MCAT Biology cont.

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  1. Layers of the Digestive Tract
    • Entire digestive tract (esophagus to rectum) has basic structure:
    • -Mucosa: composed of moist epithelial surface (cell type depends on organ) over connective tissue (lamina propria) and muscular layer (muscularis mucosa)
    • -Sub mucosa: connective tissue layer containing blood vessels; some may contain exocrine glands; nerves and ganglia of Meissner's/submucosal plexus (parasympathetic)
    • -Muscular layer: inner and outer circular smooth muscle layers that allow for propulsion of food
    • --Auerbach's/myenteric plexus: parasympathetic and sympathetic; between two muscle layers
    • -Adventitia/serosa: connective tissue layer containing lymphatics, blood vessels, nervesw
    • --Some regions contain an outer peritoneal covering
  2. The Stomach
    • Peristalic action of esophagus propels bolus into stomach through lower esophageal sphincter (prevents refluxing)
    • Stomach is a highly vascular organ with may glands to secrete enzymes that aid in digestion
    • -Smooth muscle contraction churns and mixes the food, propels it into small intestine through pyloric sphincter
    • Cardiac and pyloric glands secrete mucus (protects stomach lining from caustic acids)
    • Glands of the fundus:
    • -Parietal cells: secrete hydrochloric acid (keeps pH low; necessary to convert pepsinogen -> pepsin)
    • --Parietal cells stimulated by gastrin
    • -Chief cells: secrete pepsinogen and intrinsic factor (necessary for B12 absorption)
  3. The Small Intestine
    • Deuodenum, jejunum, ileum
    • -Fats emulsified by bile (produced by liver) and pancreatic lipase
    • -Proteins digested by trypsin, chymotripsin, carboxypeptidase (secreted by pancreas, activated in small intestines)
    • --Intestinal aminopeptidases also break proteins into amino acids
    • -Intestinal and pancreatic amylase as well as disaccharidases, maltase, sucrase, and lactase turn complex sugars into glucose
    • The mucosal surface of the small intestine covered in villi and microvilli to increase surface area for absorption
    • -Rich capillary network beneath mucosa absorbs nutrients
  4. The Pancreas
    • Located in the abdomen, behind stomach
    • Endocrine and exocrine functions
    • Endocrine: cells of Isle of Langerhans produce:
    • -Insulin (beta cell): released in response to high-blood sugar, allows glucose to enter the cells, promotes lipogenesis in adipose tissue
    • --Type I diabetes: due to destruction of beta cells; patients become thin because cannot utilize glucose
    • -Glucagon (alpha cells): released in response to high insulin, low-blood sugar, promotes glyocogenolysis in liver; increases utilization of glucose
    • -Somatostatin (delta cells): inhibits glucagon release, slow pancreas's exocrine fuction
  5. The Exocrine Pancreas
    • Ducts arranged acini (clusters)
    • Secretions flow into lumen of the acinus -> intralobar ducts -> pancreatic duct -> duodenum
    • The acinar cell's basal zone is rich in rough ER and the apical part contains zymogens (precursors to active enzymes)
    • Acinar cells secrete zymogens, digest proteins into peptides/amino acids
    • -Converted to their active form in small intestine by enteropeptidase; trypsinogen -> trypsin (helps to cleave the other zymogens)
    • --Chymotripsinogen -> chymotripsen
    • --Trypsinogen -> trypsin
    • --Procarboxypeptidase -> carboxypeptidase
    • Amylase: converts starches into maltose
    • Lipase: converts fats into fatty acids and glycerol
  6. The Liver
    • In upper right quadrant of abdomen, protected by rib cage
    • Essential for synthesis/storage of glycogen, maintenance of blood glucose via gluconeogenesis (production of glucose from amino acid, lactic acid or glycerol)
    • Liver synthesizes: nonessential amino acids, fatty acids, bile, vitamins, urea (from detoxification of ammonia in blood), albumin (most important intravascular protein to maintain osmotic pressure) and coagulation proteins
    • Bile produced by hepatocytes and stored in the gallbladder
    • -Cholesterol is an essential precursor to bile salts and acids
    • -Ileum reabsorbs 95 percent of bile salts so they can by reused (enteroheptic circulation)
  7. The Large Intestine
    • Consists of cecum, ascending, transverse, and descending colon and rectum
    • Serves in water and electrolyte reabsorption to prevent dehydration
    • -If stool moves through the colon too quickly, diarrhea results
    • Contents are compacted and propelled to the rectum where stool is stored until defecation
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  8. Fat Soluble Vitamins
    • Remember ADEK
    • Vitamin A (retinol): supports eye function, essential for night vision, immune function, skin health, antioxidant activity
    • -Found in animal livers (very high source), fruits and vegetables (carrots especially)
    • Vitamin D (1,25-dihydroxycholecalciferol): needed for absorption of calcium and bone health
    • -Cholesterol derivative + UV light produce a vitamin D precursor that is sent to the liver where 25-hydroxycholecalciferol is made
    • -In the kidney, 1-alpha-hydroxylase converts it to 1,25-dihydroxycholecalciferol (active vitamin D)
    • Vitamin E: essential antioxidant, protects against anemia by preventing red cell attack by free-radials, important for CNS functioning
    • Vitamin K: essential for proper clotting, vitamin K-dependent factors (II, VII, IX, X, protein C and S), needed for gamma-carboxylation of prothrombin so that calcium can bind to it
    • -Synthesized by bacteria in the gut
    • Pancreatic function and good bowel health are needed for absorption of these essential fat-soluble vitamins
  9. The Urinary System
    • Kidneys:
    • -Bean-shaped, retroperitoneal organ
    • -Filters blood, reclaims water, sugars, electrolytes, and amino acids
    • -Maintains pH of blood, modulates bicarbonate and hydrogen ion loss and reabsorption
    • -Produces 1,25-dihydroxycholecalciferol (active vitamin D)
    • -Modulates calcium and phosphate retention in response to bones' needs
    • Ureters:
    • -Long muscular tubes that connect renal pelvis to urinary bladder
    • -Urethral sphincter at junction of bladder prevents urinary reflux
    • Bladder:
    • -Stores urine, detrusor muscles (smooth muscle) contracts during urination
    • -Lined by transitional epithelium
    • Urethra: transmits urine outside body
  10. Kidney Structure
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  11. Nephron Structure
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  12. Urine Production
    • Blood flows through the glomerulus and is filtered to form an ultrafiltrate
    • -Water and electrolytes selectively reabsorbed:
    • --Over 95 percent of amino acids and glucose reabsorbed
    • --85 percent of sodium, chloride, and bicarbonate
    • --Varied amounts of calcium, magnesium, and phosphate
    • Secretion of waste (uric acid, creatinine, drug metabolites) occurs in the proximal tubule
    • Passive diffusion occurs throughout the loop of Henle due to the countercurrent multiplier system in the kidney:
    • -Water permeability varies at different parts of the nephron, thus passive diffusion is utilized to ultimately produce hypertonic urine
    • --External osmolarity gradient is essential part of this process
  13. Nephron Function
    • Proximal convoluted tubule (isotonic to plasma):
    • -Active reabsorption of sugars, amino acids, bicarbonate, electrolytes
    • -Passive water reabsorption; secretion of uric acid
    • Descending loop of Henle (hypertonic):
    • -Passive diffusion of water, sodium, chloride in to medulla
    • -Water stays in medulla, electrolytes diffuse back out
    • Ascending loop of Henle (hypotonic):
    • -Impermeable to water
    • -Chloride actively pumped into interstitium, sodium follows
    • Distal convoluted tubule (very low pH):
    • -Reabsorption of sodium, secretion of hydrogen (causes low pH) and potassium (regulated by aldosterone)
    • Collecting ducts:
    • -Permeable to water; ADH causes reabsorption of water
  14. Renin-Angiontensin-Aldosterone System
    • Increases salt and water retention, increases BP and kidney perfusion
    • When perfusion of juxtaglomerular apparatus in kidneys is low, juxtaglomerular cells secrete renin
    • -Renin causes angiotensin -> angiotensin I, which is cleaved in lung capillaries by angiotensin-converting enzyme (ACE) -> angiotensin II
    • Angiotensin II cauces:
    • -Arterial vasoconstriction, increased BP
    • -Constricts efferent arteriole in kidney, increasing GFR
    • -Causes release of aldosterone (mineralocorticoid) from adrenals
    • -ADH release
    • Aldosterone causes:
    • -Reabsorprion of sodium (and water) in exchange for potassium at distal convoluted tubules and collecting ducts
    • -Increases thirst and appetite for salt
  15. Evolution
    • Microevolution: occurs on a small scale within a population, due to gradual changes
    • -Natural selection: environment favors survival of most adaptable phenotype, causing unfavorable characteristics to become less prevalent due to decreased survival
    • --Proposed by Darwin and Wallace
    • -Bottleneck effect: reduction of population into one which is smaller and doesn't contain all alleles originally present
    • -Mechanisms include mutations, gene flow between populations, and recombination of existing genes
    • Macroevolution: theory involving large changes and formation of new species (speciation)
    • -May be due to smaller evolutionary mechanisms occurring over long time periods
Card Set:
MCAT Biology cont.
2013-12-31 15:02:30
Biology MCAT
Barron's MCAT flashcards
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