1
Q
What is mass balance in the digestive system
A
To maintain homeostasis, the volume of fluid entering the GI tract by intake or secretion must equal the volume leaving the lumen.
2
Q
4 processes of the digestive system
A
- Digestion: Chemical and mechanical breakdown
- Absorption: Movement of material from GI lumen to ECF
- Motility: movement by muscle contraction
- Secretion: hormones and enzymes
3
Q
Primary function of the digestive system
A
–Transfer nutrients, water, and electrolytes from ingested food into body’s internal environment
–Ingested food is essential an energy source and for supplies for the building blocks of tissues
4
Q
Salivary glands
A
sublingual
submandibular glands
parotid glands
5
Q
Chyme
A
The stomach continues digestion that began in the mouth by mixing food with acid and enzymes to create chyme.
6
Q
What happens to the products that are digested and absorbed?
A
- absorbed
- move into the blood or lymph for distribution
- waste remaining leaves the body through anus
7
Q
esophagus
A
a narrow tube that travels through the thorax to the abdomen.
8
Q
stomach
A
a baglike organ that can hold as much as 2 liters of food and fluid when fully (if uncomfortably) expanded.
9
Q
The stomach is divided into three sections:
A
fundus
body
pyloric region
10
Q
pylorus
A
opening between the stomach and the small intestine is guarded by the pyloric valve.
11
Q
what is the function of pyloric valve
A
thickened band of smooth muscle relaxes to allow only small amounts of chyme into the small intestine at any one time.
12
Q
small intestine is divided into three sections:
A
- duodenum
- jejunum
- ileum
13
Q
What happens in the large intestine?
A
water and electrolytes are absorbed out of the chyme to form feces
14
Q
rectum
A
distension of the rectal wall triggers a defecation reflex.
15
Q
anus
A
Feces leave the GI tract through the anus, with its external anal sphincter of skeletal muscle, which is under voluntary control.
16
Q
The GI tract wall consists of four layers:
A
(1) mucosa
(2) submucosa,
(3) muscularis externa
(4) serosa.
17
Q
Mucosa layers
A
- mucous membrane
- lamina propria
- muscular mucosa
18
Q
The entire wall is crumpled into folds called ____ in the stomach, and ____ in the small intestine.
A
rugae
plicae
19
Q
The intestinal mucosa also projects into the lumen in small fingerlike extensions known as _____
A
villi
20
Q
Where is submucosa found and what is its function?
A
The layer of the gut wall adjacent to the mucosa, the submucosa, is composed of connective tissue with larger blood and lymph vessels
21
Q
muscularis externa consists primarily of two layers of smooth muscle: (functions)
A
- inner circular layer –> contraction decreases diameter of lumen
- outer longitudinal layer –> shortens the tube
22
Q
Motility in the gastrointestinal tract serves two purposes:
A
- moving food from the mouth to the anus
- mechanically mixing food
- mixing maximizes exposure of the particles to digestive enzymes by increasing particle surface area.
23
Q
What happens during peristalsis? how does it propagate?
A
- wave movements
- circular muscles contract just behind bolus
- contraction pushes the bolus forward
- circular muscles are relaxed
- receiving segment then contracts
24
Q
how do segmental contractions occur, which muscles are involved and what is the purpose
A
- segments of intestine alternately contract and relax.
- In the contracting segments, circular muscles contract while longitudinal muscles relax.
- Alternating segmental contractions churn the intestinal contents, mixing them and keeping them in contact with the absorptive epithelium.
25
migrating motor complex (MMC)
is a series of contractions that begin in the empty stomach and end in the large intestine.
26
What are Slow waves
are spontaneous depolarizations in GI smooth muscle
27
What are tonic contractions
that are sustained for minutes or hours occur in some smooth muscle sphincters and in the anterior portion of the stomach.
28
What are phasic contractions
with contraction-relaxation cycles lasting only a few seconds, occur in the posterior region of the stomach and in the small intestine.
29
________ deep in the gastric glands secrete hydrochloric acid into the lumen of the stomach.
Parietal cells
30
_______ in the small intestine and colon secrete an isotonic NaCl solution that mixes with mucus secreted by goblet cells to help lubricate the contents of the gut.
Crypt cells
31
Mucus is a viscous secretion composed primarily of glycoproteins collectively called _____. The primary functions of mucus are to form a protective coating over the GI mucosa and to lubricate the contents of the gut. Mucus is made in specialized exocrine cell called _______ in the stomach, ______ in salivary glands, and goblet cells in the intestine
mucins
mucous cells
serous cells
32
What is bile and where is it secreted from?
Bile is a non-enzymatic solution secreted from hepatocytes, or liver cells
33
The key components of bile are
(1) bile salts, which facilitate enzymatic fat digestion,
(2) bile pigments, such as bilirubin, which are the waste products of hemoglobin degradation, and
(3) cholesterol, which is excreted in the feces.
34
Bile salts, which act as detergents to solubilize fats during digestion, are made from
steroid bile acids combined with amino acids.
35
Bile is secreted into _____ that lead to the ______, which stores and concentrates the bile solution. During a meal, contraction of the gallbladder sends bile into the duodenum through the ______, along with a watery solution of bicarbonate and digestive enzymes from the pancreas.
hepatic ducts
gallbladder
common bile duct
36
Once absorbed, most nutrients enter capillaries within the villi. e exception is fats, which mostly enter lymph vessels called ________.
lacteals
37
The enzyme _____ breaks long glucose polymers into smaller glucose chains and into the disaccharide maltose.
amylase
38
The transporters that are used by the intestinal cells to transport monosaccharides are
identical to those found in the renal proximal tubule: the apical Na+-glucose SGLT symporter and the basolateral GLUT2 transporter
39
True/False
Fructose absorption is Na+-dependent.
False
Fructose absorption, however, is not Na+-dependent. Fructose moves across the apical membrane by facilitated diffusion on the GLUT5 transporter and across the basolateral membrane by GLUT2
40
The enzymes for protein digestion are classi ed into two broad groups: ______ and _______.
endopeptidases
| exopeptidases
41
Endopeptidases, more commonly called _______, attack peptide bonds in the interior of the amino acid chain and break a long peptide chain into smaller fragments.
proteases
42
Proteases are secreted as inactive ______ from epithelial cells in the stomach, intestine, and pancreas and are activated in the GI tract lumen. Examples of proteases include ______ secreted in the stomach, and _____ and ________ secreted by the pancreas.
proenzymes
pepsin
trypsin
chymotrypsin
43
________ release single amino acids from peptides by chopping them o the ends, one at a time.
Exopeptidases
44
The most important digestive exopeptidases are two isozymes of _________ secreted by the pancreas. ________ play a lesser role in digestion.
carboxypeptidase
| Aminopeptidases
45
How are amino acids absorbed in the intestines?
Most free amino acids are carried by Na+-dependent cotransport proteins similar to those in the proximal tubule of the kidney. A few amino acid transporters are H+-dependent.
46
What are lipases and how do they act?
Enzymatic fat digestion is carried out by lipases, enzymes that remove two fatty acids from each triglyceride molecule.
47
The bile salt coating of the intestinal emulsion complicates digestion, however, because lipase is unable to penetrate the bile salts. So what happens?
For this reason, fat digestion also requires colipase, a protein cofactor secreted by the pancreas. Colipase displaces some bile salts, allowing lipase access to fats inside the bile salt coating. Phospholipids are digested by pancreatic phospholipase. Free cholesterol need not be digested before be- ing absorbed.
48
Step-by-step fat digestion and absorption
1. Bile salts from liver coat fat droplets.
2. Pancreatic lipase and colipase break down fats into monoglycerides and fatty acids stored in micelles.
3a. Monoglycerides and fatty acids move out of micelles and enter cells by diffusion.
3b. Cholesterol is transported into cells.
4. Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons.
5. Chylomicrons are removed by the lymphatic system.
49
Gastrin (G cells)
- Stimulus for release
- Primary targets
- Primary effects
- Other information
- Peptides and amino acids; neural reflexes
- ECL cells and parietal cells
- Stimulates gastric acid secretion and mucosal growth.
- Somatostatin inhibits release.
50
Cholecystokinin (CCK)
- Stimulus for release
- Primary targets
- Primary effects
- Other information
- Fatty acids and some amino acids
- Gallbladder, pancreas, stomach
- Stimulates gallbladder contraction and pancreatic enzyme secretion; Inhibits gastric emptying and acid secretion.
- Promotes satiety. Some effects may be due to CCK as a neurotransmitter.
51
Secretin
- Stimulus for release
- Primary targets
- Primary effects
- Acid in small intestine
- Pancreas, stomach
- StimulatesHCO3- secretion; Inhibits gastric emptying and
acid secretion.
52
Motilin
- Stimulus for release
- Primary targets
- Primary effects
- Other information
- Fasting: periodic release every 1.5–2 hours
- Gastric and intestinal smooth muscle
- Stimulates migrating motor complex.
- Inhibited by eating a meal.
53
Gastric inhibitory peptide (GIP)
- Stimulus for release
- Primary targets
- Primary effects
- Glucose, fatty acids, and amino acids in small intestine
- Beta cells of pancreas
- Stimulates insulin release (feedforward mechanism); Inhibits gastric emptying and acid secretion.
54
Glucagon-like peptide-1 (GLP-1)
- Stimulus for release
- Primary targets
- Primary effects
- Other information
- Mixed meal that includes carbohydrates or fats in the lumen
- Endocrine pancreas
- Stimulates insulin release.; Inhibits glucagon release
and gastric function.
- Promotes satiety.
55
Smelling, seeing, or even thinking about food can make our mouths water and our stomachs rumble. These long reflexes that begin in the brain create a feedforward response known as the _______ of digestion
cephalic phase
56
The stomach has three general functions
1 Storage.
The stomach stores food and regulates its passage into the small intestine, where most digestion and absorption take place.
2 Digestion.
The stomach chemically and mechanically digests food into the soupy mixture of uniformly small particles called chyme.
3 Protection.
The stomach protects the body by destroying many of the bacteria and other pathogens that are swallowed with food or trapped in airway mucus. At the same time, the stomach must protect itself from being damaged by its own secretions.
57
Once food enters the stomach, stimuli in the gastric lumen initiate a series of short reflexes that constitute the _______ of digestion.
gastric phase
58
What happens during the gastric phase?
In gastric phase reflexes, distension of the stomach and the presence of peptides or amino acids in the lumen activate endocrine cells and enteric neurons. Hormones, neurocrine secretions, and paracrine molecules then influence motility and secretion.
59
Acid secretion: Gastric acid is secreted by_____ and has multiple functions that include:
Parietal cells deep in the gastric glands secrete gastric acid (HCl).
- activates pepsin, the enzyme that digests proteins, and it denatures the proteins by breaking disulfide and hydrogen bonds that hold the protein in its tertiary structure. Unfolding protein chains make the peptide bonds between amino acids accessible to pepsin.
- Gastric acid helps kill bacteria and other ingested microorganisms, and it inactivates salivary amylase, which stops carbohydrate digestion that began in the mouth.
60
Enzyme secretion: ______ in the gastric glands secrete the inactive enzyme pepsinogen. Pepsinogen is cleaved to active pepsin in the ________ by the action of _____.
Chief cells
lumen of the stomach
H+
61
_____ is an endopeptidase that carries out the initial digestion of proteins. It is particularly effective on collagen and therefore plays an important role in digesting meat.
Pepsin
62
______ is co-secreted with pepsin. However, less than 10% of fat digestion takes place in the stomach.
Gastric lipase
63
Paracrine secretion: ______ secrete histamine.
Enterochroman-like (ECL) cells
64
What is histamine?
Histamine is a paracrine signal that promotes acid secretion by parietal cells
65
Paracrine secretion: _____ secrete somatostatin (SS)
D cells
66
What is somatostatin?
also known as growth hormone-inhibiting hormone for its action on the pituitary gland. As a GI paracrine molecule, somatostatin is the primary negative feedback signal for gastric phase secretion. It shuts down acid secretion directly and indirectly and also inhibits pepsinogen secretion.
67
Hormone Secretion: ______, found deep in the gastric glands, secrete the hormone gastrin into the blood.
G cells
68
Gastrin release is stimulated by the presence of amino acids and peptides in the stomach, by distension of the stomach, and by neural reflexes mediated by ____.
gastrin-releasing peptide
69
INTEGRATION OF CEPHALIC AND GASTRIC PHASE SECRETION Step-by-Step
1. Food or cephalic re exes initiate gastric secretion.
2. Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine.
3. Acid stimulates short re ex secretion of pepsinogen.
4. Somatostatin release by H+ is the negative feedback signal that modulates acid and pepsin release.
70
How does gastric mucosa protect itself?
Under normal conditions, the gastric mucosa protects itself from acid and enzymes by a mucus-bicarbonate barrier.
71
The net result of the gastric phase is the digestion of proteins in the stomach by pepsin; the formation of chyme by the action of pepsin, acid, and peristaltic contractions; and the controlled entry of chyme into the small intestine, where further digestion and absorption can take place. Once chyme enters the small intestine, the __________ of digestion begins.
intestinal phase
72
Intestinal Phase of digestion
Sensors in the intestine trigger a series of reflexes that feed back to regulate the delivery rate of chyme from the stomach, and feed forward to promote digestion, motility, and utilization of nutrients.
73
What does chyme do when it reaches the intestines?
Chyme in the intestine activates the enteric nervous system, which then slows gastric motility and secretion. In addition, three hormones reinforce the motility feedback signal: secretin, cholecystokinin (CCK), and gastric inhibi- tory peptide (GIP)
74
What does secretin do once chyme stimulates the enteric nervous system?
Secretin is released by the presence of acidic chyme in the duodenum. Secretin inhibits acid production and gastric motility, slowing gastric emptying. In addition, secretin stimulates production of pancreatic HCO3- to neutralize the acidic chyme that has entered the intestine.
75
What happens if a fat-containing meal enters the intestines?
CCK is secreted into the bloodstream if a meal contains fats. CCK also slows gastric motility and acid secretion. Because fat digestion proceeds more slowly than either protein or carbohydrate digestion, it is crucial that the stomach allow only small amounts of fat into the intestine at one time.
76
What happens if a carbohydrate-containing meal enters the intestines?
The incretin hormones GIP and glucagon-like peptide-1 (GLP-1) are released if the meal contains carbohydrates. Both hormones feed forward to promote insulin release by the endocrine pancreas, allowing cells to prepare for glucose that is about to be absorbed. They also slow the entry of food into the intestine by decreasing gastric motility and acid secretion.
77
How does gastric emptying inhibited?
Thee mixture of acid, enzymes, and digested food in chyme usually forms a hyperosmotic solution. Osmoreceptors in the intestine wall are sensitive to the osmolarity of the entering chyme. When stimulated by high osmolarity, the receptors inhibit gastric emptying in a reflex mediated by some unknown blood-borne substance.
78
Why is bicarbonate secretion secreted into the small intestine?
neutralizes the highly acidic chyme that enters from the stomach. Most bicarbonate comes from the pancreas and is released in response to neural stimuli and secretin.
79
Chyme moving into the duodenum triggers neural and endocrine reflexes that
1. Initiate enzyme and bicarbonatesecretion;
2. Feed back to slow gastric digestion and emptying;
3. Feed forward to start insulin secretion.
80
When does bile release occur? What happens to them after digestion is done?
Bile release into the intestine occurs when CCK stimulates gallbladder contraction following ingestion of fats. Bile salts are not altered during fat digestion. When they reach the terminal section of the small intestine (the ileum), they encounter cells that transport them back into the circulation. From there, bile salts return to the liver, are taken back up into the hepatocytes, and re-secreted. is recirculation of bile salts is essential to fat digestion because the body’s pool of bile salts must cycle from two to five times for each meal. Bilirubin and other wastes secreted in bile are not reabsorbed and pass into the large intestine for excretion.
81
What happens to the digestive enzymes are produced by the intestinal epithelium once digestion is over?
e brush border enzymes, which include peptidases, disaccharidases, and a protease called enteropeptidase (previously called enterokinase), are anchored to the luminal enterocyte membrane and are not swept out of the small intestine as chyme is propelled forward.
82
Chyme enters the large intestine through the _______
ileocecal valve
83
The large intestine has seven regions.
- The cecum is a dead-end pouch with the appendix
- ascending colon
- transverse colon
- descending colon
- sigmoid colon
- rectum
- anus
84
The wall of the colon differs from that of the small intestine in that the muscularis of the large intestine has an inner circular layer but a discontinuous longitudinal muscle layer concentrated into three bands called the ______.
tenia coli
85
Contractions of the tenia pull the wall into bulging pockets called _____
haustra
86
The mucosa is divided into three layers
1. Mucous membrane-an inner epithelial layer
2. Lamina propria-thin layer of connective tissue
3. Muscularis mucosa
87
Mucous membrane
Serves as protective surface
| Modified for secretion and absorption
88
Lamina propria
Houses gut-associated lymphoid tissue (GALT)
Important in defense against disease-causing intestinal bacteria
89
Mucous membrane contains
Exocrine gland cells –secrete digestive juices
Endocrine gland cells (secretes into blood or tissue) –secrete blood-borne gastrointestinal hormones
Epithelial cells –specialized for absorbing digestive nutrients
90
Muscularis mucosa
Sparse layer of smooth muscle
91
The epithelial lining of the stomach is highly folded called _____, which are marked by depressions called ________. _______ are found below these structures and they are found in the _______ of the stomach
rugae
gastric pits (arrows)
Coiled glands
fundus and body
92
Composition of saliva
99. 5% H2O
| 0. 5% electrolytes and protein (amylase, mucus, lysozyme)
93
Functions of saliva
* Salivary amylase begins digestion of carbohydrates (serous cells)
* Facilitates swallowing by moistening food
* Mucus provides lubrication
* Antibacterial action–Lysozyme destroys bacteria–Saliva rinses away material that could serve as food source for bacteria
* Rich in bicarbonate buffers (amylase works best at slightly basic pH)
94
Pharyngoesophageal sphincter
Keeps entrance closed to prevent large volumes of air from entering esophagus and stomach
95
Gastroesophageal sphincter
Prevents reflux of gastric contents
96
Pyloric sphincter
Serves as barrier between stomach and upper part of small intestine
97
Factors in duodenum are the principles regulator of gastric emptying:
–Fat --> is digested and absorbed more slowly than other nutrients
–Acid --> The stomach secretes HCl and highly acidic chyme is neutralized in the duodenum by sodium bicarbonate (secreted by the pancreas). Unneutralized acid in duodenum inhibits further emptying of acidic gastric contents until neutralization can be accomplished
–Hypertonicity --> build up of amino acids and glucose molecules slows gastric emptying. When osmolarity of duodenal contents starts to rise closure to stomach.
–Distension --> Too much chyme in duodenum inhibits emptying of even more gastric contents
98
One of the Factors (fat, hypertonicity, acid, distension) trigger either:
1. A Neural response --> Collectively called enterogastric reflex acts to slow the peristaltic activity
smooth muscle contractions-autonomic system
2. Hormonal response --> Release of GIP (gastric inhibitory peptide), endocrine hormone that acts on gastric muscle to decrease peristalsis and thus slow passage of food into the duodenum
99
Additional factors that that influence gastric motility
–Emotions --> Sadness and fear –tend to decrease motility
Anger and aggression –tend to increase motility
–Intense pain --> tends to inhibit motility
100
Two distinct areas of gastric mucosa that secrete gastric juice:
–Oxyntic mucosa
Lines body and fundus
–Pyloric gland area (PGA)
Lines the antrum
101
60% of ulcers are caused by a bacteria called
Helicobacter pylori
102
Endocrine function of pancreas
–Islets of Langerhans
- -> Found throughout pancreas
- -> Secrete insulin (Beta cells-causes uptake of glucose) and glucagon (released when low glucose)
103
Exocrine function of pancreas
–Secretes pancreatic juice consisting of
• Pancreatic enzymes (Trypsinogen, lipases, nucleases) actively secreted by acinar cells
• Aqueous alkaline solution actively secreted by duct cells that line pancreatic ducts-important for neutralizing stomach contents in duodenum
104
Pancreatic enzymes (Trypsinogen, lipases, nucleases) actively secreted by
acinar cells
105
Aqueous alkaline solution actively secreted by ______ that line pancreatic ducts-important for neutralizing stomach contents in duodenum
duct cells
106
Exocrine secretion of pancreas is regulated by
–Secretin-(from intestinal mucosa)
–CCK (from intestinal mucosa)
–Both act to stimulate release of pancreatic juice
107
Proteolytic enzymes of pancreas
–Digest protein
•Trypsinogen -converted to active form trypsin (converting enzymes sits poised on mucosal epithelium)
•Chymotrypsinogen –converted to active form chymotrysin
108
Pancreatic amylase
Converts polysaccharides into the disaccharide amylase
109
Pancreatic lipase
only enzyme secreted throughout entire digestive system that can digest fat
110
True/False
Pancreatic lipase is the only enzyme secreted throughout the entire digestive system that can digest fat
True
111
Segmentation of the small intestine
–initiated by pacemaker cells in small intestine which produce basic electrical rhythm (BER)
–Circular smooth muscle responsiveness is influenced by distension of intestine, gastrin, and extrinsic nerve activity
112
Migrating motility complex of small intestine
Sweeps intestines clean between meals
113
Gastrointestinal motility is determined by the properties of the tract’s smooth muscle and modified by
- chemical input from nerves
- hormones
- paracrine signals.
114
The _____________ layer of the alimentary canal contains loose connective tissue, glands, blood and lymphatic vessels, and nerves.
submucosa
115
Which salivary glands produce the most viscous saliva with lots of mucus?
sublingual
116
Intrinsic factor is secreted by which cells of the gastric glands?
parietal cells
117
What hormone does the small intestine release in response to acidic chyme?
secretin
118
The outer layer of the intestines is the ____
serosa
