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  • digestion begins in mouth with alpha-amylase contained in saliva acting on carbohydrates
no digestion in the esophagus
  • peristaltic action moving bolus down esophagus (wave motion)
  • performed by smooth muscle
  • stomach reduces food to semifluid mass called chyme
the stomach
  • protein digestion begins in the stomach (by pepsin)
  • no absorption occurs in the stomach
  • low pH denatures proteins (pH=2) and kill ingested bacteria
4 major cell types in stomach:
1. mucous cells - contains rough ER and Golgi to make mucus
2. chief (peptic) cells - synthesize pepsinogen on rough ER
3. parietal (oxyntic) cells - contain many mitochondria to produce enough energy to establish proton gradient
4. G cells - secrete gastrin
mucous cells
  • secrete mucus
  • line stomach wall
  • protects epithelial lining from acidic envionment
chief cells
  • deep in exocrine glands
  • secrete pepsinogen (zymogen precursor to pepsin-activated by low pH)
  • pepsin begins protein digestion
parietal cells
  • in exocrine glands
  • secrete HCl which diffuses to lumen
  • lower pH of stomach and raise pH of blood
G cells
  • secrete gastrin into interstitium
small intestine

  • 90% of digestion and absorption

  • wall contains villi (increase SA for greater digestion and absorption)

  • within each villus is a capillar network and lymph vessel called a lacteal

  • microvilli on each villus (increase SA further)

  • microvilli covering is called brush border

  • some epithelial cells are goblet cells that secrete mucus to lubricate intestine and protect brush border

  • most chemical digestion occurs in the duodenum (first part)

brush border
contains membrane bound digestive enzymes
the pancreas
  • duodenum has pH of 6 due to bicarbonate ion secreted by pancreas
major enzymes released by the pancreas
  • trypsin
  • chymotrypsin
  • pancreatic amylase
  • lipase
  • ribonuclease
  • deoxyribonuclease
Trypsin and chymotrypsin
degrade proteins into small polypeptides
pancreatic amylase
  • hydrolyze polysaccharides to disaccharides and trisaccharides
  • degrades nearly all carbohydrates from chyme to small glucose polymers
  • digest starch
  • brush border enzymes finish degrading polymers to monosaccharides
  • degrades fat, specifically triglycerides
  • produced in liver
  • stored in gall bladder
  • emulsifies fat , breaking it up into small particles without changing it chemically
  • increasing SA
large intestine
  • when you see large intestine on MCAT, think water reabsorption
  • mutualistic symbiosis between humans and bacteria in large intestine
  • main functions: water reabsorption and electrolyte absorption
  • contains E. coli
  • amylases digest sugars
  • lipases digest fats
  • proteases digest proteins
  • formation of glycogen
  • takes place in the liver when blood glucose levels decrease --> glucose returned to blood
  • muscle cells and especially liver cells store large amounts of glycogen
usually carries 3 fatty acid molecules (from adipose tissue) but can carry up to 30.

[!] fat is insoluble in water so requires a carrier (ex: lipoprotein or albumin)

for MCAT, associate fat with efficient long-term energy storage, lots of calories (energy) with little weight.
the liver
  • receive blood from intestines, stomach, spleen and pancreas
  • second blood supply received through hepatic artery
  • all blood received by liver collects in hepatic vein, leading to vena cava
functions of liver
  • blood storage - can expand to act as blood reservoir for body
  • blood filtration - phagocytize bacteria from intestines
  • carbohydrate metabolism - maintains normal blood glucose levels through gluconeogenesis, glycogenesis, and storage of glycogen
  • fat metabolism - synthesize bile from cholesterol and convert carbs/proteins into fat. oxidize fatty acids for energy and forms most lipoproteins
  • protein metabolism - liver deaminates amino acids, forms urea from ammonia in the blood, synthesize plasma proteins (fibronogen, prothrombin, albumin, globulins, nonessential AA)
  • detoxification - detoxified chemicals excreted by the liver as part of bile or polarized so they can be excreted by kidney
  • erythrocyte destruction - kupfer cells destroy irregular erythrocytes but most irregular erythrocytes destroyed by spleen
  • vitamin storage - stores vit A, D, and B12. also stores iron
need to know MCAT:
  • prothrombin and fibrinogen are important clotting factors
  • albumin is the major osmoregulatory protein in the blood
  • globulins are group of proteins that include antibodies
  • antibodies are not made in liver but made by plasma cells
[!] when the liver mobilizes fat or protein for energy, the blood acidity increases (b/c when liver mobilizes fat for energy, it produces acids)
function of kidney
  • excrete waste products (urea, uric acid, ammonia, phosphate)
  • maintain homeostasis of body fluid volume and solute composition
  • help control plasma pH

  • urine is created by kidney and emptied into renal pelvis
  • renal pelvis emptied by the ureter, which carries urine to the bladder.

the bladder is drained by the urethra
functional unit of kidney (process)
functional unit: nephron
  • blood flows into first capillary bed of nephron called glomerulus
  • bowman's capsule and glomerulus make up the renal corpuscle
  • hydrostatic pressure forces plasma through fenestrations into bowman's capsule
  • filtrate moves from bowman's capsule to proximal tubule (most reabsorption takes place here)
antiport system
hydrogen ions are secreted through an antiport system with sodium, driven by the sodium conc. gradient

similar to transport system of glucose with sodium except proton crosses membrane in opposite direction to sodium.

uric acid, bile pigments, antibiotics, other drugs are also secreted into proximal tubule.
proximal tubule (see process of kidney also)
net result is to reduce the amount of filtrate in the nephron while changing solute composition without changing osmolarity
loop of Henle
  • from proximal tubule, filtrate flows into loop of Henle (dips into medulla)
  • function: increase solute concentrate and osmotic pressure of medula
  • low permeability to salt so filtrate osmolarity goes up
  • as filtrate rises out of medulla, salt diffuses out of the ascending loop of Henle (passively then actively)
  • concentrates medulla by net loss of solute to medulla
distal tubule
  • reabsorbs Na+ and Ca2+ while secreting K+, H+, and HCO3-.
  • aldosterone acts on distal tubule cells to increase sodium and potassium membrane transport proteins
  • net effect: lower filtrate osmolarity
  • ADH acts on collecting tubule (not collecting duct) to increase permeability of cells to water
  • in presence of ADH, water flows from tubule concentrating filtrate

collecting duct
  • carries filtrate into highly osmotic medulla
  • impermeable to water
  • sensitive to aDH
  • in presence of ADH, collecting duct becomes permeable to water allowing it to passively diffuse into the medulla, concentrating the urine.
juxtaglomerular apparatus
  • monitors filtrate pressure in distal tubule
  • cells in here secrete enzyme renin --> initiate cascade producing angiotensin I, II, III, --> stimulates adrenal cortex to secrete aldosterone
acts on distal tubule to stimulate formation of membrane proteins that absorb sodium and secrete potassium.
need to know MCAT - kidney
  • filtration occurs in renal corpuscle
  • reabsorption and secretion mostly in proximal tubule
  • loop of Henle concentrates solute in the medulla
  • distal tubule empties into collecting duct
  • collecting duct concentrates urine
  • amount of filtrate is related to hydrostatic pressure of the glomerulus
  • descending loop of Henle is permeable to water
  • ascending loop of Henle is impermeable to water and actively transports sodium into kidney
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