Chapter 33: Digestive System Flashcards
1
Q
Why do we need a digestive system?
A
- Heterotrophs must take in the energy and nutrients needed to support their metabolism
- Food that enters the mouth usually is in large pieces that can not be used directly.
- Before a meal can be absorbed into the body and used, it must be broken down into small or modest-sized molecules.
- This breakdown, and the absorption of the products of the breakdown, are the roles of the digestive system.
- Our treatment will be limited to mammalian systems, and largely to our own species.
- Many features function to increase the rate of digestion, important to energy-hungry endotherms.
2
Q
Structure of the digestive system
A
Digestive system consists of the digestive tract and associated digestive glands.
3
Q
Digestive tract
A
Tube that extends through the body from the lips of the mouth to the anus or cloaca
– Cavity of the tube is called the lumen of the gut
• Hence inner surface is luminal surface
• Glands that empty into gut are luminal glands
4
Q
Digestive tract divided into 3 regions, based on size, shape, and embryonic derivation:
A
- Buccal cavity
- Pharynx
- Alimentary canal
5
Q
Alimentary canal itself divided into 4 regions:
A
- Esophagus (Greek “entrance for eating”)
- Stomach
- Small intestine
– Duodenum
– Jejunum
– Ileum - Large intestine
6
Q
In most vertebrates, alimentary canal ends in a ___
A
Cloaca
7
Q
Cloaca
A
- Common chamber receiving digestive tract and urinary/genital tract.
- Empties via vent
8
Q
Accessory digestive glands
A
• Located outside walls of gut but secrete digestive enzymes into lumen via ducts • Principal digestive glands: -salivary glands -liver -gall bladder -pancreas
9
Q
Development remainder
A
• 2nd invagination joins anterior end of gut to form mouth (later in development)
– Invagination called stomodeum, forms buccal cavity
– Invagination carries in surface ectoderm
– So mouth lined with ectoderm, but gut lined with endoderm
– This bit of development helps explain the nature of teeth (later)
– Boundary between buccal cavity and pharynx is palatoglossal (=glossopalatine) arch, bearing the uvula
– And note how/where your anterior pituitary forms.
10
Q
Stomodeum is invagination that forms the ___
A
Buccal cavity
11
Q
Mouth is lined with __1__, but gut lined with __2__
A
- Ectoderm
2. Endoderm
12
Q
Boundary between buccal cavity and pharynx is ___1___ arch, bearing the __2__
A
- Palatoglossal (glossopalatine)
2. Uvula
13
Q
Buccal Cavity
A
• Contains teeth, tongue, and palate (roof of mouth); discuss only teeth.
14
Q
Teeth
A
– Help catch & hold prey
– In mammals especially, teeth play important role in digestion
• Serve to break up food into smaller particles, creating more surface area for digestive enzymes, thereby speeding digestion
15
Q
Tooth anatomy
A
- Crown
- Root
- Enamel
- Dentin
16
Q
Crown
A
portion above gum
17
Q
Root
A
portion below gum
18
Q
Enamel
A
forms surface of crown, hardest substance in body, of epidermal origin (98% mineral)
19
Q
Dentin
A
below enamel, bulk of tooth, similar to bone in chemical composition but harder, of dermal origin (75% mineral)
20
Q
Pharynx
A
- 2nd of 3 regions of digestive tract
- In adult mammal, little more than passageway for air.
- In embryonic mammal, pharyngeal pouches present.
- In “lower” vertebrates, remains major structural region, site of gills.
21
Q
Even in adult mammal, many features are derived from pharynx:
A
1st pharyngeal pouch: -cavity housing middle ear -eustachian tube Floor of pharynx: -thyroid -lung primordium
22
Q
Esophagus
A
– 1st region of alimentary canal
– Connects pharynx to stomach
– Expandable to accommodate large food bolus
– Mucus secreted to lubricate passage of food
– Muscles contract to squeeze the bolus towards the stomach
23
Q
____ resists entry of food into airway
A
Epiglottis
24
Q
Stomach
A
– 2nd region of alimentary canal; receives food from esophagus
– Some absorption of water, salts, vitamins, but predominant task is to begin digestion with gastric juice
• Some enzymes
• Some mucus
• Mostly HCl secreted by glands in wall of stomach
– At rest, stomach relaxes into folds called rugae
– Chyme: stomach contents that are released into small intestine.
25
Chyme
stomach contents that are released into small intestine.
26
Stomach continued
- 4 stomach regions
- 2 sphincters
• Cardiac sphincter: between esophagus and stomach
• Pyloric sphincter: between stomach and small intestine
– Gastric glands
• Mucous cells
• Parietal cells: source of HCl
• Chief cells: source of protein-digesting (proteolytic) enzymes
27
Cardiac sphincter:
between esophagus and stomach
28
Pyloric sphincter:
between stomach and small intestine
29
Parietal cells:
source of HCl
30
Chief cells:
source of protein-digesting (proteolytic) enzymes
31
Small Intestine
– “Small” in diameter but often quite long
– Luminal cells have microvilli (brush border)
– Structural elaborations to increase surface area for digestion and absorption
• Increase length of small intestine
– So small intestine is long & coiled
• Increase surface area per unit length
– Circular folds (plica circularis) - 3X increase
– Villi (not microvilli), finger-like projections 10X
– Microvilli (elaborations of plasma membrane) 30X
– Total increase: 3 X 10 X 30 = 900 fold increase
32
Structural elaborations increase surface area ____ fold
900
33
Luminal cells have ____
Mircovilli (brush border)
34
Small Intestine continued
– 3 regions of the small intestine:
• Duodenum
– Site of most intestinal digestion
– ~ 10 inches long in adult human
• Jejunum, ileum
– Site of most intestinal absorption of nutrients
– ~ 8 feet and ~12 feet long, respectively
– Ileocolic (ileocecal) valve
• Ileum-to-colon sphincter
35
Duodenum
– Dew – oh – dee – num
- “dew – aw – deh – num” as alternative, and possibly more proper pronunciation)
– Site of most intestinal digestion
| – ~ 10 inches long in adult human
36
Jejunum and ileum
– Site of most intestinal absorption of nutrients
| – ~ 8 feet and ~12 feet long, respectively
37
Large Intestine
– Larger in diameter but much shorter in length
– Mucosa lacks villi
– In mammals, divided into large gentle loop called the colon, and a shorter, straight terminal segment called the rectum.
– Rectum empties to the exterior via the anus, where there is “one” last sphincter, the anal sphincter.
• Actually it is 2 sphincters
– Internal A.S. which is smooth muscle and involuntary
– External A.S. which is skeletal muscle and voluntary
38
Large Intestine mucosa lacks ____
Villi
39
Glands and Secretions of Digestion
• See Fig 33.10 in Urry et al. (p. 673)
– Know this material
– e.g., given location, know enzyme, or given enzyme know substrate, given substrate know product(s)
40
• Digestive process must yield molecules small enough to be absorbed. For the most part, this means ....
– Proteins to peptides to amino acids
– Polysaccharides to oligosaccharides to monosaccharides
– Lipids to fatty acids and monoglycerides
41
Glands and Secretions of Digestion
| • Buccal cavity
– Secretion is saliva
• Mucus
• Salts: Na+, K+, Cl-, phosphate, bicarbonate
• Few enzymes, primarily amylase which begins digestion of starch.
• lysozyme: enzyme kills bacteria
• other antibacterial agents including immunoglobulin A
• metabolic wastes: urea & uric acid
• pH 6.8–7.0
42
Glands and Secretions of Digestion
| • Stomach
– Aside: zymogens
• In stomach and small intestine, digestive enzymes often released from digestive gland in an inactive form called a zymogen
• Zymogen then is converted to active form in the lumen of the gut, by another enzyme or some other factor.
• Name of specific zymogens end in ~ogen
– E.g., major enzyme of stomach is pepsin (proteins to polypeptides). Pepsin is released as pepsinogen.
– HCl from parietal cells converts pepsinogen to pepsin, and pepsin then can do the conversion itself.
– Stomach glands produce 2 – 3 L of fluid per day
43
Stomach glands produce ____ L of fluid per day
2 - 3 Liters
44
Glands and Secretions of Digestion
| • Small intestine
– 2 major accessory glands are:
• Pancreas
• Liver
45
Pancreas
• 2 important EXOcrine functions here
– Releases ~ 1.2 – 1.5 L/day of bicarbonate to neutralize chyme
» Enzymes of small intestine work best at neutral or near- neutral pH
– Releases many enzymes (as zymogens) that digest proteins, starch, & lipid (see Fig 33.10 in Urry et al.)
» These zymogens are activated by a membrane-bound enzyme (enterokinase) from the duodenum, and by the enzyme trypsin.
46
Pancreas releases ~ ____ L/day of bicarbonate to neutralize chyme
1.2 - 1.5 Liters
47
Liver
* Produces bile from cholesterol
* Bile is stored in the gall bladder, and released into the duodenum when chyme enters
* Bile functions to emulsify fats (break large drops of lipid into much smaller drops), thereby increasing the surface area available for fat-digesting enzymes (lipases) to work [SA to V again !!]
* maybe 900 ml bile secreted from liver cells each day, but can be concentrated 10-20X by gall bladder
48
____ functions to emulsify fats (break large drops of lipid into much smaller drops), thereby increasing the surface area available for fat-digesting enzymes (lipases) to work
Bile
49
Maybe __1__ ml bile secreted from liver cells each day, but can be concentrated __2__ by gall bladder
1. 900 ml
| 2. 10 - 20x
50
Absorption in small intestine
– Occurs primarily in jejunum and ileum
– Food must be broken down to amino acids (proteins), monosaccharides (carbohydrates), and fatty acids & monoglycerides (lipids)
51
Absorption in small intestine
| • Amino acids and monosaccharides are transported across the epithelial cell and into capillaries of the _____
Hepatic portal system
52
Absorption in small intestine
____________:
- From capillary bed to blood vessels to capillary bed again without passing through heart
- Carries absorbed nutrients directly to liver for processing (except lipids
Portal System
53
Absorption in small intestine
| • Fatty acids and monoglycerides are handled differently...
– Inside the epithelial cell, re-synthesized as triglyceride
– Triglyceride then combined with phospholipid and cholesterol, and coated with a “skin” of protein to form a water-soluble droplet called chylomicrons
– Chylomicrons are processed by Golgi apparatus for extrusion from the cell
– Chylomicrons are too large to enter capillaries and instead enter lymphatic vessels (lacteals)
54
Chylomicrons are __1__ to enter capillaries and instead enter __2__ vessels (lacteals)
1. Too large
| 2. Lymphatic
55
Absorption in large intestine
• ~99% of ~9 L of fluid passing through the human digestive tract is reabsorbed
– ~95% in the small intestine, & ~4% in large intestine
• Large intestine also absorbs some Vitamin K and other bacterial products
• Large intestine lacks villi and so has only ~ 1/10 absorptive surface area of the small intestine
56
Absorption in large intestine
• ~___% of ~___ L of fluid passing through the human digestive tract is reabsorbed
– ~___% in the small intestine, & ~__% in large intestine
• ~99% of ~9 L of fluid passing through the human digestive tract is reabsorbed
– ~95% in the small intestine, & ~4% in large intestine
57
Absorption in large intestine
| • Large intestine lacks villi and so has only ~ ____ absorptive surface area of the small intestine
• Large intestine lacks villi and so has only ~ 1/10 absorptive surface area of the small intestine
58
Hormonal Regulation of Digestion
• Won’t say anything more about neural control
– just remember the enteric nervous system, much control via reflexes outside the CNS
• Gastrin, Cholecystokinin (CCK), Secretin
59
The ____ nervous system has much control of hormonal regulation of digestion via reflexes outside the CNS
Enteric
60
Hormonal Regulation of Digestion
| • Gastrin
– Polypeptide
– Stimulated by protein food in stomach, increase in pH
– Secreted by endocrine cells in pyloric region of stomach
– Acts on gastric glands of stomach to secrete HCl and pepsinogen, and on stomach to increase contractions (unusual hormone: target is same organ that releases it)
61
• Gastrin
| – Stimulated by protein food in stomach, ____ in pH
Increase
62
• Gastrin
| – Secreted by endocrine ___ in pyloric region of stomach
Cells
63
• Gastrin is a ____ hormone: target is same organ that releases it
Unusual
64
Hormonal Regulation of Digestion
| • Cholecystokinin (CCK)
– Polypeptide, very similar structure to gastrin (gene duplication?)
– Secretion stimulated by presence of fatty acids and polypeptides in small intestine
– Secreted by (entero)endocrine cells in wall of duodenum
– 3 main functions:
• Stimulates gall bladder to release bile
• Stimulates pancreas to release digestive enzymes
• Stimulates brain to produce feeling of satiety after meal
– with secretin, inhibit stomach peristalsis & acid secretion (buy time for duodenum)
65
Hormonal Regulation of Digestion
| • Secretin
– Polypeptide (first hormone to be discovered – 1902)
– Secreted in response to food in stomach and strong acid in stomach and small intestine
– Secreted by endocrine cells in wall of duodenum
– Functions:
• Primarily to stimulate pancreas to release bicarbonate into duodenum
• With CCK, inhibit stomach peristalsis & acid secretion (buy time for duodenum)
• Aids fat digestion via increasing bile secretion
66
Together, CCK and secretin, ____ stomach peristalsis & acid secretion
Inhibit (buy time for duodenum)
67
The first hormone to be discovered in 1902 was ____
Secretin
68
CCK functions to:
• Stimulates __1__ ____ to release bile
• Stimulates __2___to release digestive enzymes
• Stimulates __3__ to produce feeling of satiety after meal
1. Gall bladder
2. Pancreas
3. Brain
69
Secretin functions to:
• Primarily to stimulate pancreas to release __1__ into duodenum
• Aids fat digestion via __2__ bile secretion
1. Bicarbonate
| 2. Increasing
70
Gastric Fermentation
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