Digestive System
MCAT Digestive System
Intracellular Digestion: Oxidation of glucose and fatty acids to make energy
Extracellular Digestion: Process by which nutrients are obtained from food. Occurs in alimentary canal.
Mechanical Digestion: The enzymatic cleavage of chemical bonds of proteins or the glycosidic bonds of starches
Peristalsis: Rhythmic contractions of gut tube (↑Parasympathetic NS and ↓ Sympathetic NS)
Digestive Pathway
Oral Cavity → Pharynx → Esophagus → Stomach → Small Intestine → Large Intestine → Rectum
Oral Cavity
o Mastication starts mechanical digestion.
o Salivary Amylase: hydrolyzes small sugars
o Lipase: hydrolyzes fats
o Chewing = ↑ Surface area if food
o Food forms bolus and swallowed
Pharynx
Connects mouth to esophagus. Epiglottis prevents food from entering the larynx.
Esophagus
o Propels food to stomach
o The top third is made of skeletal muscle, bottom third is composed of smooth muscle.
o Top under somatic control and bottom under autonomic (involuntary) control to allow for peristalsis.
o Hower certain factors can causes emesis
o Top has upper esophageal sphincter and bottom has esophageal sphincter (cardiac) that relaxes to allow food in stomach
o No mechanical/chemical digestion takes place here besides the enzymes from the mouth
Stomach
o Highly muscular
o Uses HCL and enzymes for digestion
o HCL is harsh so stomach has thick mucus lining to prevent harm
o Fundus: contain gastric glands; respond to signals from vagus nerve of PNS
o activates to sight, smell, taste of food
o Gastric Glands (3 Types)
§ Mucous Cells: make bicarbonate rich mucus to protect stomach
§ Chief Cells: secrete pepsinogen
§ Parietal Cells: secrete intrinsic factor and HCL; HCL cleave pepsinogen to pepsin
· Pepsin: cleaves peptide bonds near aromatic amino acids
· Intrinsic Factor: glycoprotein involved in absorption of Vitamin B12
o Pyloric glands contain G-cells that secrete gastrin (a peptide hormone that induces parietal cells to secrete more HCL and signals stomach to start mixing
o Body: contain gastric glands
o Antrum: contains pyloric glands
o Pylorus: contains pyloric glands
o Rugae: folds of stomach
o Digestion results in Chyme production, an acidic semifluid mixture
Small Intestine (Deodenum)
Duodenum
· Responsible for majority of chemical digestion and minor involvement in absorption
· Food leaves stomach from pyloric sphincter
· Chyme triggers release of brush border enzymes (break down dimers/trimers to monomers) located in small intestine lumen and secretes hormones secretin and CCK
o Disaccharidase
o Bacteria produce methane gas after hydrolysis of disaccharides
o Undigested disaccharides has osmotic affect; pulling water into the stool (diarrhea)
o Maltase
o Sucrase
o Peptidases
o Aminopeptidases → removes N terminal for AAs
o Dipeptidases
o Enteropeptidases → enzyme that activates trypsinogen (pancreatic protease) to trypsin
o Trypsin → initiates activation cascade
o Activates Procarboxypeptidase A and B to active forms
Hormones
Secretin: peptide hormone that causes pancreatic enzymes to be released into duodenum; regulates pH of digestive tract by reducing HCL secretion and increasing bicarbonate secretion from pancreas; also acts as an enterogastrone (slows gut motility)
CCK: released in response to chyme entry; stimulates bile/pancreatic juice release
Bile: made of bile salts, pigments, and cholesterol
o Derived from cholesterol
o Amphipathic
o Have hydrophobic and hydrophilic regions to emulsify fats and cholesterol into micelles, bridge between aqueous and lipid environments; if not, fats would separate out of aqueous solution
Pancreatic juices: mixture of several enzymes in bicarbonate rich alkaline solution; alkaline environment for digestive enzymes
o Contain enzymes that can digest carbs, fats and proteins
Accessory Organs
Pancreas
o Endocrine functions include release of insulin, glucagon, and somatostatin
o Bulk of pancreas are exocrine cells called acinar cells that produce pancreatic juices
o Pancreatic Amylase
o Pancreatic peptidases (trypsinogen, chymotrypsinogen and carboxypeptidase A and B that are activated by endopeptidase)
o Pancreatic Lipase: break fats into free FAs and glycerol
o Pancreatic juices transferred to duodenum via duct system and acinar cells secrete into these ducts call major and minor duodenal papillae
Liver
o Bile produced in liver and stored in gallbladder or secreted in duodenum
o Bile ducts used to secrete into duodenum
o Liver receives blood from the abdominal portion of digestive track through hepatic portal vein
o Takes up excess sugar to store as glycogen
o Stores fats are triaglycerols
o Liver can reverse this process via glycogenolysis and gluconeogenesis and mobilize fats to lipoproteins
o Detoxifies endogenous and exogenous toxins/compounds
o Converts ammonia to urea for excretion
o Detoxifies and metabolizes alcohol and medication
o Pigment in bile is bilirubin → a byproduct of breakdown of hemoglobin
o Bilirubin travels to liver (where it is conjugated to a protein) and secreted into bile for excretion
o If liver cant process or excrete bilirubin, jaundice can occur
o Also synthesizes albumin protein → maintains oncotic pressure and serves as carrier for many drugs/hormones and clotting factors
Gallbladder
o Stores and concentrates bile
o When CCK released, gallbladder contracts and pushes bile out of biliary tree
o Then merges with pancreatic duct and empty into duodenum
o Common site of cholesterol and bilirubin stone formation; very painful and can cause blockage of biliary tree and pancreatic duct → pancreatitis
Small Intestine (Jejunum and Ileum)
o Involved in mainly absorption of nutrients
o Contain villi and microvilli → ↑ SA
o Middle of each villus is a capillary bed for absorption of water-soluble nutrients and a lacteal (a lymphatic channel that takes up fats for transport into lymphatic system)
Sugar/AA Absorption
o Absorbs simple sugars such as glucose, fructose, and galactose and AAs via secondary active transport and facilitated diffusion in epithelial cells of small intestine
o Then move across epithelial cell membrane and diffuse into intestinal capillaries via concentration gradient
§ Blood constantly passing by epithelial cells, carrying carbs and AAs away
§ Generates concentration gradient (blood always has lower concentration of AAs and sugars than inside epithelial cells
§ Absorbed molecules go to liver via hepatic portal vein
Fats Absorption
o Short chain FA follow same process by direct diffusion into capillaries
o Don’t require transporter b/c they are nonpolar and can cross cell membrane
o Larger fats move separately into intestinal cells but reform into triglycerides
o Triglycerides and esterified cholesterol repackaged into chylomicrons and enter lymphatic circulation through lacteals
o Lacteals converge and enter venous circulation through thoracic duct
Vitamins
Absorbed in small intestine
o Fat soluble vitamins (A, D,E,and K)
o Dissolve directly into chylomicrons and enter lymphatic circulation
Water Soluble Vitamins
o Taken up along with water, AAs and carbs across endothelial cells of small intestine, passing directly into plasma
Water absorption
o Takes up most of water
o Can be transcellularly (across cell membrane)
o Or paracellularly (squeezing between cells)
Large Intestine
· Primarily involved in water absorption
· Divided into three major sections: cecum, colon, and rectum
Cecum
· Out pocketing that accepts fluid exiting small intestine through ileocecal valve, site of attachment for appendix
Colon (ascending, descending, and sigmoid colon)
· Concentrates feces by absorbing water and salts (small intestine absorbs most water)
Rectum
· Site of storage of feces
Anus
· Is opening through which waste are eliminated
· Has internal (involuntary/autonomic) and external sphincters (voluntary/somatic)