The document summarizes gastrointestinal physiology, focusing on digestion and absorption. It describes how carbohydrates, proteins, and fats are broken down into smaller fragments by enzymes in the mouth, stomach, and small intestine. The fragments are then absorbed in the small intestine. Carbohydrates are broken down into monosaccharides like glucose and fructose. Proteins become amino acids. Fats are converted into fatty acids and monoglycerides, which are absorbed with the help of bile and form micelles. Most absorption occurs in the small intestine through active transport or diffusion.

1. Gastrointestinal physiology
Digestion & absorption
Dr.M.A.M.Shaikhani.

2. Digestion
• Breakdown of the important nutrients,fat,carbohydrates &
proteins into smaller fragments so that it can be easily absorbed
• Is basically a process of hydrolysis i.e incorporating hydroxyl &
hydrogen ions of water into these nutrients so that the
proteolytic,lipolytic & carbohydrolytic enzymes can act easily to
digest their substrate.

3. Carbohydrate digestion:
• Salivary ptyalin digest 20%
• Pancreatic amylase the other 80% of complex starches
into maltose & glucose polymers
• Which are further digested by intestinal brash
border(IBB) enzymes maltase & dextrinase into
glucose ,also sucrose is digested into fructose & glucose
by (IBB) enzyme sucrase & lactos by (IBB) enzyme
lactase into glucose & galactose.
• These sugars are the final common products of
carbohydrate digestion which are easily absorbed.

4. Carbohydrate (CHO) Digestion

5. Carbohydrate (CHO) Digestion

6. Digestion of proteins:
• The proteins are first digested by stomach pepsin into
peptones,proteases & polypeptides
• Further digested by pancreatic trypsin,chemotrypsin
&carboxypolypeptidase into amino acids & polypeptides
• Amino acids & polypeptides are further digested by peptidases
on the (IBB) into amino acids which are easily absorbed.

7. Digestion of fats:
• The fats are digested in the upper SI by the aid of bile which
through the formation of water soluble complexes called micelles
as these bile salts have 2 ends a hydrophilic one & a hydrophobic
one, so they accumulate at lipid- water interfaces & thus emulsify
fats and facilitate their hydrolysis.micelles causes agitation &
detergent action on the fats leading to its emulsification
preparing the fats to be easily digested by pancreatic lipase into
monoglycerides & fatty acids which are easily absorbed.
• Sulfation of bile acids in the liver in the major way of their
intestinal excretion since the sulfated bile salts can not be
absorbed to reciculate in the enterohepatic circulation.

8. Digestion and Absorption of Fat
• Emulsification
by bile salts
• Pancreatic
lipase
• Micelles are
water-soluble

9. Digestion and Absorption of Fat
• Monoglycerides and
FFA enter cells by
diffusion
• Triglyceride synthesis
• Add protein
• Chylomicrons

10. Absorption:
•The majority of absorption occurs in the SI .
•in the stomach only alcohol & some drugs as aspirin are absorbed.
•Large intestine only water &electrolytes are absorbed.
•In the mouth no absorption occurs, except for some drugs.
•In the esophagus no any absorption.
•Absorption takes place in the villi & microvilli which increase the
surface area of SI by 600 times to make the absorption process very
efficient.

11. Water & electrolyte absorption:
•By simple diffusion as water or by active secretion as Na+ & HCO3.
•Absorption of H2O: by simple diffusion following the law of
osmosis to cause isosmotic equilibrium when ions/nutrients absorbed
• Absorption of Na+ is by active transport from inside the epithelial
cells into the intercellular spaces requiring energy supplied by
ATPase carrier enzyme in the cell membrane,part of Na+ is
absorbed with Cl- while the other is either with K+ or H+ ions
transported in the opposite direction in exchange for Na+ helped by
aldosterone when there is need for Na+ in the body as in
dehydration
• When Na+ is absorbed H+ is secreted into the SI lumen in
exchange of Na+ , excess H+ combines with excess HCO3- to form
carbonic acid which dissociates into H2O & CO2 excreted by the
lungs.
In the ileum & large intestine there is secretion of HCO3- in
exchange of CL- by exchange proteins in luminal membrane of the
epithelial cells to provide alkaline medium to neutralize acid formed
by the bacteria.

12. Absorption of carbohydrates& proteins:
• Glucose is the most rapidly absorbed than other carbohydrates
digestion end products; fructose, manose,xylose& arabinose with
the exception of galactose which is slightly more absorbed than
glucose but glucose is present in very much higher concentrations.
• Mechanism of glucose & galactose absorption is Na+-cotransport
(secondary active transport of glucose & galactose) while that of
fructose is different:dose not require energy,is by fascilated
diffusion & fructose is partly converted to glucose inside the
epithelial cells before entering the portal blood.
• Absorption of amino acids is not by Na cotransport but by
secondary active transport of amino acids & peptides requiring
special membrane transport protein of which there are at least
five.

13. Absorption of fats:
• Bile salts & pancreatic lipase are the most important factors in fat
absorption.
• The bile salts facilitate fat absorption by reducing the surface
tension of fat particles &by forming water soluble complexes
called micelles in which fatty acids & glycerol also participate.
• The fats digestion end products; fatty acids & monoglycerides are
recombined after entering the epithelial cells by the smooth
endoplasmic reticulum to form triglycerides & chylmicrones
again which are easily absorbed by the lymphatics of the villi
called the lacteals to the lymphatic system & not by the portal
blood,while some short chained fatty acids are absorbed directly
into the portal blood rather than lymphatics and this can be used
in fat malabsorbtion

14. MCQs:
1. Almost all of the carbohydrate digestion occurs by:
A. Salivary amylase.
B. Pancreatic amylase.
C. Brash boarder disacharidases.
D. Salivary& pancreatic amylase.
E. Pepsin.

15. MCQs:
2. Final digestion of proteins occurs by:
A. Pepsin.
B. Pancreatic trypsin.
C. Brash boarder peptidases.
D. Salivary& pancreatic amylase.
E. Lipase.

16. MCQs:
3. Fats that dose not absorbed by lymphatics include:
A. Monoglycerides.
B. Long-chain fatty acids.
C. Short-chain fatty acids.
D. Triglyceride.
E. All of the above.

17. MCQs:
4. The sugar type(s) that is(are) most abosrbed in quantity
include:
A. Glucose.
B. Fructose.
C. Mannose.
D. Xylose.
E. Arabinose.

18. MCQs:
5. No any type of absorption occur in:
A. Mouth.
B. Esophagus.
C. Stomach.
D. Small intestine.
E. Large intestine.

19. MCQs:
6. No any type of nutrient absorption occur in:
A. Mouth.
B. Esophagus.
C. Stomach.
D. Small intestine.
E. Large intestine.

20. MCQs:
7. No any type of nutrient digestion occur in:
A. Mouth.
B. Esophagus.
C. Stomach.
D. Small intestine.
E. Large intestine.