Chapter 23- The digestive system Flashcards
Functions of the digestive system (6)
- Ingestion
- Movement
- Mechanical breakdown
- Digestion
- Absorption
- Defecation
How does movement occur in the digestive system?
Food propelled through digestive system by peristaltic movement. Peristaltic movements are wavelike contractions moving food in one direction- uses smooth muscle tissue
How does mechanical breakdown occur in the digestive system?
Food broken down into smaller pieces to increase efficiency of digestion. Increases the surface area of food that’s exposed to digestive substances
Membranes of the digestive system (2)
- Peritoneum
2. Mesentery
Peritoneum
Serous membrane that covers organs of the abdominopelvic cavity. The visceral peritoneum covers organs, while the parietal peritoneum clings to body wall
Peritoneal cavity
Fluid filled space between visceral and parietal peritoneum. Function- prevents friction form organ/organ or organ/body wall contact
Mesentery definition
Double layer of peritoneum extending from digestive organs to body wall
Mesentery functions (3)
- Holds organs in place
- Providing passage for blood/lymphatic/nerve supply to digestive organs
- Fat storage
Alimentary canal (6 organs)
Portion of digestive system that forms one long tube in- out of the body. Includes- mouth, pharynx, esophagus, stomach, small intestine, large intestine. Has associated accessory glands
4 basic layers of the alimentary canal organs
- Mucosa- innermost layer
- Submucosa- lies external to mucosa
- Muscularis externa
- Serosa- outermost layer is the visceral peritoneum
3 subdivisions of the mucosa
- Epithelia
- Lamina propria
- Muscularis mucosa
Epithelia of the mucosa
Epithelia is mostly simple columnar. Function- secretes mucus, digestive enzymes, hormones. Exceptions- mouth, esophagus, and anus- stratified squamous
Lamina propria
Vascularized loose areolar connective tissue. Allows absorption of nutrients. Lymph supply (MALT)- tonsils and appendix- protection from infectious disease
Muscularis mucosa
Has smooth muscle cells, creates movement to increase digestion and secretion. Digestive enzymes that break down food molecules and hormones are secreted. Not the most important layer for creating movement
Submucosa
Lies external to mucosa. Areolar connective tissue with elastic fibers- organs can stretch. Rich supply of blood and lymphatic vessels, lymphoid follicles, nerve fibers- serve surrounding layers of GI tract. BV keep the submucosa healthy, but also provides blood supply to surrounding layers
Muscularis externa
Muscular layer that generates peristaltic movement of GI tract. Thickenings in muscularis externa form sphincters. Functions- controls movement from one alimentary organ to another and prevents back movement of ingested food. The 2 layers can’t contract at the same time since they have opposite functions. To produce movement, the layers contract asynchronously
2 layers of the muscularis externa
- Circular layer- contraction causes tube to constrict
2. Longitudinal layer- contraction causes tube to become wider
Serosa
Outermost layer is the visceral peritoneum. In organs not found in abdominopelvic cavity (esophagus), the serosa is replaced by adventitia. Dense connective tissue that binds organ to surrounding structures- tougher and more fibrous than visceral peritoneum
Splanchnic circulation
Blood supply to digestive organs. Arteries that branch off aorta and hepatic portal circulation
Arteries of splanchnic circulation
Arteries- branches of celiac trunk that serve spleen, liver, and stomach. Hepatic artery, left gastric, splenic arteries. Splenic artery not part of digestive system
Hepatic portal circulation
Collects nutrient rich blood draining from digestive organs and transports it to the liver. In portal circulation, the blood skips the heart and goes to another organ. The digestive organs bring in new nutrients- don’t want the heart or the brain to be exposed to unstable conditions. Blood goes to the liver so it can be processed and toxins can be removed
2 major intrinsic nerve complexes of the enteric nervous system
- Submucosal nerve plexus- found in submucosa
2. Myenteric nerve plexus- found between circular and longitudinal muscle layers
Function of ENS nerve plexuses
Function- both plexuses supply walls of entire GI tract and help regulate motility through it
ENS long reflex
Initiated by stimuli inside and outside the GI tract. Involves CNS and extrinsic autonomic nerves- sympathetic and parasympathetic. Sympathetic stimulation inhibits gastric activity, while parasympathetic stimulation enhances gastric activity
ENS short reflex
Mediated entirely by enteric nervous system in response to stimuli within GI tract. Ex- peristalsis- wavelike contractions created by muscularis externa
Components of the mouth (2)
- Lips and cheeks
2. Palate
Lips and cheeks muscles
Composed of several skeletal muscles. Orbicularis oris- lips and buccinators- cheeks
Hard palate
Rigid, ribbed surface on roof of the mouth. Function- helps break down food
Soft palate
Soft, mobile surface at back of mouth. Posterior to the hard palate, closes off the nasopharynx when we swallow
Uvula
Projects from soft palate, helps close off nasopharynx
Bundles of skeletal muscle in the tongue (2)
- Intrinsic muscles
2. Extrinsic muscle
Intrinsic muscles of the tongue
Confined within the tongue, not attached to bone. Function- allow tongue to change shape. Ex- making the tongue flatter, thinner
Extrinsic muscles of the tongue
Origin of muscle is found on bone. Function- allows tongue to change position in mouth. Ex- moving the tongue from one side to another, sticking the tongue into/out of mouth
Functions of saliva (4)
- Cleanses mouth
- Dissolves food chemicals for taste
- Moistens food
- Contains digestive enzymes salivary amylase and lingual lipase- this is the very beginning of digestion, not all of it
Salivary amylase
Begins digestion of starchy foods only
Lingual lipase
Begins digestion of triglycerides only
Major salivary glands (3)
- Parotid gland
- Submandibular gland
- Sublingual gland
Parotid gland
Largest of all the salivary glands. Empties saliva from roof of mouth (2nd upper molar)
Submandibular gland
Opens at base of lingual frenulum
Sublingual gland
Smallest of salivary glands. Opens at floor of mouth via 10-20 different ducts
Function of teeth
Tear/grind food into smaller pieces during chewing
Types of teeth (2)
- Deciduous teeth
2. Permanent teeth
Deciduous teeth
Also called “baby teeth” or “milk teeth”. Total of 20 deciduous teeth. Roots start to degenerate, so the teeth die off and fall out
Permanent teeth
Will eventually replace baby teeth that fall out during adolescence-also called “adult teeth”. Total of 32 permanent teeth. Wisdom teeth do not emerge until about 17-25 years of age
Classification of teeth (4)
- Incisors (I)- cutting teeth
- Canines (C)- tearing teeth, important for meat eaters
- Premolars (PM) and molars (M)- grinding teeth
Primary definition formula of teeth
(2I, 1C, 2M (upper jaw)/ 2I, 1C, 2M (lower jaw)) *2
Permanent definition formula of teeth
(2I, 1C, 2PM, 3M (upper jaw)/2I, 1C, 2PM, 3M (lower jaw))* 2
2 main regions of teeth
- Crown- exposed portion
2. Root- portion embedded in jaw bone
Crown
The outer part is made of enamel- tough, mineralized substance covering exposed tooth surface. Strongest substance produced in the human body, but it can still be broken down, especially exposure to acidic substances (lemons). Cavities can go down through the dentin and reaching the pulp cavity in the middle of the tooth
Pulp cavity
Contains blood vessels, connective tisse, and nerves. The root canal is a narrow opening inferior to the pulp cavity. The pulp cavity is surrounded by dentin, and dentin is covered by enamel superficially.
Root- what is it anchored to?
Portion of the tooth embedded in jawbone. Anchored to periodontal ligament- gomphoses joint
Odontoblast
Cell that continuously lays down dentin
Deglutition
The act of swallowing food or fluid. Liquids are swallowed faster than solids
Deglutition phases (2)
- Buccal phase
2. Pharyngeal-esophageal phase
Buccal phase deglutition
Voluntary portion that occurs in the mouth. Pushing food to the back of the mouth stimulates tactile receptors in the pharynx
Pharyngeal-esophageal phase of deglutition
Involuntary portion controlled by swallowing center of the brain. Vagus nerve transmits impulses from swallowing center to muscles in pharynx and esophagus. Respiration temporarily inhibited- epiglottis closes off respiratory passages
Pharynx function
Flood/fluid/air pass through oropharynx and laryngopharynx. Contractions of muscle in walls propels food into esophagus
Where does the esophagus join to the stomach?
Joins stomach at cardiac orifice
Cardiac (gastroesophageal) sphincter
Thickening of smooth muscle at cardial orifice. Function- closes off stomach when food is not entering, prevents backflow of food/stomach acid into esophagus
Acid reflux
Gastric juices wash back up into the esophagus and burn the lining
Major regions of the stomach (4)
- Cardia
- Fundus
- Body
- Pyloric portion
Cardia of the stomach
portion of the stomach that joins with the esophagus
Fundus of the stomach
dome shaped portion
Body of the stomach
midportion of stomach
Pyloric portion of the stomach
Inferior funnel shaped portion of the stomach, leads directly into small intestine
Pyloric sphincter
Thickening of tissue that controls release of food from stomach to intestine. Prevents a large influx of material into the intestine, preventing damage
Omenta
Mesenteries extending from greater/lesser curvature of the stomach. Consists of lesser and greater sections. Function- attaches/anchors stomach to other digestive organs
Lesser omentum
Runs from liver to stomach
Greater omentum
Runs from stomach to intestines. Wraps over intestines and spleen, forming a “purse” to hold them in place. Has large collections of lymph nodes- immune cells can monitor peritoneal cavity and organs
Autonomic innervation of the stomach
There are parasympathetic fibers via vagus nerve. When active- increases digestion and gastric juice secretion. Also sympathetic fibers via thoracic splanchnic nerves. Inhibits gastric activity when active
How is the muscularis layer of the stomach modified?
Stomach has 3 layers of muscle, but the muscularis has an oblique muscle layer- provides extra muscle layer to generate force. Importance- stomach plays a role in mixing and moving food, pummeling it to smaller pieces to increase surface area
How is the mucosa of the stomach modified?
Surface epithelia is simple columnar cells that secrete large amounts of mucus. Lining dotted with gastric pits, lead into gastric glands- gastric juices produced here.
Gastric gland cells (4)
- Mucous neck cells
- Parietal cells
- Chief cells
- Enteroendocrine cells
Mucus neck cell
Produces acidic, thin mucus. Function of the mucus is unclear
Parietal cell
secrete HCl and intrinsic factor
HCl
HCl makes the stomach extremely acidic- pH 1.5-3.5 is normal. Importance- activates protein-digesting enzyme pepsin, denatures proteins and breaks down plant cell walls, kills off many bacteria. Acidic conditions aren’t good for bacterial growth or survival
Intrinsic factor
Substance necessary for vitamin B12 absorption in small intestine. Without B12, people develop megaloblastic anemia- this vitamin is important for producing RBCS. Lack of B12 also makes neurons and muscle cells fire abnormally, DNA synthesis won’t occur
Chief cells
Produce pepsinogen. Pepsinogen converted to pepsin by HCl. Function- pepsin breaks down proteins to amino acids
Enteroendocrine cells
Release 4 hormones: histamine, serotonin, somatostatin, gastrin
Histamine
Stimulates parietal cells to release HCl. Useful for before you start eating
Serotonin
Stimulates contraction of muscle in stomach wall. Interacts with smooth muscle cells in muscularis externa to stimulate smooth muscle contractions
Somatostatin
Plays several inhibitory roles in the stomach and other organs. More likely to be released in between meals
Gastrin functions (4)
- Increases HCl secretion
- Stimulates contraction of intestinal muscle
- Releases ileocecal valve
- Stimulates mass movement of material.
Purpose of the mucosal barrier in the stomach
Stomach must be protected from its own secretion of gastric juice
3 mechanisms of protection of the stomach (3)
- Thick coating of alkaline mucus on the internal surface of the stomach wall
- Epithelial cells joined by tight junctions
- Damaged epithelial cells shed and are quickly replaced
Importance of tight junctions in the stomach
Importance- gastric juices cannot escape/leak from stomach or internal organs would corrode
How often are epithelial cells in the stomach replaced?
Damaged epithelial cells shed and are quickly replaced. Undifferentiated stem cells in gastric pits replace damaged cells. You produce a new stomach lining every 3-6 days
Contractile activity of the stomach
This is important because the stomach has to push food into small intestine. Smooth muscle is thickest at the bottom of the stomach (pyloris), and the strongest contractions occur here- greater mixing/churning
Retropulsion of the stomach
As food is mixed/churned in pylorus, small amounts of liquid/small food particles can pass through the pyloric valve. This mixture of gastric juices and partially digested food is called chyme. The pyloric sphincter closes mid-contraction, forcing the rest of the food backward to be mixed/churned more
What establishes the rate of contraction of the stomach?
Rate of contraction established by enteric pacemaker cells. These cells don’t control how hard the stomach contracts. The more you stretch smooth muscle cells, the harder they will contract
Rate of stomach emptying is dependent on (3)
- What’s in the stomach
- How distended the stomach is
- Contents of the first part of the small intestine
How does what’s in the stomach determine the rate of stomach emptying?
Liquids empty more quickly since they don’t need to be broken down into smaller pieces
How does distention of the stomach determine the rate of stomach emptying?
More food= stronger contractions, which means more food is pushed into the intestine at once and the stomach empties more quickly
How do contents of the small intestine determine the rate of stomach emptying?
Receptors in duodenum react to incoming food- suppress gastric secretions to slow gastric emptying. Suppression of pyloric contractions in the stomach. Incoming fatty chyme- fats take more time to digest than carbohydrates, linger in duodenum longer
Accessory organs to the alimentary canal (6)
- Liver
- Gallbladder
- Pancreas
- Salivary gland
- Tongue
- Teeth
4 lobes of the liver
right, left, caudate, and quadrate
Falciform ligament
Divides left lobe of the liver from the right, suspends liver from diaphragm
Round ligament
Is a remnant of the umbilical veins
Common hepatic ducts
Liver products drain from lobes via common hepatic ducts. The liver has more than 500 functions, not all of them are digestive. Produces bile, which is important for digestion
Microscopic anatomy of the liver
Liver composed of tightly packed, hexagon shaped liver lobules. Each liver lobule is packed with liver cells (hepatocytes). A central vein runs through the center of each lobule. The vein brings the processed blood to the hepatic vein, where it enters general circulation again. Each corner of a lobule contains a portal triad
A portal triad consists of
- Hepatic artery branch
- Hepatic portal branch
- Bile duct
Liver sinusoids
Drain blood from portal triad, emptying into the central vein
Bile canaliculi
Drain bile secreted by hepatocytes- enter bile duct of portal triad. Bile eventually drains into duodenum
Hepatocyte functions (4)
- Bile secretion
- Processes bloodborne nutrients
- Store fat soluble vitamins
- Detoxifies blood
Bile
Yellow green alkaline solution produced by the hepatocytes. Bile salts and bilirubin
Bile salts
Component of bile aiding in digestion and absorption of fats. Bile salts are not disposed of by the body- constantly recycled. Recirculate 4-12 times per day, depending on how fatty your meals are. Saves the body energy of having to produce large amounts of new bile daily
Bilirubin
Bile pigment formed by heme breakdown. Absorbed from blood in liver, excreted into bile, metabolized in small intestine by bacteria
Cirrhosis
End stage liver failure, fibrosis of the liver. Healthy liver tissue is damaged, hepatocytes replaced by scar tissue. Liver function decreases, scar tissue obstructs vessels of hepatic portal system
Causes of cirrhosis (4)
- Build up of fats in liver tissue, some things that can cause this are Type 2 diabetes, PCOS, and sleep apnea
- Too much iron stored
- Hepatitis (B, C, and D especially)- the longer it has been going on, the more likely liver damage will be
- Alcoholism
How does too much iron cause cirrhosis?
Hereditary hemochromatosis- more iron than needed is absorbed from the digestive tract. Free iron is toxic, so it goes to the liver, but liver function is eventually affected.
How does alcoholism cause cirrhosis?
Large amounts of excessive alcohol intake. Alcohol is a toxin- the liver filters the blood. Can take anywhere from a decade to 2 or 3 decades of heavy drinking to cause cirrhosis
Treatment for cirrhosis
Liver transplant. Liver is a regenerative tissue- can regenerate if part of it is lost. With cirrhosis, it can’t- replaced by nonfunctional scar tissue. Recipient can “grow” new liver from a donor piece- won’t be exactly the same, but will still have a function. Donor can regrow the piece lost to transplant
Gallbladder
Sits in a shallow fossa on the inferior surface of the liver, very green in color when filled. When empty- has honeycomb like folds. Function- concentrated bile storage. Absorption of water and ions allows for concentration of bile
Pancreas function
Secretes substances that help digest a wide variety of foodstuff. The exocrine part of pancreas secretes pancreatic juice, the endocrine part contains pancreatic islets.
Exocrine part of pancreas (2 parts)
- Acini- synthesizes, stores, and secretes digestive enzymes. In pancreas- digestive enzymes are inactive
- Ducts- transports secretions of acini
Endocrine part of pancreas (2 parts)
- Alpha cells- secrete glucagon. Increase blood glucose levels
- Beta cells- secrete insulin. Decrease blood glucose levels
Composition of pancreatic juice
Inactive while in the pancreas. Composed of water, enzymes, electrolytes (mostly bicarbonate). Alkaline in nature- neutralizes acidic chyme entering the small intestine from the stomach
Digestive enzymes found in pancreatic juice (4)
- Amylase (starches)
- Proteases (proteins)
- Lipases (fats)
- Nucleases (nucleic acids)
Where is the small intestine located?
Named for diameter, not length. The small intestine is actually longer than the large intestine. Extends from the pyloric sphincter to the ileocecal valve.
3 subdivisions of the small intestine
- Duodenum
- Jejunum
- Ileum- joins to large intestine at the ileocecal valve
Duodenum
Digestive juices of liver and pancreas empty into duodenum via major duodenal papilla. Shortest of all the segments, major digestive portion of the small intestine
Jejunum
Connects duodenum to ileum
Ileum
Joins to large intestine at the ileocecal valve
Circular folds of small intestine
Permanent folding of submucosa and mucosa. Function- forces chyme to move slowly- increases absorptive capacity since surface area is increased
Villi of the small intestine
Projections of mucosa. Function- increases surface area of small intestine to increase absorption. Center of each villus has blood supply and lacteal. Villi are largest in duodenum, become smaller through jejunum and ileum
Intestinal crypts of the small intestine
Tubular glands in wall of small intestine. Contain different types of cells that secrete products
Cells of intestinal villi and crypts (5)
- Enterocyte
- Goblet cells
- Enteroendocrine cells
- Paneth cells
- Stem cells- continuously dividing cells that replenish mucosa, can replace any of the above cells
Enterocyte
Simple columnar cells with microvilli. Function- absorbing nutrients in villi, secrete intestinal juice in crypts
Paneth cells
release defensins and lysozyme
Digestive roles of the small intestine (2)
- Enzymes for digestion
2. Regulates chyme entry
Brush border enzymes
Digestive enzymes of the small intestine that break down macromolecules into monomers
How does the small intestine regulate chyme entry?
Large amounts of acidic chyme can damage the small intestine. Acidic chyme is hypertonic to the blood plasma. Small intestine only lets small amounts of chyme enter at one time
Segmentation
Contractions in small intestine that slows rate of movement of chyme. Segmentation is forward and backward movement- not like the wave-like contractions of peristalsis. Function- contractions push chyme back and forth
What initiates contractions of the small intestine?
Pacemaker cells initiate contractions. Cells in duodenum depolarizes faster than ileum- allows for “forward” movement overall
What happens in the small intestine after a meal?
Parasympathetic nervous system enhances segmentation. Chyme is moved slowly to increase digestion and absorption
Motilin
Between meals, hormone motilin is released when most absorption has already occurred. Long peristaltic waves begin in small intestine- moves waste, debris, and sloughed off cells toward large intestine. As soon as next meal comes in- motilin release decreases, segmentation begins again
Ileocecal valve
Controls passage of materials from small intestine to large intestine. Is mostly closed, but relaxes due to 2 factors
What relaxes the ileocecal valve? (2)
- Gastroileal reflex
2. Gastrin
Gastroileal reflex
Increases force of segmentation in small intestine and relaxes valve
Gastrin
Hormone that increases small intestine motility, relaxes valve
3 structures found in the large intestine and nowhere else in the alimentary canal
- Teniae coli
- Haustra
- Epiploic appendages
Teniae coli
Bands of smooth muscle tissue from longitudinal layer. Causes puckering of wall of large intestine
Haustra
“Pockets” formed from teniae coli. Gives colon its segmented appearance. Function- contractions of individual haustra aid motility in large intestine
Epiploic appendages
Fat filled pouches that hang from the surface of large intestine. Function is unclear
6 subdivisions of the large intestine
- Cecum
- Appendix
- Colon
- Sigmoid colon
- Rectum
- Anus
Cecum
Sac-like structure making up the first part of the large intestine. Large intestine divided from small intestine at ileocecal valve
Appendix
Extends off from cecum. Lymphoid organ- MALT. Stores extra bacteria- can recolonize gut when necessary
Colon (3 parts)
- Ascending colon travels up the right side of the abdominal cavity
- Transverse colon crosses abdomen right to left
- Descending colon travels down left side of abdominal cavity
Sigmoid colon
point at which colon enters the pelvis
Rectum
Temporary holding site for feces. Contains rectal valves- prevent feces from being passed with gas
Anus
Opens to the exterior of the body (the “end” of the alimentary canal), has 2 sphincters
2 sphincters of the anus
- Internal anal spinchter- composed of smooth muscle
2. External anal spinchter- composed of skeletal muscle tissue
Microanatomy of the large intestine
Mostly simple columnar epithelia. Exception- anal canal is stratified squamous epithelia. Abundant crypts packed with goblet cells- mucus secreted reduces friction. No villi, brush border, or circular folds. Digestion is almost entirely complete before entering large intestine
Bacterial flora of the large intestine
1000+ types of bacteria that come into the body through anus or survive harsh conditions of stomach and small intestine. Important for vitamin synthesis and fermentation
Which vitamins does the bacteria in the large intestine synthesize?
B complex vitamins, some vitamin K synthesized by gut bacteria
Fermentation
Bacteria can ferment indigestible carbohydrates and mucin- produces short chain fatty acid- used for fuel by body cells. Fermentation results in 500 ml of gas produced per day. The more indigestible carbohydrates you consume, the more gas you produce
Motility of the large intestine
Slow, sluggish contraction in haustra. Contractions allow mixing of residue to increase water absorption
Mass movements in the large intestine
Force residue toward rectum. Promotes final “drying out” of feces- last water is removed. Fiber strengthens mass movement and softens feces
Defecation reflex
Initiated by stretching of rectal walls. Sigmoid colon and rectum contract, internal sphincter relaxes. Allowed by opening of external anal spinchter (voluntary portion). If not allowed- reflex ends, but will begin again in a few minutes
Primary mechanism of digestion
Primary mechanism- enzymatic hydrolysis. Enzymes break down large food molecules to monomers, and are secreted in lumen by intrinsic and accessory glands
Where is most digestion accomplished?
Most accomplished in small intestine. Pancreatic enzymes break down large molecules into smaller molecules. Brush border enzymes from small intestine break down various molecules into monomers- lipases and bile both necessary for fat digestion
How does absorption occur?
Substances must move through the enterocytes from apical membrane- basal membrane of small intestine, and must pass through interstitial fluid, diffuse into capillaries. Nonpolar substances absorbed passively, anything else must be actively transported. Nothing you eat can be absorbed in the small intestine until it is broken down to its simplest form- monomers
Sources of carbohydrates (3)
- Polysaccharides
- Disaccharides
- Monosaccharides
Polysaccharides
Starch and glycogen. Remember- starch digestion begins in the mouth. Polysaccharides initially broken down into oligosaccharides- chains of 2-8 glucose molecules linked together
Disaccharides (3)
Sucrose, lactose, maltose. Maltose found in grains, lactose in milk, sucrose in fruit
Monosaccharides (3)
Glucose, fructose, and galactose. These are monomers absorbed by the small intestine
How are polysaccharides broken down? (5 steps)
- Polysaccharides broken down into oligosaccharides (chains of 6-8 glucose molecules) and disaccharides by pancreatic amylase.
- Brush border enzymes (dextrinase and glucoamylase) break down disaccharides and oligosaccharides into monosaccharides
- Monosaccharides transported through enterocyte. Glucose and galactose- secondary active transport. Fructose- facilitated diffusion
- Monosaccharides exit enterocyte at basal membrane- facilitated diffusion
- Enter bloodstream via intercellular clefts
Sources of proteins (3)
- Dietary proteins
- Enzyme proteins secreted into GI tracts by glands
- Proteins from sloughed off/disintegrating mucosal cells
How much dietary protein do we consume per day?
125 mg
Where does the digestion of protein begin?
Digestion begins in stomach with activation of pepsin
Which pancreatic proteases break down proteins? (2)
- Trypsin and chymotrypsin- break down larger proteins into smaller fragments
- Carboxypeptidase- split off individual amino acids
Protein digestion process (4 steps)
- Proteins broken down by pancreatic proteases
- Brush border enzymes split remaining protein fragments into individual amino acids
- Amino acids are cotransported into enterocyte
- Amino acids exit enterocyte via facilitated diffusion, enter blood via intercellular clefts
Sources of lipids
Triglycerides
Where does digestion of lipids begin?
Digestion begins in small intestine- pancreatic juices containing lipases and bile necessary for fat digestion
Monomers of lipids
Monomers- fatty acids and monoglycerides
Emulsification
Bile salts from bile break down large fat globule into smaller fat droplets. Importance- smaller fat droplets have increased surface area exposed to lipases. Emulsification does not break chemical bonds
Digestion of lipids process (6 steps)
- Emulsification
- Pancreatic lipase- breaks down triglycerides to fatty acids and monoglycerides
- Micelle formation- fatty acids and monoglycerides associate with bile salts again to form micelles
- Lipids leave micelle at absorptive surface, move into enterocyte by simple diffusion
- Chylomicron formation
- Chylomicrons transported out of enterocyte by exocytosis
Micelle formation importance
Importance- without micelle formation, fatty acids and monoglycerides would float to chyme surface, not transported to enterocyte. A micelle is an aggregate of molecules
Chylomicron formation
Once inside the enterocyte, fatty acids and monoglycerides recombined to form triglycerides. Triglycerides combined with various other molecules, coated with protective, water-soluble proteins to form a chylomicron
How are chylomicrons transported out of the enterocyte?
Chylomicrons transported out of enterocyte by exocytosis. Too large to pass through plasma membrane of enterocyte or the capillary walls. Instead, chylomicrons are pumped straight into lacteals lymph system. Chylomicrons exit lymph system into venous blood
How are nucleic acids digested?
Pancreatic nucleases hydrolyze nucleic acids. Brush border enzymes break apart nucleotides into nitrogenous bases, pentose sugars, and phosphate ions.
How are nucleic acids absorbed?
Products actively transported across small intestine epithelium, enter blood
Functions of vitamins
Serve as coenzymes for reactions in the body- help enzymes to function
Which vitamins are fat soluble? (4)
Vitamins A, D, E, and K are fat soluble only- absorbed through GI tract in a similar way to lipids. Usually don’t need a multivitamin if you are healthy/have a normal diet
Which vitamins are water soluble? (2)
Vitamins B and C are water soluble only
Where in the digestive tract is water absorbed?
95% of water absorbed in small intestine, nearly the rest by large intestine- .1 L leaves body with feces
Sodium absorption
Sodium absorption coupled with movement of glucose and amino acids- moves into enterocyte on apical surface with monomers. Pumped out of enterocyte on basal surface by Na/K pump
Potassium absorption
K+ moves into mucosa by facilitated diffusion. K+ absorption influenced by water absorption- anything that inhibits water absorption usually inhibits K+ absorption
