the digestive system. functions of digestive system food intestines absorbable nutrients lumen blood...
TRANSCRIPT
The Digestive System
Functions of Digestive System
food
Intestines
Absorbablenutrients
lumen
blood
wallwall
digestionfood
Intestines
Absorbablenutrients
lumen
blood
wallwall
digestionfood
Absorbablenutrients
blood
absorption
Forms of Nutrients
In Food Absorbable by Intestine
Protein Amino acidsCarbohydrates Monosaccharides (glucose)Fat Fatty acids, glycerol DNA, RNA Bases + monosaccharides Vit B12 B12+intrinsic factor Other vitamins Original formCholesterol Original formElectrolytes Original form Water Original form
digestionfood
blood
absorption
Amino acids
Glucose
Fatty acids,
Glycerol
Bases +
monosaccharides
vitamins
Cholesterol
Electrolytes,
Water
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
The Mouth
has the following functions :
1) breaking food
2) sense of taste
3) swallowing
4) speech
Saliva and the Salivary Glands
1) moisten food and the mouth
2) clean the teeth, inhibit bacterial growth,
3) dissolve molecules so they can stimulate taste buds
4) digest a small amount of starch and fat
Esophagus
Esophagus
The Stomach
The Stomach
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
G cells: produce hormones that regulate digestion
chief cells: produce pepsinogen
parietal cells: produce HCl and intrinsic factor
mucous cells: produce mucusThe bottom part of
the pits is the gastric gland.
The gastric glands
produce 2-3 L of gastric juice
daily, containing:
pepsinogen
HCl
intrinsic factor
Pepsinogen
Pepsinogen
- is the inactive precursor of the active enzyme pepsin.
- activated by HCl or pepsin.
Pepsin: chop proteins into small fragments
Hydrochloric Acid ( HCl )
- reduces pH of
gastric juice to as low
as pH 0.8.
HCl
HCl
Functions of HCl:
(1) activates pepsinogen
into pepsin.
(2) breaks up connective
tissues and plant cell
walls.
(3) converts ferric ions to
ferrous ions.
(4) destroys ingested
pathogens.
1) a highly alkaline mucous coat.
2) rapid replacement of epithelial cells (3-6 d)
3) tight junctions between epithelial cells
HClSelf protection of the stomach from HCl and pepsin by
Intrinsic Factor
- is essential to the absorption of vitamin B12
- is the only indispensable function of the stomach.
Intrinsic Factor
Gastrinsecreted by G cells in gastric gland not into gastric juice but blood
- is a hormone - stimulates
1) the secretion of HCl and pepsinogen
2) motility of the large intestine
- relax during swallowing
- stretch further when food enters the stomach (stress-relaxation response)
Gastric Motility
- Next, pacemaker cells in the greater curvature initiate peristaltic contractions.
- Each peristaltic wave pushs ~ 3 mL of chyme into the duodenum.
Regulation of Gastric Function
Gastric secretion and motility is divided into three stages:
1) Cephalic
2) Gastric
3) Intestinal phases
The cephalic phase is stimulated by the sight, smell, taste, or mere thought of food.
1) The Cephalic Phase
- is stimulated by food in the
stomach
- accounts for two-thirds of
gastric secretion.
2) The Gastric Phase
a) the enterogastric reflex.
b) local hormones
Secretin
Cholecystokinin
gastric inhibitory peptide
3) The Intestinal Phase
After entering
small intestines, chyme inhibit
gastric secretion and mobility
via:
The small intestine receives not
only chyme from the stomach but
also secretions from the liver and
pancreas.
The Liver
has tremendous
variety of functions,
including the
secretion of bile
for digestive
purposes.
Hepatocytes secret bile into the bile canaliculi and bile ductules.
bile ductule
Bile ductules hepatic ducts common hepatic duct
cystic duct
common hepatic duct
gallbladder
common bile duct
hepatopancreatic sphincter
pancreas
duodenum
When no chyme is in the small intestine, the hepatopancreatic sphincter is closed. Bile flows into gallbladder.
cystic duct
common hepatic duct
gallbladder
common bile duct
hepatopancreatic sphincter
pancreas
duodenum
Function of Gallbladder - absorbs water and stores the bile for
later use.
gallbladder
hepatopancreatic sphincter
duodenum
- 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
Bile
facilitate fat digestion and absorption
- reabsorbed in the ileum and returned to the liver via enterohepatic circulation.
Bile acidsphospholipids
Recycle of Bile acids and Phospholipids
The Pancreas
Endocrine:
- secretes insulin, glucagon, somatostatin into the blood.
The Pancreas
Exocrine: - secretes pancreatic
juice into the lumen of the pancreatic duct
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.
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.
Pancreatic juice is an alkaline mixture containing:
- secreted in response to similar stimuli.
-Cephalic and Gastric phases
The vagus nerves stimulate pancreatic secretion.
Regulation of Bile and Pancreatic Secretion
1) relaxation of the hepatopancreatic sphincter,
2) the contraction of the gallbladder
3) secretion of pancreatic juice and enzymes.
The Intestinal Phase
- Chyme with acid and fat, stimulate the duodenal mucosa to secrete cholecystokinin (CCK).
secretion of bicarbonate
by both the hepatic and pancreatic ducts
The Intestinal Phase
- Acidic chyme also stimulates the duodenum to release secretin.
The Small Intestine
Overview
- Nearly all chemical
digestion and nutrient
absorption occur in the
small intestines.
1) The duodenum
- ~ 25 cm
2) The jejunum
- comprises the next 2.5 m.
3) The ileum
- forms the last 3.6 m.
- The surface area inside the small intestine is greatly increased by circular folds, villi, and microvilli.
villi
Microvilli (brush border)
brush border enzymes
brush border enzymes
- activates zymogens- complete digestion of carbohydrates and proteins
- 1-2 L of intestinal juice per day.
- The duodenum endocrine cells secret cholecystokinin (CCK) and secretin. (Both are hormones.)
Secretion by the small intestines
Chemical Digestion and Absorption of Nutrients
Chemical Digestion and Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water
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.
- Starch is digested to oligosaccharides (3-8 glucose residues), disaccharide maltose, and glucose.
starcholigosaccharides glucose
pancreatic amylaseBrush borderenzymes
Intestinal lumen Intestinalepithelialcells
blood
glucose glucose
Glucose is absorbed by: - sodium-dependent glucose transporter (SGLT).
- solvent drag
Digestion and Absorption of Proteins
- Proteins are digested by proteases and peptidases.
- Protein digestion starts in the stomach.
Protein digestion continues in the small intestine by pancreatic enzymes trypsin and chymotrypsin.
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.
proteins short peptides amino acids
Gastric pepsin &pancreatic proteases
Brush borderenzymes
Intestinal lumen Intestinalepithelialcells
blood
Digestion and Absorption of Lipids
- Lipids are digested by enzymes called lipases.
- Most fat digestion occurs in the small intestine via several steps.
1) Fats are first broken up into smaller emulsification droplets by lecithin and bile salts (acids) in the bile.
2) When lipase digests fats, the products are two fatty acids (FFAs) and a monoglyceride.
3) Bile salts coat these and other lipids and form droplets called micelles.
Micelles release their lipids, which diffuse freely across the plasma membrane.
Resynthesis of triglycerides. Coating with protein, forming droplets called chylomicrons.
Intestinal absorptive cell
Chylomicrons are too large to enter blood capillaries and must be first transported in the lymphatic lacteal.
Fat globule
emulsification droplets micelles
chylomicrons
Intestinal lumen Intestinalepithelialcells
lacteal blood
Bilesalts
lipase &bile salts
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.
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.
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.
Absorption of Water
- The digestive tract receives about 9 L of water per day
0.7 L in food1.6 L in drink6.7 L in gastrointestinal secretions
- ~ 8 L absorbed by the small intestine via osmosis;
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)
- 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.
The Large Intestine
- 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.
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.
Strong contractions
called mass
movements occur
one to three times a
day, last about 15
minutes each, and
occur especially an
hour after breakfast.
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
Diarrhea
too little water absorbed
Constipation
too much water absorbed, causing difficulty in defecation
In the absence of bile, a fat-rich diet causes diarrhea.
Accumulation of fat molecules
osmolarity of intestinal content
retain water
diarrhea
People lacking lactase have diarrhea after drinking milk.
Lactose (a dimer)
glucose galactose
lactase
People lacking lactase have diarrhea after drinking milk.
Accumulation of Lactose
osmolarity of intestinal content
retain water
diarrhea
The Stomach
The Liver
The Gallbladder
The Pancreas
The Small intestine
The Large intestine
Digestion and Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water
The Organs
SUMMARY