Digestion, Absorption, and Transport

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process by which food is broken down into absorbable units

digestion

absorption

passage of nutrients from GI tract into blood or lymp

order of events in digestion

ingestion -> digestion -> absorption -> transport -> metabolism -> excretion

Gastrointestinal tract (GI tract)

flexible muscular tube from mouth, through the esophagus, stomach, small intestine, large intestine, and rectum to the anus

principle organs of the GI tract

stomach, small intestine (about 10ft in length), large intestine

lumen

inner space w/in the GI tract

mouth

begins digestive process; chews and mixes food w/ saliva

salivary glands

secrete saliva; contribute little to digestion

saliva

contains starch digestive enzymes

enzyme

made of protein; facilitate catalyze chemical reactions; end in -ase

examples of enzymes

carbohydrase, protease, lipases

epiglottis

protects airway from closing during swallowing (closes when swallowing)

esophagus

passageway for food from mouth to stomach

bolus

name for food unit after swallowing

upper esophageal sphincter

prevents backflow of food from esophagus back into mouth

cardiac/lower esophageal sphincter

prevents backflow of food from stomach back into esophagus

sphincter

circular muscle surrounding and able to close a body opening; help regulate flow of food particles

stomach

portion of digestive tract that grinds and churns swallowed food, mixing it with acid and enzymes to form chyme; adds HCL, enzymes, and fluid to bolus; major site of protein digestion

HCL

keeps pH of stomach acidic (pH <2)

mucus

secreted by goblet cells in stomach to protect stomach from HCL's acidic pH

Chyme

term for bolus after it is churned, mixed, and ground into semi-liquid mass

how is chyme released?

released into small intestine in small amounts by pyloric sphincter

pyloric sphincter

separates stomach from small intestine and regulates the flow of partially digested food into the small intestine; prevents backflow from small intestine back into stomach; opens about 3x per minute

pH

measure of acidity/alkalinity of a substance

small intestine

10 ft length; major site of carb and lipid digestion and of nutrient absorption into blood stream and lymphatic system

3 sections of small intestine

duodenum, jejunum, ileum

vascular system

blood stream

pancreatic juice

contains enzymes and bicarbonate

bicarbonate

strong base that keeps pH of small intestine basic/alkaline

pancreatic duct

conducts pancreatic juice from pancreas to duodenum of small intestine

bile duct

conducts bile from gallbladder to small intestine

pancreas

gland that secretes digestive enzymes and juices into the duodenum

reabsorbs water & minerals from chyme and leaves semi-solid waste composed of fiber, bacteria, and unabsorbed nutrients; passes waste & unabsorbed nutrients like fiber on to rectum for elimination

large intestine

3 sections of large intestine

ascending colon, transverse colon, descending colon

ileocecal valve

separates small and large intestine; lower right side of abdomen

anus

final sphincter muscle

motility

movement of food through GI tract

peristalsis

wavelike muscle contractions that push contents through GI tract

segmentation

periodic squeezing/contracting or partitioning of intestine at intervals along its length

what is the purpose of segmentation?

to mix chyme and promote close contact with digestive juices & absorptive cells of intestinal wall before contents move along

how do digestive enzymes catalyze hydrolysis reactions?

by splitting reactant into 2 products (with addition of water) then molecule is broken into smaller pieces

hormones

usually made of protein; act as chemical messengers; regulate enzyme action

examples of hormones

gastrin, secretin, gastric inhibitory peptide

organs with glands that provide digestive secretions

salivary glands, stomach, pancreas, liver (via gallbladder), small intestine

gland

cell/group of cells that secretes materials

exocrine glands

secrete materials out of the body (into digestive tract or onto skin)

endocrine glands

secrete materials inside body (into blood)

where is bile produced?

in the liver

where is bile stored?

in the gallbladder

bile

emulsifier that prepares fats/oils for digestion by bringing fats into suspension with water so that enzymes can break them down into their component parts

villi

fingerlike projections from the folds of the small intestine that increase surface area

microvilli

tiny, hairlike projections on each villus; trap nutrients and transport them into cells

crypts

tubular glands that lie between the intestinal villi and secrete intestinal juices into the small intestine

simple diffusion

nutrients can cross into intestinal cells freely

facilitated diffusion

when nutrients need specific carrier to transport them from one side of cell membrane to other

active transport

when some nutrients must be absorbed actively-these nutrients move against a concentration gradient, which requires energy

what nutrients enter vascular system directly?

amino acids, monosaccharides, & small lipid particles

what nutrients have to enter vascular system via lymphatic system?

larger lipids (monoglycerides, long chain FAs, and fat sol. vit.)

vascular system

blood circulatory system through which blood lows continuously, picking up and delivering materials as needed

lymphatic system

loosely organized system of vessels/ducts that move fluids toward thorasic duct then to vena cave of the heart

lymp

clear yellowish fluid almost identical to blood BUT does not contain RBCs or platelets; transports fat and fat soluble vit. to bloodstream

how do long chain FAs and fat-sol vit. absorb into vascular system?

small intestine -> lymp -> vascular system

homeostasis

maintenance of constand internal conditions by body's control systems; constantly react to external forces to maintain limits set by body needs

enterogastrones

any hormone that slows motility and inhibits gastric sections

gastrin

hormone secreted by cells in stomach wall; turned on by presence of protein in stomach; turned off when stomach reaches pH of 1.5

what is the target organ of gastrin?

glands of stomach

where is secretin produced?

produced in cells in duodenum wall

what causes secretin production?

presence of chyme in small intestine triggers release-pyloric sphincter opens, releasing chyme into duodenum

what is the target organ of secretin?

target organ is pancreas

how does secretin get to pancreas?

travels from small intestine to pancreas via vascular system

what does secretin produce?

bicarbonate-rich pancreatic juice

what causes cholecystokinin (CCK) production?

produced when fat is present in small intestine

where is cholecystokinin (CCK) produced?

produced by intestinal wall cells

what is the target organ of cholecystokinin (CCK)?

target organ is gallbladder

what happens when CCK reaches the gallbladder?

causes the release of bile into the small intestine

what happens when CCK reaches pancreas?

it stimulates it to secrete its juices, releasing bicarbonate and enzymes into small intestine which slows gastric motility

how does CCK travel?

travels via vascular system to gall bladder

gastric inhibitory peptide

slows digestion to allow time for fat digestion; slows GI motility; slows gastric secretions

where is gastric inhibitory peptide produced?

produced by intestinal wall cells (like CCK)

how does gastric inhibitory peptide travel?

travels from small intestine (via vascular system) to the stomach

what does bicarbonate do?

neutralizes chyme

where is bicarbonate released?

released by pancreas

what causes secretion of bicarbonate?

secretion turned on by presence of chyme in duodenum

what stops bicarbonate secretion?

secretion turned off by neutral to basic pH in duodenum

how does bicarbonate travel?

travels from pancreas (via vascular system) to small intestine (near pyloric sphincter)

what organs release proteases, lipases, carbohydrases?

secreted by both small intestine (duodenum) and pancreas

what is the target organ of proteases, lipases, and carbohydrases?

target organ is duodenum of small intestine

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