digestive system and stomach

Distention of the stomach and small intestine stretching of the abdominal wall, the hormone cholecystokinin and the mechanical activity of chewing and swallowing all inhibit the hunger center.

Swallowing (deglutation) ; primer and seconder peristatic movements along the osophagus
Osophagal sphincter
Stomach; contraction is more stronger in antrum than fundus.
Cardiac and pyloric sphincter
Small intestine; peristalsis, segmentation and penduler contraction
Large intestine; segmentation, mass movement and antiperistalsis.
Anal sphincter; internal and external and defecation occur.

Vitamin absorbtion

Vitamins are very important helpers for human being survival . Vitamins are absorbed in small intestines. The two types of vitamins : first water-soluble vitamins (all the B vitamins and vitamin C) and second fat-soluble vitamins (vitamins A, D, E, and K).
Fat-soluble vitamins are stored in the liver and fatty tissue of the body. Water-soluble vitamins are not easily stored and excess amounts go away from body in urine.

Information sources for Digestive and nutrions

Information source about digestive systems and feedings , check the below foundation's websites.

American Dietetic Association
120 South Riverside Plaza, Suite 2000
Chicago, IL 60606–6995
Phone: 1–800–877–1600 or 312–899–0040
Fax: 312–899–4739
Email: hotline@eatright.org
Internet: www.eatright.org

Protein digestion

Protein is very critical biological components of life. I mean we are all of us need proteins to maintain our daily body systems. Foods such as meat, eggs, milk, cheese, beans contains lots of molecules of protein. we will use proteins to repair our body. An enzyme in the juice of the stomach starts the digestion of swallowed protein. Then in the small intestine, several enzymes from the pancreatic juice and the lining of the intestine complete the breakdown of huge protein molecules into small molecules called amino acids. These small molecules can be absorbed through the small intestine into the blood and then be carried to all parts of the body.

Why is digestion important?

So you can ask why is digestion is very important. Let me explaing : when you eat some foods—such as bread, meat, fruits, pizza etc. The body cant use them as in this form. It must be suitable for body cells. Food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. So this mean that we need digestion process. Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy.

large intestine and functions

large intestine: 

With a diameter or contour of between 7 and 10 centimeters, the large intestine is indeed thicker than the thin, and it is almost the last stop making food in the digestive tract. Like the small intestine is retreated about himself on the inside of your body, and, if it spreads completely, would measure about 1.5 meters long. The large intestine is a tube with a closed end that stands out and that is called the appendix. Although the appendix is part of the digestive tract, does not appear to play any role, but it can give problems because sometimes becomes infected and must be removed (ie, drawn by an operation). 

As we have said, once it has removed most of the nutrients from the liquid mixture of food, is what is known as waste products, the material that I could not use your body and you have to be expelled to the outside. Can you guess where it ends? Well, here's a hint: to lose sight of, you have to pull the string. 

Before being expelled, waste products pass through the portion of the large intestine called the colon, which is where the body is the last chance to absorb water and some minerals, discharges into the bloodstream. As the waste products are losing water, are tightening while moving the large intestine into a solid. Yes, it is the cacas (more finely called stool, stools or bowel movements). 

The large intestine is pushing the stool until he arriving in the rectum, the last portion of the digestive tract. The products in solid waste remain here until you have wanted to go to the bathroom. When you go to the bathroom, you emerge from these expelling waste products through the anus. And now is when the chain comes into action! 
Check a hand to your digestive system 

You can help your digestive system by drinking water and taking a healthy diet containing foods rich in fiber. The fiber-rich foods such as fruit, vegetables and whole grains, helping to move stool through the digestive tract. The digestive system is a very important part of your body.

liver and functioncs

The next stop for nutrients is the liver! And the waste products - the leftovers of the foods that your body can not use - will continue moving towards the large intestine. 
Loves your liver 

The blood is rich in nutrients directly to the liver, where it is processed. The liver is responsible for filtering harmful substances or waste products, transforming some of these wastes more bile. The liver helps to determine what amount of nutrients will be distributed to the rest of the body, and how much will be saved as a reserve. For example, the liver stores certain vitamins and a type of sugar that the body uses for energy.

small intestine - another organ of digestion system

More than 6 meters of small intestine 

The small intestine is a long tube with a diameter or contour between 3.5 and 5 centimeters, which retreated on himself inside yourself, below the stomach. If you extend your computer's small intestine, measured about 6.7 meters long - as 22 notebooks lined one after another! 

The small intestine plays an important role in breaking down the mix of food from the stomach even more, so your body can absorb all the nutrients it contains vitamins, minerals, proteins, carbohydrates and fats. The roast chicken contains many proteins - and a little fat - and the small intestine will help you absorb them, as long as their friends - the pancreas, liver and gall bladder - you throw a hand. 

The bodies just mentioned various juices to send the first portion of the small intestine. These juices help to digest food and allow the body to absorb the nutrients they contain. The pancreas produces some juices that help the body to digest fats and proteins. Which secretes a juice called bile, the liver helps to absorb fats in the bloodstream. And the gallbladder is like a store bile, which stores this juice to your body when you need it. 

The foods you eat can stay for up to 4 hours in your small intestine until it becomes a liquid mixture and watery. It is a time well spent, because at the end of the trip, nutrients from the pizza, chicken, oranges and milk may move from the intestine into the blood. Once in the blood, your body will be much closer to harness the complex carbohydrates in the body of the pizza, the vitamin C of oranges, chicken protein and calcium from milk.

stomach and digestion occurs

Once the food enters the esophagus, it does not go directly into the stomach. Instead, the muscles in the walls of the esophagus moves describing a crushing wave movement to get the food while they do fall down the esophagus. This takes about 2 or 3 seconds. 

The stomach is tied to the lower end of the esophagus. This is a "bag" elastic that is shaped like the letter "j". Important plays three roles: 
Store the food you eat 
Decomposed food in a liquid mixture 
Slowly emptying the liquid to the small intestine 

The stomach acts as a mixer, stirring and breaking balls all the food from the esophagus into ever smaller fragments. This was done with the help of the strong muscles you have in your walls and gastric juices they secrete. Apart from fragmenting and break down food, gastric juices also help to destroy germs and bacteria that may contain the food they eat.

esophagus tube

The esophagus is an elastic tube, which measures about 25 centimeters long. Leads the food from the back of the throat to the stomach. But in the back of the throat is also the trachea, which allows air to enter and exit your body. When you swallow a ball of food and crushed or softened some liquid, a barb of a special fabric called epiglottis closes the opening in the trachea to ensure that the food enters the esophagus, rather than the trachea. 

If you've ever drunk a little too fast, you've started coughing and someone said that the drink "you'll be gone by the other side", which meant that person is the liquid you had come into the trachea by mistake . This happens when the epiglottis does not give you time to close, and you'll be coughing involuntarily (without thinking about it) to clear the trachea.

The digestion begins in the mouth

The digestion begins in the mouth 

Before that first mouthful of food at a palatable when you smell, what you see or think about him, starts digestion. You are starting to form saliva in the mouth. When you eat, saliva begins the process of decomposition of the chemicals they contain food and aid to soften to make it easier to swallow. The language helps push food through your mouth while you chew with the teeth. When you're ready to swallow, the tongue pushes a piece of food and crushed softened, called a food bolus, toward the back of the throat, to enter through the opening of the esophagus, the second part of the digestive tract.

your digestive system

Your digestive system began working even before they drive the tooth to the pizza. And will continue to busy digesting your food freshly chewed over the next few hours - or sometimes days, depending on what you've eaten. This process, called digestion, allows your body get the nutrients and energy it needs from your diet. Now let's find out what's happening with your pizza, your chicken, your orange and your milk.

eating and mouth

As you sit at mealtime, enjoy your pizza, grilled chicken to you and your orange. After devouring all these delicacies, the auction fatigues with a glass of milk, you clean your mouth and you go to the next class. Within a few minutes thinking about your city or your favorite work of science. You've completely forgotten what you just eat. But you'll still in the stomach - a kind of scientific experiment that happens constantly.

Digestive System in Mammals

General anatomy of digestive system
Ozofagus;1/3 is striated muscle, 2/3 is smooth muscle.
Small intestine; duodenum, jejenum, ileum.
The end of ileum is appendix (Secum in horse) in human.
Large intestine; colon, rectum, anusThe structure of digestive tract
Muscularis mucosa
Mucus layer
Submucosa
Plexus myentericus
In the large intestine, longitiudinal muscles concentrated as longitiuidinal line called tenia coli.
Concentrated circular muscle in large intestine produce haustra formation.
The length of digestive tract depends on type of food ingested by animals.
Longest in herbivours, shorter in carnivoursIngesting of food
Hunger drive is controlled by hypothalamus
Glucostatic theory of hunger suggests that an increase in blood glucose level increases the activity of the satiety center and decreases the activity of hunger center.
Low blood glucose has the opposite effects.
Aminoacid levels and lipid levels have also been suggested to influence hunger and satiety.
The hepatostatic theory of hunger control argues for a role of liver as an important sensor and modulator of the body energy stores. Distension of the stomach and small intestine, stretching of the abdominal wall, the hormone cholecystokinin and the mechanical activity of chewing and swallowing all inhibit the hunger center.
Swallowing (deglutation); primer and seconder peristaltic movements along the osophagus
Osophagal sphincter
Stomach; Contraction is more stronger in antrum than fundus
Cardiac and pyloric sphincter
Small intestine; peristalsis, segmentation and penduler contractionLarge intestine;Segmentation, Mass movement and antiperistalsis.
Anal sphincter; internal and external.
Defecation.

Liver Lobules

Classic liver lobule in digestive system This model is based on the direction of blood flow. In sections, liver substructure exhibits a pattern of interlocking hexagons; each of these is a classic lobule. Whereas lobules in pigs are defined by a sheath of connective tissue, there is less connective tissue in humans and the lobule boundaries are indistinct. The boundaries of human lobules can be estimated, however, by noting the positions of the portal triads at the lobule periphery, the central vein at its center, and the alternating hepatocyte plates and sinusoids that lie between them. a. Portal triad. One triad occupies a potential space (portal space) at each of the 6 corners of the lobule. Each triad contains 3 main elements surrounded by connective tissue: a portal venule (a branch of the portal vein), a hepatic arteriole (a branch of the hepatic artery), and a bile ductule (a tributary of the larger bile ducts). A lymphatic vessel may also be seen. b. Central vein. A single vein marks the center of each lobule. This vessel is easily distinguished from those in the portal triad by its larger opening and lack of a connective tissue investment. c. Hepatocyte plates and hepatic sinusoids. Many such plates radiate from the central vein toward the lobule periphery (like the spokes of a wheel). The plates are separated by hepatic sinusoids, which receive blood from the vessels in the triads, converging on the lobule center to empty directly into the central vein.

LIVER

A. General Structure: The liver is the body's largest gland. It is partly covered by a thin capsule (Glisson's capsule) and has a sparse, delicate, reticular connective tissue stroma accompanying the blood vessels as they penetrate the parenchyma. Its predominant cell type is the hepatocyte. These cells are arranged in one- or 2-cell-thick plates that are separated by the hepatic sinusoids. The liver has a dual blood supply, the portal vein and the hepatic artery; it also has 3 drainage systems, the hepatic veins, lymphatic vessels, and bile ducts.


B. General Functions: The liver has several important functions, most of which are carried out by hepatocytes. Its main role in digestion involves the enzymatic processing (metabolism) of nutrients absorbed by the intestines to provide the body with the chemical building blocks and fuel needed to support life. Some hepatocyte enzymes aid in detoxification by modifying potentially dangerous chemicals and drugs and rendering them harmless. Hepatocytes synthesize many important proteins; albumin, prothrombin, fibrinogen, lipoproteins) and secrete them into the blood, thus acting as an endocrine gland. They also synthesize bile from the wastes of erythrocyte destruction and secrete it into the biliary tract, acting as an exocrine gland. The liver also serves as a storage site for glucose, fats, and vitamin A


nC. Blood Supply:
n1. Hepatic portal vein. 2. Hepatic artery. 3. Hepatic sinusoids. 4. Central veins. 5. Hepatic veins.

nD. Cell Types:
n1. Hepatocytes. 2. Kupffer's cells. Monocyte-derived members of the mononuclear phagocyte system. 3. Fat-storing cells.

Cell Types : Pancreatic acinar cells 2. Endocrine cells.

Digestive System
1. Pancreatic acinar cells. Each acinus consists of several pyramid-shaped, enzyme secreting cells whose apices border on a small lumen and whose bases abut a basal lamina. Acinar cells synthesize a wide variety of enzymes that can hydrolyze proteins (proteases, such as trypsin, chymotrypsin, and elastase), lipids (lipases, such as triacylglycerol lipase and phospholipase A,), carbohydrates (amylase), and nucleic acids (ribonuclease and deoxy ribonuclease). Enzymes are packaged and stored in the acidophilic apical region as membrane-bound zymogen granules. Here they await exocytosis in response to stimulation by cholecystokinin, which is produced by enteroendocrine cells in the small intestine, or parasympathetic stimulation via the vagus nerve. The enzymes in the granules are zymogens, which are inactive before the release. One such zymogen, trypsinogen, is enzymatically converted to the active protease trypsin in the small intestine by enterokinase, an enzyme that is secreted by enterocytes.


2. Endocrine cells.

PANCREAS

General Structure and Function of Pancreas : The pancreas is a serous, compound acinar gland that resembles the parotid gland in its microscopic appearance. important member of digestive system.

It differs in that it lacks striated ducts and contains islets of Langerhans.

The lobules of the pancreas contain serous adenomeres that secrete a variety of digestive enzymes into a branched duct system that empties into the duodenum.

SALIVARY GLANDS

A. General Structure and Function: Three major pairs of glands, the parotid, submandibular, and sublingual, surround the oral cavity. The lobules of each gland contain numerous adenomeres that empty their secretions (saliva) through a series of intercalated, striated, and interlobular ducts into the oral cavity. The saliva moistens the food, lubricates the digestive tract, and begins the enzymatic digestion of carbohydrates. The glands also excrete certain salts; they protect against bacterial invasion through the mouth by releasing lysozyme and IgA into the saliva.


B. Cell Types:

1. Serous and mucous cells are the predominant secretory cells of salivary adenomeres of digestive system. The key to identifying the 3 types of salivary glands in tissue sections lies in knowing the differences in the staining properties of the cells, their organization, and the proportion of each type found in each gland. a. Serous cells. These relatively small basophilic cells produce a protein-rich, watery secretion and usually form acinar (spheric) adenomeres. b. Mucous cells. Larger and more acidophilic than the serous cells, these may have a foamy appearance. They produce a thick glycosaminoglycan-rich secretion (mucus) and usually form tubular adenomeres.


C. Parotid Glands: These branched acinar glands contain almost exclusively serous secretory cells. The granules in these cells are PAS-positive (owing to their polysaccharide content) and are rich in protein. Parotid secretions, about 25% of the total salivary volume.

D. Submandibular (Submaxillary) Glands: These branched tubuloacinar glands, which produce about 70% of the salivary volume, contain both serous and mucous adenomeres (mostly serous). The serous acini are composed of small basophilic cells with PAS-positive cytoplasm and basal membrane infoldings.

E. Sublingual Glands: These are also branched tubuloalveolar glands containing both mucous and serous cells (mostly mucous). While only mucous adenomeres are present, many are capped by serous demilunes. These glands produce about 5% of the salivary volume.

ANAL CANAL

APPENDIX (VERMIFORM APPENDIX)

-This is a narrow fingerlike evagination of the inferior end of the cecum. Histologically, it resembles the colon except that it has a smaller lumen, fewer and shorter crypts, many more lymphoid nodules, and no teniae coli.

-ANAL CANAL
-In humans, this canal is about 4 cm long and connects the rectum and the anal opening. The mucosa of the first 2 cm has typical colonic epithelium with very short crypts. This is replaced by stratified squamous epithelium, which continues to the anal opening.

LARGE INTESTINE (COLON)

This includes the cecum; the ascending, transverse, descending, and sigmoid colon; and the rectum. It converts undigested material received from the small intestine into feces by removing water and adding mucus. The colon is shorter than the small intestine and has a wider lumen. The colon's lining has no folds, except in the rectum. No villi are present. The epithelium is simple columnar with a great abundance of goblet cells. The mucosa has many deep crypts of Leiberkuhn, containing abundant goblet cells and few enteroendocrine cells.

Parts of Small Intestine

1. Duodenum. The major distinguishing feature of this C-shaped first part of the small intestine is the presence of duodenal (Brunner's) glands in the submucosa. The mucous cells of these glands produce an alkaline secretion. It protects the duodenal lining from the acidity of the chyme and raises the luminal pH to the optimum level for pancreatic enzyme activity. It is also the point of entry for the bile and pancreatic ducts, which penetrate the full thickness of the duodenal wall. It typically exhibits fingerlike or leaflike villi and relatively few goblet cells.

2. Jejunum. An intraperitoneal organ, the jejunum has long leaflike vilii, many plicae circulares, and an intermediate number of goblet cells. The key to its identification, however, is that although it has villi (and is thus part of the small intestine), it contains neither Brunner's glands nor Peyer's patches.

3. Ileum. This intraperitoneal organ has fewer villi, which are short and broad-tipped (clublike), and relatively abundant goblet cells.

Small Intestine Properties

1. Villi. The presence of these epithelium-covered fingerlike mucosal projections into the lumen is the most diagnostic feature of small intestine structure. The villi increase the mucosal surface area about 10-fold and thus enhance absorption; their shape and abundance differ according to the region where they are located.
2. Intestinal glands (crypts of Lieberkuhn), These simple tubular glands (often coiled) extend into the lamina propria below the bases of the villi. They are lined by absorptive, goblet, Paneth's, enteroendocrine, and undifferentiated cells. Their secretions enter the lumen via small openings between the villi. Similar glands are seen in the large intestine, where they contain many more goblet cells.
3. Enterocytes (absorptive cells). These are the predominant cell type covering the villi. They occur in small numbers in the crypts. The approximately 3000 microvilli per cell give the cell-lumen border a striped appearance, referred to as a striated border.

4. Goblet cells. These lie between the absorptive cells, with more in the surface epithelium than in the crypts. They gradually increase in number from the duodenum to the ileum.

5. Paneth's cells. Lying in the bases of the crypts, these cells synthesize a protein polysaccharide complex. In addition to RER and Golgi complexes, they have many large acidophilic secretory granules that contain lysozyme, an antibacterial enzyme that may help control the intestinal flora.

6. Enteroendocrine cells. Most known types of enteroendocrine cells are found in the crypts of the small intestine. Those that occur mainly in this area produce hormones and amines such as secretin, which increases pancreatic and biliary bicarbonate and water secre tion; cholecystokinin, which increases pancreatic enzyme secretion and gallbladder contraction; gastric inhibitory peptide, which decreases gastric acid production; and motilin, which increases gut motility.

7. Undifferentiated cells. Mucosal epithelial cells undergo continual turnover. Replacement occurs through the mitosis of undifferentiated (stem) cells located near the base of the crypts. Products of these divisions differentiate into all the cell types described above; by a mechanism that is still unclear, they move toward the crypt base or toward the tips of the villi, from which they are finally sloughed into the lumen.

SMALL INTESTINE

The small intestine, which includes the duodenum, jejunum, and ileum, receives chyme from the stomach, bile from the liver, and digestive enzymes from the pancreas. Here, nutrients are hydrolyzed into an absorbable form; they are absorbed and transferred to blood and lymphatic capillaries. Undigested material is moved to the large intestine by peristalsis. The word small refers to diameter, not length: the small intestine is longer and narrower than the large intestine.
A. General Structure: The walls of the small intestine have the same layers as do the rest of the tract. A series of permanent folds, the plicae circulares (valves of Kerckring), composed of both submucosa and mucosa, extend into the lumen and increase the surface area about 3-fold. The main distinguishing features of the small intestine (as viewed through the microscope) are in the composition and organization of the mucosa.

B. Mucosa of the Small Intestine: This consists of simple columnar epithelium with goblet cells, underlain by a lamina propria and separated from the submucosa by a muscularis mucosae.

Histology Of STOMACH

This dilated portion of the digestive tract temporarily holds ingested food, adding mucus, acid, and the digestive enzyme pepsin, Muscular contractions of the stomach blend these components into a viscous mixture called chyme, The chyme is then divided into parcels for further digestion and absorption by the intestines.
A. General Structure: The stomach wall has the same layers as the rest of the tract. The complex mucosa contains numerous gastric glands, a 2-3-layer muscularis mucosae that helps empty the glands, and an intervening lamina propria. When the stomach is empty and contracted, the mucosa and underlying submucosa are thrown into irregular, temporary folds called rugae, that flatten when it is full. The smooth muscle of the muscularis externa is arranged in 3 layers: outer longitudinal, middle circular, and inner oblique. The stomach has 4 major regions: cardia, fundus, body, and pylorus.
B. Gastric Mucosa: The stomach lining of simple columnar epithelium is perforated by numerous small holes called foveolae gastricae, The foveolae are the openings of epithelial invaginations, the gastric pits, which penetrate the lamina propria to various depths. The pits serve as ducts for the branched tubular gastric glands, Each gland has 3 regions: an isthmus at the bottom of the pit, a straight neck that penetrates deeper into the lamina propria (perpendicular to the surface), and a coiled base that penetrates deeper still and ends blindly just above the muscularis mucosae. The mucosa is characterized by the following epithelial cell types.
1. Surface mucous cells. They secrete a neutral mucus that protects the stomach's surface from the acidity of the gastric fluid.
2. Undifferentiated cells. Low columnar cells with basal ovoid nuclei are found scattered in the neck of the gastric glands.
3. Mucous neck cells occur singly or in clusters between the parietal cells in the neck of the gland. They differ from the surface mucous cells by secreting acidic mucus.
4. Parietal (oxyntic) cells secrete HCI and intrinsic factor. Parietal cell secretion is stimulated by cholinergic nerve endings. Acid production is greatly enhanced by histamine and gastrin produced by enteroendocrine cells in gastric glands (and elsewhere).
5. Chief (zymogenic) cells secrete pepsinogen and some lipase. These cells are smaller than parietal cells. They are basophilic owing to the ribosomes associated with their RER. They also contain membrane-limited pepsinogen-filled zymogen granules. Pepsinogen is an inactive proenzyme or zymogen that is converted to the active protease pepsin when exposed to the acidic environment of the stomach lumen. Gastric lipase has only weak lipolytic activity. 6. Enteroendocrine cells. In the stomach, these cells occur mainly in the base of gastric glands. They produce various endocrine and paracrine amines leg, histamine, serotonin) and peptide hormones leg, gastrin).

C. Regional Differences:
1. Cardia. the cardia surrounds the point of entry of the esophagus. Here, the lamina propria contains simple or branched tubular cardiac glands like those in the terminal part of the esophagus. The basal portions of these glands are often coiled, with wide lumens.
2. Fundus and body. The glands in these regions are similar in structure and function. The body is the stomach's largest region, extending from the cardia to the pylorus. The fundus is a smaller, roughly hemispheric region that extends above the cardia. Gastric glands--termed fundic glands in both regions--are characterized by shallow pits and long glands. The pits extend about a third of the distance from the mucosal surface to the base of the glands. Fundic glands contain abundant parietal and chief cells. Parietal cells are concentrated in the neck and upper part of the base, while chief cells predominate in the lower portion. Serotonin (5-hydroxytryptamine)-secreting cells are typically found at the bases of these glands.
3. Pylorus. This makes up the distal 4-5 cm of the stomach, leading to the small intestine. Pyloric glands are characterized by deep pits and short glands. Chief cells are especially scarce in this region. Gastrin-secreting cells (G cells) are typical of the bases of these glands. At the pylorus-small intestine junction, a thickened band of the middle circular layer of the muscularis externa, the pyloric sphincter, controls the passage of chyme.

PHARYNX

A short, broad, muscular tube that lies behind the tongue and soft palate, the pharynx is shared by the respiratory and digestive tracts. Its superior portion, the respiratory pharynx, lies above the soft palate; it communicates with the nasal cavity and is lined by respiratory epithelium. The inferior portion, the oral pharynx (oropharynx), lies below the level of the soft palate.
It communicates with the oral cavity and is lined by nonkeratinized stratified squamous epithelium. The pharynx also communicates with both the esophagus and the larynx. During swallowing, the back of the tongue helps close the epiglottis to direct food away from the larynx and into the esophagus.

ORAL CAVITY

The upper end of the digestive tract is bounded anteriorly by the teeth and lips, posteriorly by the oral pharynx, laterally by the teeth and cheeks, superiorly by the hard and soft palate, and inferiorly by the tongue and floor of the mouth.

A. Wall Structure: The mucosa includes the lining epithelium and the underlying lamina propria. Nonkeratinized stratified squamous epithelium (mucous membrane) covers all internal surfaces of the oral cavity and pharynx except the teeth. The lamina propria is a vascular connective tissue with papillae like those of the dermis. The papillae contain capillaries that nourish the epithelium. The oral cavity has no muscularis mucosae. The submucosa is a more fibrous connective tissue than the lamina propria; it contains many blood vessels and small salivary glands. The oral cavity lacks a standard muscularis externa. Skeletal muscle underlies the submucosa in the lips, cheeks, tongue, floor of the mouth, oral pharynx, soft palate, and its downward extension, the uvula. Bone underlies the thin submucosa of the hard palate and gums (gingiva).

B. Lips: Here, there is a transition from nonkeratinized mucous membrane to the keratinized stratified squamous epithelium of the skin. The thin keratinized layer covering the lips' ver million border allows the reddish color of blood in vessels of the lamina propria to show through. Hair follicles, keratin, and additional pigment help distinguish the outer lip surface from the inner in tissue sections.

C. Tongue: This is a mass of skeletal muscle covered by a mucosa. The mucosa is bound tightly to the muscle by the lamina propria, which penetrates between the bundles of muscle fibers. There is little or no submucosa. The muscle is arranged in bundles of many sizes; these are separated by connective tissue and cross each other in 3 planes. This gives the tongue the flexibility required for speech, positioning food, chewing, and swallowing. The mucosa differs on the dorsal (upper) and ventral (lower) surfaces. The ventral surface has a thin nonkeratinized stratified squamous epithelium underlain by a lamina propria. The epithelium covering the dorsal surface is partly keratinized. The anterior two-thirds of the dorsal surface is separated from the posterior third by a V-shaped groove. Behind this, the epithelium invaginates to form the crypts of the lingual tonsils. Cryptless patches of lymphoid tissue in the lamina propria cause surface bulges in this region. The anterior two-thirds of the dorsal surface has many papillae-projections of the mucosal surface. There are 4 types of papillae.

1. Filiform papillae are the most numerous. They are sharp, often partly keratinized, conical projections that lack taste buds.
2. Fungiform papillae resemble mushrooms. Each has taste buds on its expanded upper surface but not on its narrow stalk. Fungiform papillae occur singly and are scattered among the filiform papillae.
3. Foliate papillae are poorly developed in humans. They occur in rows separated by furrows into which serous glands in the lamina propria drain. The furrow walls (sides of the papillae) harbor many taste buds.
4. Circumvallate papillae are the largest and least numerous, with only 7-12 occurring near the V-shaped groove at the back of the tongue. Each is surrounded by a ringlike ridge of mucosa from which it is separated by a circular furrow, whose walls contain taste buds on both sides. As with the foliates, ducts from serous (von Ebner's) glands empty into the furrow and periodically wash the chemical stimuli from the taste buds, allowing new tastes to be sensed

Functions of the digestive tract

C. General Functional Features: The main functions of the digestive tract are the absorption of nutrients and water and the excretion of wastes and toxins.

1. Digestion. Enzymatic degradation of foods is a prerequisite for absorption; enzymes act mainly at food surfaces. Chewing exposes more surface area. Lip, cheek, and tongue muscles help position food between the teeth. Saliva dissolves water-soluble particles and contains enzymes that attack carbohydrates. Taste buds check for contaminants toxins, and nutrients. The tongue moves chewed food back into the oral pharynx and closes the epiglottis to protect the airway. The esophagus adds mucus to reduce friction, but mainly moves material to the stomach. Glands in the stomach wall add acid (HCI), a protease (pepsin), and mucus to the mixture (now called chyme). Smooth muscles in the stomach wall mix and pulverize the chyme and move it to the small intestine (duodenum), where pancreatic enzymes and bile are added. The enzymes hydrolyze nutrients to an absorbable form. The detergent action of bile disperses water-insoluble lipid into tiny droplets, increasing the surface area available to pancreatic lipases. The lining epithelial cells (enterocytes) of the small intestine have additional enzymes on their luminal surfaces to complete the hydrolysis of certain nutrients.

2. Absorption. This primary function of the digestive tract occurs mainly in the intestines: the small intestines absorb nutrients, and the large intestines absorb water. To maximize the absorptive surface, the small intestine's lining has multiple permanent folds including plicae circulares and villi. Intestines are lined by absorptive cells (enterocytes) whose apical microvilli further increase the surface area. These cells absorb and transfer amino acids and sugars to capillaries in the lamina propria, whose blood carries them to the liver for further process ing. Enterocytes assemble chylomicrons from absorbed lipids and transfer them to lymphatic capillaries (lacteals) in the lamina propria. From here, lipids reach the blood through the lymphatic vascular system.

3. Excretion. Metabolic wastes are excreted by the liver as bile and emptied into the duodenal lumen by the bile duct. Smooth muscles in the walls of the small intestine move undigested material and waste products to the large intestine (colon). Here, more mucus is added and most of the water is extracted. This concentrates and solidifies the intestinal contents, forming feces. This material is further dehydrated and stored in the rectum and finally expelled through the anal canal.

4. Endocrine function. Individual cells with characteristics of the diffuse neuroendocrine system are scattered among the epithelial cells lining the tract's mucosal glands and crypts. These enteroendocrine cells were formerly called argentaffin, ar gyrophilic, and enterochromaffin cells because of their affinity for stains containing silver and chromium. They secrete hormones and amines leg, serotonin, secretin, gastrin, somatostatin, cholecystokinin, glucagon) that regulate such local gastrointestinal functions as gut motility and the secretion of acid, enzymes, and hormones by other cell types. 5. Innervation. Distributed along and in the walls of the tract are the myenteric (Auerbach's) and submucosal (Meissner's) autonomic nerve plexuses. These include postsynaptic sympathetic fibers, pre- and postsynaptic parasympathetic fibers, parasympathetic ganglion cell bodies, and some visceral sensory fibers. After voluntary swallowing, these autonomic plexuses coordinate peristaisis-wavelike contractions of the muscularis externa that propel ingested material through the tract. They also control the independent activity of the muscularis mucosa, which maintains contact between the mucosa and the contents of the tract and help empty mucosal glands. These plexuses also modulate the secretory activity of certain DNES-like cells. In general, sympathetic action inhibits gut motility and parasympathetic action has the opposite effect. 6. Blood supply. Mesenteric branches of the abdominal aorta branch further in the mesenteries to form a series of arcades. Small arteries penetrate the tract walls to feed capillaries of the lamina propria. Amino acids, sugars, small fatty acids, and any toxins absorbed in the intestine thus travel directly to the liver to be metabolized, stored, or detoxified before reaching the general circulation. 7. Protection. The extensive absorptive surface of the digestive tract increases the risk of infection. The risk is reduced by immunoreactive cells--including IgA-secreting plasma cells--in the lamina propria and submucosa. Other defenses include lysozyme secreted by Paneth's cells, digestive enzymes in the lumen, the layer of mucus covering the epithelium, and the tight junctions between absorptive cells. Toxic substances that do reach the blood are carried directly to the liver for detoxification in the SER of the hepatocytes.

GENERAL FEATURES OF THE DIGESTIVE TRACT

-A. Components: The digestive tract is a series of organs forming a long muscular tube whose continuous lumen opens to the exterior at both ends. The organs include the oral cavity, oral pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum and appendix; ascending, transverse, and descending colon), rectum, and anal canal.
-B. General Structural Features: The walls of each organ consist of 4 concentric layers: the mucosa, submucosa, muscularis externa, and serosa or adventitia
-1. Mucosa, This layer borders the lumen and has 3 parts. The epithelium (mucous membrane) derives from endoderm. It is stratified squamous in the oral cavity, oral pharynx, esophagus, and anal canal; it is simple columnar in the stomach, intestines, and rectum. The lamina propria is a layer of loose connective tissue beneath the endothelium; it contains small blood and lymphatic vessels. The muscularis mucosae is a thin layer of smooth muscle bordering the submucosa.
2. Submucosa, This dense, irregular connective tissue layer contains blood and lymphatic vessels and the submucosal (Meissner's) plexus of nerves. Some organs are characterized by glands and lymphoid nodules in this layer.
3. Muscularis externa, This consists of 2 layers of smooth muscle--an inner circular and an outer longitudinal-through most of the tract. Between them lies the myenteric (Auerbach's) plexus. The muscle around the oral cavity is skeletal; where it is absent leg, hard palate, gingiva) the submucosa binds tightly to bone. In the upper esophagus, this layer contains mainly skeletal muscle, which is replaced by smooth muscle in the lower portion. The stomach's muscularis externa has 3 layers: outer longitudinal, middle circular, and inner oblique. The colon's outer longitudinal layer is gathered into 3 bands, the taeniae coli. The smooth and skeletal muscles encircling the anal canal form involuntary and voluntary sphincters, respectively.
4. Serosa and adventitia. The tract's outer covering differs by location. The esophagus and rectum are surrounded and held in place by a connective tissue adventitia like that around blood vessels. Intraperitoneal organs (stomach, jejunum, ileum, transverse and sigmoid colon) are suspended by mesenteries and covered by a serosa composed of a thin layer of loose connective tissue covered by simple squamous epithelium (mesothelium). Retro peritoneal organs (duodenum, ascending and descending colon) are bound to the posterior abdominal wall by adventitia and covered on their free (anterior) surfaces by serosa.