1. The Digestive System

2. Functions of Digestive System
food
blood
Absorbable
nutrients
wall
lumen
wall
Intestines

3. digestion
food
blood
Absorbable
nutrients
wall
lumen
wall
Intestines

4. digestion
food
blood
Absorbable
nutrients
absorption

5. Forms of Nutrients
In Food Absorbable by Intestine
Protein Amino acids
Carbohydrates Monosaccharides (glucose)
Fat Fatty acids, glycerol
DNA, RNA Bases + monosaccharides
Vit B12 B12+intrinsic factor
Other vitamins Original form
Cholesterol Original form
Electrolytes Original form
Water Original form

6. digestion absorption food blood Amino acids Glucose Fatty acids,
Glycerol
Bases +
monosaccharides
vitamins
Cholesterol
Electrolytes,
Water
absorption

7. The Organs of the Digestive System
The Mouth
The Salivary Glands
The Esophagus
The Stomach
The Liver
The Gallbladder
The Pancreas
The Small intestines
The Large intestines

8. The Mouth has the following functions : 1) breaking food
2) sense of taste
3) swallowing
4) speech

9. Saliva and the Salivary Glands
moisten food and the mouth
clean the teeth, inhibit bacterial growth,
dissolve molecules so they can stimulate taste buds
digest a small amount of starch and fat

10. Esophagus

11. Esophagus

12. The Stomach

13. The Stomach Gastric pits Functions food storage host defense
minor role in digestion and
absorption of nutrients
- The above functions are performed via gastric secretion and motility
Gastric pits

14. The bottom part of the pits is the gastric gland.
mucous cells: produce mucus
The bottom part of the pits is the gastric gland.
parietal cells: produce HCl and intrinsic factor
G cells: produce hormones that regulate digestion
chief cells: produce pepsinogen

15. The gastric glands produce 2-3 L of gastric juice daily, containing:
pepsinogen
HCl
intrinsic factor

16. Pepsinogen - is the inactive precursor of the active enzyme pepsin.
- activated by HCl or pepsin.
Pepsinogen
Pepsin: chop proteins into small fragments

17. HCl Hydrochloric Acid ( HCl )
- reduces pH of gastric juice to as low as pH 0.8.

18. HCl Functions of HCl: activates pepsinogen into pepsin.
(2) breaks up connective tissues and plant cell walls.
(3) converts ferric ions to ferrous ions.
(4) destroys ingested pathogens.
HCl

19. Self protection of the stomach from HCl and pepsin by
a highly alkaline mucous coat.
rapid replacement of epithelial cells (3-6 d)
tight junctions between epithelial cells

20. - is essential to the absorption of vitamin B12
 Intrinsic Factor
- is essential to the absorption of vitamin B12
- is the only indispensable function of the stomach.
Intrinsic Factor

21. Gastrin secreted by G cells in gastric gland not into gastric juice but blood
is a hormone
stimulates
the secretion of HCl and pepsinogen
motility of the large intestine

22. Gastric Motility relax during swallowing
stretch further when food enters the stomach (stress-relaxation response)
- Next, pacemaker cells in the greater curvature initiate peristaltic contractions.
- Each peristaltic wave pushs ~ 3 mL of chyme into the duodenum.

23. Regulation of Gastric Function
Gastric secretion and motility is divided into three stages:
1) Cephalic
2) Gastric
3) Intestinal phases

24. 1) The Cephalic Phase
The cephalic phase is stimulated by the sight, smell, taste, or mere thought of food.

25. 2) The Gastric Phase
- is stimulated by food in the stomach
- accounts for two-thirds of gastric secretion.

26. 3) The Intestinal Phase After entering small intestines, chyme inhibit gastric secretion and mobility via:
the enterogastric reflex.
local hormones
Secretin
Cholecystokinin
gastric inhibitory peptide

27. The small intestine receives not only chyme from the stomach but also secretions from the liver and pancreas.

28. The Liver
has tremendous variety of functions, including the secretion of bile for digestive purposes.

29. Hepatocytes secret bile into the bile canaliculi and bile ductules.

30. Bile ductules  hepatic ducts 
common hepatic duct
Bile ductules  hepatic ducts 
cystic duct
common hepatic duct
common bile duct
gallbladder
hepatopancreatic
sphincter
pancreas
duodenum

31. When no chyme is in the small intestine, the hepatopancreatic sphincter is closed. Bile flows into gallbladder.
cystic duct
common hepatic duct
common bile duct
gallbladder
hepatopancreatic
sphincter
pancreas
duodenum

32. Function of Gallbladder
- absorbs water and stores the bile for later use.
gallbladder
hepatopancreatic
sphincter
duodenum

33. Bile
- is constantly produced by the liver (500-1,000 mL of bile per day).
- is a yellow-green fluid containing :
minerals
bile pigments
bile acids
phospholipids
cholesterol
neutral fats
facilitate fat digestion and absorption

34. Recycle of Bile acids and Phospholipids
- reabsorbed in the ileum and returned to the liver via enterohepatic circulation.
Bile acids
phospholipids

35. The Pancreas

36. The Pancreas Exocrine:
- secretes pancreatic juice into the lumen of the pancreatic duct
Endocrine:
secretes insulin, glucagon, somatostatin into the blood.

37. Exocrine
secretes 1,200-1,500 mL of pancreatic juice per day into the main pancreatic duct.
It empties into duodenum when hepato-pancreatic sphincter opens.

38. Pancreatic juice is an alkaline mixture containing:
1) sodium bicarbonate: neutralize HCl
2) inactive digestive enzymes and zymogens which are activated after secreted into duodenum.
The activated enzymes digest carbohydrates, lipids, RNA, DNA, and proteins.

39. Regulation of Bile and Pancreatic Secretion
secreted in response to similar stimuli.
Cephalic and Gastric phases
The vagus nerves stimulate pancreatic secretion.

40. The Intestinal Phase
- Chyme with acid and fat, stimulate the duodenal mucosa to secrete cholecystokinin (CCK).
1) relaxation of the hepatopancreatic sphincter,
2) the contraction of the gallbladder
3) secretion of pancreatic juice and enzymes.

41. The Intestinal Phase
- Acidic chyme also stimulates the duodenum to release secretin.
secretion of bicarbonate
by both the hepatic and pancreatic ducts

42. The Small Intestine

43. Overview
- Nearly all chemical digestion and nutrient absorption occur in the small intestines.

44. 1) The duodenum
- ~ 25 cm
2) The jejunum
- comprises the next 2.5 m.
3) The ileum
- forms the last 3.6 m.

45. - The surface area inside the small intestine is greatly increased by circular folds, villi, and microvilli.
villi

46. Microvilli (brush border)
brush border enzymes

47. brush border enzymes
activates zymogens
complete digestion of carbohydrates and proteins

48. Secretion by the small intestines
- 1-2 L of intestinal juice per day.
- The duodenum endocrine cells secret cholecystokinin (CCK) and secretin. (Both are hormones.)

49. Chemical Digestion and Absorption of Nutrients

50. Chemical Digestion and Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water

51. Digestion and Absorption of Carbohydrate
- Most digestible dietary carbohydrate is starch.
The starch digestion begins in the mouth by salivary amylase.
But fully digestion of starch occurs in the small intestines.

52. - Starch is digested to oligosaccharides (3-8 glucose residues), disaccharide maltose, and glucose.

53. oligosaccharides glucose glucose glucose starch Intestinal lumen
pancreatic amylase
Brush border
enzymes
oligosaccharides
glucose
glucose
glucose
starch
Intestinal
epithelial
cells
Intestinal lumen
blood

54. Glucose is absorbed by:
- sodium-dependent glucose transporter (SGLT).
- solvent drag

55. Digestion and Absorption of Proteins

56. - Proteins are digested by proteases and peptidases.
- Protein digestion starts in the stomach.

57. Protein digestion continues in the small intestine by pancreatic enzymes trypsin and chymotrypsin.

58. Protein digestion is completed in the small intestine by brush border enzymes carboxypeptidase, aminopeptidase, and dipeptidase.
Amino acid absorption is similar to that of monosaccharides, via several sodium-dependent amino acid cotransporters.

59. proteins short peptides amino acids Intestinal lumen Gastric pepsin &
pancreatic proteases
Brush border
enzymes
proteins
short peptides
amino acids
Intestinal
epithelial
cells
Intestinal lumen
blood

60. Digestion and Absorption of Lipids
Lipids are digested by enzymes called lipases.
Most fat digestion occurs in the small intestine via several steps.

61. Fats are first broken up into smaller emulsification droplets by lecithin and bile salts (acids) in the bile.

62. 2) When lipase digests fats, the products are two fatty acids (FFAs) and a monoglyceride.

63. 3) Bile salts coat these and other lipids and form droplets called micelles.

64. Micelles release their lipids, which diffuse freely across the plasma membrane.

65. Resynthesis of triglycerides.
Coating with protein, forming droplets called chylomicrons.
Intestinal absorptive cell

66. Chylomicrons are too large to enter blood capillaries and must be first transported in the lymphatic lacteal.

67. emulsification droplets
Bile
salts
lipase &
bile salts
Fat globule
emulsification droplets
chylomicrons
micelles
Intestinal
epithelial
cells
Intestinal lumen
lacteal
blood

68. Digestion and Absorption of Nucleic Acids
- The pancreatic nucleases hydrolyze nucleic acids to their component nucleotides.
- The brush border nucleosidases and phosphatases further break them down, and the products are transported across the intestinal epithelium by membrane carriers.

69. Absorption of Vitamins
- Vitamins are absorbed without digestion.
- The fat-soluble vitamins are absorbed with other lipids.
Water soluble vitamins are absorbed by simple diffusion, with the exception of vitamin B12.
Vitamin B12 is an unusually large molecule that can only be absorbed with the help of intrinsic factor.

70. Absorption of Minerals
- Minerals (electrolytes) are absorbed without digestion.
- Iron and calcium are unusual in that they are absorbed in proportion to the body's need.
- Other minerals are absorbed at fairly constant rates regardless of need.

71. Absorption of Water
- The digestive tract receives about 9 L of water per day
0.7 L in food
1.6 L in drink
6.7 L in gastrointestinal secretions
- ~ 8 L absorbed by the small intestine via osmosis;

72. Intestinal Motility serve three functions:
1) to mix chyme with intestinal juice, bile, and pancreatic juice (segmentation)
2) to churn chyme and bring it in contact with the brush border for digestion and absorption;
(segmentation)
3) to move residue toward the large intestine. (peristalsis)

73. - Segmentation is the most common type of movement of the small intestine.
When nutrients have been absorbed, segmentation slows and peristalsis begins.
- The intensity of the contractions is modified by nervous and hormonal influences.

74. The Large Intestine

76. - No further chemical digestion
- Water (~ 1L) is further absorbed.
The feces consists of:
75% water
25% solid matter, of which 30% is bacteria, and 30% undigested fiber.

77. Bacterial Flora
- refer to several species of useful bacteria in large intestine.
ferment cellulose and other undigested carbohydrates
synthesize B vitamins and vitamin K, which are absorbed by the colon.

78. Strong contractions called mass movements occur one to three times a day, last about 15 minutes each, and occur especially an hour after breakfast.

79. Neural Control of Defecation
1. Filling of the rectum
2. Reflex contraction of rectum & relaxation of internal anal sphincter
3. Voluntary relaxation of external sphincter

80. Diarrhea
too little water absorbed
Constipation
too much water absorbed, causing difficulty in defecation

81. In the absence of bile, a fat-rich diet causes diarrhea.
Accumulation of fat molecules
osmolarity of intestinal content
retain water
diarrhea

82. People lacking lactase have diarrhea after drinking milk.
Lactose (a dimer)
lactase
glucose galactose

83. People lacking lactase have diarrhea after drinking milk.
Accumulation of Lactose
osmolarity of intestinal content
retain water
diarrhea

84. Digestion and Absorption of Nutrients
SUMMARY
Digestion and Absorption of Nutrients
The Organs
The Stomach
The Liver
The Gallbladder
The Pancreas
The Small intestine
The Large intestine
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water