Medical Physiology/Gastrointestinal Physiology/Anatomy

Anatomy of the GI Tract
The GI tract is essentially a hollow tube connecting the mouth to the anus. The GI tract has a similar layout through out its length: This is illustrated graphically here:
 * An inner mucosal layer with an epithelial lining
 * A submucosal layer
 * A thin layer of muscle, the Muscularis Mucosa is at the junction of the mucosal and sub mucosal layers, outside this are the nerves of the mucosal plexus
 * A muscular layer with an inner circular muscle layer and an outer longitudinal layer
 * Between the muscles are the nerves of the myenteric plexus
 * A serosal layer which is continuous with the mesentry



Mucosa
The mucosal layer consists of a epithelial layer, and its underlying supportive tissue, the Lamina Propria. It is separated from the submucosal layer by the Muscularis Mucosa. The epithelial layer varies from section to section of the gut. In the oesophagus it is a non-cornified stratified squamous epithelium; in the stomach it is mainly mucosal cells; the small intestine and large intestine are characterized by absorptive cells, with many mucous producing goblet cells. In the absorptive sections of the intestine, the surface are for absorption is greatly increased by finger-like projections into the lumen called villi, and the absorptive cells themselves also have small projections of microvilli, giving them the appearence of a brush border when viewed with a light microscope.

The lamina propria contains lymph and blood vessels which drain into larger vessels in the submucosal layer. Also in the lamina propria, particularly in the absorptive layers are numerous immune cells, wandering macrophages and lymphocytes, as well as aggregations of lymphoid tissue called called Peyers patches. By some estimates 80% of the body's lymphoid tissue is in the intestine.

The cell-to-cell junctions are of the epithelia are mainly tight junctions. In the stomach there are very little or no gaps between the epithelial cells, but in the absorptive sections there is a certain amount of 'leakiness' so that water and some solutes can go between the cells rather than through them. The degree of leakiness is variable and is to some extent under hormonal control.

Submucosa
The submucosa consists of connective tissue with larger blood and lymph vessels. It is separated from the mucosa by the muscularis mucosa. Also in the submucosal layer is the submucosal plexus, part of the enteric nervous system. The muscularis mucosa probably acts to propel the contents of the mucosal glandular lumens and crypts (see below) into the lumen and also to enhance contact of the cells with the contents of the lumen.

Muscular Layers
Consists of an inner circular muscle, and an outer longitudinal muscle. Between the two layers is the mesenteric plexus, also part of the enteric nervous system. In the stomach there is also an oblique layer of muscle fibers interior to these two. The musclar layers work in harmony to produce peristaltic contractions and segmental contractions.

Serosa
The serosa is a continuation of the peritoneal membrane. It is useful to think of the gut as being envaginated into the peritoneal membrane until it completely surrounds it. The double layer of membrane as it attaches to the gut is called the mesentery, and it contains the main vessels and the non-intrinsic nerve supply to the gut.



Each section of the intestine has a variation on this theme. A brief review of the anatomy and function of each section follows. 

Mouth & Pharynx
Digestion starts in the oral cavity. Food is taken into the mouth; as it is masticated by the teeth, it is mixed with saliva from the parotid, sub-mandibular, and sublingual glands. Saliva contains amylase and lipase enzymes, which are mixed with the food. Because these enzymes are deactivated on reaching the stomach, salivary action is greater when food is more thoroughly masticated. When mastication is complete, the food is swallowed; this is a three-part process, but only the first part is under voluntary control. (see Motility). The epithelial lining of the mouth and pharynx is non-cornified squamous epithelium.





Esophagus
The esophagus extends from the pharynx to the stomach and is about ten inches in length. It traverses three regional anatomical areas: the neck, the thorax, and the abdominal cavity. At the upper and lower ends of the esophagus the muscular layers act like sphincters; they are in tonic contraction, and are known as the esophageal and the cardiac sphincters. Although the esophagus is outside the abdomen, and thus does not have a serosal layer, it has the same basic layout as the rest of the system.

The epithelium of the mucosal layer is non-cornified stratified squamous epithelium. The lamina propria is not copious and contains aggregates of lymphoid tissue.

 The oesophageal glands are scattered throughout the length of the esophagus and are located in the submucosa. There are also cardiac glands, which are similar in microscopic appearance to those in the stomach but have no enzymes in their secretions, at the proximal and distal ends of the esophagus. These glands are confined to the mucosal layer. The longitudinal rugae disappear on swallowing. The sub-mucosal layer consists of loose fibrous tissue and elastic tissue that allow expansion. No new digestive enzymes are added nor does absorption take place in the esophagus.

Clinical Note - Barrett's esophagitis
In Barrett's esophagitis the stratified squamous epithelium at the lower end of the esophagus is replaced by intestinal-type lining (columnar epithelium), due to reflux esophagitis. It is considered a pre-malignant condition as about 0.5% per year of patients will go on to develop esophageal cancer (adenocarcinoma of the esophagus). 

Gross Anatomy


1. Body of stomach 2. Fundus 3. Anterior wall 4. Greater curvature 5. Lesser curvature 6. Cardia 9. Pyloric sphincter 10. Pyloric antrum 11. Pyloric canal 12. Angular notch 13. Gastric Canal 14. Rugal folds

Food enters the stomach from the esophagus at the cardia and passes into the stomach. In the fasting state the stomach is kept in a state of contraction, but the presence of food causes it to expand. The rugae of the stomach are folds in the mucosa seen in the fasting state; unlike the small intestine they are not there to increase surface area for absorption. Numerous pores are seen: the openings to the gastric glands which secrete enzyme pepsinogen and hydrochloric acid as well as mucous. The hydrochloric acid produces a pH of about 2. This highly acidic environment serves two purposes: first, to provide an environment hostile to bacteria and other pathogens; second, to denature protein and cause it to unfold, thereby increasing the area that pepsins can attack (see Digestion below)

The stomach is divided into three parts: the fundus, the main body, and the pyloric antrum. Stored food is mixed with enzymes and HCl to form chyme. The muscles feed the chyme down to the pyloric antrum, where it is thoroughly mixed, and fed in small amounts into the small intestine by relaxation of the pyloric valve.

The arterial supply to the stomach is from the coeliac artery. The venous drainage of the stomach drains into the venous portal system 

Microscopic Anatomy


The basic layout pattern holds true in the stomach, although the stomach has a third inner layer of oblique muscle fibers. The mucosal layer has numerous pits opening into the lumen, the mouths of the gastric glands. Some of these glands penetrate down into the submucosal layer.

Gastric Glands
The gastric glands secrete mucous, hydrochloric acid and enzymes into the stomach. They are located for the most part in the mucosal layer of the stomach, but some of the deeper gland penetrate into the submucosal layer, and secrete into the lumen via ducts. Several different secretory cells are found: mucous; parietal; chief cells; D cells; enterochafin cells; and G cells.

Mucous cells secrete both mucous and bicarbonate, substances that protect the stomach from auto-digestion. Parietal cells secrete hydrochloric acid (1-3 liters a day) which cause the pH in the stomach to fall as low as 1. Chief cells secrete the enzyme pepsinogen, which is activated to pepsin by the pH of the stomach. G cells produce gastrin, a hormone that promotes gastric acid secretions and stimulates the growth of the gastric mucosa.



The composition of the gastric glands varies throughout the stomach. In the fundus, the glands are more branched and contain all the different kinds of secretory cells. In the pyloric part of the stomach the glands are deeper and contain more mucous cells. Chief cells are found only in the fundus of the stomach, G cells are found mainly in the antrum, and the other cells are found in all areas. Not much absorption of nutrients take place in the stomach, although many fluids -- particularly alcohol -- can be absorbed from here. 

Small Intestine
The small intestine, where most of the absorption occurs, is divided into three sections, the duodenum, the jejunum, and the ileum. In all three sections the layers follow the general pattern:  

The surface for absorption is increased in many ways: the mucosa of the small intestine is thrown into folds called rugae; the mucosa itself has numerous finger-like projections called villi, and the epithelial cells are also covered with numerous projections called microvilli. The microvilli give the appearance of a 'brush' on light microscopy; hence the term brush border. The following illustration shows the anatomy of a villi: 

Mucous is secreted by numerous goblet cells; other cells are specialized for absorption and are known as absorptive cells. In the base of the crypts are numerous secretory cells which secrete the digestive enzymes of the small intestine. Some of the crypts penetrate into the submucosal layer, forming digestive glands which will communicate with the mucosa via a secretory duct.

The length of the small intestine is about 12-13 ft. in the living adult, although after death this length doubles, due to the loss of longitudinal muscle tonicity.

Duodenum
Release of chyme into the duodenum is controlled by a number of factors to ensure a controlled release into the small intestine for processing. The duodenum is itself divided into three parts: the first, second, and third part. Small portions of highly acidic chyme is released into the first part of the duodenum. This part secretes the hormone secretin from the mucosal cells (as do the other parts of the doudenum) in response to the presence of acidic chyme leaving the stomach. This stimulates the pancreas to secrete copious amounts of neutralizing sodium bicarbonate, so the pH of the chyme is nearer 7 when it leaves the duodenum. The digestive enzymes of the pancreas and small intestine operate maximally at this pH. This also means that any pepsin from the stomach is also deactivated. The first part of the duodenum does not have plicae and folds, indicating that this part is probably not important for absorption.

In the second part of the duodenum bile and pancreatic enzymes are secreted via the common duct into the lumen see (Biliary System & Pancreas) below. Absorption of food starts in the second part of the duodenum.

Jejunum & Ileum
The jejunum comprises about the upper two fifths and the ileum the lower three fifths of the small intestine past the duodenum. The jejunum and ileum are classified according to various anatomical differences. These differences include the arrangement of the cascade of vessels in the mesentery; the distribution of Peyers patches (more numerous in the ileum); thicker mucosa with greater blood supply in the jejunum; and increased incidence of plicae for greater surface area in the jejunum. Physiologically the two areas have more or less the same functions, although probably more digestive enzymes are produced in the proximal portions of the small intestine. In any case, when we consider absorption and digestion we shall just consider them all to be 'small intestine'.



Epithelial Regeneration
Epithelial cells have a lifetime of 5–7 days. New cells are continuously being generated in the crypts, and migrate up the sides of the villi. These cells differentiate into either goblet cells (10–25%) or absorptive cells. Old cells are shed from the tips of the villi, migrating upwards and forming new villi.

Biliary System & Pancreas
Bile and bile salts are manufactured by the liver and fed into the second part of the duodenum via the common duct. Bile salts are important in fat digestion.

The pancreas is a retro-peritoneal gland that is both an endocrine gland (producing the hormones insulin and glucagon) and an exocrine gland (producing digestive enzymes). The enzymes are secreted in a deactivated form - to prevent auto-digestion - and are activated in the lumen of the duodenum.

The following illustration indicates the relationships of the pancreas and biliary systems. 

Large Intestine
The large intestine extends from the ileo-caecal junction to the rectum and anus. The microscopic appearance is similar to that of the small intestine. In is divided into the caecum, ascending, transverse, descending and sigmoid colon. In the colon electrolytes and water are removed, and faeces are formed. These are propelled forwards by a form of movement called mass movement. (see Motility). The colon is populated with bacteria, which usually operate in a symbiotic mode. Any food that has not been processed may be digested by these bacteria, and the products will then be absorbed by the large intestine.

The microscopic appearance is similar to the small intestine with the following exceptions:  
 * Goblet cells are more numerous.
 * Villi are few or absent.
 * The muscularis mucosa has both a circular and a longitudinal layer.
 * The Crypts of Lieberkuhn are larger and do not branch.
 * The longitudinal muscle layer is condensed in the colonic portion of the large intestine into three longitudinal bands called the teniae coli.

Rectum and Anus
The faeces are now passed to the rectum where they await elimination. The process of elimination is controlled by two sphinctae, the Internal and external anal sphinctae. The former is under involuntary control, the latter under voluntary control. The functioning of these are discussed in the section on Motility 

Splanchnic Circulation
This includes the blood supply and drainage - via the portal system - of the gut as well as the pancreas, liver and spleen. The liver is drained by the hepatic vein.

Gastrointestinal blood supply
Blood supply comes from three arteries branching off the aorta: the coeliac artery, the superior mesenteric artery, and the inferior mesenteric artery.

Coeliac Artery
Supplies blood to the stomach, liver, pancreas and spleen as well as the duodenum.



Superior Mesenteric Artery
Supplies blood to the small intestine as well as the superior part of the colon. Notice the system of arching anastemoses that is a feature of the splanchnic arteries.



Inferior Mesenteric Artery
Supplies blood to the colon. Again notice the system of arching anastomoses.



Hepatic Portal System
All the blood from the GI system below the esophagus drains into the hepatic portal vein. The portal system also drains the spleen and pancreas. Blood is conveyed to the liver for processing where it passes through millions of liver sinusoids. This allows the reticuloendothelial cells that line the sinusoids to remove bacteria and other particulate matter. Blood finally leaves the liver via the hepatic vein where it drains into the inferior vena cava.

The total blood flow to the liver is about 1.5 liters per minute, of which two-thirds is carried by the portal vein. The blood in the portal vein is more oxygen-saturated than blood in the systemic venous system; it is about 80-90% saturated and provides about 70% of the oxygen requirements of the liver.

