Digestive System : the bottomless pit

• the digestive system is responsible for the intake and breakdown of nutrients necessary for the process of cellular respiration
• there are two main sections of the digestive system : the alimentary canal, which runs from the mouth to the anus, and several accessory organs, which make enzymatic and other secretions to aid in the breakdown of the food
• the alimentary canal (gastrointestinal tract) includes: mouth, pharynx, esophagus, stomach, small intestine, and large intestine
• the accessory organs include: salivary glands, liver, gallbladder, and pancreas

• the wall of the canal is composed of four layers.
• The innermost layer is a mucosa, which is formed of surface epithelium, usually simple columnar with many goblet cells.
• The submucosa is just outside of it, and composed of moderately dense connective, elastic fibers, blood vessels, lymph vessels, and nerves.
• Two coats of smooth muscle tissue make up the muscular layer; the inner coat is the circular fiber layer, and the outer is the longitudinal fiber layer.
• The outermost layer of the alimentary canal is the serous layer, and is composed of the visceral peritoneum.
• The alimentary canal has two basic motor functions associated with it: mixing and movement. Mixing occurs mostly in the stomach. Movement, which is mostly a wavelike motion called peristalsis, propels food along the canal.

Functional Anatomy of the Digestive System

The Mouth: Ingestion and initial breakdown

• oral or buccal cavity
• the mucosa is composed of stratified squamous instead of simple columnar; the gums’ epithelium is slightly keratinized
• the lips and the cheeks are have a core of skeletal muscle covered by skin. The orbicularis oris makes up the lips and the buccinators are the cheek muscles. Both of these keep food in the teeth during chewing and have a small function during speech
• the palate is the uppermost boundary of the oral cavity. The hard palate (anterior) is the palatine bone; the soft palate is mostly skeletal muscle.
• The palatine tonsils are found at the back of the mouth, on either side of the tongue. They are lymphatic tissue, and are commonly infected. The adenoids (pharyngeal tonsils) are on the posterior wall of the pharynx
• the tongue is a large muscular organ that occupies most of the buccal cavity when the mouth is closed. It houses most of the taste buds, and has a core of skeletal muscle. While chewing, the tongue is responsible for constantly repositioning the food and eventually forming it into a bolus (lump) which is pushed to the back of the throat to be swallowed. Lastly, the tongue is also important for speech.
• the lingual frenulum is a small fold of mucosa that holds the tongue to the floor of the mouth and restricts its posterior movements. If the frenulum is too short, speech distortions (ankyloglossia) result.
• the salivary glands secrete saliva, which has a variety of functions:
• cleansing the mouth
• dissolving food chemicals for tasting purposes
• moistening food for aid in bolus formation
• salivary amylase (digestion of starches)
• there are several sets of salivary glands, both intrinsic (in the cheeks) and extrinsic (outside the oral cavity).
• when the parotid salivary glands are inflamed, this could be an indication of mumps.
• if salivary glands are inhibited, halitosis can result
• the teeth are involved in a mechanical breakdown of food. We go through two natural sets of teeth in a lifetime -- the 20 milk teeth and the 32 (or so) permanent teeth
• impacted teeth are quite painful. Wisdom teeth (the third molars) are most commonly impacted
• there are four types of teeth, classified by their shape and function:
• incisors: chisel-shaped, used for cutting
• canines: fang-like, used for tearing and piercing
• premolars: also called bicuspids, have two cusps "tips" for grinding
• molars: have 4-5 cusps for grinding and crushing
• a tooth has two major regions: the crown and the root. the crown is enamel-covered and is the exposed part above the gum (gingiva). Enamel is produced by cells that die after the tooth emerges from the gum, which is why breaches cannot be repaired.
• the embedded portion of the tooth is the root. Canines, incisors and premolars have one root, but molars have two or three (lower vs. upper). The root is connected to the crown by the neck of the tooth. Cementum (a calcified connective tissue) around the root connects it to the periodontal ligament, which anchors the tooth in the jaw, forming the gomphosis joint (peg-in-socket).
• dentin (a bonelike material) underlies the enamel in the crown, and surrounds the central pulp cavity. The pulp of the pulp cavity is made up of blood vessels, nerve fibers and connective tissue. In the root, the pulp cavity becomes the root canal.
• if the tooth’s nerve is killed, the tooth darkens and becomes infected. It must then be removed, in a procedure called a "root canal"
• note that dentin, cementum and enamel, while they resemble bone, are not bone. They are avascular.
• gum and tooth disease :
• cavities (caries) : begin with plaque. Bacteria in the plaque eating sugar produces lactic acid (and other acids), which dissolve the calcium salts of the tooth, leaving the organic material behind for bacterial ravaging.
• if plaque is not removed regularly, it can calcify, forming calculus, otherwise known as tartar. If the tartar forms in the area where the gum borders a tooth (gingiva sulcus), it can disrupt the gum-tooth seal and cause an infection known as gingivitis. Gingivitis is reversible if the calculus is removed.
• if the tartar is not removed, the infection worsens, becoming periodontal disease (periodontitis), which accounts for 80-90% of all tooth loss in adults. Even periodontal disease is reversible in early stages with aggressive treatment: scaling the teeth, antibiotic therapy and cutting the gums after cleaning the infected sulci.

Pharynx : a connecting passageway

• the part of the pharynx that food passes through would be the oropharynx and the laryngopharynx
  
• like the mouth, the pharynx’s mucosa is stratified squamous
• the muscular layer includes the circular pharyngeal constrictor muscles and an outer longitudinal skeletal muscle layer, which help to propel food into the esophagus

Esophagus : down the straightaway!

• the name literally means "carry food"
• roughly 10in (25cm) long
• at the esophageal hiatus, the esophagus "pierces" the diaphragm to move from the thoracic cavity into the abdominal.
• just past the hiatus, it joins to the stomach at the cardiac orifice (aka, cardiac sphincter, hiatal sphincter, or gastroesophageal sphincter). The diaphragm helps to keep this sphincter closed when not in use.
• of note is that the mucosa of the alimentary canal switches from stratified squamous in the esophagus to simple columnar at the cardiac orifice.
• the submucosa of the esophagus secretes a special mucus that further moisturizes the bolus as it moves down the tube
• the muscular layer is completely skeletal in the superior third, half skeletal/half smooth in the middle third and all smooth in the inferior third
• instead of a serous membrane, the esophagus has a fibrous adventitia made of connective tissue
• heartburn occurs when stomach acid refluxes into the esophagus. It may occur in period of overeating, and in obese and pregnant people as well, due to the pressures on the stomach. A hiatal hernia is a condition where the acid refluxes into the esophagus due to the fact that part of the stomach protrudes a bit above the diaphragm.
• peristalsis -- wavelike contractions of the muscular layer -- move the food through the pharynx and the esophagus. It usually takes 4-8 seconds for chewed food to go from oropharynx to the stomach (liquids only take 1-2 seconds).

Stomach : what chyme is it?

• the stomach is a temporary storage chamber for food, and is the place where the initial breakdown of proteins takes place. It is relatively "J" shaped, and can hold about 4 liters of food and liquid when extremely distended (and it will possibly stretch all the way down to the pelvis in this case).
• there are three main regions of the stomach -- the cardiac region, the fundus and the pyloric region
• food enters the stomach through the cardiac orifice, and leaves through the pyloric valve
• the mucosa of the stomach is specially designed for this organ’s role in digestion. It is entirely composed of goblet cells in a simple columnar epithelium.
• the mucosa is dotted with millions of gastric pits which lead to the gastric glands. These secrete the gastric juice.
• several types of cells are found in the gastric glands. mucous neck cells (secrete mucus with an unknown function), parietal cells (secrete HCl), chief cells (secrete pepsin-precursor), and enteroendocrine (hormone-releasing cells) are among the most common
• the pepsin-precursor (pepsinogen) is catalyzed to become pepsin by the hydrochloric acid, and by other pepsin molecules. It can then break down protein molecules.
• the pepsin and HCl in the stomach could destroy the organ, if not for the alkaline mucosal barrier
• gastric ulcers are caused by persistent damage to the underlying stomach tissues. One known cause of stomach ulcers is Helicobacter pylori, a bacterium that works to destroy the mucosal layer, allowing the gastric juices to work on the stomach lining directly.
• the stomach is not well-suited to absorption of food nutrients, though it is responsible of a few fat soluble substances such as aspirin, alcohol and some other drugs
• the stomach’s main job is to continue the mechanical digestion of food started by mastication
• peristaltic waves mix food with the gastric juices, eventually forming a thick, pasty goo called chyme. Chyme will accumulate near the pyloric sphincter, basically waiting for the small intestine to empty enough for the sphincter to open. Only a small amount of chyme is allowed through at a time.
• carbohydrates move into the intestines more quickly than do fats; proteins fall somewhere in between those two
• food may leave the stomach in another way. Emesis, otherwise known in high school circles as "barfing," "spewing," "hurling," or "throwing chunks," is caused by irritation or a distension of the stomach. Irritation could be caused by toxins, excessive alcohol, spicy foods or some drugs. Impulses trigger the emetic center of the medulla and the stomach (and sometimes the duodenal) contents are sent upward.

The Small Intestine : every meal is a winding road

• the longest section of the alimentary canal, the three sections of the small intestine combined stretch out to 20-25 feet.... in a cadaver. In a living human, the small intestines would only stretch maybe 6 feet, due to muscle tone. Its diameter is about 1 inch at most.
• there are three sections of the small intestine. In the order that food passes through them, they are :
• duodenum: "12 finger widths long," is about 10 inches long. It is here that bile and pancreatic juices are delivered
• jejunum : "empty," is about 8 feet long.
• ileum : "twisted," is about 12 feet long.
• the mesentery is a connective tissue that holds the intestines in place against the posterior abdominal wall. Blood and lymphatic vessels run through the mesentery, supplying the intestines. Venous blood from the small intestine, laden with nutrients, is drained into the hepatic portal vein.
• the vagus and splanchnic nerves -- parasympathetic and sympathetic, respectively -- innervate the intestines
• the duodenum is where the last part of chemical digestion will take place. Bile and pancreatic juices are secreted into the duodenum through a common duct in the major duodenal papilla, which is controlled by a sphincter valve called the sphincter of Oddi (also called the hepatopancreatic sphincter)
• bile is not an enzyme. It is a chemical composed of bile salts, bile pigments, cholesterol, neutral fats, phospholipids (esp. lecithin) and a variety of electrolytes. Only the bile salts and the phospholipids work in digestion.
• bile salts -- cholic acid and chenodeoxycholic acids -- are cholesterol derivatives. They emulsify fats. This is so that fat digesting enzymes have more surface area to work on. Bile salts also aid in the absorption of fats and lipids.
• bile pigments include bilirubin, which is broken down to urobilinogen and stercobilin in the large intestine by bacteria. Without bile secretion, feces would be white with fatty streaks. The fatty streaks would be due to the fact that virtually no fats could be digested or absorbed without the bile salts).
• pancreatic juice is a mixture of water, enzymes and electrolytes. Water is the primary component.
• the pH of pancreatic fluid is around 8. This is due to the fact that epithelial cells in the pancreatic duct add bicarbonate ions to the juice as it passes. This alkalinity allows the juice to neutralize the acidity of the chyme as it moves through, and also allows for the activation of some pancreatic enzymes.
• some of the enzymes released by the pancreas include: amylase (starches/carbohydrates), trypsin (proteins), lipase (fats), and nucleases (nucleic acids).
• trypsin is initially released as trypsinogen, which is activated to trypsin form by enterokinase, found in the microvilli of the intestine (the "intestinal brush border").
• the small intestine is, in whole, designed for nutrient absorption, and the wall of the intestine shows this function. The "velvety appearance" of the intestinal lumen border is due to the presence of seemingly infinite projections of the innermost tissue layer, forming small villi, which are most numerous in the duodenum and the jejunum. Each villi is in turn covered with hundreds of microvilli, or a "brush border."
• The villi, at their longest, are about 1 millimeter in length, and are formed of the simple columnar epithelium of the mucosal layer. They are large and "leaflike" in the duodenum, where most absorption takes place, and gradually become smaller as you move into the jejunum and ileum.
• In the center of each finger-like projection of the intestinal wall, there is a dense capillary bed for the intake of food nutrients to be distributed through the body. There is also a structure called a lacteal, which is a modified lymph capillary designed to absorb fats.
• there are also special structures in the small intestine called the circular folds. These deep, permanent folds of the intestine force food through in a spiral method. Instead of mechanically mixing the food as chewing or "squishing" in the stomach does, the spiraling of chyme through the duodenum effectively slows it down enough to be affected by the digestive juices, and exposes more surface area to the digestive enzymes. In addition, the circular folds will allow more time for nutrient absorption.
• peristalsis will move the chyme into and through the duodenum, but there usually isn’t much demand to "rush" the food through the rest of the intestine and into the large intestine.
• If there is overstimulation of peristaltic waves, diarrhea may result. Irritation of the intestinal wall (due to bacteria, virus, or chemicals) may cause diarrhea.
• the four major layers of the GI tract are present here, but modified.
• in addition to the simple columnar cells, there will be goblet cells and enteroendocrine cells as well.
• between villi, there are intestinal crypts pits that secrete intestinal juice which would further moisturize that food. Deep in these pits, Paneth cells have been found which secrete lysozyme as a protection from bacteria.
• the submucosa is a typical areolar connective tissue, which contains lymphoid nodules called Peyer’s patches. The closer to the large intestine, the more Peyer’s patches. Early on, in the duodenum, special mucus-secreting glands called Brunner’s glands, are found. They also help to reduce the acidity of the chyme as it enters the small intestine.
• the muscle layer is typical, bilayered, as is the serous membrane.
• the small intestine ends at the junction between it and the large intestine, called the ileocecal valve. Once food has moved through this valve, it is not allowed back into the ileum.

Large Intestine : the home stretch

• except in the terminal end, the longitudinal muscle layer in the large intestine is reduced to three bands of smooth muscle called teniae coli ("ribbons of the colon"). The tone of these muscles causes the characteristic haustra, or pocketlike sacs, of the large intestine.
• one other unique feature of the large intestine includes the epiloic appendages, which are small, fat-filled pouches of visceral peritoneum that hang from the surface of the intestine. They have unknown significance.
• There are several subdivisions of the large intestine:
• the cecum is the first part of the intestine after the ileocecal valve. The vermiform appendix is attached to the cecum.
• the colon, made of distinct regions, is the part of the intestine that travels up and around the small intestine. It includes the ascending, transverse, sigmoid and descending colons.
• the sigmoid colon attaches to the rectum, which in turn leads to the anal canal.
• except in the anal canal, the mucosa of the large intestine is simple columnar. In the anal canal, the mucosa hangs in long ridges or folds called anal columns, which contain stratified squamous.
• many types of bacteria live in the large intestine. They somehow survive the attacks of HCl, lysozyme, and proteases, and will constitute the bacterial flora of the intestine. They help to breakdown indigestable material such as cellulose.
• intestinal bacteria often produce gases as a by-product of fermentation. about 500ml of flatus is produced a day, and some is quite foul -- hydrogen sulfide and methane, especially.
• some intestinal bacteria synthesize B complex vitamins and vitamin K, which is absorbed.
• the primary focus of the large intestine is expulsion of food wastes in the form of feces. A secondary function is absorbing water, electrolytes and vitamins from the food waste. Therefore, the large intestine is not even vital for life.
• Peristalsis in the large intestine is all but unknown. Wave-like movement of the haustra (haustral contractions) occur every 30 minutes, and mass movements (mass peristalsis) occur three or four times daily, typically just after eating.
• bulk (fiber) in the diet increases the strength of colon contractions and softens the stool
• without enough fiber in our diets, the result is usually constipation
• semi-solid feces are pushed to the rectum. This includes food residues, mucus, sloughed off epithelial cells, bacteria, and a tad bit of water to allow smooth passage.
• every 500ml of food residue that enters the cecum results in about 150ml of fecal matter.
• the defecation reflex -- a spinal cord mediated parasympathetic reflex -- occurs when the rectal wall is stretched by feces being forced into by mass movements. The defecation reflex causes the sigmoid colon and rectal walls to contract and the anal sphincters to relax. As the fecal material enters the anal canal, our brains decide if the external (voluntary) anal sphincter should open or not.
• You can only delay the inevitable for so long. If you resist the urge at first, the next mass movement of feces into the rectum will cause and even stronger reflex and desire. Eventually, the urge will become unavoidable.
• defection is a combination of involuntary and voluntary muscles. The muscles of the rectum contract on their own, but we voluntarily close our glottis and contract our diaphragm to increase the intra-abdominal pressure. We also contract the levator ani muscle to pull the anal canal superiorly to "leave" the feces below the anal canal and therefore outside the body.
• diarrhea occurs in any condition that rushes food through the intestine before it can absorb the remaining water in the food residue. The most common cause is intestinal irritation due to bacteria.

Chemical Digestion : enzymatic hydrolysis of foodstuffs

• a catabolic process involving the addition of a water molecule in order to break the chemical bonds of the food molecules.
• carbohydrates -- about 200-600 grams/day are ingested by the average person
• monomer is the monosaccharide (glucose, fructose, galactose)
• digestible disaccharides include sucrose, lactose and maltose; digestible polysaccharides include glycogen and starch
• indigestible carbohydrates, such as cellulose, act as bulk
• salivary amylase begins carbohydrate breakdown, splitting starch into oligosaccharides. Pancreatic amylase picks up where the stomach’s pepsin forced the salivary amylase to leave off. Intestinal brush border enzymes (most importantly dextrinase and glucoamylase on oligosaccharides of more than 3 units, and maltase, sucrase and lactase) complete the breakdown to monosaccharides.
• chemical digestion of carbohydrates officially ends in the small intestine, though the bacterial flora of the large intestine does work to metabolize cellulose and some other "indigestable" carbohydrates
• lactose intolerance is a problem due to the lack of lactase. It results in water loss (due to the undigestable lactose sugar causing an abnormal osmotic gradient), and diarrhea, as well as high levels of flatulence, bloating and cramps due to bacterial metabolism of the lactose.
• carbohydrate absorption involves the movement of monosaccharides by protein carriers into the epithelial cells. They then move into the blood stream.
• proteins -- we eat about 125 grams/day, but there is an additional 15-25 grams/day of enzymes of our own that we digest
• monomer is the amino acid
• digestion begins in the stomach, with pepsin. Pepsin preferentially cleaves bonds that include tyrosine and/or phenyalanine.
• pepsin is inactivated in the high pH of the duodenum (its optimal range is 1.5-3.5). Other proteolytic enzymes attack the peptide fragments. Trypsin and chymotrypsin (pancreas) make even smaller peptide fragments, and carboxypeptidase (intestinal brush border) works on these fragments by taking off one amino acid at a time. Aminopeptidase and dipeptidase (intestinal brush border) also liberate single amino acids.
• absorption is carried out by one of several carrier proteins (differenct carriers pick up different classes of amino acid). Rarely, whole proteins are picked up via endocytosis and released into the bloodstream by way of exocytosis. This can provoke an allergic response.
• lipids -- varies between 30 and 150+ grams/day.
• monomers are fatty acids and monoglycerides
• the small intestine is the sole spot for lipid digestion. Lipids need to be "pre-treated" with emulsifying agents in bile before the pancreatic lipases can break the triglyceride apart.
• digested lipids form micelles, collections of fatty elements along with bile salts (especially lecithin). Micelles can easily diffuse between the microvilli to come in contact with the mucosal surface. The fatty elements then leave the micelle and via simple diffusion, enter the epithelial cell. Inside the cell, the fatty elements are reassembled into triglycerides, cholesterol and phospholipids, then "coated" with proteins to form a chylomicron. The chylomicron is processed by the Golgi complex for exocytosis and sent out into the lacteal, mainly due to their size (they are too big to move through the basement membrane into the capillary). Once in the bloodstream, the chylomicron is re-hydrolyzed by lipoprotein lipase. The glycerol and fatty acids can then diffuse out of the bloodstream to where they are needed.
• nucleic acids -- only small amounts are ingested
• monomers are nucleotides
• pancreatic nucleases, along with intestinal brush border enzymes (phosphatases and nucleosidases) break down DNA and RNA into their monomers
• absorption involves special carriers for each part of the strand (pentose, base and phosphate).
• vitamins -- absorption of ingested vitamins takes place in the small intestine. Bacterially made vitamins are absorbed in the large intestine.
• electrolytes -- iron and calcium absorption are limited to the duodenum, but other electrolytes are absorbed pretty much anywhere along the alimentary canal.
• absorption of sodium is coupled in the small intestine to the absorption of monosaccharides and amino acids
• chloride ions are actively transported, while potassium enters by way of simple diffusion
• water -- about 9L enters the small intestine daily, but most of it is derived from GI tract secretions
• 95% of it is absorbed in the small intestine via osmosis