Transcript Digestion, Absorption, and Transport

Chapter 3
Part 2
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Digestion is the process of breaking down
foods into nutrients to prepare for absorption
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Visual, olfactory, and auditory senses
Taste (smell)
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Presence of food in the mouth
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◦ Sweet sour, salty, bitter, umami (savoriness)
◦ Activate secretory responses from
 Salivary glands
 Stomach
 Pancreas
◦ Contraction of gall bladder
 Bile
 Anatomy
tract
– Gastrointestinal (GI)
 Flexible muscular tube from mouth to
anus
 Lumen - inner space of the tract.
 Anatomy
–GI tract
◦ Mouth - beginning of digestive system and
digestion
 Mastication (chewing) -stimulates taste buds,
 Swallowing - epiglottis closes to prevent food from
entering the pharynx.
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Anatomy - GI tract
◦ Esophagus - tube carrying bolus to stomach.
◦ Sphincters – control release, keeps food moving
forward
 Peristalsis
 Upper and lower esophageal sphincters (cardiac
sphincter - LES)
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Anatomy - GI tract
◦ Stomach – HCl and enzymes turn food into a fluidy
paste
 Holds 2 to 4 cups of food (6 L)
 Grinds the bolus to a semiliquid mass (chyme)
 Holding “tank”
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Emptying closely regulated
5 ml/min (teaspoon)
1-4 h to empty
Pyloric sphincter
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Anatomy - GI tract
◦ Stomach
 Prevention of autodigestion
 Mucus lining
 HCO3 Prevents HCl in the stomach from burning through the
stomach lining
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Stomach Acid (HCl)
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Destroys biological activity of protein
Activates digestive enzymes
Partially digests dietary protein
Assists in vitamin B12 absorption
 B12 requires gastric acid to be released from food
◦ Improves absorption of minerals
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Gastrin stimulates secretion of gastric hydrochloric
acid, performing many functions.
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Activates pepsin
Stimulates the release of hydrochloric acid
Kills pathogenic organisms
Improves absorption of iron and calcium
Inactivates hormones of plant and animal origin
Denatures food proteins, making them more vulnerable to
enzyme action
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Anatomy - GI tract
◦ Small intestine - digestive juices
 Gallbladder (Bile) and pancreas (enzymes that break
down CHO, fats and proteins)
 Three parts of small intestine
 Duodenum
 Ileum
 Cecum
◦ Large intestine (colon)
 Ileocecal valve to rectum and anus.
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Anatomy – GI tract
◦ Approximately 10 feet long
 Narrow
◦ Duodenum (10 in)
 Where enzymes and bile are added
 Regulatory center that senses:
 Nutrient content (particularly fat)
 pH (HCO3- from pancreas raises pH of chyme from
stomach)
◦ Jejunum (4 ft)
◦ Ileum (5 ft)
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Anatomy – GI tract
◦ Physical structure – intestinal wall
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Folded walls
Villi – made up of absorptive cells
Increase surface area greatly
Rapid turnover absorptive cells (daily)
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The Muscular Action of Digestion – under
autonomic control
◦ Peristalsis - pushes the digestive contents along.
 Muscles are circular, longitudinal, and diagonal
muscles.
◦ Sphincter contractions open and close passageways
 Prevents reflux, controls the passage of contents.
Longitudinal
Circular
Diagonal
Fig. 3-2, p. 75
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Salivary glands
Pancreas
Liver (gall bladder)
Stomach
Small intestine
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Begins in the mouth
Saliva
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Electrolytes (Na, Cl)
Solvent
Enzymes (salivary amylase)
Mucus
Saliva moistens food
◦ Begins breakdown of CHO
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Small Intestine
◦ Pancreatic amylase (duodenum)
 Smaller CHO molecules
◦ Brush border enzymes
 Maltase, sucrase, lactase
 Disaccharides to monosaccharides
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Maximum rate of glucose absorption
◦ 50 – 80 g/h (70 kg)
◦ 4 g CHO/min or 240 g/h
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Absorbed into circulation
◦ Hepatic portal vein
 Liver
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Large intestine
◦ Water reabsorption
◦ Some digestion
 Mostly bacterial
 Vitamin absorption
 Vitamins K, B12, thiamin and riboflavin
 Compacts and prepares feces for defecation
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Mouth
◦ Lingual lipase
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Stomach
◦ Gastric lipase
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Pancreas
◦ Pancreatic lipase
◦ Bile
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Cholecystokinin (CCK)
◦ Released from wall of duodenum
◦ In response to fat and protein in the chyme
◦ Causes the release of bile and pancreatic enzymes
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Secretin
◦ Stimulates bile secretion by liver and release of
bicarbonate into duodenum
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Micelles
◦ Monoglyceride, FFA + bile salts
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Micelles are formed after bile acts on
large fat droplets
Pancreatic and brush border enzymes
breakdown micelles to fatty acids
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These fatty acids
are then
repackaged as
chylomicrons,
which are largely
absorbed into
the lymphatic
system
They then travel
to the heart and
enter the
Pepsinogen - precursor
 Pepsin
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◦ Released from stomach wall
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Gastrin
◦ Release of HCL
◦ Controls pepsin release
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Trypsinogen
 Secreted by pancreas
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Trypsin
 Tripeptides, dipeptides, and
single amino acids.
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Brush border enzymes
Small intestine into portal
vein
 Active absorption
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Vitamin absorption
 Passive diffusion
 In the jejunum and ileum.
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Fat-soluble vitamins
 With dietary lipids
 Transport
 Chylomicrons, lipoproteins
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Water-soluble vitamins
 Diffuse into the blood
 Excess (exceeds renal capacity)
 Pass into the urine
 Vitamin B12.
 Intrinsic factor
 Intestine absorbs by endocytosis.
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Dietary factors
 Type of food source impacts absorption
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Cellular factors
 The body does not absorb minerals very well.
 Availability depends on its chemical form
 Heme vs nonheme
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Passive process of osmosis in small intestine
9 L of water each day.
◦ 72% absorbed in the proximal small intestine
◦ 20% absorbed from the distal segment of the small
intestine
◦ 8% absorbed from the large intestine