gastrointestinal (GI) tract Flashcards
what does the GI tract do
allows the body to extract necessary nutrients, vitamins, minerals and water the body requires from the external environment to maintain cellular energy and functioning
what are the 4 main layers of the GI tract from the outermost layer
- serosa
- muscularis externa
- submucosa
- mucosa
what is the serosa layer made of
has 2 parts
- visceral peritoneum- the serous lining covering most of the viscera within the abdominopelvic cavity. the walls of the cavity are lined with the parietal peritoneum. in between these layers is the peritoneal cavity, filled with serous fluid that allows free, frictionless movement of the viscera within the abdominopelvic cavity
- adventitia- a layers of fibrous connective tissue surrounding the GI tract and attaching it to adjacent structures at the pharynx, esophagus and rectum
what is the muscularis externa layer made of
a double layer of smooth muscle- inner circular layer and external longitudinal layer that produces varied movements of the GI tract. this layer also forms sphincters throughout various locations of the tract. the stomach contains a third, smooth layer that enhances movements for it to mix food with gastric secretions
what is the submucosa made of
a layer of sense irregular connective tissue that supports the mucosa. it is rich in blood vessels, nerves and lymphatic vessels and follicles.
what is the mucosa made of
it is the innermost lining found throughout the GI tract.
- it secretes mucus, digestive enzymes and hormones
- absorbs the products of digestion and protects against pathogens.
it is further subdivided into 3 sub-layers
- epithelium
- lamina propria- CT basement membrane for the epithelium containing blood vessels, nerves and lymph vessels
- muscularis mucosa- thin layer of smooth muscle underlying the basement membrane
what is ingestion
putting food/ drink into the digestive tract (eating)
what is mechanical digestion
physically breaking food into smaller pieces
what is propulsion
moving substances along the length of the tract
what is chemical digestion
breaking food into its smallest chemical components
what is absorption
movement of nutrients into the blood/ body fluids
what is defecation
elimination of indigestible material from digestive tract
what does chemical digestion require
the presence of enzymes which catabolize food molecules into their metabolic building blocks which are then able to be absorbed into the blood or lymph capillaries residing in the submucosa.
how does propulsion occur
via peristalsis- waves of contractions that occur behind the food which push it forward
what movements does mechanical digestion include
movements that alternately squeeze different segments of food to physically break it smaller including;
- chewing
- grinding/ churning- stomach movements in alternating directions that break up and mix food
- segmentation- contractions of segments of the small intestine that break up and mix food
how does defecation occur
via the opening and closing of sphincters in the anus. the internal sphincter opens under involuntary control so feces moves into the anal canal and the external sphincter opens under voluntary control
what are structures and functions of the oral cavity
lips, cheeks, palate, tongue
- these structures are lines with the mucosa of the digestive tract to resist abrasion which also secretes the antimicrobial peptides called defensins to keep the mouth healthy and clean
what digestive processes occur in the oral cavity
- ingestion- food is placed into the oral cavity
- mechanical digestion- chewing
- chemical digestion- salivary amylase breaks down carbohydrates
- propulsion- swallowing
what are the functions of the salivary glands (accessory gland of the oral cavity)
- cleanses the mouth
- dissolves food chemicals for taste
- moistens food and aids in bolus formation
- chemical digestion of carbohydrates via the digestive enzyme salivary amylase
- immune protection
what is the digestion process in the pharynx and esophagus
propulsion- the continuing of swallowing. the 2nd step and is involuntary via peristalsis
what is the esophagus
a passageway for food from the pharynx to the stomach. when it is not conducting ingested food, it is collapsed and lies flat against the posterior of the trachea
what is the gastroesophageal sphincter
it is at the end of the esophagus and controls the entrance of food into the stomach
what are the 3 regions of the pharynx
nasopharynx, oropharynx and larngyopharynx . they conduct food and drink from the mouth into the esophagus
what is the structure of the stomach
a muscular pouch in the upper left quadrant of the abdomen. it has a volume of 50ml but can expand to 4L due to the presence of rugae (folds of the mucosa and submucosa) which flatten out as the stomach fills.
what does the stomach wall do
it enhances the ability for the stomach to perform its digestive processes. the muscularis externa contains a third innermost muscular layer (oblique layer) which enhances the motility of the stomach, while the mucosa contains numerous deep gastric pits which extend and open into gastric glands
what are gastric pits and glands
gastric pits are lined with mucous cells which secrete a cloudy, alkaline mucus. the gastric glands are composed of 4 different secretory cells (mucous neck cells, parietal cells, chief cells and enteroendocrine cells) whose secretions collectively form gastric juice, which mixes with food in the stomach to form chyme. each chemical secretion has its own digestive functions
what do mucous neck cells secrete
acidic mucus- which lubricates the stomach and moistens food
what do parietal cells secrete
- hydrochloric acid- which lowers the pH of the stomach killing most bacteria, unfolds proteins and converts pepsinogen into pepsin
- intrinsic factor- required for intestinal absorption of vitamin B12
what do chief cells secrete
pepsinogen- which inactivates digestive enzymes converted into pepsin to chemically digest proteins
what do enteroendocrine cells secrete
hormones- which regulate stomach secretion and motility and effect other digestive processes or overall digestive regulation
what are the digestive processes in the stomach
- propulation and retropulsion- chyme is propelled from the fundus towards the pylorus of the stomach and then most is propelled back
- mechanical digestion- chyme is broken into smaller droplets due to churning muscular actions of the stomach
- chemical digestion- pepsin chemically digests proteins, breaking them into polypeptides and olipeptides
what are the GI tract structures
esophagus, large intestine, pharynx, oral cavity/ mouth, small intestine, stomach
what are the GI accessory glands
salivary glands, pancreas, liver
what digestive process is unique to the oral cavity
ingestion
what is the small intestine and what are the accessory glands of it
a long tube connecting the stomach to the large intestine.
pancreas- produces digestive enzymes
liver- produces bile and bile salts which emulsify fats (prevents fat molecules from remerging)
gallbladder- store and concentrate bile
what are the digestive processes of the small intestine
- absorption- 100% of macronutrients, 80-90% of water, minerals and vitamins absorbed
- chemical digestion- pancreatic amalase break down carbohydrates, proteases break down to proteins, lipases break down lipids and brush border enzymes break down carlos and proteins
- propulsion- via peristalsis
- mechanical digestion- segmentation
what are the 3 parts of the small intestine
- duodenum- first and shortest region extending from the stomach and curving around the pancreas
- jejunum- middle region arising from the duodenum and continuous with the ileum
- ileum- distal region connecting to the large intestine via the ileocecal valve.
what do the circular folds in the small intestine do
increase the contact surface by slowing chyme passage to spiral down the length of the small intestine
what does the villi do in the small intestine
increase the absorptive surface area
what does micro-villi do in the small intestine
enhance the surface area and hold the brush border enzymes which do the final chemical breakdown of carbs and proteins prior to absorption
what do the intestinal glands/ crypts of lieberkuhn do in the small intestine
secrete intestinal juice composed of mucus, alkaline chemicals and hormones which regulate activity of the small intestine
what are the functions of the liver
- blood filtration
- metabolism of nutrients and other chemical products
- production of bile
- detoxification
- storage of various substances
- production of plasma proteins
what function of the liver is directly associated with the digestive functions of the small intestine
bile production
what is the structure of the liver
liver cells are stacked into hexagonal shaped lobules that liver sinusoids (capillaries) run through to form part of the hepatic portal system. This system carries poorly oxygenated, but nutrient rich blood arriving at the liver from the GI tract
what does the hepatic portal system do
- removal of debris, microbes and dead/damaged cells from the blood by macrophages
- removal or metabolism of toxins
- regulation of solute and nutrient levels through absorption and secretion by hepatocytes
- secretion of plasma proteins by hepatocytes into the bloodstream
what do hepatocytes produce
bile that they secrete into bile canaliculi passing throughout the lobule and converging at one of the six bile ducts at the corners of each hexagonal lobule.
what is bile from hepatocytes made of
- bile salts
- bile pigments and other waste products
- cholesterol
- triglycerides
- phospholipids
- electrolytes
of the substances found in bile, what one is associated with the digestive functions of the small intestine
bile salts- these emulsify to aid with lipid digestion
what do bile ducts do
drain bile into the left or right hepatic ducts which merge to form the common hepatic duct. it will drain bile from the liver into either the common bile duct opening into the duodenum or the cystic duct opening into the gallbladder
what does the gallbladder do and where is it located
it is located on the posterior-inferior aspect of the liver and stores and concentrates bile when it is not passing directly into the duodenum of the small intestine
what does the pancreas exocrine function produce
pancreatic juice which is excreted into the duodenum of the Gi tract. it is produced by the acinar cells of the pancreas and consists of digestive enzymes, water, ions and bicarbonate
what macronutrients are broken down by the pancreatic digestive enzymes
carbs, proteins and lipids
what is the pancreatic amylase pancreas digestive enzyme
chemically digests carbs from polysacchrides into smaller oliosaccharides and disaccharides
what is the lipases pancreas digestive enzyme
a class of enzymes that chemically breaks down lipids from triglycerides into monoglycerides and fatty acids
what is the proteases pancreas digestive enzyme
a class of enzymes that chemically breaks down proteins from larger polypeptides into smaller polypeptides and peptides
what is the nucleases pancreas digestive enzyme
break down DNA and RNA
propulsion in the small intestine
peristalsis moves the intestinal mixture from the duodenum into and through the jejunum and ileum
mechanical digestion in the small intestine
segmentation breaks the mixture up into smaller droplets throughout the jejunum and ileum
chemical digestion in the small intestine
all 3 macronutrients are chemically digested in the small intestine:
- lipids- emulsification occurs to allow lipases to breakdown lipids into fatty acids
- carbs- pancreatic amylase continues the job started in the mouth by breaking polysaccharides and oligosaccharides into disaccharides. brush border enzymes then break the disaccharides into monosaccharides
- proteins- proteases break up polypeptides into smaller polypeptides and peptides. brush border enzymes then break the peptides to amino acids
absorption in the small intestine
- 100% of macronutrients
- monosaccharides and amino acids are absorbed into the capillaries in villi
- glycerides and fatty acids are packaged into micelles and chylomicrons which are absorbed into the lacteals and travel in lymph before being deposited back into bloodstream - most vitamins
- 80-90% of water and minerals
what is emulsification
in order for lipids to be chemically digested by lipases, they must first undergo emulsification to make the chemical digestion easier and more effective. Emulsification is accomplished by bile salts.Lipids (fat) and water don’t mix, and thus fat droplets tend to coalesce. To prevent this from happening, and keep fat droplets small, bile salts surround fat droplets, preventing them from coalescing.
what is the structure of the large intestine
appendix, cecum, ascending colon, transverse colon, descending colon, sigmoid colon and anus/rectum
what are the functions of the normal flora in the large intestine
- metabolism of some molecules and fermentation of indigestible carbs
- synthesis of B complex vitamins and vitamin K
- immune protection via out competing other microorganisms.
what are the digestion processes of the large intestine
- absorption- the remaining water, minerals and vitamins are absorbed
- propulsion- occurs via slow, mass peristaltic contractions that occur 3-4 times daily and which compact the material into feces.
- defecation- propulsion of feces into the rectum intitates the defecation reflex
what is the defecation reflex of the large intestine
- stretch of the rectal wall initiates involuntary signals to enhance peristalsis in the colon and to relax the internal anal sphincter
- as more feces stretches the rectal wall, signals are sent to the brain which give a conscious awareness of the need to expel the fecal matter
- expulsion of feces occurs due to the voluntary signals to open the external anal sphincter and allow the feces to pass out of the body
