorus – lower, narrow section which joins the first part of the small intestine. Structure of stomach Structures and functions The lining of the stomach has numerous folds when empty called rugae; they allow for expansion when we eat. Has numerous gastric glands that secrete “gastric juice” into the stomach. Most common cells found in the glands are: Parietal cells - produce HCl acid that denatures proteins. Chief cells - produce pepsin that begins protein digestion. Mucus cells - produce mucus the protects from HCl action. Gastric pits and gland Structures and functions The stomach wall has 3 layers of muscle that allow mixing of the food with gastric juice, breaking it down into a semisolid mixture called chyme. Pyloric sphincter (muscle) holds food in the stomach or allows for emptying of chyme into the small intestine. Controlled by the small intestine. LS of stomach Structures and functions Small intestine Roughly 7 meters in length. Smaller in diameter compared to the large intestine. Divided into three sections: Duodenum Jejunum Ileum Structures and functions Small intestine Mucous lining, contains thousands of microscopic glands which secrete intestinal digestive juices. Intestinal lining has multiple circular folds, plicae, that increase the surface area available for absorption of nutrients. The surface is covered with tiny finger-like projections called villi. LS of small intestine Structures and functions Liver Fills the entire upper right section of the abdominal cavity and extends part way into the left side. The largest gland in the body secretes bile into hepatic ducts. Functions: Produces bile Stores vitamins and minerals Plays a part in maintaining blood sugar (glycogen) and blood pressure (blood proteins). Structures and functions Liver cont’d Bile breaks up (emulsifies) large fat globules into smaller particles to increase the surface area for digestion. Bile gives the feces its color (no bile - stools become clay colored). Bile absorbed into the blood gives the skin a yellowish color = jaundice or icterus. Liver – anterior view Structures and functions Gall Bladder Stores bile Cystic duct arising from it joins with hepatic duct. When fat enters the duodenum, the mucosal cells release a hormone called cholecystokinin (CCK). CCK stimulates the contraction of the gallbladder and forces bile into the duodenum via the common bile duct. Gall bladder Structures and functions Pancreas Lies behind the stomach in the C-shaped curve of the duodenum. Endocrine portion produces insulin and glucagon. Exocrine portion (ascini) secretes pancreatic juice into ducts that connect to the duodenum at the same area as the bile duct. Pancreatic juice contains enzymes such amylase, trypsin, chymotrypsin and lipase and also bicarbonate that neutralizes the HCl in the chyme. Pancreas Structures and functions Large Intestine About 5 feet in length, forms the lower portion of the digestive tract. Undigested and unabsorbed food material enter the large intestine after passing through a sphincter-like structure called the ileocecal valve. L/S of Large intestine Structures and functions Large Intestine One primary function of the large intestine is to remove water from the feces. A few vitamins and minerals are also absorbed. Bacteria living in the large intestine break down some of the remaining material. Bacteria also produce some of the B-complex vitamins and vitamin K. Structures and functions Large Intestine Inner surface has no folds or villi – not well suited for absorption. Large Intestine Cecum – ileocecal valve opens into this pouchlike area. Ascending colon – right side of body Transverse colon – extends across the front of the abdomen Descending colon – left side of the body Sigmoid colon – leads to the rectum Large intestine Digestion Complex process that occurs in the alimentary canal, consists of mechanical and chemical changes that prepare food for absorption. Mechanical digestion Breaks food into tiny particles, mixes them with digestive juices, moves them along the alimentary canal and finally eliminates the digestive wastes from the body. Includes mastication, deglutition, peristalsis and defecation. Digestion Chemical digestion Breaks down large, non-absorbable food molecules into smaller, absorbable molecules that can easily pass through intestine into blood and lymph. Process includes numerous chemical reactions catalyzed by specific enzymes in saliva, gastric, pancreatic, and intestinal juices. Digestion Carbohydrates Digestion begins in mouth (salivary amylase) Salivary and pancreatic amylase converts starches into disaccharides (double sugars). Intestinal enzymes (sucrase, maltase, lactase) break down disaccharides into monosaccharides. Polysaccharide salivary amylase maltose &small polysaccharide Undigested polysaccharides pancreatic amylase maltose & diaaccharides maltse,sucrase, lactase monosaccharides Digestion Proteins Starts in the stomach and ends in the small intestine. Pepsin in stomach,trypsin and chymotrypsin in pancreatic juice digest protein. Aminopeptidase present in intestinal secretion finally digest protein into amino acids. Protein pepsin short polypeptides Short polypeptides trypepsin, chymotrypsin small polypeptides & peptides carboxypeptidase, peptidases,dipeptidases amino acids Digestion Lipids (fat) Bile produced by liver is poured into the duodenum. It brings about emulsification. Pancreatic lipase splits the lipid molecules into fatty acids and glycerol. Fat bile salts emulsified fat droplets pancreatic lipase fatty acids & glycerol Absorption Stomach absorbs only a few substances Main absorption occurs in small intestine Absorption occurs by a combination of simple diffusion, facilitated diffusion and active transport Amino acids and glucose are directly transported to liver by hepatic portal vein Fatty acids and glycerol are absorbed by intestinal cells Resembled as triglycerides in ER Packed into protein covered fat droplets-chylomicrons Chylomicrons enter the lacteals and enter into blood circulation Hormonal control Gastrin(stomach) causes gastric glands in stomach to secrete pepsinogen. Stimulated by distention of stomach by food, partially digested proteins and caffine Secretin(duodenum) causes the pancreas and liver to secrete sodium bicarbonate and bile respectively. Stimulated by the presence of acidic chyme in the Duodenum. CCK/cholecytokinin(duodenum) Stimulates pancreas to release digestive enzymes and gall bladder to empty bile Stimulated by the presence of fatty acids and partially digested protein in duodenum. Gastric inhibitory peptide/GIP(duodenum) Decrease stomach churning, thus slowing emptying. Stimulated by the presence of fatty acids or glucose in duodenum
