Digestive System

Takes in food, breaks it down into nutrient molecules, absorbs them into the bloodstream, and then rids the body of the indigestible remains.
Organs fall into 2 main groups: those in the alimentary canal & those in the accessory digestive organs.

Also called gastrointestinal tract
Continuous, muscular digestive tube that winds throughout the body
F: digests food (into smaller fragments; dissolves), absorbs the fragments through its lining into the blood
Organs: mouth, pharynx, esophagus, stomach, small and large intestines (large leads to the terminal opening; the anus).
Food in this canal is technically outside the body because it is open to the environment at both ends.

Organs: teeth, tongue, gallbladder, and large dig. glands (salivary, liver, and pancreas).
The glands and gallbladder lie outside of the GI tract and connect to it by ducts.
F: produce a variety of secretions that contribute to the break down of food.

The tract is like a disassembly line: food becomes less complex at each step of processing and as the nutrients become available to the body.
1. Ingestion: taking in food into the tract, usually via mouth.
2. Propulsion: movement of the food through the alim. canal; can be voluntary (swallowing) and involuntary (peristalsis). Peristalsis: major means of propulsion; alternate waves of contraction and relaxation from muscles in organ walls. Squeezes food along, but also mixes some; food and fluid waves are powerful.
3. Mechanical digestion: physically prepares food for the chem. dig. by enzymes. Chewing, mixing of food with saliva by tongue, churning in the stomach, and segmentation (rhythmic constrictions of the s. intestine; mixes food w/dig. juices and increases absorption from repeated movement of food over the walls).
4. Chemical digestion: Complex food molecules broken down into their chem. elements by enzymes which are secreted into the lumen of the alim. canal. Begins in the mouth and is normally complete in s. intestine.
5. Absorption: passage of dig. end products from the lumen of the GI tract through the mucosal cells by active or passive transport into the blood or lymph. S. intestine is the major absorptive site.
6. Defecation: elimination of indigestible substances from the body via anus in the form of feces.
*Some processes occur only in one organ(ex:ingestion in mouth, defecation in l. intestine), but for the most part, the dig. system activities require several organs and occur bit by bit as they move through the tract.

*Responds to changes in environment by working to change the lumen environment to be the most functional it can be so that digestion and absorption can occur there effectively.
Two facts:
1. Digestive activity is provoked by a range of mechanical and chemical stimuli:
-There are sensors involved in the controls of GI tract activity located in the walls of the tract organs.
-Sensors respond to several stimuli including stretching of the organs from food in the lumen, osmolarity(solute concentration) and pH of contents, and the presence of substrates and end products of digestion.
-Stimulation of the receptors initiate reflexes which 1) activite or inhibit glands that secrete dig. juices into the lumen or hormones into the blood or 2) stimulate smooth muscle of GI tract walls to mix the lumen contents and move it along the tract.
2. Controls of digestive activity are both intrinsic and extrinsic.
-Many are intrinsic (nerve plexuses or homone-producing cells in the tract).
-Gut brain: bt muscle layers of wall of the alim. canal; made up of nerve plexuses that spread along entire GI tract length and influence each other in the same and in diff. organs.
-Two types of reflec activity occur: short and long reflexes. Short:mediated entirely by the gut plexuses in response to stimuli in GI tract. Long:initiated by stimuli inside or outside the tract and involve CNS centers and extrinsic autonomic nerves.
-Nerve fibers that excite smooth muscle secrete acetylcholine or substance P; those that inhibit release vasoactive intestinal peptide or nitric oxide.
-Stomach and s. intestine have hormone producing cells, that when stimulated release their products to the i.s. fluid in the ECS. Hormones are distributed via blood and i.s. fluid to their target cells.

*Most all lie within the abdominopelvic cavity: serous membranes (parietal & visceral peritoneum, and peritoneal cavity with serous fluid). Also mesentaries (double peritoneum layers) which extend to the dig. organs from the body wall. Peritonitis=inflammation of the peritoneum, peritoneal coverings stick together around the inflammation site.
*Splanchnic circulation: includes the arteries that branch from the abdominal aorta to serve the dig. organs and the hepatic portal circulation. This arterial supply normally receives 1/4th of the cardiac output (increases after a meal). Hepatic portal circulation: collects nutrient-rich blood draining from the dig. viscera and delivers it to the liver(for metabolic processing or storage before release back to bloodstream).
*Same 4 basic layers from esophagus to anal canal of alim. canal walls, each playing a role in the food breakdown: 1)Mucosa: innermost; moist epithelial membrane lining the lumen from the mouth to the anus. F: secrete mucus, dig. enzymes, and hormones, absorb end products into the blood, and protect against infectious disease. Has 3 sublayers: epithelium (simple columnar, rich with mucus secreting cells to ease food through and protect the organs from being digested), the lamina propria (loose areolar CT, under epithelium, wtih capillaries to nourish epith. and absorb digested nutrients), and the muscularis mucosae (smooth muscle cells that produce local movements of the mucosa).
2)Submucosa:areolar CT, lots of blood/lymphatic vessels & lots of elastic fibers. F: vessels supply the tissues around the GI tract wall, and the fibers enable stomach to regain normal shape after storing a large meal.
3) Muscularis Externa: F: segmentation and peristalsis. Has smooth muscle cells, inside circular layer can form sphincters to act as valves that prevent backflow of food from one organ to the next.
4) Serosa:protective outermost layer, is the visceral peritoneum.Areolar CT with single layer of squamous ep. cells.
*The alimentary canal nerve supply=enteric neurons that communicate widely with one another to regulate dig. system activity. 2 major intrinsic nerve plexuses: submucosal occupies the submucosa & regulates activity of the glands and smooth muscle in the mucosa, and the myenteric controls GI tract motility, within the muscularis layer.

Only part of alim. canal involved with ingestion.
Also called oral cavity, is mucosa-lined. Boundaries: lips, cheeks, palate, and tongue. Oral orifice is the ant. opening and posteriorly it is continuous with the oropharynx.
Walls are lined with thic stratified squam. ep. and can withstand lots of friction; some parts keratinized for extra protection during eating; responds to injury by producing defensins keeping it really healthy.
Lips and Cheeks: have a core of skeletal muscle covered by skin. Orbicularis oris muscle forms lips and buccinators form the cheeks. Contains a vestibule. The oral cavity proper is the area within the teeth and gums. Lips are larger than thought; what we see is call the red margin which is poorly kertinized, allowing the red color of the underlying capillaries to show through. It lacks glands so it must be moistened with saliva to not become dry.
The Palate: forms roof of the mouth. Has the hard anterior portion, with palatine bones underneath, gives a rigid surface to help the tongue with chewing. The soft palate is made up of skeletal muscle and rises to close off the nasopharynx when we swallow; it's anchored to the tongue and walls of oropharynx to form the fauces(throat); uvula is the dangly thing.

Occupies the floor of the mouth and the majority of oral cavity when mouth is closed, made up of skeletal muscle fibers, mixes food with saliva to form a bolus, and intitiates swallowing by pushing bolus into pharynx. Also helps us form consonants.
Intrinsic muscle fibers: in tongue, not attached to bone. Allow tongue to change shape for speech and swallowing, but not its position.
Extrinsic muscle fibers: alter the tongue's position and originate from points of origin in the skull or soft palate.
Lingual frenulum: mucosa that attaches tongue to the floor of mouth, limiting post. movements.
Papillae are on sup. surface and are projections of the mucosa underneath.
-Filiform papillae: give roughness to aid in licking semisolid foods like ice cream, & provide friction for manipulating foods. Smallest and most numerous of all the papillae, contain keratin which stiffen then and give a white look.
-Fungiform papillae: scattered on surface, mushroom-shaped, vascular cores giving red hues.
-Vallate papillae: 10-12, large, in v shape at back of tongue.
-Foliate papillae:on lateral parts of the post. tongue.
-All papillae house taste buds, but those on the foliates function in taste in infancy.
Terminal Sulcus: groove post. to vallate papillae, separating the portion of tongue in the oral cavity and oropharynx(origin).

Saliva functions: cleanse the mouth, dissolves food chemicals so they can be tasted, moistens foods and aids in making a bolus, and contains enzymwa that begin chem. breakdown of starchy foods.
Most saliva produced by glands outside of oral cavity that empty their secretions into it: extrinsic saliv. glands. These develop from oral mucosa and are connected to it by ducts. Intrinsic, or buccal glands, are scattered throughout the oral cavity mucosa.
Parotid gland: large, ant. to the ear, surgery to it can result in facial paralysis because there are branches of facial nerve throughout it.
Submandibular gland: walnut sized. Sublingual gland: lies ant. to the submand. gland, is under the tongue, and opens via 10-12 ducts into floor of mouth.
Salivary glands are composed of 2 types of secretory ells: mucous (mucus) and serous (water secretion). Sublingual have mostly mucus cells, while the other glands have equal #'s.
Saliva composition: largely water (97-99%) so it is hypoosmotic.Slightly acidic(6.75-7). Function in protection against microbes through antibodies, lysozyme which inhibits bacterial growth in mouth, cyanide compound, and defensins.Highly toxic nitric oxide is formed from defensins in acidic places.
Control of Salivation: intrinsic salivary glands secrete saliva continuously, but in amounts small enough to keep mouth moist. Food entering mouth triggers the extrinsics so copious amounts poor out. Parasympathetic division of ANS controls salivation primarily: food in mouth sends signals to the salivatory nuclei in the brain stem, so PNS activity increases and the facial & glossopharyngeal nerves trigger *watery saliva release. Chemoreceptors activated by acidic substances, mechanoreceptors by any stimulus in mouth. Nausea increases salivation to try and neutralize the irritants. SNS causes release of thick mucin rich saliva.
Disease inhibiting saliva release can cause cavities and difficulty in swallowing, talking, etc.Halitosis(bad breath) can occur from flourishing bacteria.

Lie in sockets (alveoli) in gum-covered margins and tear and grind food to break them into smaller segments. Masticating-chewing.
Primary Dentitions: deciduous (baby) teeth. First appear at 6 mths, all have come in by 24 mths.
Permanent Dentitions: deep-lying and make the primary teeth loosen and fall out bt 6-12 years. Wisdom teeth=emerge bt 17-25 years, sometimes don't develop at all. Usually 32 teeth in full set.
Impacted tooth=one that remains embedded in the jawbone; ex:wisdoms.
Incisors: chisel-shaped, used for cutting/nipping off pieces of food. Canines: fanglike, tear and pierce.
Premolars and molars: broad crowns, roundeds tips, best for grinding. Upper and lower molars lock together during chewing to generate crushing force.
Dental Formula: indicates the #'s and relative positions of the diff. types of teeth in the mouth. Written for 1/2 of the mouth, top over bottom, since the other side is the same; so you multiply it by 2.
Tooth structure: crown is the exposed portion about the gingiva(gum) and the root is embedded in the jawbone. Enamel-brittle, ceramic-like material thich as a dime which bears the force of chewing; hardest substance in the body; must be artifically filled if broken.
*The neck: in bt crown and root; outer surface covered by cementum which attaches the tooth to the periodontal ligament around teeth.
*Dentin: protein-rich bonelike material under enamel, forms bulk of tooth; acts as a shock-absorber for forces on the enamel during chewing; contains ubules with odontoblast which maintains/forms the dentin continuously. Surrounds a central pulp cavity with pulp(supplies nutrients and helps with tooth sensation).
Root canal: where pulp extends into the root. Apical foramen: allows bv's, nerves, etc to enter the pulp cavity.
Enamel, dentin, and cementum are all calcified and resemble bone, but are different because they're avascular.

Root canal: needed if a tooth's nerve dies from a blow to the jaw.
Dental caries/cavities: demineralization of enamel and dentin by bacterial action. Decay begins when plaque (sugar, bacteria) adheres to the teeth. Acids can then be produced and dissolve the calcium salts on the teeth, allowing the rest of the tooth to be digested by enzymes on bacteria.
Calculus(tartar): unremoved plaque on the gums, as plaque accumulates is calcifies and forms calculus. Gingivitis can occur where the gums are red/swollen from the gums being exposed to bacteria.
Periodontal disease: affects up to 95% of those over 35, tooth loss can occur from bone dissolving away.This disease can increase the risk of heart disease and stroke bc bacteria can enter the blood from infected gums and create clots.

Food passes from the mouth into the oropharynx and then the laryngopharynx. These are common passageways for food, air, and fluids. Nasopharynx has no role in digestion.
Histology= resembles oral cavity. Friction-resistant stratified squam. ep. with lots of mucus-producing glands. External muscle layer has 2 layers, their contractions propel food into the esophagus.

Collapsed when not involved in food propulsion; after food moves through the laryngopharynx, it is routed to the esophagus while the epiglottis closes off the larynx.
Takes a straight course through the mediastinum of thorax and pierces the diaphragm at the esophageal hiatus to enter into the abdomen. It joins the stomach at the cardiac orifice which is surrounded by a cardiac sphincter to act as a valve. Muscular diaphragm keeps the sphincter closed when food is not being swallowed.
Heartburn: burning, radiating substernal pain that occurs when the acidic gastric juice regurgitates into the esophagus.
Hiatal Hernia: abnormal relaxation of the gastroesophageal sphincter, where the superior part of the stomach protrudes slightly above the diaphragm.
Esophagus contains all 4 of the alim. canal layers described earlier, with some other notable features: a) mucosa has a nonkeratinized strat. squam. ep and is abrasion resistant, but changes to simple columnar at the stomach junction for secretions b)when esophagus is empty, the mucosa & submucosa are in folds, but are flat when food is in it. c) submucosa has esophageal glands that secrete mucus as bolus passes through to aid the passage d) muscularis is different muscle types throughout e) has an adventitia made up of CT instead of a serosa.

Mouth and its accessory dig organs are involved in most dig processes. The mouth: ingests, mechanically digests through mastication, initiates propulsion by swallowing, and start chem. breakdown of polysaccharides; essentially no absorption occurs in mouth; salivary amylase in saliva digests starch and glycogen allowing smaller fragments of glucose to be freed. Lingual lipase in saliva digests fat in the stomach as well.
Single dig. function of pharynx and esophagus is propulsion through their role in swallowing; they just pass food from mouth to stomach.
Mechanical processes:
-Mastication:cheeks and lips hold food bt the teeth, tongue mixes food with saliva to soften it, and teeth cut and grind food smaller. Partly voluntary, partly reflexive.
-Deglutition (swallowing): tongue first compacts food into a bolus and it is swallowed. Swallowing is complicated, involves over 22 muscle groups,2 major phases. 1)Buccal: occurs in mouth, is voluntary, tip of tongue against hard palate pushes bolus to the oropharynx. 2) Pharyngeal-esophageal: involuntary, controlled by brain stem swallowing center(medulla and lower pons). Tongue blocks off the mouth, soft palate closes off nasopharynx, larynx rises so epiglottis closes it, and upper esophageal sphincter relaxes to let food into the esophagus. Contractions push food from pharynx into esophagus. Solid foods pass from oropharynx to stomach in 8 secs and fluids in 1-2.Gastroesophageal spincter relaxes at end of esophagus to let food to the stomach.
*Food can enter the resp. passageways if we try to talk/inhale while swallowing.

Temporary storage tank where chem. breakdown of proteins begins and food is converted to a paste called chyme.
Diameter and volume depend on how much food is in the stomach. Empty stomach has volume of 50 ml, but when distended it can hold about 4 L of food. Rugae=the folds of the stomach when it is empty.
Cardiac region=small, surrounds portion where food enters. Fundus= dome-shaped, beneath diaphragm.
Body=midportion, continuous inferiorly with the Pyloric region=goes from wider to narrower and forms the canal and pylorus. Pylorus=continuous with the duodenum(first part of s. intestine) through the pyloric sphincter(controls stomach emptying).
Greater and lesser curvatures, which have mesentaries attached to connect stomach to other dig. organs.
Stomach is served by the ANS; sympathetic fibers from splanchnice nerves are relayed through the celiac plexus while paras. fibers are supplied by the vagus nerve.
Arterial supply of stomach= branches of the celiac trunk.