Digestion Lecture 3 Flashcards
(57 cards)
1
Q
The Small Intestine
Intestinal Juice
A
- Secreted by glands of the small
intestine - 1-2 L daily
- Contains water and mucus, slightly
alkaline - Provide liquid medium aiding
absorption
2
Q
The Small Intestine
Brush Border Enzymes
A
- Inserted into plasma membrane of absorptive cells
- Some enzymatic digestion occurs at apical surface
rather than just in lumen - Examples:
- Carbohydrates: α-dextrinase, maltase, sucrase, lactase
- Proteins: aminopeptidase, dipeptidase, phosphatases
- Nucleosides: nucleosidases, phosphatases
The Small Intestine
3
Q
Digestion in the Small Intestine
* Mechanical Digestion
Segmentation
A
Localized, mixing contractions
* Causes sloshing of the chyme back and forth
* Mix chyme with digestive juices and brings it in
contact with mucosa for absorption
* No set pattern of contractions, so no particular
direction of
4
Q
Digestion in the Small Intestine
Chemical Digestion of Carbohydrates
Mouth:
A
Must break down polysaccharides to monosaccharides before they can
be absorbed
* Mouth:
* So far, carbs have been acted on by salivary amylase but the acidic pH of the
stomach has inactivated it
5
Q
Digestion in the Small Intestine
Chemical Digestion of Carbohydrates
Small Intestine:
A
Must break down polysaccharides to monosaccharides before they can
be absorbed
* Small Intestine:
* From pancreas: pancreatic amylase will act on any remaining starch
* Along with brush border enzymes on the apical surface of the small intestine
Digestion in the Small Intestine
6
Q
Digestion in the Small Intestine
Chemical Digestion of Carbohydrates
Small Intestine (brush border enzymes):
A
- brush border enzymes act to break down small starches (disaccharides) into monosaccharides
- -dextrinase: clips off one glucose molecule at a time
- Sucrase: breaks sucrose into glucose and fructose
- Lactase: breaks lactose into glucose and galactose
- Maltase: breaks maltose and maltotriose into 2 or 3 molecules of glucose
- Monosaccharides can then be absorbed
7
Q
Digestion in the Small Intestine
Lactose Intolerance
A
- Mucosal cells of the small intestine fail to produce lactase
- Undigested lactose in chyme causes fluid to be
retained in the feces - Bacterial fermentation of the undigested lactose results in the production of gases
- Symptoms include diarrhea, gas, bloating and
abdominal cramps after consumption of milk and other dairy products - Either avoid dairy or take lactase supplement with dairy (LACTAID)
8
Q
Digestion in the Small Intestine
Chemical Digestion of Proteins
Stomach:
A
- Must break down proteins to amino acids before they can be absorbed
- Stomach:
- So far, proteins have been broken down into smaller peptide molecules in the stomach
(by pepsin)
9
Q
Digestion in the Small Intestine
Chemical Digestion of Proteins
Small Intestine:
A
- Small Intestine:
- From pancreas: trypsin, chymotrypsin, carboxypeptidase, and elastase from pancreas
continue to break down proteins into small peptides - Aminopeptidase and dipeptidase in brush border help complete the break down process
by cleaving off single amino acids (which can then be absorbed) - Amino acids can then be absorbed
10
Q
Digestion in the Small Intestine
Chemical Digestion of Lipids
Mouth and Stomach:
A
- Mouth and Stomach:
- So far, some lipid digestion has occurred by lingual lipase and gastric lipases
11
Q
Digestion in the Small Intestine
Chemical Digestion of Lipids
Small Intestine:
A
- Small Intestine:
- From pancreas: pancreatic lipase most important in triglyceride digestion which occurs in the small intestine
- From liver and gallbladder: bile salts emulsify fats into small lipid globules so pancreatic lipase can act more efficiently
- Fatty acids, glycerol, and monoglycerides can then be absorbed
12
Q
Digestion in the Small Intestine
* Chemical Digestion of Nucleic Acids
A
- Small Intestine:
- From pancreas: Ribonuclease and deoxyribonuclease in pancreatic juice
- Nucleosidases and phosphatases in brush border
13
Q
Absorption in the Small Intestine
A
- All digestion to this point is to produce small molecules that can be absorbed through the epithelial cells lining the lumen
- This includes:
- Monosaccharides (glucose, fructose, galactose)
- Single amino acids, dipeptides, and tripeptides
- Fatty acids, glycerol, and monoglycerides
- Different molecules are absorbed differently (diffusion, facilitated diffusion,
osmosis, and active transport) - About 90% of absorption occurs in the small intestine and the remaining
10% occurs in the stomach and large intestine
14
Q
Absorption in the Small Intestine:
Monosaccharides
A
- Absorbed by facilitated diffusion or
active transport into the absorptive cell - Monosaccharides will move out of the
absorptive cells and into capillaries via
facilitated diffusion
15
Q
Absorption in the Small Intestine: Amino Acids
A
- Most absorbed as amino acids via active
transport into blood - Half of absorbed amino acids come from
proteins in digestive juice and dead mucosal
cells and the other half comes from food - Amino acids will move out of the absorptive
cells and into the capillaries via diffusion
16
Q
Absorption in the Small Intestine: Lipids
A
- All dietary lipids absorbed by simple diffusion
- Short-chain fatty acids are small enough that they can be absorbed via
simple diffusion and then pass into the capillaries via diffusion - Long-chain fatty acids and monoglycerides must be transported to the apical
surface via micelles because they are large and hydrophobic
17
Q
Absorption in the Small Intestine: Lipids
* Long-chain fatty acids and
monoglycerides
A
- Bile salts form micelles to ferry them to
absorptive cell surface where they are
absorbed and the micelle remains in the
lumen - Reform into triglycerides and are coated
with a protein forming chylomicrons - Leave cell by exocytosis
- Enter lacteals to eventually enter blood
with protein coat of chylomicron
keeping them suspended and separate - An enzyme called lipoprotein lipase
(it is attached to the apical surface of
capillary endothelial cells) acts to
break down the triglyceride in the
chylomicron into fatty acids and
glycerol - These products will then diffuse into
hepatocytes and adipose cells to
reform triglycerides again!
18
Q
Absorption in the Small Intestine
Electrolytes
A
- From GI secretions or food/liquid
- Electrolytes absorbed by active transport
19
Q
Absorption in the Small Intestine
Vitamins
A
- Fat-soluble vitamins A, D, E, and K are absorbed by simple diffusion and transported with
lipids in micelles - Most water-soluble vitamins also absorbed by simple diffusion except for B12 which
combines with intrinsic factor and is absorbed by active transport
20
Q
Absorption in the Small Intestine
Water
A
- 9.3L: comes from ingestion (2.3L) and GI secretions (7.0L)
- Most absorbed in small intestine, some in large intestine
- Only 100ml excreted in feces
21
Q
The Large Intestine
Major functions
A
- Reabsorbing water (and some electrolytes) and compacting intestinal contents into feces
- Produce and absorb some vitamins (vitamin K, vitamin B7)
- Storing fecal material prior to defecation
The Large Intestine
22
Q
The Large Intestine
ileocecal valve
A
Starts at the junction of the ileum
and the cecum via the ileocecal
valve
23
Q
The Large Intestine
Four segments
A
- Cecum
- Colon
- Rectum
- Anal canal
24
Q
The Large Intestine
* Large intestine segments
* Cecum
A
- Expanded pouch distal to the ileum
- Collects and stores material
- Begins the process of compaction
- Compression into feces
- Opening between the cecum and ileum is the ileocecal valve
25
The Large Intestine
Cecum (continued)
Attached to the
appendix (also called the vermiform appendix)
* Contains numerous lymphoid nodules
* Appendicitis is inflammation of the appendix
* **Something to consider if someone is experiencing RLQ pain during treatment**
26
The Large Intestine Colon
Subdivided into four regions
1. Ascending colon
2. Transverse colon
3. Descending colon
4. Sigmoid colon
| * Ascending and descending colon are retroperitoneal
## Footnote
* Transverse and sigmoid colon are suspended by
mesocolon
27
The Large Intestine
1. Ascending colon
* From the cecum along the right margin of
the peritoneal cavity to the inferior surface
of the liver
* Bends sharply to the left at the right colic
(hepatic) flexure
28
The Large Intestine
2. Transverse colon
* Crosses the abdomen from right to left
* Supported by the transverse mesocolon
* Makes a 90º turn at the left colic (splenic)
flexure
29
The Large Intestine
3. Descending colon
* Moves inferiorly along the body’s left
side to the iliac fossa
* Ends at the **sigmoid flexure**
30
The Large Intestine
4. Sigmoid colon
* S-shaped segment
* About 15 cm (6 in.) long
* Empties into the rectum
31
The Large Intestine
* Rectum
* Forms last 15 cm (6 in.) of digestive tract
* Expandable for temporary feces storage
* Movement of fecal material into the rectum
triggers the urge to defecate
32
The Large Intestine
* Histology of the Large Intestine
* Mucosa
* Lacks villi
* No circular folds
* Contains distinctive intestinal glands
* Dominated by mucin-secreting goblet
cells
* Mucus lubricates feces as it becomes
drier and more compact
* Mucosa does not produce enzymes
33
The Large Intestine
* Histology of the Large Intestine
* Muscularis
* Has an outer longitudinal layer and an
inner circular layer
* Portions of the longitudinal muscles are
thickened, and form **teniae coli**
* Forms **haustra:** pouches which give the
large intestine a puckered appearance
| teniae coli & haustra (point these out on diagram)
34
The Large Intestine
* Histology of the Large Intestine
* Serosa and Andventitia
* Ascending and descending colon are
retroperitoneal = adventitia
* Transverse and sigmoid colon = serosa
* Omental (fatty) appendices
* Teardrop-shaped sacs of fat in the serosa of
the colon
35
The Large Intestine
* Histology of the Large Intestine
* Internal anal sphincter
* Inner circular smooth muscle layer
* Not under voluntary control
36
The Large Intestine
* Histology of the Large Intestine
* External anal sphincter
* Outer skeletal muscle layer
* Under voluntary control
37
The Large Intestine
* Anus (exit of the anal canal)
* Epidermis here becomes keratinized
38
Digestion in the Large Intestine
* There is very little digestion occurring in the large intestine, mechanical or chemical
* Mucus is secreted by the glands in the LI but no enzymes are secreted
* **Final stage of digestion through bacterial action**
* Bacteria acts to ferment any remaining carbohydrates, decomposes bilirubin which
gives feces its brown color, produces some B vitamins and vitamin K
* Remaining water absorbed along with ions and some vitamins
39
Absorption in the Large Intestine
* Accounts for <10 percent of all nutrient absorption in the digestive tract
* Vitamin absorption
* Biotin, vitamin K, vitamin B5
* Produced by normal bacteria in the colon
40
Absorption in the Large Intestine
Water reabsorption
* Prevents dehydration
* ~1500 mL of material enters colon
* Over 1 L of water is reabsorbed through osmosis
* ~200 mL of feces are excreted
41
Movement Through the GIT
* Peristalsis
* Discussed earlier, peristalsis is rhythmic contractions that keeps chyme moving through the digestive tract from esophagus all the way to the rectum
* The movement of chyme from the ileum into the cecum is regulated by the ileocecal sphincter
42
Movement Through the GIT
Haustral Churning
* Once chyme has accumulated in the ascending colon, **haustral churning occurs**
* This is where the haustra become distended as they fill up, once they are distended the walls contract and squeeze the contents into the next haustrum
43
Movement Through the GIT
Mass peristalsis
* is a strong peristaltic wave that begins in the middle of the transverse colon and drives the contents of the colon into the rectum
* This is initiated by food in the stomach so it usually occurs 3-4x per day
44
Movement Through the GIT
* Migrating motility complexes (MMC)
* Type of peristalsis occurring between meals
* Happens every 90-240 minutes
* Begins in lower portion of stomach and slowly migrates down the small intestine and
once it reaches the ileum another MMC begins
* Keeps undigested food and bacteria moving into the large intestine
* “rumbling of your stomach”
45
Defecation Reflex
* Begins with distension of rectal
wall after arrival of feces
* Triggered by stretch receptors in
the rectal wall
46
Defecation Reflex
Involves two positive feedback
loops
* Begins with distension of rectal wall after arrival of feces
* Triggered by stretch receptors in the rectal wall
1) Long reflex
* Coordinated by sacral
parasympathetic system
2) Short reflex
* Stimulates the myenteric plexus in
the sigmoid colon and rectum
47
Defecation Reflex
Steps
* The mass peristalsis pushes fecal material into the rectum and this distension initiates
the defecation reflex
* Sensory impulses are sent to the sacral spinal cord
* Motor impulses from the cord will travel with parasympathetic nerves back to the
colon, rectum, and anus
* This causes contraction of the longitudinal muscles which shortens the rectum,
increasing the pressure
* This pressure, voluntary contractions and parasympathetic stimulation acts to open
the internal anal sphincter
48
Regulation of Digestion
Local factors
* Primary stimulus for digestive activities
* Examples:
* Changes in pH of contents in lumen
* Physical distortion of digestive tract wall
* Presence of chemicals (specific nutrients or chemical messengers released by the mucosa)
49
Regulation of Digestion
Neural control mechanisms
* **Short reflexes (myenteric reflexes)**
* Triggered by chemoreceptors or stretch receptors in digestive tract walls
* Controlling neurons located in the myenteric plexus
* **Long reflexes**
* Higher level of control involving interneurons and motor neurons of the CNS
* Generally control large-scale peristalsis, moving material from one region of the tract to another
* May involve parasympathetic motor fibers that synapse in the myenteric plexus
50
Regulation of Digestion
Hormonal control mechanisms
* Involve at least 18 hormones that affect digestive function
* Some affect other systems as well
* Hormones are peptides produced by enteroendocrine cells (endocrine cells in the
epithelium of the digestive tract)
* Most digestive hormones are produced by the duodenum
51
Regulation of Digestion
Hormonal control mechanisms
* CCK (Cholecystokinin)
* Secreted from CCK cells in the small intestine
* is secreted in response to chyme containing amino acids from partially digested
proteins and fatty acids from partially digested triglycerides
* Stimulates secretion of pancreatic juice that is rich in digestive enzymes and causes
contraction of the wall of the gallbladder and relaxation of the hepatopancreatic sphincter
* Also slows gastric emptying by promoting contraction of the pyloric sphincter, producing
satiety by acting on the hypothalamus
Regulation of Digestion
52
Hormonal control mechanisms
Secretin
* Secreted from S cells in the small intestine
* stimulates flow of pancreatic juice that is rich in bicarbonate ions to buffer the
acidic chyme that enters the duodenum
* Also inhibits secretion of gastric juice
* overall, causes buffering of acid in chyme and slows the production of acids in the
stomach
53
Phases of Digestion
Gastric secretion occurs in
three phases:
1. Cephalic phase
2. Gastric phase
3. Intestinal phase
54
Phases of Digestion
Cephalic phase
* Begins when you see, smell, taste, or think of food
* Directed by the CNS to prepare mouth and stomach to receive
food
* Parasympathetic NS sends impulses down the facial nerve
(CNVII) and glossopharyngeal nerve (CN IX)
* Increases salivation in the mouth from salivary glands
* Parasympathetic NS also sends impulses down the vagus nerve
(CNX) stimulates the submucosal plexus
* Postganglionic fibers innervate gastric gland cells
* Gastric juice production increases (~500 mL/h)
* Phase generally lasts only minutes
55
Phases of Digestion
Gastric phase
* Begins with stimuli as food arrives in the stomach
* Stimuli include:
1. Distention of stomach
2. Increase in gastric content pH
3. Presence of undigested materials (especially proteins and
peptides) in stomach
* Gastrin secretion has both neural and hormonal control
* Neural: Vagus nerve (CNX); hormonal: Gastrin
* Increases mixing waves of stomach muscle contraction
* Increases secretion by parietal and chief cells
* Phase may last 3–4 hours
56
Phases of Digestion
Intestinal phase
* Begins when chyme enters duodenum (usually
after several hours of mixing)
* Distention of the duodenum initiates the
enterogastric reflex
* Slows exit of chyme from stomach to prevent the duodenum from being overloaded with more chyme than it can handle
57
Phases of Digestion
Intestinal phase (continued)
* Inhibits gastrin production
* Decreases gastric motility and secretion
* Stimulates contraction of pyloric sphincter, decreasing chyme release into duodenum
* Mucus production is increased in the duodenum
* Neural:
* parasympathetic NS to stomach inhibited and stimulates sympathetic nerves to decrease motility and increases contraction
of the pyloric sphincter which slows gastric emptying
* Hormonal:
* regulated by CCK and secretin
