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Lecture 5

BIOL 2P97 Lecture Notes - Lecture 5: Gastric Glands, External Anal Sphincter, Skeletal Muscle


Department
Biological Sciences
Course Code
BIOL 2P97
Professor
dr.
Lecture
5

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Gastrointestinal Physiology
Overview of Digestive system
-gastrointestinal tract= alimetary tract=GI tract
-GI tract: opens to the outside world (considered part of the external environment)
-a tube passing through the body
-closed o by a skeletical sphincter at each end
Challenges of the GI tract: Don’t digest yourself
-powerful enzymes to break down food and can also damage our own cells
(=autodigestion; breaking down cells)
-there are protective mechanisms against autodigestion
:Don’t get dehydrated Fig. 21.1
-match 'uid intake with output
-total GI secretion of the gut is 7L per day
-must be reabsorbed very rapidly to prevent dehydration and reabsorption is very
e*cient
-only 100mL of 'uid is lost in the feces
-conditions that lead to vomiting and diarrhea can quickly cause dehydration +
unable to maintain blood pressure
:Don’t bug me
-prevent foreign invaders (bacteria, fungi, viruses)
-large surface area of GI tract= about the size of a tennis court
-bringing nutrients, water, and electrolytes in, but keeping bugs out is di*cult
Defense Mechanisms
-mucus, digestive enzymes,
-gut-associated lymphoid tissue=GALT
-largest collection of the lymphoid tissue in the body
-80% of the body’s lymphocytes
Challenges of the Gut
-we need to absorb nutrients, yet not absorb invaders
-need to secrete digestive enzymes, yet ensure we don’t get dehydrated
-need the digestive enzymes, et we have to make sure they don’t digest us
Meeting these challenges Fig. 21.2
-by coordinating the 4 basic processes of the GI tract:
-Digestion: chemical and mechanical breakdown of foods into smaller units
that can be taken
across the intestinal epithelium into the body
-Absorption: the active or passive transfer of substances from the lumen of
the GI tract to the
extracellular 'uid

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-Motility: movement of material in the GI tract as a result of muscle
contraction
-Secretion: the transepithelial transfer of water and ions from the ECF to the
digestive tract
lumen + to the release of substances synthesized by GI epithelial cells
Autonomy of the Digestion system Fig. 21.3
-oral cavity: receptacle for food
-place for physical digestion (chewing)
-lubrication and moistening of food through saliva
-salivary glands
-submandibular: under the mandible
-sublingual: under the tongue
-parotid: near hinge of the jaw
-saliva: hypsomotic 'uid secreted in the oral cavity = 1L a day!
-consists of: water, ions, mucus, digestive enzymes, immune proteins
-esophagus: begins as skeletal muscle; transition to smooth muscle 2/3 down; ends
at stomach
-stomach
-fundus: the area that protrudes out a little bit
-body: main part
-antrum: as it starts to narrow
-pylorus: band of smooth muscle that controls release of broken down food
(chime)
-pyloric valve: thickened band of smooth muscle
-relaxes to allow small amounts of chime through at a time
-stomach acts as an intermediary between eating + physiologic events of
digestion/absorption
-ensures the intestine isn’t overwhelemed
-small intestine: most digestion happens here
-3 sections: duodenum, jejunum, ileum (DJ Illeum)
-contains intestinal enzymes
-accepts secretions from 2 glandular organs: pancreas + gallbladder
-these secretions enter the duodenum through small openings (ducts)
-a sphincter regulates the passage of the secretions: sphincter of Oddi
-digestion is essentially complete in small intestine
-all digested nutrients are absorbed
-secreted 'uids are absorbed
-1.5L chime per day passes on to colon
-colon (large intestine)
-water and electrolytes are absorbed; semisolid feces is produced
-rectum: when feces is in the rectum it triggers the defecation response
-anus: external anal sphincter – under voluntary control
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