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CHAPTER 66
Digestion and Absorption in the Gastrointestinal Tract

General Overview of Digestion

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◦ These nutrients cannot be absorbed in their natural form
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◦ Digestion involves enzymes and a process called hydrolysis to break down large molecules
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What is Hydrolysis?


































◦ Hydrolysis is the process where water molecules (H₂O) are used to break down bonds in large molecules
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g
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▪ Fats → Glycerol + Fatty Acids
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Carbohydrate Digestion

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Lactose: Found in milk (disaccharide)
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Cellulose: Found in plants, but humans cannot digest it (it acts as fiber instead)
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Where and How Carbohydrates Are Digested:
◦ In the Mouth:
▪ Saliva contains an enzyme called ptyalin (α-amylase), which begins breaking down starch into
maltose (a disaccharide)
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◦ In the Stomach:
▪ Starch digestion continues for up to an hour until stomach acid deactivates ptyalin
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◦ In the Small Intestine:
▪ The pancreas secretes pancreatic amylase, a powerful enzyme that completes the digestion of starch
into maltose and small glucose polymers
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▪ Sucrose → Glucose + Fructose
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▪ End Result: Most of the carbohydrates are converted to glucose, which is water-soluble and absorbed
into the bloodstream
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What Are Proteins?
◦ Proteins are long chains of amino acids linked by peptide bonds
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▪ Pepsin breaks proteins into smaller fragments (proteoses, peptones, and polypeptides)
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◦ In the Small Intestine:
▪ Pancreatic enzymes (e
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, trypsin, chymotrypsin) further break polypeptides into smaller peptides
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▪ Elastase digests elastin, a protein in connective tissues
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▪ Amino acids are absorbed into the blood
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What Are Fats?
◦ The main dietary fats are triglycerides (composed of 1 glycerol + 3 fatty acids)
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◦ In the Small Intestine:
▪ Step 1: Emulsification:
▪ Bile (produced by the liver and stored in the gallbladder) contains bile salts and lecithin
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▪ Step 2: Digestion by Pancreatic Lipase:
▪ Pancreatic lipase splits triglycerides into free fatty acids and monoglycerides
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▪ Micelles transport these fat products to the intestinal wall for absorption
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Digestion of Other Fats:
◦ Cholesterol Esters: Broken down by cholesterol ester hydrolase
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◦ Micelles also help transport these fats for absorption
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Enzymes Are Specific:
◦ Different enzymes are required for digesting carbohydrates, proteins, and fats
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Proteins → Amino acids
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These digested products are absorbed into the blood or lymph for transport throughout the body
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Importance of Bile:
◦ Bile plays a critical role in fat digestion by emulsifying fats and forming micelles for fat absorption
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Unabsorbed Substances:











































◦ Some substances like cellulose (fiber) cannot be digested or absorbed and are excreted
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The small intestine absorbs the majority (about 7 to 8 liters), with a small amount (around 1
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The small intestine has specialized structures—folds of Kerckring (valvulae conniventes), villi, and microvilli—
that significantly increase the absorptive surface area, enhancing nutrient absorption
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The presence of actin filaments in microvilli helps keep them constantly exposed to intestinal fluid, aiding
absorption
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Water follows osmotic gradients,
moving from areas of lower solute concentration (like dilute chyme) to areas of higher concentration (the blood in
villi)
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Sodium absorption is critical for maintaining fluid balance, and aldosterone increases sodium absorption in

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cases of dehydration
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Nutrient Absorption:
Carbohydrates: Carbohydrates are primarily absorbed as monosaccharides, mainly glucose (about 80% of

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carbohydrate absorption)
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Proteins: After digestion, proteins are absorbed as dipeptides, tripeptides, or amino acids through co-transport with

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sodium ions
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These micelles transport

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lipids to the brush border of intestinal cells, where they diffuse into the cells
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Absorption in the Large Intestine:
Electrolyte and Water Absorption: The large intestine absorbs significant quantities of sodium and chloride ions,

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and the osmotic gradient they create facilitates water absorption
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Maximum Absorption Capacity: The large intestine can absorb up to 5-8 liters of water and electrolytes daily
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Composition of Feces:

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Feces are typically composed of 75% water, with the remaining 25% being solid matter, including undigested food,
bacterial cells, fats, proteins, inorganic matter, and bile pigment derivatives like stercobilin and urobilin, which
give feces their characteristic brown color
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These absorption processes are essential for maintaining fluid balance, electrolytes, and nutrient availability, supporting







the body’s metabolic needs

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