Introduction to Gastrointestinal Physiology
What is the GI system?
Long tube open at both ends (lips and anus), the gut
Associated glands
How is it organized?
Some of what we ingest gets into our bloodstream
Some comes out the other end
GI system converts what we ingest into usable forms. It’s a batch
processor. Batch processors hold contents in a chamber, do
something to it, move it to another chamber, do something else to
it, etc.
What does the GI system do?
Breaks ingested chunks into small pieces.
Converts large molecules to small molecules (digestion, by
hydrolysis) that can be absorbed.
Secretes fluids into lumen of gut.
Absorbs nutrients from lumen of gut into circulation.
Mixes and propels contents.
What will we deal with in the GI section?
How it propels and mixes contents (motility)
What gets secreted into the lumen
What gets digested (hydrolyzed) in the lumen
What gets absorbed from the lumen
Maybe most important, how are those things coordinated in time?
Gut is layered through it’s entire length
Skeletal muscle to about one-third of the way down the length of
the esophagus, again at the external anal sphincter. Everything
else is smooth muscle
Splanchnic Circulation
All venous blood from stomach, intestines, and pancreas go to liver
before entering the general circulation.
Therefore, liver gets exposed to higher levels of anything in venous
effluent from those organs than anyplace else does.
Guess what? Liver regulates plasma concentrations of stuff
absorbed from stomach and intestines, largely under the influence
of hormones secreted from the pancreas
Guess what else? Liver is an important site of detoxification of
ingested compounds that are absorbed in the small intestine
Basic GI Functions
Motility (mixing and propulsion of gut contents)
Secretion (of various fluids into gut lumen)
Digestion (hydrolysis of macromolecules into absorbable units)
Absorption (of fluids and of nutrients)
Motility
The important functions of muscle activity in the gut (“motility”)
are mixing secretions with contents and exposing all contents to
mucosa for absorption, and propulsion of contents.
When longitudinal muscle contracts, gut segment shortens. When
circular muscle contracts, gut diameter decreases.
There’s one muscle layer that is only in the small intestine, the
muscularis mucosae. We’ll deal with it when we get to that.
Most propulsion of gut content is done by circular muscle.
Peristalsis
Peristalsis is a ring of contraction of circular muscle that travels
(not extends) analward in the gut. Although not shown in this
cartoon, some gut contents are propelled in each direction.
Note that gut diameter ahead of peristaltic ring is larger than it is
behind. Therefore, flow resistance is lower in analward direction
(resistance varies as fourth power of diameter!). Therefore,
there is propulsion as well as mixing by peristalsis.
Secretion
Sources:
Glands in wall of gut that secrete into gut lumen via ducts
Secretory cells in gut mucosa
External glands that secrete into the gut lumen via ducts
Two major kinds of secretions are serous (watery) and mucous
(viscous, containing mucus) fluids. Mucous secretions are
lubricants. One source is goblet cells in the mucosa of every part of
the GI tract.
Serous fluids serve a number of functions:
Some contain digestive enzymes
Some contain strong acid
Some are alkaline, neutralizing the acid
Some are essentially salt and water, contributing to the fluidity
Digestion
Most of what we eat is in the form of macromolecules (proteins,
polysaccharides, nucleic acids, fat) that are too big to absorb. They
are hydrolyzed to amino acids, monosaccharides (simple sugars),
nucleotides, fatty acids and glycerol, respectively, mostly through
hydrolysis by digestive enzymes.
Absorption
Nutrient, electrolyte and water reabsorption involve a number of
specialized transport systems in the mucosa.
Control of GI Functions
Efficiency requires activities be coordinated. Why expend energy
by using muscles in or secreting fluids into an empty gut segment?
How are activities controlled?
1. Local stimulation by gut contents. Local mechanical stimulation
by gut contents is how goblet cells are stimulated to secrete
mucous, for example.
2. “Short reflex” neural mechanisms involving receptors on
mucosal surface that synapse with one or both plexuses, which
transmit information along length of the gut and elicit
responses. Plexuses in gut are sometimes called the “little
brain” because they are so independent of the CNS.
3. “Long reflex” mechanisms involving the autonomic nervous
system. Sympathetic stimuli (splanchnic nerve) are usually
inhibitory, parasympathetic stimuli (vagus nerve) usually
excitatory.
4. Humoral control via polypeptide gut hormones. Neural
mechanisms act nearly instantaneously and are local. Humoral
mechanism have gradual onset and end, act globally.
The Oral Cavity - Motility
Chewing – Why bother?
1. The typical bite of food is too big to swallow safely because
esophagus is behind trachea; tracheal cartilage is in front
and sides only. So, reducing particle size is essential
2. The nutritious part of some vegetable material is enclosed
in a cellulose hull and can’t be digested unless that hull is
fractured.
3. Chewing brings food into contact with entire surface of the
oral cavity, which initiates a number of GI reflexes as well as
stimulating salivary secretion.
The Chewing Reflex
Chewing is more formally called mastication. Feel free to titter.
“Chewing reflex” is really two consecutive reflexes:
1. Pressure of bolus of food on palate stimulates pressure
receptors on palate. Pressure receptor stimulation reflexively
relaxes jaw muscles; lower jaw drops. That’s the first reflex arc.
2. Drop of lower jaw stretches jaw muscles, stimulating stretch
receptors. Stretch receptors reflexively cause jaw muscles to
contract. This is the second reflex arc.
Contraction of jaw muscles raises jaw. If there is still food in the
mouth, it re-establishes pressure on the palate, starting the
sequence again.
Salivary Secretion
Three pairs of salivary glands: parotid, submandibular, sublingual
Resting rate of secretion of around 0.5 ml/min keeps mouth lining
moist. Important for speech, and opposes sensation of thirst.
Saliva is alkaline, important to neutralize acid produced by bacteria
around base of teeth, which can erode tooth enamel.
Stimuli to salivary secretion:
Local mechanical pressure (why do you suppose tobacco chewers
generate so much saliva?)
Low pH in oral cavity (acidity in mouth promotes secretion of
alkaline solution into it)
Stimuli to salivary secretion (cont’d)
Neural stimuli – cholinergic and catecholaminergic transmitters
both stimulate.
Cholinergic is more important because parasympathetic
innervation of salivary glands is much more extensive than
sympathetic.
Parasympathetic stimulation results from any sensation
associated with eating (Pavlov’s discovery of conditioned
reflexes is a good example)
Saliva
Resting level of secretion = 0.5 ml/min; up to 5 ml/min. Average =
1 to 2 liters/day, nearly all of which enters gut.
Kinds of saliva
1. Serous (watery) fluid, maintains normal oral pH and
contributes to fluidity of ingested food.
2. Mucous fluid, lubricates.
3. Digestive, one enzyme that
breaks down starch to smaller pieces, including simple
sugars. Doesn’t do much oral cavity – not enough time.
Sublingual and submandibular glands both produce serous and
mucous fluids. Parotid gland produces salivary amylase (also
called ptyalin).
Swallowing
Formally = deglutition. Not nearly as titter-worthy as mastication.
Includes two major phases, voluntary and involuntary (Sherwood
uses oropharyngeal and esophageal, respectively)
Voluntary phase consists of front of tongue pressing hard palate,
which rolls the bolus of food toward the pharynx. Everything
beyond this is involuntary. That’s why you can give a dog a pill by
throwing it into the back of his mouth
Involuntary phase of swallowing:
Muscles in back of throat force food into pharynx.
Larynx elevates and entrance is blocked by epiglottis.
Vocal cords narrow to a slit, limiting the size of the opening to
anything that got past the epiglottis.
Respiration is inhibited.
Pharyngeal muscles contract, forcing food toward esophagus.
Note that respiration is inhibited when entrance to trachea is
blocked. Overriding this inhibition can allow food into trachea.
Esophagus
Sphincters at pharyngeal and gastric ends. Upper, normally closed,
prevents swallowing air during breathing. Kids learn how to force
air past it, and can burp at will. Lower is normally constricted and
prevents reflux of gastric contents.
When pharyngeal muscles force food toward esophagus, upper
sphincter relaxes and a peristaltic wave going to stomach begins.
Food still in the esophagus? Distension initiates secondary, tertiary
and quaternary waves until esophagus is empty. It normally covers
the length of the esophagus in about 10 seconds
Esophagus (cont’d)
Skeletal muscle can’t transmit action potentials, so myenteric
plexus must control rate and direction of peristalsis from pharynx
through upper third of esophagus.
In fact, it controls rate and direction of peristalsis over the entire
length of esophagus. Rate of conduction of action potentials in
smooth muscle is much too slow to account for rate of peristaltic
travel in esophagus.
Heartburn
Heartburn is caused by gastric reflux. Gastroesophageal sphincter
is normally tightly closed. That’s why you can dive head first
without gastric contents getting into your mouth.
Increasing abdominal pressure by contracting abdominal muscles
doesn’t force gastric content into esophagus. Why? Because
gastroesophageal sphincter is below diaphragm, in abdominal
cavity, so increased pressure tends to force it closed.
Hiatal hernia is a condition in which opening in diaphragm is large
enough to let upper end of stomach (and gastroesophageal
sphincter) poke up into thorax.
When abdominal pressure rises, it can force gastric content into
esophagus. That’s heartburn.
Esophageal Secretion
The only secretion in the esophagus is mucous, secreted by goblet
cells.
Esophageal Digestion and Absorption
There is no digestion except a trivial amount of starch hydrolysis
catalyzed by salivary amylase. There is no absorption in the
esophagus.