Digestive System

Chapter 23

 

Objectives for Digestive System

 

1.            Identify the organs of the alimentary canal and the accessory organs.

 

2.            Define the six essential activities of digestion.

 

3.            Describe the basic structure of the wall of the alimentary canal.

 

4.         For the following organs of the digestive system describe the structure and digestive processes occurring there:  stomach, small intestine (3 sections), large intestine.

 

5.            Describe the digestive processes associated with the salivary glands.

 

6.            Describe the structure and function of the liver and gallbladder.

 

7.            Describe the digestive processes associated with the pancreas.

 

8.            Describe the contributions of the bacterial flora.

 

Introduction

The organs of the digestive system can be grouped into:

1. alimentary canal

2. accessory organs

 

The alimentary canal (gastrointestinal tract ‑ G.I.) is a continuous, coiled, hollow, muscular tube that winds through the ventral body and is open Cal both ends. It digests food and absorbs the fragments through its lining into the blood.

Organs include:

mouth

pharynx

esophagus

stomach

small intestine

large intestine

anus

 

Accessory digestive organs include:  teeth, tongue,  gallbladder, salivary glands, liver, pancreas. The accessory digestive glands produce saliva, bile, and enzymes.

 

Digestive Processes

 

"Disassembly line" in which food becomes less complex at each step of processing and its nutrients become available to the body.

 

Six essential activities occur:

            Ingestion

            Propulsion

            Mechanical digestion

            Chemical digestion

            Absorption

            Defecation

 

Ingestion is simply the process of taking food into the digestive tract.

Propulsion is the process that moves food through the alimentary canal, and includes swallowing and peristalsis.

Peristalsis is the major means of propulsion involving alternate waves of contraction and relaxation of muscles in the organ walls. Its net effect is to squeeze food along the pathway from one organ to the next with some mixing occurring along the way.

Mechanical digestion includes chewing, mixing of food with saliva by the tongue, churning food in the stomach and segmentation of the intestines.

Segmentation mixes food with digestive juices and increases the rate of absorption by repeatedly moving different parts of the food mass over the intestinal wall.

Chemical digestion is a catabolic process wherein molecules are broken down to their monomers which are small enough to be absorbed by the GI tract. Accomplished by enzymes secreted by various glands. Begins in the mouth and is essentially complete in the small intestine.

Absorption is the transport of end products from the GI tract to the blood or lymph. Substances must first enter the mucosal cells by active or passive transport processes. The small intestine is the major site of absorption.

Defecation is the elimination of indigestible substances from the body.

 

Basic Functional Concepts

 

The digestive system creates an optimal environment for its functioning in the lumen of the GI tract, an area that is actually outside the body, and essentially all digestive tract regulatory mechanisms act to control luminal conditions. Sensors involved in control of digestive activities are located in the walls of the tract organs. They respond to a limited number of stimuli including stretch, osmolarity and pH, and the presence of substrates and end products.

The receptors initiate activities such as secretion of digestive juices into the lumen, secretion of hormones into the blood, and mixing lumen contents.

 

Digestive System Organs

 

Most digestive system organs reside in the abdominopelvic cavity. All body cavities contain serous membranes. Connecting the visceral and parietal peritoneums is a fused double layer of parietal peritoneum called the mesentary. It provides a route for conducting blood vessels, lymphatics, and nerves to the digestive viscera; helps hold the organs in place; and stores fat. In most places the mesentery is dorsal and attaches to the posterior abdominal wall.

 

The hepatic portal circulation collects nutrient‑rich venous blood draining from the digestive tract and delivers it to the liver.

 

From the esophagus to the anal canal, the walls of every organ of the alimentary canal are made up of the same four basic layers, or tunics.

 

From the lumen outward, these layers are the:

1. mucosa            3. muscularis externa

2. submucosa            4. serosa

 

The mucosa lines the lumen from mouth to anus. Its major functions are: Secretion of mucus, digestive enzymes, and hormones. Absorption of end products. Protection against infectious disease. The mucus protects certain digestive organs from being digested themselves by enzymes, and eases food passage along the tract.

The submucosa contains blood and lymphatic vessels, lymph nodules, nerve fibers, and a rich supply of elastic fibers.

The muscularis externa is responsible for segmentation and peristalsis. This tunic typically has an inner circular layer and an outer longitudinal layer of smooth muscle cells. In several places along the tract, the circular layer thickens to form sphincters that act as valves to prevent backflow and control food passage.

The serosa is the protective outermost layer.

 

 

Functional Anatomy of the Digestive System

The mouth is the only part of the alimentary canal involved in ingestion. The mouth also begins the propulsive process of swallowing, during which food is carried through the pharynx and esophagus to the stomach. The tongue contains most of the taste buds and some mucous and serous glands, and during chewing, the tongue grips the food and constantly repositions it between the teeth. It also mixes the food with saliva and forms it into a compact mass (bolus) and then initiates swallowing. A number of glands produce and secrete saliva. Saliva cleanses the mouth, dissolves food chemicals so that they cam be tasted, moistens food and aids in compacting it into a bolus, and contains enzymes. Saliva is mostly water and includes electrolytes, salivary amylase, mucin, lysozyme, and IgA and metabolic wastes.

 

Digestive Processes

 

The mouth ingests and begins mechanical digestion by chewing and initiates propulsion by swallowing. Salivary amylase starts the chemical breakdown of polysaccharides. Essentially no absorption occurs in the mouth.

 

Both the pharynx and the esophagus serve only as conduits to pass food from the mouth to the stomach. Their single function is propulsion of food.

 

The stomach is a temporary "storage tank" where the chemical breakdown of protein as begins and in which food is converted to a creamy paste called chyme.

 

The major regions of the stomach are: 1. Cardiac region (near the heart) -  surrounds the cardiac orifice through which food enters the stomach. 2. Fundus ‑ dome shaped, tucked beneath the diaphragm 3. Body ‑ midportion 4. Pylorus ‑ inferior terminus, continuous with the duodenum.

 

 

The stomach has an extra layer of smooth muscle in its walls that allows the stomach not only to move food along the tract,, but also to churn, mix, and pummel the food, physically breaking it down into smaller fragments. The lining epithelium (mucosa) produces large amounts of protective mucus. The lining is covered with millions of deep gastric pits which lead into the gastric glands that produce the stomach secretion called gastric juice.

The glands contain a variety of secretory cells:

1. Parietal cells ‑ secrete HCl and intrinsic factor. HCl makes the stomach contents very acid (pH 1. 5 ‑ 3.5), necessary for activation of pepsinogen and to kill ingested bacteria. Intrinsic factor is required for the absorption of Vit. B12 in the small intestine.

2. Chief cells ‑ produce pepsinogen

3. Enteroendocrine cells ‑ release a variety of hormones including gastrin, which plays essential roles in regulating stomach secretion and mobility.

 

Besides serving as a holding area for ingested food, the stomach continues the demolition job begun in the mouth. It then delivers chyme into the small intestine at an appropriate rate.

Protein digestion is essentially the only type of enzymatic digestion that occurs in the stomach. The most important protein‑digesting enzyme produced by the gastric mucosa is pepsin.

 

The only stomach function essential to life is secretion of intrinsic factor which is required for intestinal absorption of Vit. B12 needed for the production of mature red cells.

 

 Hormonal control is largely the province of gastrin, which stimulates the secretion of both enzymes and HCl

Stimuli acting at three distinct sites ‑‑ head, stomach, and small intestines ‑provoke or inhibit gastric secretory activity. The three phases of gastric secretion are called the: cephalic, gastric, and intestinal.

The cephalic phase occurs before food enters the stomach and is triggered by the sight, aroma, taste, or thought of food. The brain gets the stomach ready for its upcoming digestive chore.

In the gastric phase, once food reaches the stomach neural and hormonal mechanisms are initiated. About 2/3's of the gastric juice is released. Chemical stimuli provided by partially digested proteins and caffeine directly activate gastrin‑secreting cells. Gastrin’s main target is the HCL‑producing parietal cells. Gastrin secretion is inhibited when the gastric contents become highly acidic (pH < 2). When protein foods are in the stomach, the pH of the gastric contents generally rises. The rise in pH stimulates gastrin and subsequently HCl release.  This provides the acidic conditions needed for protein digestion. The more protein in the meal, the greater the amount of gastrin and HCl released. As proteins are digested, the gastric contents gradually become more acidic, which again inhibits the gastrin‑secreting cells. This negative feedback mechanism helps maintain an optimal pH.

 

 

The intestinal phase has two components ‑ one excitatory and the other inhibitory. The excitatory aspect is set into motion as partially digested food begins to fill the initial part of the small intestine. This stimulates the release of a hormone that encourages the gastric glands to continue their secretory activity. As the intestine is distended with chyme, the inhibitory component is triggered in the form of the enterogastric reflex. This reflex causes the pyloric sphincter to tighten and prevent farther food entry into the small intestine. As a result, gastric secretory activity declines.

 

Gastric Motility and Emptying

 

Stomach contractions not only cause its emptying but also compress, knead, twist and continually mix the food with gastric juice to produce chyme.

 

The mixing movements are accomplished by a unique type of peristalsis. Each peristaltic wave reaching the pyloric muscle squirts 3ml or less of chyme into the small intestine. Since the contraction closes the pyloric valve, the rest of the chyme is propelled backward into the stomach where it is mixed further. The stomach usually empties completely within 4 hours after a meal. The larger the meal and the more liquefied its contained food, the faster the stomach empties. Fluids pass through the stomach very quickly; solids remain until they ewe well mixed with gastric juice and converted to the liquid state. As a rule, a meal rich in carbohydrates moves through the duodenum rapidly, but fats form an oily layer at the top of the chyme and are digested more slowly.

 

 

Small Intestines

The body's major digestive organ. Digestion is completed and virtually all absorption occurs here. The small intestine has three subdivisions:

            duodenum

            jejunum

            ileum

The duodenum curves around the head of the pancreas and is about 25cm long. The bile duct joins close to the duodenum. The jejunum is about 2.5 m long and extends from the duodenum to the ileum. The ileum is approximately 3.6 m long and joins the large intestine at the ileocecal valve. The small intestine is highly adapted for nutrient absorption. Its length provides a huge surface area and its walls have three structural modifications: plicae circulares, villi, and microvilli These structures amplify to: absorptive surface greatly.

 

Plicae circulares are deep, permanent circular folds that extend either entirely or part way around the circumference of the small intestine. They force chyme to spiral through the lumen continually mixing the chyme with intestinal juices and slowing its movement, allowing time for full nutrient absorption.

 

The villi are finger like projections of the mucosa; over 1 mm high. Within the core of each villus are a dense capillary bed and a modified lymphatic capillary called a lacteal. Digested foodstuffs are absorbed through the epithelial cells into both the capillary blood and the lacteal. The villi are large and leaflike in the duodenum and gradually become narrower and shorter along the length of the small intestine.

 

The microvilli are tiny projections of the plasma membrane of the absorptive cells of the mucosa (brush border). In addition to enhancing absorption, the plasma membranes of the microvilli bear the intestinal digestive enzymes, referred to collectively as brush border enzymes. The enzymes are mainly disaccharidases and peptidases, which complete the digestion of carbohydrates and proteins.

 

Liver and Gallbladder

 

These organs am accessory organs associated with the small intestine. The liver has many metabolic and regulatory roles. Its only digestive function is to produce bile for export to the duodenum.

 

Bile is a fat emulsifier; it breaks fats into tiny particles so they can be more accessible to digestive enzymes.

 

The liver also processes nutrient‑laden venous blood delivered to it from the digestive organs. This is a metabolic rather than a digestive role.

 

The gallbladder is chiefly a storage organ for bile.

 

Bile leaves the liver through several bile ducts that ultimately fuse to form the hepatic duct. The hepatic duct fuses with the cystic duct draining the gallbladder to form the common bile duct.

 

The liver is composed of structural and functional units called liver lobules. Each lobule is a hexagonal, roughly cylindrical structure consisting of plates of hepatocytes. The hepatocyte plates, radiate outward from a central vein.

 

At each of the six corners of a lobule is a portal triad containing three basic structures ‑‑ a branch of the hepatic artery, a branch of the hepatic portal vein and a bile duct.

Between the hepatocyte plates are enlarged blood‑filled capillaries, or sinusoids. Blood from both the hepatic portal vein and the hepatic artery percolates from the triad regions through the sinusoids and empties into the central vein. Inside the sinusoids are hepatic macrophages (kupffer cells) which remove debris and worn‑out red cells.

 

Besides producing bile, the hepatocytes process the blood‑borne nutrients in various ways: store glucose and use amino acids to make plasma proteins, store fat‑soluble vitamins, detoxification (convert ammonia to urea). The blood leaving the liver contains fewer nutrients and waste materials than the blood entering it.

 

Bile entering the bile ducts eventually leaves the liver via the hepatic duct and is stored in the gallbladder when digestion is not occurring. Bile is a yellow‑green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and a variety of electrolytes. Only the bile salts and phospholipids aid the digestive process.

Bile's role is to emulsify fits, but it also facilitates fat and cholesterol absorption. Bile salts are conserved by means of recycling (enterohepatic circulation). Bile salts are reabsorbed into the blood by the distal part of the small intestine (ileum) and returned to the liver via the hepatic portal blood.

 

 

Gall Bladder – Stores and concentrates bile that is not immediately needed for digestion. When the muscular wall contracts, bile is expelled into the cystic duct and then flows into the common bile duct. The liver makes bile continuously and stores it in the gallbladder. The major stimulus for release is cholecystokinin which is released into the blood when acidic, fatty chyme enters the duodenum.

 

Pancreas - An accessory digestive organ that produces a broad spectrum of enzymes. This exocrine product (pancreatic juice) drains from the pancreas via centrally located pancreatic duct which generally fuses visa the common bile duct. The pancreas also has endocrine function (insulin, glucagon). Pancreatic juice contains mainly water, enzymes and electrolytes. It has a pH of 8.0 which enables the pancreatic fluid to neutralize the acid chyme entering the duodenum and provides the optimal environment for the operation of intestinal and pancreatic enzymes.

 

 

Digestive Processes ‑ Small Intestine

 

Although food reaching the small intestine is unrecognizable, it is far from digested chemically. Carbohydrates and proteins are partially degraded, but virtually no fat digestion has occurred up to this point. During the journey through the small intestine virtually all absorption lakes place. Most substances required for chemical digestion (bile, enzymes, etc.) are provided by the liver. Optimal digestive activity also depends on a slow, measured delivery of chyme from the stomach, since the small intestine is only able to process small amounts of chyme at one time.

Additionally, the low pH of the chyme must be adjusted upward and the chyme must be well mixed with bile and pancreatic juice so that digestion can continue. Food movement into the small intestine is carefully controlled by the pumping action of the pylorus. Intestinal smooth muscle mixes chyme thoroughly and moves food residues through the ileocecal valve into the large intestine. Segmentation is the most common motion of the small intestine. The contents are simply moved backward and forward in the lumen a few centimeters at a time by alternating contraction and relaxation of rings of smooth muscle. Segmentation also produces a slow steady movement of the contents toward the ileocecal valve at a rate that allows ample time to complete digestion and absorption.

 

Large Intestine

Frames the small intestine on three sides and extends from the ileocecal valve to the anus. Its diameter is larger, but its length is less than the small intestine.

Its major function is to absorb water and to eliminate indigestible food residues from the body as semisolid feces.

The large intestine has the following subdivisions:

Cecum, appendix, colon, rectum, anal canal

 

The sac like cecum lies below the ileocecal valve and is the first part of the large intestine. Attached is the vermiform appendix which contains masses of lymphoid tissue.

The colon has several distinct regions. The ascending colon travels up the right side of the abdominal cavity, the transverse colon travels across the abdominal cavity. It then turns acutely and continues down the left side as the descending colon and enters the pelvis, where it becomes the S‑shaped sigmoid colon. The sigmoid colon joins the rectum. The rectum has three lateral curves or bends, represented internally as three transverse folds called the rectal valves, which separate feces from flatus (gas).

The anal canal is about 3 cm long and opens to the body exterior at the anus. The anal canal has two sphincters which act to open and close the anus.

The wall of the large intestine differs in several ways from the small intestine. Because most food is absorbed before reaching the large intestine, there are no plicae circulares, no villi, and virtually no cells that secrete digestive enzymes. The mucosa is thicker and there are tremendous numbers of goblet cells. The lubricating mucus cases the passage of feces and protects the intestinal wall from irritating acids and gases.

 

Bacterial Flora

Most bacteria entering the cecum from the small intestine are dead, although some remain alive. Together with bacteria that enter the GI tract from the anus, these constitute the bacterial flora of the large intestine. These bacteria colonize the colon and ferment some of the remaining indigestible carbohydrates, releasing irritating acids and a mixture of gases. The bacterial flora also synthesize B complex vitamins and most of the vitamin K the liver requires.

The large intestine harvests vitamins and reclaims most of the remaining water and some of the electrolytes.

However, the primary concerns of the large intestine are propulsive activities that force the fecal material toward the anus and then eliminate it from the body.