3- digestion + absorption physio

3 main processes of digestion

1. secretion: enzymes, mucus, ions into digestive tract + hormones into blood

2. absorption: water, ions, nutrients

3. motility: contractions of smooth muscle to crush, mix, propel

what’s the main organ of digestion + absorption

small intestine

what other 2 organs are involved w/ digestion + absorption

1. pancreas: releases pancreatic juice containing enzymes

2. gallbladder: releases bile

how many liters of fluid pass though the GI system everyday

9 L

how many liters are ingested each day

2 L, remaining 7 L comes from the GI system

how many liters are eliminated/lost each day to stool

only 100 mL

5 important cell types of small intestine

1. enterocyte

2. goblet cell: mucin

3. enteric endocrine cell: gastrin, CCK, secretin

4. stem cell

5. Paneth cell: lysozyme

which small intestine cell is involved with absorption

enterocytes, absorbs amino acids, glucose, monoglycerides, membrane enzymes

4 features of the small intestine that increase its surface area

1. intestinal folds

2. villi

3. microvilli

4. crypts

T/F: villi only live for a few days, die + shed into lumen

true

which part of the small intestine has the largest amount of digestion + significant amount of nutrient absorption

jejunum

which nutrients are not absorbed in the small intestine

1. iron

2. vitamin B12

3. bile acids

how is vitamin B12 absorbed

intrinsic factor from stomach parietal cells binds to B12 in duodenum → binds to receptor in distal ileum to be absorbed into portal blood

3 types of absorption diffusion mechanisms of GI tract

1. simple

2. carrier-mediated

3. channel-mediated

how is starch/carbs digested

1. salivary amylase breaks down food → polysaccharides

2. pancreatic amylase in duodenum breaks down polysaccharides → disaccharides

3. disaccharidase in small intestine epithelium breaks down disaccharides → monosaccharides (simple sugars)

4 examples of disaccharides

1. lactose: glucose-galactose

2. sucrose: glucose-fructose

3. maltose: glucose-glucose

4. starch: glucose-glucose

3 examples of monosaccharides (simple sugars)

1. glucose

2. fructose

3. galactose

how are monosaccharides (simple sugars) absorbed

• via Na⁺ dependent transporter: glucose + lactose

• via non-Na⁺ dependent transporter: fructose

in capillaries

once the monosaccharides (simple sugars) are absorbed, where does it go

carried to liver where they’re stored

how are pancreatic proteases (enzymes that cleave peptide bonds) activated

small intestine secretes enterokinase, which cleaves trypsinogen (secreted by pancreas) → trypsin, which allows all other proteases to be activated

how is protein digested

1. stomach: denatures + partially hydrolyzes proteins using HCl + pepsin

2. small intestine: proteases breakdown proteins → amino acids

how is protein absorbed

Na⁺ dependent cotransporters via active transport in small intestine enterocytes

how are lipids digested

bile acids combine w/ fats → emulsion droplets that are degraded by lipase, breaking the fat → monoglycerides

how are lipids absorbed

micelles containing monoglycerides penetrate + release fats into unstirred layer next to small intestine epithelium for absorption

how does pancreatic lipase break down fat

hydrolyzes glycerol-fatty acid bonds in triglyceride → 2 monoglycerides + free fatty acids

how are fats absorbed

after entering the enterocyte, the monoglycerides + fatty acids are reassembled into triglycerides → triglycerides are repackaged into chylomicrons in smooth ER → chylomicrons are secreted from the enterocyte via exocytosis into the lymphatics

main function of colon

reabsorb water via Na⁺ gradient