UNIT OVERVIEW

This unit was on the digestive system. It described the process involved in the body’s treatment of food, as well as summarized the groups of molecules encountered during digestion. Our digestive system is made up of two major parts, and it has two main functions: digestion and absorption. This complex system of breaking down food and absorbing the vital nutrients is something we should have a good understanding of in order to understand human physiology.
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WOMAN.jpg

3 MAJOR CONCEPTS


Function of the Digestive System


The function of the digestive system is digestion and absorption. Digestion is the breakdown of food into small molecules, which are then absorbed into the body.

The digestive system is divided into two major parts:
The gastrointestinal (GI) tract is a continuous tube with two openings, the mouth and the anus. It includes the mouth, pharynx, esophagus stomach, small intestine, and large intestine. Food passing through the internal cavity (lumen) of the GI tract does not technically enter the body until it is absorbed through the walls of the GI tract and passes into blood or lymphatic vessels.

Accessory organs include the teeth and tongue, salivary glands, liver, gallbladder, and pancreas.

The body’s treatment of food in the digestive system involves the following processes:
Ingestion is the process of eating.

Propulsion is the movement of food along the digestive tract. The major means of propulsion is peristalsis, a series of alternating contractions and relaxations of smooth muscle that lines the walls of the digestive organs and that forces food to move forward.

Secretion of digestive enzymes and other substances liquefies, adjusts the pH of, and chemically breaks down the food.

Mechanical digestion is the process of physically breaking down food into smaller pieces. This process begins with the chewing of food and continues with the muscular churning of the stomach. Additional churning occurs in the small intestine through muscular constriction of the intestinal wall. This process, called segmentation, is similar to peristalsis, except that the rhythmic timing of the muscle constriction forces the food backward and forward rather than forward only.

Chemical digestion is the process of chemically breaking down food into simpler molecules. The process is carried out by enzymes in the stomach and small intestines.

Absorption is the movement of molecules (by passive diffusion or active transport) from the digestive tract to adjacent blood and lymphatic vessels. Absorption is the entrance of the digested food into the body.

Defecation is the process of eliminating undigested material through the anus.

Large Intestine


The large intestine is about 1.5 m long and is characterized by the following components:

The cecum is a dead-end pouch at the beginning of the large intestine, just below the ileocecal valve.

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The appendix is an 8 cm long fingerlike attachment to the cecum that contains lymphoid tissue and serves immunity functions.


The colon, representing the greater part of the large intestine, consists of four sections: the ascending, transverse, descending, and sigmoid colons. At regular distances along the colon, the smooth muscle of the muscular is layer causes the intestinal wall to gather, producing a series of pouches called haustra. The epithelium facing the lumen of the colon is covered with openings of tubular intestinal glands that penetrate deep into the thick mucosa. The glands consist of absorptive cells that absorb water and goblet cells that secrete mucus. The mucus lubricates the walls of the large intestine to smooth the passage of feces.

The rectum is the last 20 cm of the large intestine. The mucosa in the rectum forms longitudinal folds called anal columns.

The anal canal, the last 3 cm of the rectum, opens to the exterior at the anus. An involuntary (smooth) muscle, the interior anal sphincter, and a voluntary (skeletal) muscle, the external anal sphincter, control the release of the feces through the anus.

The functions of the large intestine include:

Mechanical digestion. Rhythmic contractions of the large intestine produce a form of segmentation called haustral contractions in which food residues are mixed and forced to move from one haustrum to the next. Peristaltic contractions produce mass movements of larger amounts of material.
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large_intestine.jpg

Chemical digestion. Digestion occurs as a result of bacteria that colonize the large intestine. They break down indigestible material by fermentation, releasing various gases. Vitamin K and certain B vitamins are also produces by bacterial activity.

Absorption. Vitamins B and K, some electrolytes (Na+ and Cl-), and most of the remaining water is absorbed by the large intestine.

Defecation. Mass movement of feces into the rectum stimulates a defecation reflex that opens the internal anal sphincter is voluntarily closed, feces are evacuated through the anus.

Three Phases of Digestive Regulation


There are three phases of digestive regulation:

The cephalic phase comprises those stimuli that originate from the head: sight, smell, taste, or thoughts of food, as well as emotional states. In response, the following reflexes are initiated:
Neural response. Stimuli that arouse digestion are relayed to the hypothalamus, which in turn, initiates nerve impulses in the parasympathetic vagus nerve. These impulses innervate nerve networks of the GI tract (enteric nervous system), which promote contraction of smooth muscle (which causes peristalsis) and secretion of gastric juice. Stimuli that repress digestion (emotions of fear or anxiety, for example) innervate sympathetic fibers that suppress muscle contraction and secretion.
General effects. The stomach prepares for the digestion of proteins.

The gastric phase describes those stimuli that originate from the stomach. These stimuli include distention of the stomach (which activates stretch receptors), low acidity (high pH), and the presence of peptides. In response, the following reflexes are initiated:
Neural response. Gastric juice secretion and smooth muscle contraction are promoted.
Hormonal response. Gastrin production is promoted.
General effects. The stomach and small intestine prepare for the digestion of chyme, and gastric emptying is promoted.

The intestinal phase describes stimuli originating in the small intestine. These include distention of the duodenum, high acidity (low pH), and the presence of chyme (especially fatty acids and carbohydrates). In response, the following reflexes are initiated:
Neural response. Gastric secretion and gastric motility are inhibited (enterogastric reflex). Intestinal secretions, smooth muscle contraction, and bile and pancreatic juice production are promoted.
Hormonal response. Production of secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP) is promoted.
General effects. Stomach emptying is retarded to allow adequate time for digestion (especially fats) in the small intestine. Intestinal digestion and motility are promoted.


CAREER APPLICATION


Gastroesophageal reflux disease (GERD or acid reflux) is a very common condition in which the acidified liquid contents of the stomach backs up into the esophagus. It is said to affect 1/3 of the adult population of the US to some degree at least once a month. Almost 10% of adults experience GERD weekly or daily, and not just adults are affected…even infants and children can have GERD, so it is very commonly seen by doctors. GERD is caused by an abnormal lower esophageal sphincter, hiatal hernia, abnormal esophageal contractions, and/or slow emptying of the stomach. GERD may damage the lining of the esophagus, causing inflammation (esophagitis). Some of the symptoms of GERD are heartburn, regurgitation, and sometimes nausea. Complications include ulcers and strictures of the esophagus, Barrett’s esophagus, cough and asthma, throat and laryngeal inflammation, inflammation and infection of the lungs, and collection o fluid in the sinuses and middle ear. Barrett’s esophagus is a pre-cancerous condition that requires periodic endoscopic surveillance for the development of cancer. GERD is diagnosed or evaluated by a trial of treatment, endoscopy, biopsy, x-ray, examination of the throat and larynx, 24 hour esophageal acid testing, esophageal motility testing, emptying studies of the stomach, and esophageal acid perfusion. It is treated with life-style changes, antacids, histamine antagonists (H2 blockers), proton pump inhibitors (PPIs), pro-motility drugs, foam barriers, surgery, and endoscopy. So if you are suffering from the symptoms of GERD, get to your doctor for treatment. If ignored, it can have very serious complications

ESSENTIAL QUESTIONS


Choose two organs of the digestive system. Explain how the structure of that organ contributes to the function of that organ.
The stomach is a J-shaped, baglike organ that expands to store food, mixes the food with water and gastric juice, breaks down food into smaller particles, chemically breaks down proteins, and moves chyme into the small intestine via the pyloric sphincter. Typical of that of the entire digestive tract, the wall of the stomach contains four layers. However, the inner layer, the mucosa, is modified for the specialized f
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unctions of the stomach. In particular, the innermost layer of the mucosa (facing the lumen) contains a layer of simple columnar epithelium consisting of goblet cells. Gastric pits on the surface penetrate deep into the layer, forming ducts whose walls are lined with various gastric glands. The exocrine glands (mucous surface cells, mucous neck cells, parietal cells, and chief cells) secretions, collectively called gastric juice, enter the stomach and mix with food. An endocrine gland (enteroendocrine cells) secretes various hormones that diffuse into nearby blood vessels.

The secretions of the pancreas, called pancreatic juice, include various enzymes, including pancreatic amylase (digestion of starch), trypsin, carboxypepiydase, and chymotrypsin (proteases), as well as
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pancreas.png
pancreatic lipase (digestion of fats). Sodium bicarbonate is also produced, making the pancreatic juice alkaline. An alkaline solution neutralizes the HCl in the chyme and provides an optimal environment for the action of these enzymes. Pancreatic juice is produced in clusters of exocrine cells called acini. The remaining cells in the pancreas also form clusters (islets of Langerhans). These are the endocrine cells that produce the hormones insulin, glucagon, somatostatin, and pancreatic polypeptide. Pancreatic juice collects in small ducts that merge to form two large ducts. The main pancreatic duct exit’s the pancreas and merges with the common bile duct from the liver and gallbladder. This combined duct, called the ampulla of Vater, then enters the duodenum. A smaller, second duct that exit’s the pancreas, the accessory pancreatic duct, joins the duodenum directly.


The three molecules, carbohydrates, proteins, and lipids, are important molecules in food that need to be physically and chemically digested by the digestive system. For each nutrient (carbohydrates, proteins, and lipids):
  • Describe where the nutrient is digested in the digestive system
  • Describe the enzymes that help to hydrolyze the molecules
  • Describe how the nutrient is absorbed
  • Describe how the body uses the molecule

Carbohydrate digestion begins in the mouth through the action of salivary amylase. Pancreatic amylase digests starch into disaccharides and short-chain oligosaccharides. Complete digestion into monosaccharides is accomplished by brush border enzymes.
Protein digestion begins in the stomach through the action of pepsin. Pancreatic juice contains the protein-digesting enzymes trypsin and chymotrypsin, among others. The brush border contains digestive enzymes that help to complete the digestion of proteins into amino acids. Amino acids, like monosaccharides, are absorbed and secreted into capillary blood entering the portal vein.
Lipids are digested in the small intestine after being emulsified by bile salts. Free fatty acids and monoglycerides enter particles called micelle, formed in large part by bile salts, and they are absorbed in this form or as free molecules. Once inside the mucosal epithelial cells, these subunits are used to resynthesize triglycerides. Tryglycerides in the epithelial cells, together with proteins, form chylomicrons, which are secreted into the central lacteals of the villi. Chylomicrons are transported by lymph to the thoracic duct and from there enter the blood.
This video further explains the digestion of carbohydrates, proteins, and lipids.



References
All information was obtained from Human Physiology; Stuart Ira Fox; 12th Edition, Burton’s Microbiology For The Health Sciences; Paul G. Engelkirk and Janet Duben-Engelkirk; 9th Edition, http://www.wikipedia.org/_, http://www.howstuffworks.com/,_ http://www.physioweb.org/, and http://www.webmd.com/.
All sources for photos can be accessed directly by clicking on the photos.