Physiology 3120 Lecture 57: Phys 3120 - Lecture 57
2 May 2018
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Human Physiology Lecture 57
Gastrointestinal Physiology
Four processes of the GI tract
1. Digestion
- Chemical and mechanical
o Mechanical = Absolute crushing of food into smaller fragments
▪ Mouth and stomach
▪ Even if you are a bad chewer, the stomach will pulverize the food (crush it into
smaller pieces)
o Chemical = Take particles of food that we are eating (usually in big forms – complex
carbohydrates, intact proteins) and break them down into individual monosaccharaides
(carbs) amino acids, smaller fragments of fat
▪ Via enzymes from the digestive tract – take the products and chemically digest
them
2. Absorption
- Happens the best in the small intestine
o Majority of nutrient absorption occurs
o Important region for absorption
o Maximized in its structure and the type of cells that it contains so that we can absorb the
majority of the nutrients that we eat
- Little absorption in the large intestine
- Little absorption in the mouth
- Little absorption in the stomach (minimal)
- So minimal in the mouth and stomach, we often ignore it
3. Motility
- Essential – do not want food staying in one segment of the GI tract
- Move from mouth, down the esophagus
o Food stays in the stomach for the right amount of time – stomach knows how long food
should stay there. Stomach motility depending on the:
▪ 1. Composition of the food
▪ 2. Volume of the food
▪ 3. What is happening in the rest of the tract (whether or not there are things
in the small intestine)
- Move out of the stomach, to the first part of the small intestine where other organs make
secretions into the small intestine
o Other organs important for digestive tract:
▪ Liver: makes bile (stored in gallbladder for when it wants to be secreted)
▪ Exocrine pancreas
▪ DO NOT make contact with the food but make important secretions
- Need to make room for the next meal, therefore want to ensure motility is rapid but not too rapid
o Motility is carefully controlled by neurons and hormones
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4. Secretion
- Exocrine + endocrine secretions
- Exocrine secretion: goes into the digestive tract
o Water, mucous, ions, enzymes
- Endocrine secretion: goes into the blood
o Various hormones triggered for release into the blood due to what is going on in the
digestive tract
Organs of the GI Tract
- Organs of the GI tract = organs that physical come into contact with food
o Mouth
o Pharynx
▪ Connects the esophagus to the trachea, common passage that closes off when
you are eating food so only food goes down the esophagus
o Stomach
▪ Holding food for the right period of time
o Small intestine
▪ BIG DEAL OF DIGESTIVE TRACT (chemical digestion + nutrient absorption)
o Large intestine/colon
▪ Bacteria that live in there regulate health in general + digestive health
- Accessory glands = structures and organs that connect to the digestive tract
o Make important secretions into the digestive tract that help us with nutrient digestion
- Along the tract we h ave sphincters that will permit or close off entry
o Upper esophageal sphincter: top part of the esophagus
▪ Needs to open when you are eating food
▪ Closed for the rest of the time, so air does not get into our digestive tract
o Lower esophageal sphincter: boundary for the stomach
▪ Contents of the stomach stay in the stomach (acid + food)
▪ Only triggered to open when food is coming down the esophagus
▪ Stays shut for most of the time
▪ IF it opens inappropriately, you get acid reflux (lower esophageal sphincter is
opening when it should not be opening)
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o Pyloric sphincter
▪ Usually closed in the stomach
▪ As soon as food enters, the sphincter is triggered to open to allow for small
particles of pulverized, liquidized food to pass into the small intestine
▪ Control the rate of emptying out of the stomach
▪ Ex. eat a high fat meal, do not want it to exit the stomach too quickly, therefore
the pyloric sphincter will stay shut more frequently
o Ileocecal valve: separates the small intestine from the large intestine
▪ Only opens when you want to empty the contents of the small intestine into the
large intestine
▪ DO NOT want large intestinal contents to go back into the small intestine
because the large intestine is full of bacteria (most are good for you). Small
intestine is not designed to have a large volume of bacteria inside of it can
cause sickness if the small intestine becomes TOO inhabited with bacteria
• Some bacteria in the small intestine but not a lot
o Anal sphincter (internal and external)
▪ As adults we know how to control it
▪ Young child does not develop ability to control these until 3 years old (potty
training is coincident)
• DO NOT want to potty train our children too early because they might
not be physiologically ready
General Layers of the GI Tract
- Digestive tube: tube from the mouth to the anus
o Main components of the tube are about the same
- Lower part of the esophagus, stomach, small intestine, majority of the large intestine Look the
same!
o Lumen can be narrow – small intestine
o Lumen ca n be large – stomach
- Top part of your esophagus is structurally different
o Striated epithelium
o Skeletal muscle
- Last part of the large intestine has skeletal muscle too
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find more resources at oneclass.com
