Download Unit One: Introduction to Physiology: The Cell and General Physiology

Chapter 65: Digestion and Absorption
In the Gastrointestinal Tract
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Digestion of the Various Foods by Hydrolysis
• Hydrolysis of Carbohydrates
a. When cbhs are digested, the water molecule that
was removed during the synthesis reaction, is
added back to split the polysaccharide
Digestion of the Various Foods by Hydrolysis
• Hydrolysis of Fats
a. The fat digesting enzymes return 3 water molecules
to the triglyceride and break the fatty acids away
from the glycerol
•
Hydrolysis of Proteins
a. Reverse of the condensation synthesis reaction also;
the hydrogen from water goes to one of amino acid
in a peptide bond and the hydroxyl ion goes to the
other amino acid
Digestion of the Various Foods by Hydrolysis
• Digestion of Carbohydrates
a. Cbh foods of the diet- consists of sucrose, lactose,
and starches
b. Digestion in the mouth and stomach-ptyalin (alphaamylase)-eventually blocked by the acid of the
stomach
d. Small intestine-pancreatic amylase; usually
converted to maltose or other small glucose polymers
Digestion of the Various Foods by Hydrolysis
• Digestion of Carbohydrates
e. Hydrolysis by intestinal epithelial enzymesenterocytes containing lactase, sucrase, maltase,
and alpha-dextrinase
Fig. 65.1 Digestion of carbohydrates
Digestion of the Various Foods by Hydrolysis
• Digestion of Proteins
a. In the stomach- pepsin is the most important peptic
enzyme of the stomach; active at pH 2-3 but
inactive at pH5
Fig. 65.2 Digestion of proteins
Digestion of the Various Foods by Hydrolysis
• Digestion of Proteins
b. Most protein digestion results from actions of
pancreatic proteolytic enzymes: trypsin,
chymotrypsin, carboxypeptidase and proelastase
c. Digestion of peptides by peptidases in the enterocytes that line the small intestinal villi; cells have a
brush border containing hundreds of microvilli
d. More than 99% of the absorbed products are
individual amino acids
Digestion of the Various Foods by Hydrolysis
• Digestion of Fats- the most abundant fats in the diet
are triglycerides
Fig. 65.3 Hydrolysis of neutral fat catalyzed by lipase
Digestion of the Various Foods by Hydrolysis
• Digestion of Fats
a. Triglycerides or neutral fats are more commonly
found in foods of animal origin
b. Digestion of fats in the stomach- small amount
(<10%) by a lingual lipase (lingual glands of the
tongue)
c. First step of fat digestion is emulsification by
bile acids and lecithin
Digestion of the Various Foods by Hydrolysis
• Digestion of Fats
d. Triglycerides are digested by pancreatic lipase
e. End products of fat digestion are free fatty acids
Fig. 65.4 Digestion of fats
Digestion of the Various Foods by Hydrolysis
• Digestion of Fats
f. Bile salts from micelles that accelerate fat digestion
g. Digestion of cholesterol esters and phospholipidstwo other lipases in the pancreatic juice;
cholesterol ester hydrolase and phospholipase A
Basic Principles of GI Absorption
• Anatomical Basis of Absorption- total quantity of
fluid absorbed each day by the intestines is
equal to the ingested fluid plus that secreted
by the various gastrointestinal secretions (8-9
liters); about 7 is reabsorbed in the small
intestine.
Only about 1.5 passes through the ileocecal
valve into the colon each day; stomach is a
poor absorptive area (except alcohol and
aspirin for example)
Basic Principles of GI Absorption
• Anatomical Basis of Absorption
a. Folds of Kerckring, villi, and microvilli increase the
intestinal mucosal absorptive area by 1000 fold
Fig. 65.5 Longitudinal section of the small intestine, showing the valvulae conniventes covered by villi
Basic Principles of GI Absorption
• Anatomical Basis of Absorption
b. The folds are called valvulae conniventes and
extend most of the way around the intestine
c. Also, there are millions of villi all the way down to
the ileocecal valve
Basic Principles of GI Absorption
• Anatomical Basis of Absorption
Fig. 65.6 Functional organization of the villus; longitudinal (A) and cross-sections (B)
Basic Principles of GI Absorption
• Absorption in the Small Intestine
a. Absorption of water by osmosis
b. Absorption of ions- active transport of sodium;
sodium is also co-transported by several specific
carrier proteins:
1) Sodium-glucose co-transporter
2) Sodium-amino acid co-transporters
3) Sodium-hydrogen exchanger
Basic Principles of GI Absorption
Fig. 65.8 Absorption of sodium, chloride, glucose, and amino acids through the
intestinal epithelium; note the osmotic absorption of water
Basic Principles of GI Absorption
c. Aldosterone greatly enhances the absorption of
sodium.
d. Absorption of choride ions in the small intestinemainly by diffusion following an electrochemical
gradient caused by sodium flow
e. Absorption of bicarbonate ions in the duodenum
and jejunum-when sodium is absorbed, small
amounts of hydrogen are secreted into the lumen of
the gut in exchange for some of the sodium.
Basic Principles of GI Absorption
Hydrogens combine with bicarbonate to form carbonic
acid which then dissociates to form water and carbon
dioxide. Water remains as part of the chyme, but the
carbon dioxide is absorbed into the blood, and expired
through the lungs—this is called the “active absorption
of bicarbonate ions” and is the same mechanism that
occurs in the tubules of the kidney.
f. Secretion of bicarbonate ions in the ileum and large
intestine—simultaneous absorption of chloride ions
Basic Principles of GI Absorption
g. Active absorption of calcium, iron, potassium,
magnesium, and phosphate
•
Absorption of Nutrients
a. Cbh are mainly absorbed as monosaccharides
b. Proteins are absorbed as dipeptides, tripeptides, or
amino acids
c. Fats are absorbed as micelles into lacteals; small
amounts are absorbed as fatty acids into the portal
blood
Absorption in the Large Intestine-Formation of Feces
•
Absorption and Secretion of Electrolytes and Water
a. High capacity for the active absorption of sodium
and chloride (due to electrochemical gradient of
sodium)
b. Creates an osmotic gradient which in turn causes
absorption of water
c. Mucosa secretes bicarbonate to counter the acidity
of bacterial end products
Absorption in the Large Intestine-Formation of Feces
•
Maximum Absorption Capacity of the Large Intestine
a. 5-8 liters of fluids and electrolytes each day
b. Bacterial action in the colon-vitamin K, vitamin B-12,
thiamine, riboflavin, and gases (carbon dioxide,
hydrogen gas, and methane)
c. Composition of the feces- 75% water and 25% solid
matter; solid matter is 30% dead bacteria, 19-20% fat,
10-20% inorganic matter, 2-3% protein, 30% undigested
roughage; color is due to bilirubin by-products; odor
due to bacterial actions