Digestive System Flashcards
1
Q
Digestive functions
A
- ingested food yields energy molecules (ATP) that can be used by body cells for: transportation, contraction, synthesis and secretion
- food serves a source of building supplies for the body tissues
2
Q
Enzyme that converts carbohydrates (CHO) into glucose
A
amylase
3
Q
Enzyme that converts proteins into amino acids
A
trypsin
4
Q
Enzyme that converts lipids into fatty acids
A
lipase
5
Q
How do you liberate energy from available food
A
food undergoes digestion, biochemical breakdown and absorption in GI tract
6
Q
4 digestive processes
A
- Motility
- Secretion
- Digestion
- Absorption
7
Q
2 types of motility
A
- propulsive motility
- mixing movements
8
Q
Propulsive motility
A
- muscular contractions propel the food contents forward in the digestive tract
- allows an appropriate velocity to a segment of the GI tract to perform its function in digestion
- ex: protein-rich food slows down in the stomach more than the mouth and esophagus (takes longer to breakdown)
9
Q
Mixing movements
A
- mixes the food with digestive juices
- exposes all portions of the food contents to the absorbing surface of the digestive tract
10
Q
Secretion
A
- the digestive juices from the exocrine glands into the digestive tract lumen
- comprise of water, electrolytes, and specific organic constituents such as enzymes, bile salts and mucus
- is under appropriate neural and/or hormonal stimulation
11
Q
Absorbable units of CHO
A
monosaccharides
12
Q
Absorbable units of proteins
A
amino acids
13
Q
Absorbable units of lipids/fats
A
fatty acids
14
Q
Carbohydrates (CHO)
A
are in the form of polysaccharides which consists of chains of interconnected glucose molecules
15
Q
Types of CHO and their sources (polysaccharide)
A
- starch= plants
- cellulose= plant cell wall
- glycogen= body muscles (meat)
- dietary CHO are in the form of sucrose, lactose (milk sugar)
16
Q
Digestion of carbohydrates
A
- Mouth-salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide
- As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.
- Duodenum- chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose
- the monosaccharides glucose and galactose are absorbed into the interior of the cell and eventually enter the blood by means of Na+ and energy-dependent secondary active transport
- the monosaccharide fructose is absorbed into the blood by passive facilitated diffusion
17
Q
Absorption
A
- complete digestion and absorption take place in the small intestine
- small absorbable units along with water, vitamins and electrolytes from the digestive tract –> digestive tract lumen –> blood/lymph
18
Q
Digestion of proteins
A
- Stomach- enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides
- Duodenum- other enzymes— trypsin, elastase, and chymotrypsin—act on the peptides reducing them to smaller peptides
- Duodenum- further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides)
- The amino acids are absorbed into the bloodstream through the small intestines
19
Q
Digestion of lipids
A
- Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase
- However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase
- When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification
- pancreatic lipases can then act on the lipids more efficiently and digest them. Lipases break down the lipids into fatty acids and glycerides
- These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining
20
Q
4 major tissue layers of the digestive tract
A
- mucosa- innermost layer
- submucosa
- muscularis externa
- serosa- outer layer
21
Q
Mucosa
A
- lines luminal surface of the digestive tract
- highly folded surface greatly increases the absorptive area
- 3 layers: mucous membrane, lamina propria and muscularis mucosa
22
Q
Mucous membrane
A
- protective surface
- modified for secretion and absorption
- contains: exocrine gland cells (digestive juices), endocrine gland cells (blood-borne gastrointestinal hormones), epithelial cells (absorbing digestive nutrients)
23
Q
Lamina propria
A
houses gut-associated lymphoid tissue (GALT): important in defence against disease-causing intestinal bacteria
24
Q
Muscularis mucosa
A
a thin layer of smooth muscle
25
Submucosa
- provides the distensibility and elasticity to GI tract
- blood vessels and lymph vessels
- a local neural network known as as submucosal plexus
26
Muscularis externa
- forms smooth muscle coat of the GI tract
- consists of two layers: inner circular layer (decreases the diameter of the lumen) and outer longitudinal layer (contraction shortens the GI)
- myenteric plexus: lies between the two muscle layers
27
Serosa
- secretes serous fluid that lubricates and prevents friction between digestive organs and surrounding viscera
- continuous with mesentery (a fold of membrane that attaches the intestine to the wall around the stomach area and holds it in place) throughout much of the tract
28
Purpose for the serosa being continuous with the mesentery
1. the attachment provides relative fixation
2. secure the digestive organs in proper place
3. allow the freedom for mixing and propulsive movements
29
Saliva
produced largely by 3 pairs of salivary glands
30
Composition of saliva
- 99.5% water
- 0.5% electrolytes and protein (amylase, mucus, lysozyme)
31
Functions of saliva
- salivary amylase: carbohydrate digesting enzyme
- facilitates swallowing by moistening food
- mucus provides lubrication
- antibacterial action: lysozyme destroys bacteria, saliva rinses away material that could serve as a food source for bacteria
- the solvent for molecules that stimulate taste buds
- aids speech by movements of lips and tongue
- help keep mouth and teeth clean
- rich in bicarbonate buffers
32
Autonomic influence on salivary secretion
- both sympathetic and parasympathetic NS act synergistically to increase salivary secretion
- parasympathetic: stimulates prompt and abundant saliva is rich in enzymes
- sympathetic: elicits smaller saliva volume; feeling mouth drier than usual (e.g. during stress)
33
Stomach structure
- j-shaped sac-like chamber lying between esophagus and small intestine
- divided into 3 sections: (1)fundus, (2) body and (3) antrum
34
Stomach functions
- food storage
- secretion of hydrochloric acid (denatures proteins) (HCl) and enzymes for protein digestion
- pulverizes semi-solid food into chyme
35
Pyloric sphincter
serves as a barrier between the stomach and upper part of small intestine
36
4 aspects of gastric motility
1. gastric filling
2. gastric storage
3. gastric mixing
4. gastric emptying
37
Gastric filling
Involves receptive relaxation:
- enhances the stomach's ability to accommodate the extra volume of food
- triggered by an act of eating
- mediated by the vagus nerve
38
Gastric storage
the body of the stomach
39
Gastric mixing
the antrum of the stomach
40
Gastric emptying
controlled by factors in the duodenum
41
Gastric emptying and mixing as a result of antral peristaltic contractions
1. a peristaltic contraction originates in the upper fundus and sweeps down toward the pyloric sphincter
2. the contraction becomes more vigorous as it reaches the thick-muscled antrum
3. the strong antral peristaltic contraction propels the chyme forward
4. a small portion of chyme is pushed through the partially open sphincter into the duodenum. The stronger the antral contraction, the more chyme is emptied with each contractile wave
5. when the peristaltic contraction reaches the pyloric sphincter, the sphincter is tightly closed and no further emptying takes place
6. when chyme that was being propelled forward hits the closed sphincter, it is tossed back into the antrum. Mixing of chyme is accomplished as chyme is propelled forward and tossed back into the antrum with each peristaltic contraction, a process called retropulsion
42
Factors in stomach that regulate gastric emptying
the amount of chyme in the stomach is the main factor that influences the strength of contraction
42
Factors in duodenum that regulate gastric emptying
- fat: fat digestion and absorption takes place only within the lumen of the small intestine, the presence of fat in the duodenum prevents further gastric emptying into the stomach
- acid: the unneutralized acid in duodenum inhibits the gastric emptying
- hypertonicity: osmolarity in the duodenal contents
- distension: too much chyme in duodenum
43
Factors that triggers gastric motility
- neural response
- hormonal response
- emotions
44
Factors that triggers gastric motility: neural response
- mediated through both intrinsic nerve plexuses and autonomic nerves reflexes
- collectively called enterogastric reflex
45
Factors that triggers gastric motility: hormonal response
involves the release of hormones from duodenal mucosa, collectively known as enterogastrones (secretin, CCK)
46
Factors that triggers gastric motility: emotions
- sadness, fear and intense pain= decrease motility
- anger and aggression= tend to increase motility
47
Gastric secretions
- 2 distinct areas of gastric mucosa: oxyntic mucosa and pyloric gland area (PGA)
- gastric pits at the base of gastric glands
- 3 types of gastric exocrine secretory cells: (1) mucous cells, (2) chief cells and (3) parietal (oxyntic) cells
48
Oxyntic mucosa
lines body and fundus
49
Pyloric gland area (PGA)
lines the antrum
50
Exocrine cells: Mucous cells
Function of cell:
Product secreted:
Function(s) of secretory product:
Function of cell:
- lines gastric pits and the entrance of glands
- secrete thin, watery mucus
Product secreted:
- alkaline mucus
Function(s) of secretory product:
- protects mucosa against mechanical, pepsin, and acid injury
51
Exocrine cells: Chief cells
Function of cell:
Product secreted:
Function(s) of secretory product:
Function of cell:
- secrete enzyme precursor, pepsinogen
Product secreted:
- pepsinogen
Function(s) of secretory product:
- when activated, begins protein digestion
52
Exocrine cells: Parietal (oxyntic) cells
Function of cell:
Product secreted:
Function(s) of secretory product:
Function of cell:
- secrete HCl and intrinsic factor
Product secreted:
- Hydrochloric acid (HCl)
- intrinsic factor
Function(s) of secretory product:
- HCl= activates pepsinogen, breaks down connective tissue, denatures proteins, kills microorganisms
- Intrinsic factor= facilitates absorption of vitamin B12
53
Endocrine/Paracrine cells: ECL (enterochromaffin-like) cells
Product secreted:
Function(s) of secretory product:
Product secreted: histamine
Function(s) of secretory product: stimulates parietal cells
54
Endocrine/Paracrine cells: G cells
Product secreted:
Function(s) of secretory product:
Product secreted: gastrin
Function(s) of secretory product: stimulates parietal, chief, and ECL cells
55
Endocrine/Paracrine cells: D cells
Product secreted:
Function(s) of secretory product:
Product secreted: somatostatin
Function(s) of secretory product: inhibits parietal, G, and ECL cells
56
Secretin
- presence of acid in the duodenum stimulates the release of secretin into the blood
- it inhibits emptying to prevent acid from entering the duodenum
- it inhibits gastric secretion to reduce the normal acid production
- it stimulates the pancreatic duct cells to produce a large volume of aqueous bicarbonate (NaHCO3) secretion in the duodenum to neutralize the acid
- along with CCK, secretin is tropic to the exocrine pancreas
57
Gastrin
- presence of protein-rich food in the stomach causes gastrin release
- increases HCl (parietal cells) and pepsinogen (chief cells) secretion
- HCl and pepsinogen promote protein digestion
- enhances gastric motility, stimulates ideal motility
- relaxes the ileocecal sphincter and induces mass movements of the colon
- has trophic effects on stomach and small intestine to keep the digestive tract lining viable
58
Major pancreatic proteolytic enzymes in protein digestion
1. trypsinogen
2. chymotrypsinogen
3. procarboxypeptidase
59
Protein digestion and absorption
1. dietary and endogenous proteins are hydrolyzed to their constituent amino acids and a few small peptide fragments by gastric pepsin and pancreatic proteolytic enzymes
2. amino acids are absorbed into the small intestine epithelial cells and eventually enter the blood by means of Na+ and energy-dependent secondary active transport
3. the small peptides, which are absorbed by a different type of carrier, are broken down into their amino acids by aminopeptidases in the epithelial cells brush borders or by intracellular peptidases
60
Pancreas
- a "mixed" gland- has both endocrine and exocrine functions
- located below the stomach
61
Endocrine function of pancreas
Islets of langerhans- secrete insulin and glucagon
62
Exocrine function of pancreas
- secrete pancreatic juice consisting of:
pancreatic enzymes actively secreted by acinar cells, aqueous alkaline solution (NaHCO3) actively secreted by duct cells that line pancreatic ducts
- regulated by: secretin, CCK
63
Types of pancreatic enzymes
proteolytic enzymes, pancreatic amylase and pancreatic lipase
64
Proteolytic enzymes
- digest protein
- trypsinogen: converted to active form trypsin
- chymotrypsinogen: converted to active form chymotrypsin
- procarboxypeptidase: converted to active form carboxypeptidase
65
Pancreatic amlyase
converts polysaccharides into disaccharides
66
Pancreatic lipase
a fat-digesting enzyme secreted throughout the entire digestive system
67
Functions of the liver not related to digestion
- metabolically processes the major categories of nutrients
- detoxifies or degrades body wastes, hormones, drugs, and other foreign compounds
- synthesizes plasma proteins
- it stores glycogen, fats, iron, copper, and many vitamins
- activates vitamin D
- removes bacteria and worn-out RBCs
- excretes cholesterol and bilirubin
68
Bile
- actively secreted by the liver and diverted to the gallbladder between meals
- stored and concentrated in the gallbladder
- bile consists of: bile salts, cholesterol, lecithin, bilirubin
- after the meal, bile enters the duodenum
69
Bile salts
- derivatives of cholesterol
- convert large fat globules into a liquid emulsion
- after participation in fat digestion, bile salts are reabsorbed back into the blood via an active transport mechanism
- pancreatic lipase cannot act on large fat globules to make fatty acids so liver/bile salts needs to break it down
70
Enterohepatic circulation
recycling of bile salts between the small intestine and liver
71
Emulsification
bile salts convert the large fat globules into small fat globules by a liquid emulsion
72
Action of bile salts
- emulsification is necessary for the action of pancreatic lipase to convert triglycerides into monoglycerides and fatty acids
- because monoglycerides and fatty acids are insoluble in watery chyme, these products are enclosed in a water-soluble, hydrophilic shell that encloses the fatty acids and vitamins called a micelle
73
Micelle
- consists of a hydrophilic (water-soluble) shell and a hydrophobic (lipid-soluble) core
- because the outer shell is water soluble, the products of fat digestion, which are not water-soluble, can be carried through the watery luminal contents to the absorptive surface of the small intestine by dissolving in the micelle's lipid-soluble core
74
Process of fatty acid absorption across small intestinal membrane
1. dietary fat in the form of large fat globules composed of triglycerides is emulsified by the detergent action of bile salts into a suspension of smaller fat droplets. This lipid emulsion prevents the fat droplets from coalescing and thereby increases the SA available for attack by pancreatic lipase
2. lipase hydrolyzes triglycerides into monoglycerides and free fatty acids
3. these water-insoluble products are carried in the interior of water-soluble micelles, which are formed by bile salts and other bile constituents, to the luminal surface of the small intestine epithelial cells
4. when a micelle approaches the absorptive epithelial surface, the monoglycerides and fatty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes
5. the monoglycerides and free fatty acids are resynthesized into triglycerides inside the epithelial cells
6. these triglycerides aggregate and are coated with a layer of lipoprotein to form water-soluble chylomicrons, which are extruded through the basal membrane of the cells by exocytosis
7. chylomicrons are unable to cross the basement membrane of blood capillaries, so instead they enter the lymphatic vessels
75
Absorption of water and sodium
- sodium is absorbed passively and actively down to its electrochemical gradient from the lumen into the blood through intestinal epithelial cells
- the movement of sodium is an energy-dependent process
- sodium either uses sodium-channels or co-transport molecules located in the epithelial cells across the luminal border
- water is absorbed from the lumen into the villus down the hydrostatic pressure into the capillary network
76
Absorption of iron
1. only a portion of ingested iron is in a form that can be absorbed
2. dietary iron that is absorbed into the small intestine epithelial cells and is immediately needed for RBC production is transferred into blood
3. in the blood, absorbed iron is carried to the bone marrow bound to transferrin, a plasma protein carrier
4. absorbed dietary iron that is not immediately needed is stored in the epithelial cells as ferritin, which cannot be transferred into the blood
5. excess iron in the blood can be dumped into the ferritin pool
6. this unused iron is lost in the feces as the ferritin-containing epithelial cells are sloughed
7. dietary iron that was not absorbed is also lost in the feces
77
CCK
- the presence of fat and other nutrients causes the release of CCK from the duodenal mucosa
- it inhibits gastric motility and secretion, allowing more time for the nutrients to be digested and absorbed
- stimulates pancreatic enzymes secretion of (fat digesting enzyme) by pancreatic acinar cells
- causes contraction of the gallbladder and relaxation of the sphincter of Oddi to empty the bile into the duodenum
- involved in long-term adaptive changes by increasing the secretion of pancreatic enzymes in response to prolonged changes in diet
- stimulates food intake (satiety)
78
Small intestine
- a site where most digestion and absorption take place
- only a small amount of water absorption takes place in this organ
- 3 segments: duodenum, jejunum, ileum
- motility includes: segmentation and migrating motility complex
79
Segmentation
- primary method of motility in small intestine
- consists of ring-like contractions along the length of small intestine
- this action mixes chyme throughout the small intestine lumen
80
Initiation and control of segmentation
- refers to mixing and propelling the chyme
- consists of oscillating, ring-like contraction of circular smooth muscles along the length of the small intestine
- contraction and relaxation of smooth contractile muscles propels the chyme from one end to another within the small intestine
- initiated by pacemaker cells in the small intestine that produce basic electrical rhythm (BER)
81
Functions of segmentation
- mixing chyme with digestive juices secreted into small intestine lumen
- exposing all chyme to absorptive surfaces of the small intestinal mucosa
82
Migrating motility complex
sweeps intestines clean between meals
83
Secretion in small intestine
- juice secreted by small intestine does not contain any digestive enzymes
- synthesized enzymes act within the brush-border membrane of epithelial cells: enterokinase (activates trysinogen- protein digestion), disaccharidases (carb digesting enzyme) and aminopeptidases (amino acid digesting enzyme)
84
Digestion in small intestine
- pancreatic enzymes from brush-border continue carbohydrate and protein digestion
- fat is digested entirely within the small intestine lumen by pancreatic lipase
85
Absorption in small intestine
- absorbs almost everything presented to it
- most occurs in duodenum and jejunum
- adaptations that increase small intestine's SA: microscopic finger-like projections called villi, brush border (microvilli) arise from the luminal surface of epithelial cells
- lining is replaced about every 3 days
- products of fat digestion undergo transformations that enable them to be passively absorbed - eventually enter lymph
86
Large intestine
- primarily a drying and storage organ
- not a large role in digestion itself
- consists of: colon, cecum, appendix, rectum, taeniae coli, haustra
- contents from the small intestine consist of indigestible food residues and unabsorbed biliary components, and remaining fluid
87
Colon
- extracts more water and salt from contents
- feces- what remains to be eliminated
88
Taeniae coli
longitudinal bands of muscle
89
Haustra
- pouches or sacs
- actively change location as result of contraction of circular smooth muscle layer
- haustral contractions: main motility, initiated by autonomous rhythmicity of colonic smooth muscle cells
90
Mass movements
- massive contractions
- moves colonic contents into the distal part of large intestine
91
Gastrocolic reflex
- mediated from stomach to colon by gastrin and by autonomic nerves
- most evident after first meal of the day
- often followed by the urge to defecate
92
Defecation reflex
1. contents from the colon into the rectum
2. distention of the rectum stimulates stretch receptors in the rectal wall initiating reflex
3. causes external anal sphincter to relax and defecation to take place
- voluntary (skeletal muscles)
- intra-abdominal pressure + forcible expiration against a closed glottis causes defacation
93
Delayed defecation may result in__
constipation
94
Constipation causes
- retention of colonic contents longer than normal
- more than the usual amount of water is absorbed from the feces; stool becomes hard and dry
- delayed in defecation frequencies beyond normal
95
Constipation symptoms
- abdominal discomfort, dull headache, loss of appetite followed by nausea and mental depression
- symptoms promptly disappear after relief from distension
96
Appendicitis
- deposition of hardened fecal material in the appendix causes inflammation of the appendix or appendicitis
- the inflamed appendix often becomes swollen and filled with pus, and tissue may die as a result of local circulatory interference
