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GI Digestion and Absorption

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