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Nutrition is the science of nourishment.
The body needs six classes of nutrients. Name them.
Upper digestive tract
- oral cavity
Lower digestive tract
- small intestine
- large intestine
Accessory organs of the digestive tract
The accessory organs provide or store secretions that ultimately are delivered to the lumen of the digestive tract and aid in the digestive and absorptive processes.
The four main tunics (layers) of the gastrointestinal tract:
- muscularis externa
- serosa, or adventitia
release a mixture of water, mucus, and enzymes
mechanical breakdown, moistening, and mixing of food with saliva
The mouth and pharynx (or throat) constitute the oral cavity and provide the entryway to the digestive tract.
The pharynx (throat) propels food from the back of the oral cavity into the esophagus
transports food from the pharynx to the stomach
produces bile, an important secretion needed for lipid digestion
stores and reeases bile, needed for lipid digestion
muscular contractions mix food with acid and enzymes, causing the chemical and physical breakdown of food into chyme
releases pancreatic juice that neutralizes chyme and contains enzymes needed for carbohydrate, protein, and lipid digestion
major site of enzymatic digestion and nutrient absorption
receives and prepares undigested food to be eliminated from the body as feces
Four sublayers of the small intestine
- Serosa or adventitia: Outer cover that protects the GI tract made of connective tissue
- Muscularis: Two layers of smooth muscle (longitudinal and circular muscle) responsible for GI motility
- Submucosa: Connective tissue that contains blood vessels, lymphatic vessels, and nerves
- Mucosa: Innermost mucous membrane layer that produces and releases secretions needed for digestion (it's what comes in direct contact with the food and lines the Lumen/tube)
Name and discuss the three pairs of small, bilateral saliva secreting Salivary Glands
Secretions (about 1 L/day) from the Parotid, Submandibular, and Sublingual glands constitute saliva.
Saliva is primarily (99.5%) water, which helps dissolve foods. The other 0.5% contains mucous, eectrolytes, and antibacterial and antiviral compounds.
Parotid Glands: secrete water, electrolytes (sodium, potassium, chloride), and enzymes.
Submandibular and Sublingual glands: secrete water, electrolytes, enzymes, and mucus.
These glands are affected by the actions of the parasympathetic and sympathetic nervous systems.
The principal enzyme of saliva is _____.
α amylase (also called ptyalin). This enzyme hydrolyzes internal α 1-4 bonds within starch.
A second digestive enzyme, lingual lipase, is produced by lingual serous glands on the tongue and in the back of the mouth. This enzyme hydrolyzes dietary triacylglycerols (triglycerides) in the stomach but diminishes with age.
compounds consisting of both carbohydrates and proteins
Mucus secretions found in saliva...
lubricate food and coats and protects the oral mucosa. Mucus secretions found in saliva contain glycoproteins.
Food from the mouth that is now mixed with saliva is caled a bolus.
The passage of the bolus of food from the oral cavity into the esophagus via the pharynx constitutes swallowing.
Swallowing, which can be divided into several stages--voluntary, pharyngeal, and esophageal--is a reflex response inititated by a voluntary action and regulated by the swalowing center in the medulla of the brain.
a progressive wavelike motion that moves the bolus through the esophagus into the stomach.
AKA Heartburn (burning sensation in the midchest) caused by the movement of substances from the stomach back into the esophagus.
The high tonic pressure at the gastroesophageal sphincter keeps the sphincter closed to prevent gastroesophageal reflux.
Foods and food-related substances can indirecty affect gastroesophageal sphincter pressure and cause reflux. Smoking, chocolate, high-fat foods, acohol, and carminatives such as peppermint and spearmint, promote relaxation of the gastroesophageal sphincter and increase the likelihood of acid reflux into the esophagus.
A carminative, also known as carminativum (plural carminativa), is a herb or preparation that either prevents formation of gas in the gastrointestinal tract or facilitates the expulsion of said gas, thereby combating flatulence. Carminatives have been shown to decrease lower esophageal pressure, which on the other hand increases the risk of Gastroesophageal Reflux Disease (GERD) or heartburn.
- a drug causing expulsion of gas from the stomach or bowel.
- adjective 2. expelling gas from the body; relieving flatulence.
The antrum or distal pyloric portion of the stomach grinds and mixes food with the gastric juices, thus forming a semiliquid chyme or partially digested food existing as a thick semiliquid mass.
The volume of the stomach when empty (resting) is about _____
The volume of the stomach when empty (resting) is about 50 mL (~ 2 oz) and folds called rugae present in all but the antrum section of the stomach are visible.
The volume of the stomach when full can expand to accommodate from ___ to approximately ___ or more and the _____ disappear.
The volume of the stomach when full can expand to accommodate from 1 L to approximately 1.5 L (~ 37 to 52 oz) or more and the rugae disappear.
Neck (mucus) cells
Mucus-secreting neck cells on the surface of the gastric pit produce an alkaline mucus that forms the gastric mucosal barrier.
This protects the mucosal lining from the acidity of the gastric juice.
Chief (peptic or zymogenic) cells
Secrete pepsinogens -- these cells produce enzymes needed for protein digestion
Parietal (oxyntic) Cells
produce hydrochloric acid (HCl) and intrinsic factor, which is needed for the absorption of vitamin B12
produce the hormone gastrin, which stimulates parietal and chief cells.
Hydrochloric acid (HCl) has several functions in gastric juice, including:
- converting or activating the zymogen pepsinogen to form pepsin
- denaturing proteins, which results in the destruction of the tertiary and secondary protein structure and thereby opens interior bonds to the proteoytic effect of pepsin
- releasing various nutrients from organic complexes
- acting as bactericide agent, killing many bacteria ingested along with food
Name and describe the three enzymes found in gastric juice.
Pepsin, α amylase, and gastric lipase.
Pepsin: the main enzyme made by the chief cells, functions as the principal proteolytic enzyme in the stomach. Pepsin is dervived from either Pepsinogen I (found primarily in the body of the stomach) or Pepsinogen II (found in both the body and the antrum of the stomach). Pepsinogen I correlates positively with acid secretion and has been associated with an increased incidence of peptic ulcers.
α amylase: originates from the salivary glands in the mouth. This enzyme, which hydrolyzes starch, retains some activity in the stomach until it is inactivated by the low pH of gastric juice.
gastric lipase: made by chief cells. Gastric lipase hydrolyzes primarily short- and medium-chain triacylglycerols and is thought to be responsible for up to about 20% of lipid digestion in humans.
breakdown of proteins: the breakdown of proteins or peptides into amino acids
Nourishment and survival are possible without the stomach as long as a person receives injections of ____. Why is this true?
Though gastric juice contains several important compounds that aid in the digestive process, very little chemical digestion of nutrients occurs in the stomach except for the initiation of protein hydrolysis by the protease pepsin and the limited coninuation of starch hydrolysis by salivary α amylase. The only absorption that occurs in the stomach is that of water, alcohol, a few fat-soluble drugs such as aspirin, and a few minerals.
The HCl and intrinsic factor generated in the stomach are important for absorbing nutrients such as iron and especially vitamin B12 respectively.
Thus, a healthy stomach makes attaining adequate nourishment much easier but doing so without a stomach is also possible.
The most common cause of peptic ulcers is the bacterium _____.
helicobacter pylori; however, chronic use of many substances, including aspirin, alcohol, and nonsteroidal anti-inflammatory drugs (NSAID) like ibuprofen, can disrupt the mucus-rich and bicarbonate-rich barriers that protect the mucosa and eeper layers of the gastrointestinal tract and can promote the development of ulcers.
Gastric emptying following a meal usually takes between ___ and ___ hours.
2 and 6 hours
The main site for nutrient digestion and absorption is the _____.
Digestion of nutrients is usually completed on the _____ but may be completed within the cytoplasm of the enterocytes.
What helps to neutralize the acid released from the stomach?
The pancreatic secretions released into the duodenum are rich in bicarbonate, which heps to neutralize the acid released from the stomach.
What two types of active cells are found in the pancreas?
- ductless endocrine cells: secrete hormones, primarily insulin and glucagon, into the blood
- acinar exocrine cells: produce the digestive enzymes, which get packaged in secretory structures called granules and released by exocytosis into pancreatic juice.
Pancreatic juice, produced by the ____ cells, contains:
Pancreatic juice, produced by the acinar cells, contains:
- bicarbonate: important for neutralizing the acid chyme passing into the duodenum from the stomach and for maximizing enzyme activity within the duodenum
- electrolytes: including the cations sodium, potassium, and calcium and the anion chloride
- pancreatic digestive enzymes: in watery solutions
The enzymes released by the pancrease digest approximately half (50%) of all ingested _____, half (50%) of all _____, and almost all (80% to 90%) of ingested _____.
50% carbohydrates, 50% protein, 80-90% fat.
The portal vein takes blood rich in nutrients away from the _____ and _____ to the _____.
The portal vein takes blood rich in nutrients away from the digestive tract and pancreas to the liver.
The central veins direct blood from the _____ into general circulation through _____ veins and then ultimately into the _____.
The central veins direct blood from the liver into general circulation through hepatic veins and then ultimately into the inferior vena cava.
The gallbladder concentrates and stores the bile made in the _____ until it is needed for _____ in the small intestine.
The gallbladder concentrates and stores the bile made in the liver until it is needed for fat digestion in the small intestine.
The hormone _____, secreted into the blood by enteroendocrine cells, stimulates the gallbladder to contract and release ____ into the _____.
The hormone cholecystokinin, secreted into the blood by enteroendocrine cells (called I-cells) of the proximal small intestine, stimulates the gallbladder to contract and release bile into the duodenum.
_____, inhibits gallbladder contraction.
Gallstones are thought to form when _____ becomes supersaturated with _____.
Gallstones are though to form when bile becomes supersaturated with cholesterol.
How/where is bile stored and secreted?
During the interdigestive periods, bile is sent from the liver to the gallbladder, where it is concentrated and stored.
The gallbladder concentrates the bile so that as much as 90% of the water, along with some of the electrolytes, is reabsorbed by the gallbladder mucosa. The fluid reabsorption thus leaves the remaining bile constituents (i.e. bile acids and salts, cholesterol, lecithin, bilirubin, and biliverdin) in a less dilute form. Concentration of the bile permits the gallbladder to store more of the bile produced by the liver between periods of food ingestion.
Cholecystokinin, released in response to chyme entering the duodenum, stimulates gallbladder contraction.
Bile is secreted into the duodenum through the sphincter of Oddi.
Bie acids and bile salts act as ...
Bile acids and bile salts act as detergents to emulsify lipids, that is, to break down large fat globules into small fat droplets.
Bile acids and salts, along with phospholipids, help to absorb lipids by forming small spherical, cylindrical, or disklike complexes called micelles.
Micelles can contain as many as 40 bile salt molecules.
The circulation of bile.
The human body contains a total bile acid pool of about 2.5 to 5.0 g. New bile, mixed with recirculated bile, is sent through the cystic duct to be stored in the gallbladder. The pool of bile is thought to recycle at least twice per meal.
What is the purpose of cholestyramine (Questran) type drugs and phytostanols and phytosterols functional foods? How do they work?
The purpose is to lower cholesterol by binding to bile as well as dietary and endogenous cholesterol in the gastrointestinal tract to enhance fecal excretion from the body. The increased fecal excretion of the bile, decreased recirculation of the bile, and decreased absorption of cholesterol requires the body to use cholesterol to synthesize new bile acids. The increased use of cholesterol to make more bile diminishes the body's cholesterol concentrations.
Thus, the goal of using such medications and functional foods is to lower blood cholesterol concentrations and reduce risk of cardiovascular disease.
5 steps in enterohepatic circulation of bile
1. Bile is made in the liver, and is transported to the gallbladder where it is stored.
2. When the gallbladder contracts, bile is released into the cystic duct. The cystic duct joins the common bile duct.
3. Bile aids in lipid digestion by enabling large lipid globules to disperse in the watery environment of the small intestine.
4. After aiding in lipid digestion, the bile constituents are reabsorbed from the ileum and returned to the liver via the hepatic portal vein.
5. The liver uses thes constituents to resynthesize bile, which is then stored in the gallbladder.
Nutrient digestion occurs both in the _____ of the gastrointestinal tract and on the ____.
Nutrient digestion occurs both in the lumen of the gastrointestinal tract and on the brush border
Digestion is accomplished through enzymes from the ....
mouth, stomach, pancreas and small intestine and with the help of bile from the liver.
Once digested, nutrients must move into the cells of the gastrointestinal tract by a process known as absorption.
Although some nutrient absorption may occur in the stomach, the absorption of most nutrients begins in the duodenum and continues throughout the jejunum and ileum.
Bacteria adapted to living in a specific environment, such as the intestines.
An anaerobic process that facilitates the breakdown of carbohydrate and protein by bacteria.
What type of diet is needed for people with liver disease?
A low protein diet.
foods that contain live cultures of specific strains of bacteria.
The intent of consuming probiotics is for the bacteria to survive the passage through the upper digestive tract and then establish themselves in the lower GI tract.
The most common probiotic bacteria are
lactic acid bacteria such as bifidobacteria and lactobacilli.
Prebiotics are food ingredients that are not digested by human digestive enzymes but can benefit the host by acting as substrate for the growth and activity of one or more selected species of bacteria in the colon and thus improve the health of the host.
Enteric Nervous System
The nervous system of the gastrointestinal tract.
The enteric nervous system, which is connected to the central nervous system largely through the vagus nerve and other pathways out of the spinal cord, can be divided into two neuronal networks, or plexuses: the myenteric pexus (or plexus of Auerbach) and the submucosal plexus (or plexus of Meissner).
Controls peristaltic activity and gastrointestinal motility
Lies in the muscularis externa between longitudinal and circular muscles of the muscularis propria.
Controls mainly gastrointestinal secretions and local blood flow.
Lies in the submucosa (mostly in the intestines)
cells of the small intestine