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breakdown of large molecules into small molecules that can be absorbed
polysaccharides are converted to monosaccharides, proteins are degraded to amino acids, lipids are converted to free fatty acids and monoglyceride molecules
hydrolysis is key to breakdown
enzymes are either luminal or membrane bound


transfer of water and solutes (digested food) from the lumen of the digestive tract into blood and lymph
occurs primarily in small intestine
occurs through brush border by active transport, passive diffusion, facilitated diffusion, and solvent drag


digestion via salivary amylase
no nutrient absorption and only some drug absorption i.e. nitroglycerin


digestion via HCl, pepsin
no nutrient absorption; only alcohol and aspirin absorption

carbohydrate digestion

initiated by salivary alpha amylase from salivary glands
majority by pancreatic beta amylase in small intestine
optimum pH is 7; activated by chloride ions
alpha 1,4 bonds give straight chains and alpha 1,6 bonds give branched chains; amylase can only hydrolyze 1,4 bonds

protein digestion

begins in stomach
pepsin becomes active at a pH of 2-3 and secretion is stimulated by ACh or acid; only protease that can break down collagen
action terminated by neutralization by bicarbonate in duodenum

peptide absorption

active transport by carrier
mostly dependent on sodium gradient; co-transport similar to that for glucose
some amino acids are absorbed by facilitated diffusion
affected by pancreatitis, congential protease deficiencies, and deficiencies of specific transporters

fat digestion

fat stimulates CCK release which causes gallbladder contraction
bile salts emulsify fats; lecithin is important for emulsification
pancreatic lipase is water soluble, acid labile, and extremely active and secreted in large quantities


indigested fat in feces
due to a lack of lipases or severe lack of bile

intestinal absorption

virtually all absorption occurs in the small intestine; most substances absorbed before mid-jejunum reached
vitamin B12 and bile salts absorbed in terminal ileum
large intestine primarily absorbs additional water and ions
surface area available for absorption is greatly increased by villi; microvilli of epithelial cells form brush border membrane; contains digestive enzymes and transport systems

water and NaCl absorption

absorption of water along entire length of small intestine is passive process secondary to absorption of dissolved materials (osmosis)
Na+ absorption has greatest effect on water absorption; active transport of Na+ and water follows passively

intestinal obstruction

net movement of water and ions into lumen may decrease blood volume and blood pressure

regulation of water and NaCl absorption

regulation by neural and hormonal signals
sympathetic innervation increases and parasympathetic innervation decreases
adrenal steroid hormones (aldosterone and glucocorticoids) increase absorption

bicarbonate absorption

secreted by upper small intestine
H+ is secreted into lumen in exchange for Na+; H+ combines with carbonic acid which dissociates to form water and CO2
ileum secretes bicarbonate in exchange for Cl-

sugar absorption

carbohydrates absorbed primarily as monosaccharides
glucose (80%) and galactose are actively transported by carrier-mediated transporter system
fructose absorbed passively by facilitated diffusion

absorption of simple sugars

limiting step is the rate of absorption; large excess in small intestine
majority absorbed in duodenum and jejunum
digested at membrane so available for transport
fructose absorbed by facilitated diffusion
glucose/galactose absorbed passively under anaerobic conditions and actively absorbed by some carrier when O2 is available

disruption of carbohydrate absorption

deficiency in brush border enzymes such as lactase deficiency or maltase deficiency cause osmotic diarrhoea
GI infection/disease i.e. celiac disease, bacterial infections, and protozoan infections can all cause inflammation and interference with brush border absorption; can also cause osmotic diarrhoea

lactose intolerance

lack of lactase causes diarrhea and other symptoms; diarrhea and fluid loss due to osmosis, gas from lactose metabolism by bacteria in large intestine where it is too late to be absorbed

fluid absorption disorders

consequence is diarrhoea
hyperosmotic chyme i.e. high intake of artificial sugars or high acid content
infection i.e. cholera
the colon can absorb 7 liters of water per day, but if the small intestine secretes more than this, the result is diarrhoea

protein absorption

absorption in luminal membranes of intestinal epithelial cells in form of di, tri, and free amino acids
co-transported similar to glucose via Na+ co-transport mechanism

large intestine

most absorption occurs in the proximal half of the colon
high capacity for active absorption of Na+; electrical gradient for Na+ absorption causes Cl- absorption
Na/Cl absorption creates osmotic gradient sufficient for water absorption

vitamin B12

released from food protein in stomach HCl
bound by R proteins in the stomach and intrinsic factor in the small intestine
intrinsic factor and dietary intake of B12 are needed for red blood cell maturation; absence causes pernicious anemia

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