The Digestive System

Part 1: Overview of the Digestive System

Digestive system organs fall into two main groups: the alimentary canal and the accessory organs.

Alimentary canal, or the gastrointestinal (GI) tract, is the continuous muscular digestive tube that winds through the body digesting and absorbing foodstuff; its organs include: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

Accessory digestive organs aid digestion physically and produce secretions that break down foodstuff in the GI tract; the organs involved are the teeth, tongue, gallbladder, salivary glands, liver, and pancreas.

Digestive Processes

Ingestion is the simple act of putting food into the mouth.

Propulsion moves food through the alimentary canal and includes both swallowing and peristalsis.

Mechanical digestion is the physical process of preparing the food for chemical digestion and involves chewing, mixing, churning, and segmentation.

Chemical digestion is a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by enzymes.

Absorption is the passage of digested end products from the lumen of the GI tract through the mucosal cells into the blood or lymph.

Defecation eliminates indigestible substances from the body via the anus as feces.

The digestive system creates an optimal internal environment for its functioning in the lumen of the GI tract, an area that is technically outside of the body. Two important basic functional characteristics are:

Digestive activities within the GI tract are triggered by a variety of mechanical and chemical stimuli. Chemoreceptors, osmoreceptors, and stretch receptors in the walls of the GI tract respond to digestion products, pH, osmolarity, and stretch.

Control digestive juice or hormone secretion, mixing, and movement

Controls of the digestive activity are both extrinsic and intrinsic (nervous and hormonal).

Intrinsic controls include short reflexes mediated by local nerve plexuses (the enteric nervous system) and local hormone producing cells that secrete in response to chemical and neural stimuli.

Extrinsic contols include long reflexes involving CNS centers and extrinsic autonomic nerves.

Digestive System Organs: Relationship and Structural Plan

Relationship of Digestive Organs to the Peritoneum

The visceral peritoneum covers the external surfaces of most of the digestive organs, and the parietal peritoneum lines the body wall of the abdominopelvic cavity.

Peritoneal cavity is located between the visceral and parietal peritoneums and is filled with serous fluid.

Mesenteries are a double layer of peritoneum that extends to the digestive organs from the body wall. They allow blood vessels, lymphatics, and nerves to reach the digestive organs, and hold the organs in place as well as store fat.

Retroperitoneal organs are found posterior to the mesentery, lying against the dorsal abdominal wall.

The splanchnic circulation serves the digestive system and includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation.

Histology of the Alimentary Canal

Mucosa is the innermost, moist, epithelial membrane that lines the entire digestive tract. It (1) secretes mucus, digestive enzymes, and hormones; (2) absorbs digestive end products into the blood; and (3) protects against infectious disease.

Consists of a lining epithelium, a lamina propria, and a muscularis mucosae.

Epithelium - simple columnar epithelium and goblet cells

Lamina propria - areolar C.T. with capillaries and lymphoid follicles (MALT)

Muscularis mucosae - thin layer of smc, produces local movements of the mucosa

Submucosa is a moderately dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers.

Contains the submucosal nerve plexus of Meissner, one of the 2 major intrinsic nerve plexuxes, which regulates glands and muscle of the muscularis mucosae.

Muscularis externa typically consists of smooth muscle and is responsible for peristalsis and segmentation.

Contains the myenteric plexus of Auerbach, the other major intrinsic nerve plexus. Located between the two layers of smooth muscle, controls motility of the G.I. tract.

Serosa, the protective outer layer of the intraperitoneal organs, is the visceral peritoneum.

Part 2: Functional Anatomy of the Digestive System

Mouth, Pharynx, and Esophagus

The mouth is a stratified squamous epithelial mucosa-lined cavity with boundaries of the lips, cheeks, palate, and tongue.

The lips and cheeks have a core of skeletal muscle covered externally by skin that helps to keep food between the teeth when we chew and plays a small role in speech.

The palate forms the roof of the mouth and has two parts: the hard palate anteriorly and the soft palate posteriorly.

The tongue is made of interlacing bundles of skeletal muscle and is used to reposition food when chewing, mix food with saliva, initiate swallowing, and help form consonants for speech.

Salivary glands produce saliva, which cleanses the mouth, dissolves food chemicals for taste, moistens food, and contains chemicals that begin the breakdown of starches.

Intrinsic - buccal

Extrinsic - parotid, submandibular, sublingual

parotid - only serous cells (no mucin)

submandibular and buccal - about equal amounts serous and mucous cells

sublingual - only mucous cells

Salivia contains:

  1. serous fluid
  2. mucin
  3. salivary amylase
  4. lingual lipase
  5. lysozyme
  6. IgA
  7. defensins
  8. some antimicrobial cyanide compound
  9. some nitrogenous wastes

The teeth tear and grind food, breaking it into smaller pieces.

The pharynx (oropharynx and laryngopharynx) provides a common passageway for food, fluids, and air.

The esophagus provides a passageway for food and fluids from the laryngopharynx to the stomach where it joins at the cardiac orifice.

Digestive Processes Occurring in the Mouth, Pharynx, and Esophagus

Mastication, or chewing, begins the mechanical breakdown of food and mixes the food with saliva.

Deglutition, or swallowing, is a complicated process that involves two major phases.

The buccal phase is voluntary and occurs in the mouth where the bolus is forced into the oropharynx.

The pharyngeal-esophageal phase is involuntary and occurs when food is squeezed through the pharynx and into the esophagus.

 

The Stomach

The stomach is a temporary storage tank where the chemical breakdown of proteins is initiated and food is converted to chyme.

The adult stomach varies from 15–25 cm long, but its diameter and volume vary depending on the amount of food it contains. About 50 mls when empty and can expand to hold a gallon or more. (milk challenge 1; milk challenge 2)

The major regions of the stomach include the cardiac region, fundus, body, and the pyloric region.

The convex lateral surface of the stomach is its greater curvature, and its convex medial surface is its lesser curvature.

Extending from the curvatures are the lesser omentum and the greater omentum, which help to tie the stomach to other digestive organs and the body wall.

Microscopic Anatomy

The surface epithelium of the stomach mucosa is a simple columnar epithelium composed of goblet cells, which produce a protective two-layer coat of alkaline mucus.

The gastric glands of the stomach produce gastric juice, which may be composed of a combination of mucus, hydrochloric acid, intrinsic factor, pepsinogen, and a variety of hormones.

Chief cells produce pepsinogen and small amounts of gastric lipase.

Parietal cells produce HCl and intrinsic factor (required for absorption of vitamin B12).

Enteroendocrine cells include G-cells, which produce gastrin, enterochromaffin-like cells (ECLs), which produce histamine, and others that produce somatostatin and serotonin.

Mucous neck cells produce a thin, watery, acidic mucus, the purpose of which is an ongoing area of investigation.

 

Gastric Ulcer

Digestive Processes Occurring in the Stomach

Gastric secretion is controlled by both neural and hormonal mechanisms and acts in three distinct phases: the cephalic phase, the gastric phase, and the intestinal phase.

The reflex-mediated relaxation of the stomach muscle and the plasticity of the visceral smooth muscle allow the stomach to accommodate food and maintain internal pressure.

The interstitial cells of Cajal establish the stomach’s basic electrical rhythm of peristaltic waves.

The rate at which the stomach empties is determined by both the contents of the stomach and the processing that is occurring in the small intestine.

Small Intestine and Associated Structures

The small intestine is the site of the completion of digestion and absorption of nutrients.

It extends from the pyloric sphincter to the ileocecal valve where it joins the large intestine. It has three subdivisions: the duodenum, the jejunum, and the ileum.

It is highly adapted for absorption with three microscopic modifications: plicae circulares, villi, and microvilli.

The plicae circulares are circular folds of mucosa and submucosa that impart a spiral movement to chyme, allowing more mixing with intestinal secretions and greater absorption.

Villi are finger-like projections of mucosa covered with absorptive columnar cells. The villi have capillaries and lacteals in the lamina propria for nutrient absorption (most dietary fat is absorbed by the lacteals, specialized lymphatic capillaries).

Microvilli (the brush border) are projections from the apical surface of each epithelial cell which further increase the surface area for absorption and also contain enzymes (brush border enzymes) that complete digestion of nutrients.

Intestinal crypts (glands), or the crypts of Lieberkühn, lie between the villi and secrete intestinal juice that serves as a carrier fluid for absorbing nutrients from chyme. Paneth cells deep in the crypts secrete lysozyme, an enzyme that degrades bacterial cell walls. The number of crypts decreases along the length of the small intestine.

The submucosa contains Peyer's patches, aggregated lymph nodules (MALT), which increase in number along the length of the small intestine (there are more in the large intestine).

The submucosa also contains duodenal glands (Brunner's glands), which secrete alkaline mucus to raise the pH and protect the wall of the duodenum. Distension or irritation of the mucosa by hypotonic or acidic chyme stimulates the release of intestinal juice, around 1 - 2 liters per day.

 

The liver and gallbladder are accessory organs associated with the small intestine.

The liver is the largest gland in the body and has four lobes.

The liver is composed of liver lobules, which are made of plates of liver cells (hepatocytes).

The digestive function of the liver is to produce bile, which is a fat emulsifier.

Bile is a yellow-green, alkaline solution containing bile salts, bile pigments (primarily bilirubin), cholesterol, neutral fats, phospholipids, and a variety of electrolytes.

The gallbladder stores and concentrates bile that is not needed immediately for digestion.

Bile does not usually enter the small intestine until the gallbladder contracts when stimulated by cholecystokinin.

 

 

The pancreas is an accessory gland that is retroperitoneal.

Pancreatic juice consists mainly of water and contains enzymes that break down all categories of foodstuffs and electrolytes.

Secretion of pancreatic juice is regulated by local hormones and the parasympathetic nervous system.

Digestive Processes Occurring in the Small Intestine

Food takes 3 to 6 hours to complete its digestive path through the small intestine, the site of virtually all nutrient absorption.

Most substances required for chemical digestion within the small intestine are imported from the pancreas and the liver.

Optimal digestive activity in the small intestine depends on a slow, measured delivery of chyme from the stomach.

Segmentation is the most common motion of the small intestine.

The Large Intestine

The large intestine absorbs water from indigestible food residues and eliminates them as feces.

The large intestine exhibits three unique features: teniae coli, haustra, and epiploic appendages, and has the following subdivisions: cecum, appendix, colon, rectum, and anal canal.

The teniae coli is the longitudinal layer of smooth muscle reduced down to 3 bands. This causes puckering of the wall, which forms the haustra. Epiploic appendages are small pockets of visceral peritoneum filled with fat.

 

 

Mesenteries of the Abdominal Digestive Organs

The mucosa of the large intestine is thick and consists of simple columnar epithelium except in the anal canal. The mucosa of the anal canal is stratified squamous epithelium. The anal epithelium hangs in long folds (anal columns) in the superior portion of the anus. The anal sinuses are the recesses between the anal columns; they secrete mucus when compressed by feces, which aids passage of feces out of the anus.

There are no modifications for absorption like in the small intestine and no cells that produce digestive enzymes.

There are numerous deep crypts with a large number of mucus-producing goblet cells to protect the mucosa from irritants produced as by-products of bacterial metabolism.

Bacteria entering the colon via the small intestine and anus colonize the colon and ferment some of the indigestible carbohydrates, producing acids and gases. Bacteria also produce biotin and vitamin K, which are absorbed through the intestinal wall.

Digestive Processes Occurring in the Large Intestine

The primary movements seen in the large intestine include haustral contractions and mass movements, which occur mostly in the ascending and transverse colon.

Haustral contractions are slow contractions that occur about every 30 minutes and last approximately 1 minute. They are stimulated by stretch when food remnants fill the haustra.

Mass movements are long, slow moving, powerful contractions that move over the colon 3 or 4 times per day, typically after meals. This gastrocolic reflex accompanies the gastroileal reflex stimulated by gastrin release when the stomach recieves food.

In addition to these movements some segmentation occurs in the descending and sigmoid colon to increase water absorption before mass movements propel the feces into the rectum.

Feces forced into the rectum by mass movements stretch the rectal wall and initiate the defecation reflex.

Part 3: Physiology of Chemical Digestion and Absorption

Chemical digestion is a catabolic process in which large food molecules are broken down to chemical building blocks (monomers), which are small enough to be absorbed by the GI tract lining.

Chemical digestion is accomplished by enzymes, secreted by intrinsic and accessory glands of the alimentary canal, used in hydrolysis reactions.

Carbohydrates

Monosaccharides are simple sugars that are absorbed immediately (glucose, galactose, and fructose).

Disaccharides are composed of two monosaccharides bonded together (maltose, lactose, and sucrose).

The digestible polysaccharide found in the diet is starch; other polysaccharides, such as cellulose, are not able to be broken down by humans.

Chemical digestion of carbohydrates begins in the mouth where salivary amylase breaks large polysaccharides into smaller fragments.

Proteins digested into amino acids in the GI tract include not only dietary proteins but also enzyme proteins secreted into the GI tract lumen.

Pepsin, secreted by the chief cells, begins the chemical digestion of proteins in the stomach.

Rennin is produced in infants and breaks down milk proteins.

Pancreatic enzymes, such as trypsin and chymotrypsin, further break down proteins in the small intestine.

The brush border enzymes carboxypeptidase, aminopeptidase, and dipeptidase work on freeing single amino acids in the small intestine.

The small intestine is the sole site for lipid digestion.

Lipases are secreted by the pancreas and are the enzymes that digest fats after they have been pretreated with bile.

Nucleic acids (both DNA and RNA) are hydrolyzed to their nucleotide monomers by pancreatic nucleases present in pancreatic juice.

 

Protein Absorption

 

Fat Emulsification and Fatty Acid Absorption

 

Absorption occurs along the entire length of the small intestine, and most of it is completed before the chyme reaches the ileum.

Absorption of Specific Nutrients

Glucose and galactose are transported into the epithelial cells by common protein carriers and are then moved by facilitated diffusion into the capillary blood.

Several types of carriers transport the different amino acids before entering the capillary blood by diffusion.

Monoglycerides and free fatty acids of lipid digestion become associated with bile salts and lecithin to form micelles, which are necessary for lipid absorption.

Pentose sugars, nitrogenous bases, and phosphate ions are transported actively across the epithelium by special transport carriers in the villus epithelium.

The small intestine absorbs dietary vitamins, while the large intestine absorbs biotin and vitamin K porduced by intestinal bacteria.

Electrolytes are actively absorbed along the entire length of the small intestine, except for calcium and iron which are absorbed in the duodenum.

Water is the most abundant substance in chyme and 95% of it is absorbed in the small intestine by osmosis.

Monosaccharide Absorption

Malabsorption of nutrients can result from anything that interferes with the delivery of bile or pancreatic juices, as well as factors that damage the intestinal mucosa.

Developmental Aspects of the Digestive System

Embryonic Development

The epithelial lining of the developing alimentary canal forms from the endoderm with the rest of the wall arising from the mesoderm.

The anteriormost endoderm touches the depressed area of the surface ectoderm where the membranes fuse to form the oral membrane and ultimately the mouth.

The end of the hindgut fuses with an ectodermal depression, called the proctodeum, to form the cloacal membrane and ultimately the anus.

By week 8 the alimentary canal is a continuous tube stretching from the mouth to the anus.

Aging

GI tract motility declines, digestive juice production decreases, absorption is less efficient, and peristalsis slows resulting in less frequent bowel movements and often constipation.

Diverticulosis, fecal incontinence, and cancer of the GI tract are fairly common problems in the elderly.