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Review the A & P of the Gastrointestinal (GI) system. The general purpose of the digestive system is to efficiently process ingested food and fluids and the various secretions from glands.
The digestive system, sometimes called the gastrointestinal tract, alimentary tract, or gut, consists of a long hollow tube which extends through the trunk of the body, and its accessory structures: the salivary glands, liver, gallbladder, and pancreas.
The digestive tract is divided into two sections, the upper tract, consisting of the mouth, esophagus, and stomach, and the lower tract, consisting of the intestines.
Although variations occur along the tube, the wall of the gut basically has five continuous layers:
FIGURE 17-1 Anatomy of the digestive system with associated events.
The peritoneal cavity refers to the potential space between the parietal and visceral peritoneum.
A small amount of serous fluid is present in the cavity to facilitate the necessary movement of structures such as the stomach.
Numerous lymphatic channels drain excessive fluid from the cavity.
Because serous membranes are normally thin, somewhat permeable, and highly vascular, the peritoneal membranes are useful as an exchange site for blood during peritoneal dialysis in patients with kidney failure.
However, such an extensive membrane may also facilitate the spread of infection or malignant tumor cells throughout the abdominal cavity or into the general circulation.
The mesentery is a double layer of peritoneum that supports the intestines and conveys blood vessels and nerves to supply the wall of the intestine.
The mesentery attaches the jejunum and ileum to the posterior (dorsal) abdominal wall.
This arrangement provides a balance between the need for support of the intestines and the need for considerable flexibility to accommodate peristalsis and varying amounts of content.
The greater omentum is a layer of fatty peritoneum that hangs from the stomach like an apron over the anterior surface of the transverse colon and the small intestine.
The lesser omentum is part of the peritoneum that suspends the stomach and duodenum from the liver.
Upper Gastrointestinal Tract Oral Cavity Mouth
Food begins its journey through the digestive system in the mouth, also known as the oral cavity.
Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands.
Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx.
Teeth. The teeth are 32 small, hard organs found along the anterior and lateral edges of
The pharynx also plays an important role in the respiratory system, as air from the
nasal cavity passes through the pharynx on its way to the larynx and eventually
the lungs.
Because the pharynx serves two different functions, it contains a flap of tissue
known as the epiglottis that acts as a switch to route food to the esophagus and air
to the larynx.
Esophagus
The esophagus is a muscular tube connecting the pharynx to the stomach that is part of the upper gastrointestinal tract.
It carries swallowed masses of chewed food along its length.
At the inferior end of the esophagus is a muscular ring called the lower
esophageal sphincter or cardiac sphincter.
The function of this sphincter is to close of the end of the esophagus and trap food in the stomach.
Stomach
The stomach is a muscular sac that is located on the left side of the abdominal cavity, just inferior to the diaphragm.
In an average person, the stomach is about the size of their two fists placed next to
each other.
This major organ acts as a storage tank for food so that the body has time to
digest large meals properly.
The stomach also contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth.
Small Intestine
The small intestine is a long, thin tube about 1 inch in diameter and about 10 feet long that is part of the lower gastrointestinal tract.
It is located just inferior to the stomach and takes up most of the space in the
abdominal cavity.
The entire small intestine is coiled like a hose and the inside surface is full of
many ridges and folds.
These folds are used to maximize the digestion of food and absorption of
nutrients.
Liver and Gallbladder
The liver is a roughly triangular accessory organ of the digestive system located to the right of the stomach, just inferior to the diaphragm and superior to the small intestine.
The liver weighs about 3 pounds and is the second largest organ in the body.
The liver has many different functions in the body, but the main function of the liver in digestion is the production of bile and its secretion into the small intestine.
Gallbladder
The gallbladder is a small, pear-shaped organ located just posterior to the liver.
The gallbladder is used to store and recycle excess bile from the small intestine so that it can be reused for the digestion of subsequent meals.
Pancreas
The pancreas is a large gland located just inferior and posterior to the stomach.
It is about 6 inches long and shaped like short, lumpy snake with its “head” connected to the duodenum and its “tail” pointing to the left wall of the abdominal cavity.
The pancreas secretes digestive enzymes into the small intestine to complete the chemical digestion of foods.
- Swallowing. Swallowing is the process of using smooth and skeletal muscles in the mouth, tongue, and pharynx to push food out of the mouth, through the pharynx, and into the esophagus.
- Peristalsis. Peristalsis is a muscular wave that travels the length of the GI tract, moving partially digested food a short distance down the tract. It takes many waves of peristalsis for food to travel from the esophagus, through the stomach and intestines , and reach the end of the GI tract.
Digestion
Digestion is the process of turning large pieces of food into its component chemicals.
Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces.
This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines.
Bile produced by the liver is also used to mechanically break fats into smaller globules.
While food is being mechanically digested it is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb.
Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates.
The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine thanks to the action of the pancreas.
The pancreas secretes an incredibly strong digestive cocktail known as pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids.
By the time food has left the duodenum , it has been reduced to its chemical building blocks—fatty acids, amino acids, monosaccharides, and nucleotides.
Absorption
Once food has been reduced to its building blocks, it is ready for the body to
absorb.
Absorption begins in the stomach with simple molecules like water and alcohol being absorbed directly into the bloodstream.
Most absorption takes place in the walls of the small intestine, which are densely
folded to maximize the surface area in contact with digested food.
Small blood and lymphatic vessels in the intestinal wall pick up the molecules and
carry them to the rest of the body. The large intestine is also involved in the
absorption of water and vitamins B and K before feces leave the body.
Excretion
The final function of the digestive system is the excretion of waste in a process known as defecation.
Defecation removes indigestible substances from the body so that they do not
accumulate inside the gut.
The timing of defecation is controlled voluntarily by the conscious part of the
brain, but must be accomplished on a regular basis to prevent a backup of
indigestible materials.
FIGURE 17-2 The oral cavity.
into two systems: the autonomic nervous system, which regulates involuntary actions such as breathing and digestion, and the somatic nervous system, which governs voluntary action and body reflexes. Some of your peripheral nervous system (PNS) is under your voluntary control
○ Enteric nervous system
The gut has a mind of its own, the "enteric nervous system".
Just like the larger brain in the head, researchers say, this system sends and receives impulses, records experiences and respond to emotions.
Its nerve cells are bathed and influenced by the same neurotransmitters. The gut can upset the brain just as the brain can upset the gut.
The gut's brain or the "enteric nervous system" is located in the sheaths of tissue lining the esophagus, stomach, small intestine and colon.
Considered a single entity, it is a network of neurons, neurotransmitters and proteins that zap messages between neurons, support cells like those found in the brain proper and a complex circuitry that enables it to act independently, learn, remember and, as the saying goes, produce gut feelings.
The gut's brain is reported to play a major role in human happiness and misery. Many gastrointestinal disorders like colitis and irritable bowel syndrome originate from problems within the gut's brain.
● peripheral (local hormonal control) ● It has now been well established that gut hormones have a key role in controlling food intake and energy expenditure.
● The gut is the body’s largest hormone-producing organ, releasing more than 20 different peptide hormones, some of which target the brain to regulate appetite and influence the pleasure of eating.
● The gut hormones work in association with the gut’s extensive nervous system (enteric nervous system) and play a co-ordinating role in the control of appetite, the digestion of food, the regulation of energy balance and the maintenance of blood glucose levels.
● The gut continuously sends information to the brain regarding the quality and quantity of the food that is consumed.
○ gastrin ○ Secreted by mucosal cells (stomach) in response to distention of stomach or partially digested substances
■ Increases gastric motility, relaxes pyloric and ileocecal sphincters—promotes stomach emptying
○ cholecystokinin- Inhibits gastric emptying; stimulates contraction of gallbladder
○ secretin ○ Decreases gastric secretions
Answered some of these above. ● Intake ????
○ nutrient content (macromolecules) ● It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across cell membranes.
● Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles before they can be absorbed by the digestive epithelial cells.
● Different organs play specific roles in the digestive process.
● The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance.
● Digestive enzymes are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their
into pepsin.
Intrinsic factor (IF): Intrinsic factor is produced by the parietal cells of the stomach. Vitamin B12 (Vit. B12) is an important vitamin that requires assistance for absorption in terminal ileum.
Mucin: The stomach has a priority to destroy the bacteria and viruses using its highly acidic environment but also has a duty to protect its own lining from its acid.
The way that the stomach achieves this is by secreting mucin and bicarbonate via its mucous cells, and also by having a rapid cell turn-over.
Gastrin: This is an important hormone produced by the "G cells" of the stomach.
G cells produce gastrin in response to stomach stretching occurring after food enters it, and also after stomach exposure to protein.
Gastrin is an endocrine hormone and therefore enters the bloodstream and eventually returns to the stomach where it stimulates parietal cells to produce hydrochloric acid (HCl) and Intrinsic factor (IF).
Pancreatic enzymes help break down fats, proteins and carbohydrates.
hepatic bile salts ○ Bile salts (bile acids) are the major organic component in bile.
○ The liver uses active transport to secrete bile salts into the canaliculus, the cleft between adjacent hepatocytes.
○ Once secreted, bile salts draw other bile components (particularly Na and water) into the canaliculus by osmosis.
○ Bile salts are also biologic detergents that enable the body to excrete cholesterol and potentially toxic compounds (eg, bilirubin, drug metabolites).
○ The function of bile salts in the duodenum is to solubilize ingested fat and fat-soluble vitamins, facilitating their digestion and absorption.
○ From the liver, bile flows from the intrahepatic collecting system into the right or left hepatic duct, then into the common hepatic duct.
● Absorption
Functional Chest Pain
The Large Intestine (Colon) Constipation Diarrhea Irritable Bowel Syndrome (IBS)
● integrity of tissues ○ inflammation
○ spleen (RBC destruction and bilirubin metabolism) ● The spleen is an organ in the upper far left part of the abdomen, to the left of the stomach.
● The spleen varies in size and shape between people, but it’s commonly fist-shaped, purple, and about 4 inches long.
● Because the spleen is protected by the rib cage, you can’t easily feel it unless it’s abnormally enlarged.
● The spleen plays multiple supporting roles in the body. It acts as a filter for blood as part of the immune system.
● Old red blood cells are recycled in the spleen, and platelets and white blood cells are stored there.
● The spleen also helps fight certain kinds of bacteria that cause pneumonia and meningitis.
Common Manifestations of the GI Alterations
Anorexia, Nausea, Vomiting May be signs of digestive disorder or other condition elsewhere in the body Systemic infection Uremia Emotional responses Motion sickness Pressure in the brain Overindulgence of food, drugs Pain
Anorexia and vomiting Can cause serious complications Dehydration, acidosis, malnutrition
Anorexia Often precedes nausea and vomiting
Nausea Unpleasant subjective feeling Simulated by distention, irritation, inflammation of digestive tract Also stimulated by smells, visual images, pain, and chemical toxins and/or drugs
Vomiting (emesis) Vomiting center located in the medulla Coordinates activities involved in vomiting Protects airway during vomiting
Forceful expulsion of chyme from stomach Sometimes includes bile from intestine
● Diarrhea ● Excessive frequency of stools o Usually of loose or watery consistency
● May be acute or chronic ● Frequently with nausea and vomiting when infection or inflammation develops ● May be accompanied by cramping pain ● Prolonged diarrhea may lead to dehydration, electrolyte imbalance, acidosis, malnutrition
Fluid and Electrolyte Imbalances- complication of diarrhea
Dehydration and hypovolemia are common complications of digestive tract disorders.
Electrolytes
Lost in vomiting and diarrhea
Acid-base imbalances Metabolic alkalosis
- Results from loss of hydrochloric acid with vomiting
Metabolic acidosis
● Malnutrition ● May be limited to a specific nutrient or general ● Examples of limited malnutrition—specific problem o Vitamin B 12 deficiency o Iron deficiency o Folic Acid (Folate) deficiency o Celiac Disease o Cystic Fibrosis
General Terms ● Atresia Atresia is a condition in which an orifice or passage in the body is abnormally closed or absent. Examples of atresia include: Imperforate anus, malformation of the opening between the rectum and anus.
● fistula ● An abnormal connection between organs.
● stricture Benign esophageal stricture is a narrowing of the esophagus (the tube from the mouth to the stomach). It causes swallowing difficulties.
● dysphagia- Difficulty swallowing
● Causes
○ Neurological deficit
○ Muscular disorder
○ Mechanical obstruction
● Results and presentation
○ Pain with swallowing
○ Inability to swallow larger pieces of solid material
○ Difficulty swallowing liquids
● Ascites the accumulation of fluid in the peritoneal cavity, causing abdominal swelling.
● jaundice Jaundice causes your skin and the whites of your eyes to turn yellow. Too much bilirubin causes jaundice. In other words yellowing of the skin.
● hyperbilirubinemia
Hyperbilirubinemia is a condition in which there is too much bilirubin in the blood. When red blood cells break down, a substance called bilirubin is formed. Babies are not easily able to get rid of the bilirubin and it can build up in the blood and other tissues and fluids of the baby's body.
● icteric sclera Jaundice/ yellowing of the eyes (sclera of the eyes)
● gall bladder calculi stones- cholesterol
● steatorrhea ● Steatorrhea—“fatty diarrhea” ○ Frequent bulky, greasy, loose stools ○ Foul odor ○ Characteristic of malabsorption syndromes
■ Examples: Celiac disease, cystic fibrosis ○ Fat usually the first dietary component affected
■ Presence interferes with digestion of other nutrients. ○ Abdomen often distended
● melena
Dark-colored, tarry stool, May result from significant bleeding in upper digestive tract
● occult blood ■ Small hidden amounts, detectable with stool test ■ May be caused by small bleeding ulcers
● frank blood Red blood—usually from lesions in rectum or anal canal
● H. pylori Helicobacter pylori, previously Campylobacter pylori, is a gram-negative, microaerophilic bacterium found usually in the stomach
● Hematemesis the vomiting of blood.
Diagnostic tests
Barium swallow
