Digestive System Notes PDF

Digestive System Asst. Prof. A. Taha Demirbas  The digestive system is essentially a disassembly line Its primary purpose is to break nutrients down into forms that can be used by the body, and to absorb them so they can be distributed to the tissues Most of what we eat cannot be used...

Digestive System Asst. Prof. A. Taha Demirbas  The digestive system is essentially a disassembly line Its primary purpose is to break nutrients down into forms that can be used by the body, and to absorb them so they can be distributed to the tissues Most of what we eat cannot be used in the form found in our food Nutrients must be broken down into smaller components, such as amino acids and monosaccharides, that are universal to all species Digestive system functions Ingestion, the selective intake of food Digestion, the mechanical and chemical breakdown of food into a form usable by the body Absorption, the uptake of nutrients into the blood and lymph Compaction, absorbing water and consolidating the indigestible residue into feces; and finally Defecation, the elimination of feces  The digestive system has two anatomical subdivisions, the digestive tract and the accessory organs The digestive tract is a muscular tube extending from mouth to anus It is also known as the alimentary canal or gut It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine Part of it, the stomach and intestines, constitutes the gastrointestinal (GI) tract The accessory organs are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas The mouth The mouth is also known as the oral (buccal) cavity Its functions include ingestion (food intake), taste and other sensory responses to food, mastication (chewing), chemical digestion, swallowing, speech, and respiration The mouth is enclosed by the cheeks, lips, palate, and tongue The salivary glands There are two kinds of salivary glands, intrinsic and extrinsic The intrinsic salivary glands are an indefinite number of small glands dispersed amid the other oral tissues They include lingual glands in the tongue, labial glands on the inside of the lips, and buccal glands on the inside of the cheeks They secrete saliva at a fairly constant rate whether we are eating or not, but in relatively small amounts This saliva keeps the mouth moist and inhibits bacterial growth  The extrinsic salivary glands are three pairs of larger, more discrete organs located outside of the oral mucosa They communicate with the oral cavity by way of ducts The parotid gland, located just beneath the skin anterior to the earlobe Its duct passes superficially over the masseter, pierces the buccinator, and opens into the mouth opposite the second upper molar tooth Mumps is an inflammation and swelling of the parotid gland caused by a virus The submandibular gland, located halfway along the body of the mandible, medial to its margin, just deep to the mylohyoid muscle Its duct empties into the mouth at a papilla on the side of the lingual frenulum, near the lower central incisors The sublingual gland, located in the floor of the mouth It has multiple ducts that empty into the mouth posterior to the papilla of the submandibular duct The pharynx The pharynx is a muscular funnel that connects the oral cavity to the esophagus and the nasal cavity to the larynx Thus, it is a point where the digestive and respiratory tracts intersect The osophagus The esophagus is a straight muscular tube 25 to 30 cm long, posterior to the trachea Its superior opening lies between vertebra C6 and the cricoid cartilage of the larynx It ends at the point of stomach in the abdominal region The stomach The stomach is a muscular sac in the upper left abdominal cavity immediately inferior to the diaphragm It functions primarily as a food storage organ The stomach mechanically breaks up food particles, liquefies the food, and begins the chemical digestion of proteins and fat This produces a soupy or pasty mixture of semidigested food called chyme Most digestion occurs after the chyme passes on to the small intestine  Pylorus is the end of the stomach The pylorus is surrounded by a thick ring of smooth muscle, the pyloric (gastroduodenal) sphincter, which regulates the passage of chyme into the duodenum The small intestine The small intestine is a coiled mass filling most of the abdominal cavity inferior to the stomach and liver It is divided into three regions: the duodenum, jejunum, and ileum Nearly all chemical digestion and nutrient absorption occur here To perform these roles efficiently, the small intestine must have a large surface area exposed to the chyme This surface area is imparted to it by extensive folding of the mucosa, and by the great length of the small intestine  The end of the small intestine is the ileocecal junction, where the ileum joins the cecum of the large intestine The muscularis of the ileum is thickened at this point to form a sphincter, the ileocecal valve, which protrudes into the cecum and regulates the passage of food residue into the large intestine and prevents feces from backing up into the ileum The large intestine The large intestine receives about 500 mL of indigestible food residue per day, reduces it to about 150 mL of feces by absorbing water and salts, and eliminates the feces by defecation It is named for its relatively large diameter, not its length It consists of four regions: the cecum, colon, rectum, and anal canal  The outer layer of the colon is unusual. Its longitudinal fibers are especially concentrated in three thickened, ribbonlike strips called the taeniae coli The muscle tone of the taeniae coli contracts the colon lengthwise and causes its wall to bulge, forming pouches called haustra In the rectum and anal canal, however, the longitudinal muscle forms a continuous sheet and haustra are absent The anus is normally held shut by two muscular rings—an internal anal sphincter composed of smooth muscle of the muscularis externa, and an external anal sphincter composed of skeletal muscle of the pelvic diaphragm The internal anal sphincter is under involuntary control and relaxes automatically when the rectum is distended with feces The external anal sphincter is under voluntary control and enables one to postpone defecation when appropriate The liver The liver is a reddish brown gland located immediately inferior to the diaphragm It is the body’s largest gland It contributes to digestion—the secretion of bile Bile acids emulsify fat Breaking globules of dietary fat into smaller droplets with more surface area exposed to enzyme action Emulsification greatly enhances the efficiency of fat digestion  The pancreas Most digestion is carried out by pancreatic enzymes The pancreas is a spongy digestive gland posterior to the greater curvature of the stomach The pancreas is both an endocrine and exocrine gland Its endocrine part is the pancreatic islets, which secrete the hormones insulin and glucagon The pancreatic duct runs lengthwise through the middle of the gland and joins the bile duct at the hepatopancreatic ampulla The pancreas secretes 1,200 to 1,500 mL of pancreatic juice per day This fluid is an alkaline mixture of water, sodium bicarbonate, other electrolytes, enzymes