Background: main Digestion and Absorption site?
?Small Intestine is Main Digestion and Absorption Site
Has plicae, villi, and microvilli to increase surface area (remember)
Path of food (after digestion): Review
Lumen --> mucosal epithelium (absorptive cells) --> lamina propria --> capillary beds in lamina propria --> venule --> hepatic portal vein --> liver sinusoids for filtering --> central vein of lobule --> hepatic veins --> inferior vena cava
Typical Meal contains?
*A typical meal contains carbohydrates, proteins, lipids, water, electrolytes, and vitamins
-Each component of meal is handled differently by the digestive system
-Large molecules must be broken down by digestion before absorption occurs**only final small
Carbohydrate digestion
1. Salivary amylase (from salivary acinar cells-denatured by stomach HCL)
-Starch hydrolysis occurs in upper stomach
-not effective
**starch/disac->oligosac/disac
2. Pancreatic amylase (from pancreatic acinar cells into duodenum)
*More effective than sali
What enzymes break oligosaccharides to disac?
Dextrinase and glucoamylase
Disaccharides:
? Lactose
? Maltose
? Sucrose
what enzyme break di to monosaccharides?
Lactase, maltase, and sucrase
Monosaccharides:
? Glucose
? Galactose
? Fructose
? What we want! Can be absorbed!
Lactase products?
Lactase - breaks up lactase --> galactose and glucose
Maltase products?
Maltase - breaks up maltose --> 2 glucose molecules
Surcrase Products?
Sucrase - breaks up sucrose --> glucose and fructose
True or False. Fructose and Galactose is aborbed by the same way
False. Galactose and Glucose are absorbed by the same way via Na+ cotransporters. Na+ has to be present for this transporter to work!
Fructose uses faciliated tranporter, which can bring in Fructose without Na+ being there
**they all then enter blood thro
What transporter brings glu and galactose into cell? How?
Glucose and galactose enter the jejunal epithelial cells against a concentration gradient via SGLT1 (sodium/ Glucose tranporter 1)-very effective!
*glu or gal is coupled with Na, which provide driving force/energy when Na moves into the cell, down its con
Name Frutose transporter on luminal side
Fructose (less efficiently) moves across the brush border by GLUT5 (glucose transporter 5)
How does different monosaccharides get transported into blood? which monosaccharide is more special?
All three monosaccarides (glu, gal, fru) leave the cell at the basolateral membrane by facilitated transport via GLUT2 (glucose transporter 2)
**if block GLUT2, flucose still can enter blood, therefore, "there should be another fructose transporter beside
How does the low efficiency of fructose transporter (GLUT5) affect a person with high disaccharide diet?
Much less efficient GLUT2 causes
? some fructose may remain in the lumen and not be fully absorbed
-May keep some water in the lumen --> diarrhea
Where does most of water absorption occur in GI?
When particles are absorbed from the lumen, water is pulled along with them
SO most of the water absorption occurs in the jejunum as well (where all the solutes are being absorbed)
Protein Digestion
-Pepsin from chief cell in stomach starts digestion- but Gastric hydrolysis is minor (<15%)
**Protein-> large polypeptide
-Pancreatic peptidases are active within the intestinal lumen (3)**see next slide
***large polypeptide-small polypep/ peptides
-Intes
Pepsin is important for what type of digestion?
collagen digestion (meat connective tissue)
Name pancreatic enzymes (3)
Trypsin
Chymotrypsin
Carboxypeptidase
Name brush border enzymes (3)
Aminopeptidases
Carboxypeptidases
Dipeptidases
True or False: Most aa are cotransported with NA+ ions
True
What are transporters for amino acid and short peptides on luminal side?
aa transport proteins-transport aa
peptide transport proteins-transport di-and tripeptides
Fat digestion: major site of digestion? what enzymes involved? what else aid in fat digestion?
-The majority of lipolysis occurs in small intestine
-Pancreatic juice contains 3 important lipolytic enzymes (pancreatic lipase)
-Bile acids aid in emulsifying lipids to form micelles
True or False. Lingual and gastric Lipase play minor role in fat digestion.
True. Gastric Lipase inactivated at high acidity
Lipase in the stomach (released via salivary glands or stomach glands)
Bile acid increases surface area of fat dropplets through emulsification by how many times?
600-1000x increase in SA when forming create small lipid droplets/ micelles
True or False. The most effective way to digest fat is through lipolytic ezymes.
False. most effect when increase in surface area of fat by bile acid
What is micelle?
Small fat droplet + bile salt
How does fat come into absorptive cell?
*cross luminal membrane via simple diffusion b/c FAT likes FAT
?No transporters needed
How does bile help with fat absorption (2 ways)
1. emulsify fat
2. increase rate of absorption (break down faster by enzyme)
What is in Bile?
bile acids
phopholipids
cholesterole
Fat-soluble vitamins (from diet)
Free fatty acids
2 monoglycerides
Besdies fatty acids, what else require micelle for absorption?
Cholesterol (steroid hormones) and fatsoluble vitamins require
micelles to be absorbed
what are the fat soluble vitamines?
Vitamin D,E,K,A =DEKA
True or False. Bile stays in lumen until it gets recycled to hepatocytes
True. Stays in lumen, then cross transporter in ileum to blood, go back via hepatic porter vein->sinusoids->hepatocytes
**BILE stimulated BILE release!
What happens to Fat once it enters the cell?
Within the cell, it is reesterified into a triglyceride, which then get wrapped up by protein to form a chylomicron (protein coat + fat)
Protein coat is provided by the golgi apparatus in absorptive cell to prevent fat from forming huge globule again once
How does chylomicron cross basolateral membrane?
A vesicle surrounds the chylomicron coat and fuses with the basolateral membrane to exocytose it
Once cross absorptive cell, how is fat circulation different from protein and glucose?
Glucose+aa->enters blood circulation
Fat Enters the lacteals (lymphatic capillaries of small intestine right next to blood vessel) at villi.
*Transported to systemic circulation via the lymph in the RIGHT thoracic duct
lacteals channeled into lymphatic ve
aa/ glucose and fat enter liver via different components of portal triads. How?
aa/glucose enter via hepatic portal vein
fat enters via hepatic artery
Lipids are transported in blood in what form?
Transported as LIPOPROTEINS
Because fat is so hydrophobic, it needs help being transported in the watery blood
Will be surrounded by proteins to help dissolve it
? Forms micelles
? Hydrophobic tails face in, hydrophilic heads face out
4 Types of Lipoproteins:
1. Chylomicron
2. LDL - Low Density Lipoprotein
3. HDL - High Density Lipoprotein
4. VLDL - Very Low Density Lipoprotein
Chylomicron: source?
Chylomicron - the proteins + fat from the absorptive cells
?From DIETARY fat
?See above
Which type of Liporotein is Bad?
LDL - Low Density Lipoprotein is bad cholesterol
?Much lower protein:fat ratio
Which type of liprotein is good? What ratio reduces risk of heart disease?
?"Good" cholesterol
?Much higher protein: fat ratio
?High HDL:LDL ratio --> lower risk for heart disease
Want high HDL, low LDL
What is optimal level of LDL?
less than 100 mg/dL
What is optimal level of HDL?
greater than 40 mg/dL
Where does Fat processing occurs?
?Fat processing occurs in the liver
In the hepatocytes
May be made into a steroid, cholesterol, bile acid, etc.
Gastric Absorption (In stomach)
-Very little absorption of nutrients
-With poorly absorbed substances, the absorption rate depends upon gastric emptying (how full stomach is)
-Solubility of some substances is pH dependent
Absorption of Ethyl alcohol. How long does it take to reach peak blood alcohol in empty stomach? why get drunk slower after eating?
ethyl alcohol
-small highly water-soluble molecule that is
absorbed rapidly anywhere in GI tract (including stomach)
-in the fasting state, peak blood alcohol concentration within 30 minutes
-presence of food in gut, delays absorption
by slowing gastric e
aspirin absorption. Where does it occur in GI? which triggers faster absorption?
-Unionized in low pH (ex. In the stomach acid)
? Absorbed much faster! The ion can cross the membranes more easily
? SO in an empty stomach, aspirin is mostly absorbed in the stomach
-In a full stomach, less aspirin will be absorbed
? The pH will be highe
True or False. Absorption of Aspirin is pH independent.
False. Aspirin absorption is pH dependent
True/False. Absorption of alcohol depends on rate of gastric emptying.
True. presence of food in gut, delays ETOH absorption by slowing gastric emptying
Intestinal Absorption. What is the functional unit of absorption? what are mucosal transport mechanisms?
-Most nutrients are absorbed in the small intestine
-Functional unit of absorption here is the villus
-The mucosal epithelium possesses microvilli (brush border) on the luminal side
-Mucosal transport mechanisms are
1. transcellular (through cell via tran
Colon Absorption
?Any amino acids and peptides that weren't absorbed in the small intestine can be absorbed here
?Water not absorpted in small intestine
Absorption of Water. what is total from ingestion and GI secretion? How much absorbed in SI? How much in colon? what is max colon absorptive capacity? what happens when this capacity is exceeded?
?We ingest about 1.5 L of water a day
When you add all the GI secretions, you get about 9L of water in the GI
?About 7L are absorbed in the small intestine each day (90% abs in SI via osmosis)
?The rest of the water (about 2 L) of water enters the colon e
Other Causes of Diarrhea (besides high level of water ingestion) . Examples?
Food materials that can't be absorbed will keep water with it
?WATER FOLLOWS THE NUTRIENTS
-This can lead to diarrhea because more water will remain in the lumen
Ex. High fructose diet
?Fructose isn't absorbed as well as glucose
?More will remain in the l
How is water absorbed in GI?
-Water moves by transcellular(through cell) and paracellular (between cell) mechanisms
Where is main site of Electrolyte Absorption ?
?Absorbed along the small intestine mostly via transporters
?Sodium
-Usually brought in with many other molecules via co-transporters, facilitated by sodium concentration gradient
?Calcium
-Brought in via active transport
Vitamin Absorption. which are water soluble vitamins?
Vit B's & C
How are water soluble vitamins absorbed? what is the exception?
?B and C's Absorbed mostly in the small intestine via DIFFUSION
?Concentration gradient drives the absorption - concentration is much higher in the lumen --> driven into the absorptive cells and then into the blood -> hepatic porter vein->liver
?Vitamin B
which are fat soluble vitamins?
fat soluble vitamins DEKA
How are fat soluble vitamins transported?
dissolved in & absorbed with lipids particles
Exact same path as fat
Once in the liver, they will be processed as necessary
? Delivered via the hepatic artery, NOT the hepatic portal vein (because they came from the systemic circulation, remember)
True or False. Unlike water soluble vitamins, fat soluble vitamins enter lacteals instead of blood after crossing absorptive cells
True. they travel same path as fat
?Absorption Pathway: micelle --> absorptive cells --> chymomicron in the cell (via protein coat) --> lacteals --> lymphatic system --> right thoracic duct --> blood circulation --> hepatic artery-> sinnusoids->liver