Chapter 33: Digestive System Flashcards
Why do we need a digestive system?
- 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.
Structure of the digestive system
Digestive system consists of the digestive tract and associated digestive glands.
Digestive tract
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
Digestive tract divided into 3 regions, based on size, shape, and embryonic derivation:
- Buccal cavity
- Pharynx
- Alimentary canal
Alimentary canal itself divided into 4 regions:
- Esophagus (Greek “entrance for eating”)
- Stomach
- Small intestine
– Duodenum
– Jejunum
– Ileum - Large intestine
In most vertebrates, alimentary canal ends in a ___
Cloaca
Cloaca
- Common chamber receiving digestive tract and urinary/genital tract.
- Empties via vent
Accessory digestive glands
• Located outside walls of gut but secrete digestive enzymes into lumen via ducts • Principal digestive glands: -salivary glands -liver -gall bladder -pancreas
Development remainder
• 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.
Stomodeum is invagination that forms the ___
Buccal cavity
Mouth is lined with __1__, but gut lined with __2__
- Ectoderm
2. Endoderm
Boundary between buccal cavity and pharynx is ___1___ arch, bearing the __2__
- Palatoglossal (glossopalatine)
2. Uvula
Buccal Cavity
• Contains teeth, tongue, and palate (roof of mouth); discuss only teeth.
Teeth
– 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
Tooth anatomy
- Crown
- Root
- Enamel
- Dentin
Crown
portion above gum
Root
portion below gum
Enamel
forms surface of crown, hardest substance in body, of epidermal origin (98% mineral)
Dentin
below enamel, bulk of tooth, similar to bone in chemical composition but harder, of dermal origin (75% mineral)
Pharynx
- 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.
Even in adult mammal, many features are derived from pharynx:
1st pharyngeal pouch: -cavity housing middle ear -eustachian tube Floor of pharynx: -thyroid -lung primordium
Esophagus
– 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
____ resists entry of food into airway
Epiglottis
Stomach
– 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.
Chyme
stomach contents that are released into small intestine.
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
Cardiac sphincter:
between esophagus and stomach
Pyloric sphincter:
between stomach and small intestine
Parietal cells:
source of HCl
Chief cells:
source of protein-digesting (proteolytic) enzymes
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
Structural elaborations increase surface area ____ fold
900
Luminal cells have ____
Mircovilli (brush border)
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
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
Jejunum and ileum
– Site of most intestinal absorption of nutrients
– ~ 8 feet and ~12 feet long, respectively
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
Large Intestine mucosa lacks ____
Villi
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)
• 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
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
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
Stomach glands produce ____ L of fluid per day
2 - 3 Liters
Glands and Secretions of Digestion
• Small intestine
– 2 major accessory glands are:
• Pancreas
• Liver
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.
Pancreas releases ~ ____ L/day of bicarbonate to neutralize chyme
1.2 - 1.5 Liters
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
____ 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
Maybe __1__ ml bile secreted from liver cells each day, but can be concentrated __2__ by gall bladder
- 900 ml
2. 10 - 20x
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)
Absorption in small intestine
• Amino acids and monosaccharides are transported across the epithelial cell and into capillaries of the _____
Hepatic portal system
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
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)
Chylomicrons are __1__ to enter capillaries and instead enter __2__ vessels (lacteals)
- Too large
2. Lymphatic
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
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
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
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
The ____ nervous system has much control of hormonal regulation of digestion via reflexes outside the CNS
Enteric
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)
• Gastrin
– Stimulated by protein food in stomach, ____ in pH
Increase
• Gastrin
– Secreted by endocrine ___ in pyloric region of stomach
Cells
• Gastrin is a ____ hormone: target is same organ that releases it
Unusual
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)
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
Together, CCK and secretin, ____ stomach peristalsis & acid secretion
Inhibit (buy time for duodenum)
The first hormone to be discovered in 1902 was ____
Secretin
CCK functions to:
• Stimulates __1__ ____ to release bile
• Stimulates __2___to release digestive enzymes
• Stimulates __3__ to produce feeling of satiety after meal
- Gall bladder
- Pancreas
- Brain
Secretin functions to:
• Primarily to stimulate pancreas to release __1__ into duodenum
• Aids fat digestion via __2__ bile secretion
- Bicarbonate
2. Increasing
Gastric Fermentation
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