BIOL 240M Chapter Notes - Chapter 41: Portal Vein, Bile Acid, Gastrointestinal Tract

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6 Feb 2017
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III. Organs specialized for sequential stages of food processing form the
mammalian digestive system
a. Food is pushed by peristalsis (alternating waves of contraction and
relaxation in the smooth muscles lining the canal) and separated
between compartments by ring like valves or sphincters
b. The Oral Cavity, Pharynx, and Esophagus
i. Oral Cavity - teeth of various shapes cut, mash, and grind food
to increase surface area and make small enough to swallow
1. Salivary glands deliver saliva through ducts in the oral
cavity to initiate chemical digestion
2. The enzyme amylase hydrolyzes starch and glycogen
3. Mucus - viscous mixture of water, salts, cells, and
slippery glycoproteins called mucins (protective effect
of saliva comes from mucus)
a. Lubricates food and protects lining of mouth,
acts as pH buffer and antimicrobial agents
4. Bolus - a ball is formed of saliva and food in order to
swallow
ii. Pharynx - throat region that opens into trachea and esophagus
1. Esophagus - connects to the stomach
c. Digestion in the Stomach
i. Stomach - stores food and begins digestion of proteins
1. Excretes gastric juice to mix with food and form chyme
ii. Chemical Digestion in the Stomach
1. HCl exposes proteins through denaturation and allows
proteases (protein-digesting enzymes) to attack
peptide bonds; protease is called pepsin
2. Gastric glands produce the juice
a. Parietal Cells use ATP driven pump to expel
hydrogen ions into lumen; Cl- enter lumen
through specific membrane channels
b. Chief Cells release pepsin into lumen in inactive
form called pepsinogen
c. HCl and pepsinogen combine to form pepsin
iii. Stomach Dynamics
1. Stomach churns through contractions and relaxations
every 20 seconds
2. Cardiac sphincter sometimes opens causing acid reflux
3. Peristaltic contractions empty contents of stomach into
small intestine within 2-6 hours of ingestion
d. Digestion in the Small Intestine
i. Small Intestine - the alimentary canal's longest compartment
(6m in humans)
1. First 25cm is duodenum - chyme mixes with digestive
juices of pancreas, liver, and gallbladder to neutralize
pH
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