The gastrointestinal tract, digestive tract, or the gut is the system of organs within the multicellular organisms that takes in food, digests it to extract energy and nutrients, and expels the remaining matter.
Gastroenterology is the branch of medicine where the digestive system and its disorders are studied. Diseases affecting gastrointestinal tract are the focus of this specialty. Doctors specializing in the field are called gastroenterologists. Hepatology or hepatobiliary medicine encompasses the study of the liver, pancreas, and biliary tree and is traditionally considered a subspecialty.
A nutritionist is a health professional who studies and applies the principles of the science of nutrition. A dietitian is a health professional who is trained to plan nutrition programs for the sick as well as healthy people. The digestive system prepares food for use by hundreds of millions of body cells. Food when eaten cannot is not in the proper chemical state to be used by the body cells as energy. The gut modifies food physically and chemically and disposes of unusable waste. Physical and chemical modification (digestion) depends on exocrine and endocrine secretions and controlled movement of food through the digestive tract.
The digestive tract is also called the alimentary canal and is approximately 30 feet long, beginning with the mouth and ending with the anus. The organs of the digestive system work together to prepare food for absorption into the bloodstream and to prepare foods for use by the body cells. In addition to this vital function, the digestive system is also responsible for elimination of solid wastes from the body.
The major functions of the digestive tract therefore is
- Ingestion
- Digestion
- Absorption
- Defecation
The structure of the digestive system is divided into 2 sections.The upper GI tract and the lower GI tract.
- The upper GI tract consists of the oral cavity, pharynx, esophagus, and stomach
- The lower GI tract consists of the large and small intestines. Other accessory organs of the digestive tract are the salivary glands, liver, gallbladder, and the pancreas.
THE ORAL CAVITY
The digestive tract begins with the oral cavity and is designed to receive food. The oral cavity consists of the lips, the cheeks, the hard palate, the soft palate, uvula, tongue, and the teeth. Lips surround the opening of the mouth. The cheeks form the walls of the oral cavity and are lined with mucous membranes. The hard palate forms the anterior roof of the oral cavity and soft palate forms the posterior roof.
The palate is also lined with mucous membranes. At the end of the soft palate is a small cone-shaped projection called the uvula that helps in speech. The tongue is a solid mass of strong skeletal muscle covered with mucous membranes. It is the floor of the oral cavity and is the principal organ of the sense of taste and also assists in the process of chewing or mastication and swallowing or deglutition. The upper surface of the tongue is covered with
small, rough elevations known as papillae.
These papillae contain taste buds that detect sweet, sour, salty, and bitter tastes of food and beverages. During the chewing process, the tongue aids by moving the food around to mix it with saliva, shaping it into a ball-like mass called a bolus, and moving it toward the throat to be swallowed. The teeth are the main organs of chewing that begin the digestive process in the mouth. The teeth are embedded within sockets of the upper and lower jawbones (maxillae and mandible). Each tooth is surrounded by gum or gingiva, and held in its socket by a periodontal ligament. The 20 deciduous teeth (baby teeth or milk teeth) are eventually replaced by 32 permanent teeth.
There are three types of teeth, based on shape and function: Incisors have a chisel-shaped edge suitable for biting off food. Canines (cuspids) are pointed fangs and are used for tearing food. Premolars (bicuspids) and molars have flat surfaces used for grinding and crushing.
There are three types of teeth, based on shape and function:
- Incisors have a chisel-shaped edge suitable for biting off food.
- Canines (cuspids) are pointed fangs and are used for tearing food.
- Premolars (bicuspids) and molars have flat surfaces used for grinding and crushing.
A tooth has the following structural features:
- Dentin is a calcified tissue (like bone) that composes the bulk of the tooth.
- The crown is the visible part of the teeth
- The root is the portion of the tooth embedded in the bone.
- The neck is the region at the gum line where the crown and root meet.
- Enamel is the hard, nonliving material that covers the crown. Calcium compounds make the enamel the hardest substance in the body.
- Cementum is the bone-like substance that covers the root and binds it to the periodontal ligament.
- The pulp cavity is the central cavity inside the tooth. It contains blood vessels, nerves, and connective tissue (collectively called pulp).
THE PHARYNX
The pharynx or the throat, as explained in the respiratory system, is a passageway for both the respiratory and digestive systems. The pharynx receives the bolus of food from the oral cavity and passes it on to the esophagus. During the act of swallowing, the soft palate and the uvula move upward and close off the nasal cavity so that food moves into the pharynx. Further, the epiglottis drops downward to cover the larynx, directing the bolus into the esophagus. The food is propelled in the digestive tract through peristaltic movements.
THE ESOPHAGUS
The esophagus is the food pipe which connects the throat to the stomach. The movement of food from the pharynx into the esophagus is controlled by the upper esophageal sphincter, which prevents food from moving back into the oral cavity. The passage of food from the esophagus into the stomach is controlled by the cardiac sphincter or the lower esophageal sphincter, which prevents food from refluxing into the food pipe from the stomach.
THE STOMACH
The stomach is located in the upper left quadrant of the abdominal cavity. The stomach is a J-shaped, bag-like organ that expands to store food. Typical to 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 functions 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 stomach has 3 major divisions – the fundus, the body and the pylorus. The fundus is the upper rounded portion, above the level of the esophageal opening. The body is the central part of the stomach. The lower tubular region that approaches the duodenum is the pylorus.
The stomach serves a variety of functions:
• Storage. Because of its folds (called rugae), the wall of the stomach can expand to store two to four liters of material. Temporary storage is important because you eat considerably faster than you can digest food and absorb its nutrients.
• Mixing. The stomach mixes the food with water and gastric juice to produce a creamy medium called chyme.
• Physical breakdown. Three layers of smooth muscles (rather than the usual two) in the muscularis externa churn the contents of the stomach, physically breaking food down into smaller particles. In addition, HCl denatures (or unfolds) proteins and loosens the cementing substances between cells (of the food). The HCl also kills most bacteria that may accompany the food.
• Chemical breakdown. Proteins are chemically broken down by the enzyme pepsin. The stomach cells are protected from self-digestion because they produce and secrete an inactive form of pepsin, pepsinogen. Pepsinogen is converted to pepsin by the HCl. Only after pepsinogen is secreted into the stomach cavity can protein digestion begin. Once protein digestion begins, the stomach is protected by the layer of mucus secreted by the mucous cells.
• Controlled release. Movement of chyme into the small intestine is regulated by a valve at the end of the stomach, the pyloric sphincter.
THE SMALL INTESTINE
The small intestine is approximately 20 feet long, coiling and looping as it fills most of the
abdominal cavity. The small intestine (small in diameter compared to the large intestine) is divided
into three sections.
• The jejunum, about 2.5 m (8 ft) long, is the
middle section of the small intestine.
• The ileum, about 3.6 m (12 ft) long, is the last
section of the small intestine. It ends with the
ileocecal valve (sphincter), which regulates the
movement of chyme into the large intestine and
prevents backward movement of material from the large intestine.
The functions of the small intestine include
• Mechanical digestion. Segmentation mixes the chyme with enzymes from the
small intestine and pancreas. Bile from the liver separates fat into smaller fat
globules. Peristalsis moves the chyme through the small intestine.
• Chemical digestion. Enzymes from the small intestine and pancreas break down
all four groups of molecules found in food (polysaccharides, proteins, fats, and
nucleic acids) into their component molecules.
• Absorption. The small intestine is the primary location in the GI tract for
absorption of nutrients:
Villi (singular, villus) are fingerlike projections that cover the surface of the mucosa, giving it
a velvety appearance. They increase the surface area over which absorption and digestion
occur. The spaces between adjacent villi lead to deep cavities at the bases of the villi called
intestinal crypts. Glands that empty into the cavities are called intestinal glands, and the
secretions are collectively called intestinal juice. Microvilli are microscopic extensions of the
outer surface of the absorptive cells that line each villus. Because of their brush-like
appearance (microscopically), the microvilli facing the lumen form the brush border of the
small intestine. Like the villi; the microvilli increase the surface area over which digestion and
absorption take place.
THE LARGE INTESTINE
The large intestine begins at the ileocecal valve and extends to the anus. The ileocecal valve
prevents the backflow of wastes from the large intestine into the small intestine.
The large intestine is about 1.5 m (5 ft) 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.
• The appendix (vermiform appendix) is an 8 cm (3 inches) long fingerlike
attachment to the cecum that contains lymphoid tissue and is thought to serve
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 muscularis 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 (8 inches) of the large intestine. The mucosa in the
rectum forms longitudinal folds called anal columns.
• The anal canal, the last 3 cm (1 inch) 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.
• 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 produced by bacterial activity.
•Absorption. Vitamins B and K, some electrolytes (Na+
and Cl−
), and most of the
remaining water is absorbed from the large intestine.
• Defecation. Mass movement of feces into the rectum stimulates
a defecation reflex that opens the internal anal sphincter. Unless
the external and sphincter is voluntarily closed, feces are
evacuated through the anus.
ACCESSORY ORGANS OF THE DIGESTIVE SYSTEM
The Salivary Glands
The Pancreas
The pancreas is an elongated organ approximately 6 to 9 inches long. It is located in the upper left
quadrant of the abdomen, behind the stomach. It extends horizontally across the body, beginning at
the first part of the small intestine and ending at the edge of the spleen.
Pancreas is both an exocrine and an endocrine gland. The secretions of the pancreas, called
pancreatic juice, include various enzymes,
including pancreatic amylase (digestion of
starch), trypsin, carboxypeptidase, and
chymotrypsin (protease), as well as pancreatic
lipase (digestion of fats). Sodium bicarbonate
is also produced, making the pancreatic juice
alkaline. The 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 (about 1 percent of the total)
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 (duct of Wirsung) exits the pancreas and merges with the common bile duct
from the liver and gallbladder. This combined duct, called the hepatopancreatic ampulla
(ampulla of Vater), then enters the duodenum. A smaller, second duct that exits the pancreas,
the accessory pancreatic duct (duct of Santorini), joins the duodenum directly.
The Liver
Liver is located immediately under the diaphragm in the right upper quadrant. It is the
largest internal organ in the body and weighs approximately 3 to 4 pounds. The only digestive
function of the liver is the production of bile for the emulsification of fats in the small
intestine. The liver cells are known as hepatocytes produce a yellowish-green secretion called
bile. Bile consists of bile salts, the pigments, and cholesterol. It emulsifies fats and prepares
them for further digestion. The primary bile pigment is bilirubin which is an end product of
hemoglobin destruction. The color of feces is due to the presence of bilirubin.
The liver performs numerous metabolic functions. Some of the most important are:
• Production of bile.
• Blood glucose is regulated. When blood glucose is high, the liver converts glucose
to glycogen (glycogenesis) and stores the glycogen. When blood glucose is low,
glycogen is broken down (glycogenolysis), and glucose is released into the blood.
• Proteins (including plasma proteins) and certain amino acids are synthesized.
• Ammonia (which is toxic) is converted to urea (less toxic) for elimination by the
kidneys.
• Bacteria and expended red and white blood cells are broken down. From the red
blood cells, iron and globin are recycled, and bilirubin is secreted in the bile.
• Vitamins (A, D, and B12) and minerals (including iron from expended red blood
cells) are stored.
• Toxic substances (drugs, poisons) and hormones are broken down.
The liver is composed of numerous functional units called lobules. Within each lobule,
epithelial cells called hepatocytes are arranged in layers that radiate out from a central vein.
Blood enters the liver through the hepatic artery and hepatic portal vein and is distributed to
lobules.
The Gallbladder
The gallbladder is a pear-shaped organ present on the undersurface of the liver. Gallbladder is the
temporary storage organ for bile that is produced in the liver. Bile flows from the liver through the
cystic duct. The cystic duct joins the hepatic duct to form the common bile duct that carries bile
from the gallbladder to the duodenum to aid in digestion.
SIGNS AND SYMPTOMS OF THE DIGESTIVE SYSTEM
- Anorexia – loss or lack of appetite
- Aphagia – loss of ability to swallow
- Ascites – accumulation of fluid within the peritoneal cavity
- Borborygmus – abdominal sounds produced due to peristalsis
- Constipation – difficulty in passing feces out of the body
- Diarrhea – frequent passing of watery, loose stools
- Dyspepsia – Epigastric discomfort felt after eating, what is generally referred as indigestion or heartburn
- Emesis – vomiting Eructation – expulsion of gas or air through the mouth; belching
- Flatus – expulsion of gas or air through the anus
- GE reflux – backflow of contents of the stomach into the esophagus
- Icterus – yellow discoloration of skin, eyes, nails, mucous membranes, etc., due to excessive bilirubin; jaundice
- Melena – black, tarry stools
- Nausea – unpleasant sensation leading to an urge to vomit
- Steatorrhea – presence of fat in the feces
PATHOLOGIC CONDITIONS OF THE DIGESTIVE SYSTEM
Achalasia
Achalasia is increased mobility of the lower esophagus along with constriction of the lower
esophageal sphincter. This happens because of absence of sympathetic nerve receptors. Food and
fluid accumulate in the lower esophagus.
Adhesions
Adhesions are bands of fibrous tissue that can connect the loops of the intestines to each other, or
the intestines to other abdominal organs, or the intestines to the abdominal wall. These bands can
pull sections of the intestines out of place and may block passage of food. Adhesions are a major
cause of intestinal obstruction.
Anal Fistula
An abnormal passageway in the skin surface near the anus usually connecting with the rectum is an
anal fistula and it occurs as the result of a draining abscess.
Anal fistula is treated via a fistulectomy.
Aphthous stomatitis
These are inflammatory noninfectious ulcerated lesions of unknown etiology occurring on the lips,
tongue, and inside the cheeks of the mouth. They are also called canker sores. Some of the possible
causes could be emotional stress, food and drug allergies, endocrine imbalances, viral infections,
vitamin deficiency, and stress.
Barrett’s Esophagus
This is a condition in which the esophagus, the muscular tube that carries food and saliva from the
mouth to the stomach, changes so that some of its lining is replaced by a type of tissue similar to
that normally found in the intestine.
Celiac Disease
Celiac disease is nutrient malabsorption due to damaged small bowel mucosa. Due to this, fat
digestion is affected as is vitamin and carbohydrate absorption.
Treatment includes dietary control of gluten ingestion and a gluten-free diet for life.
Chronic Polyposis
This is a condition of a large number of polyps in the large bowel. A polyp is a small growth
projecting from a mucous membrane.
Cirrhosis
Cirrhosis is disease of the liver that is chronic and degenerative causing injury to the hepatocytes.
Multisystem problems result from the liver’s obstructed blood flow and inability to metabolize. It is
a result of chronic liver disease, malnutrition, alcoholism, infection, or poisons. It ultimately leads
to portal hypertension and liver failure.
Treatment involves eliminating the cause of cirrhosis.
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