Lecture 15 Digestive Tract JAN. 26
- tube delivering + absorbing food
- things that are not absorbed will be removed from body
Visceral musculature = Muscle surrounding the gut that does the motion of transporting.
This muscle produces wave-like contractions from mouth to anus, pushing food down
the tube (=Peristalsis)
Reverse peristalsis = causes vomiting
No gravity is involved in digestion.
GI tract = produces mechanical digestion + chemical digestion where small bits of food
are further broken down at the molecular level to be absorbed through wall of gut.
Absorption = getting the stuff across through wall of gut + enter the circulatory
cardiovascular stream. Until this happens, the food is not part of us. The inside of GI
tract is just a tube and it goes right through us. So inside of it is not actually technically
part of our body. For that reason you have to get absorption. Need to have nutrients.
Gut can be divided into various parts.
- Higher vertebrates – has segmented along the gut, uniform across vertebrates
- Lower vertebrates – merging of pattern
1) Human: foregut + midgut + hindgut. This system doesn’t work well for majority of
2) Shark: No distinction b/w small + large intestines, there is a single valvular intestine.
3) In some fish (e.g. lungfish): there is no stomach
4) In cyclostomes: Everything behind post-pharyngeal region is a simple tube.
There is a great deal of variation. The point is that condition is not comparable to
humans or mammals in general. So there happens to be a landmark that is almost
always present in vertebrates w/ very few exceptions, that is the constriction at the
beginning of intestine called pylorus.
Pylorus = use that as a landmark to subdivide digestive tract into foregut & hindgut. That
is a much easier way to look at the system in further developments.
In foregut we have: pharynx, esophagus, stomach
- Pharynx: area of throat where gill slits are in the shark as well as in embryos
- Distinction b/w esophagus & stomach is gross morphology: Consider esophagus to
be a fairly thin pipe leading to a stomach (sac like structure). But the boundary b/w
2 is not always distinct. (e.g. in shark, you cannot see distinction until you cut +
know difference internally. Stomach has rugae (longitudinal ridges) + esophagus
(papillae). Therefore has a clear distinction internally but not externally.
Hindgut: follows pylorus that has valve called pyloric sphincter which can be opened &
closed & therefore can control the passage of material from stomach (from foregut) to
the hindgut. In hindgut we can usually but not always recognize various specializations.
1) Small intestine: duodenum, jejunum, ileum (no sharp distinction either
morphologically or internally, it’s a tube of similar diameter throughout)
Duodenum: first loop of small intestine + loops towards the left & has part of
pancreas adhering to surface + makes another turn & call it jejunum.
2) Large intestine
- In b/w small & large intestines or in vicinity b/w them, we often get out-pocketing
that is a blind-ended pouch = Cecum
- Cecum (plural ceca) – a small out-pocketing at the proximal end of the colon (e.g.
b/w small & large intestine)
Functions of foregut:
- Esophagus: we defined it as a thin pipe but sometimes it is not, and actually a tube
where food is being ingested to where food will be treated.
In vertebrates, there is ancestrally primitively very little digestion associated with
the whole foregut but particularly with esophagus.
- Stomach: develops as a large sac to store food. Being a part of foregut there is very
little chemical digestion. In mammalian condition, we have a sharp distinction b/w
esophagus and stomach. Esophagus being thin pipe and stomach as sac-like
structure. In fish, not distinct. Fish have gill + pharyngeal region those are
important part of foregut. Pharynx is very anterior end of GI tract (therefore part of
foregut) but once we lose, pharynx becomes narrower & esophagus becomes a
well developed pipe. Birds have a crop (a sac like ventral extension of esophagus,
therefore in throat area; used as food storage).
Stomach becomes important when you need a place to dump food into. In
cyclostomes having ancestral or primitive filter-feeding mechanism, the foods
drivels back into GI tract at a slow rate. Let the food stay in stomach until it can be
fed to intestine at a reasonable rate. That is what pyloric sphincter does
controlling passage of material in stomach (storage area) & feed intestine at a
- We have almost no chemical digestion in foregut but there are exceptions. At the
very front end, we do have glands that produce saliva. All tetrapods have salivary
glands that are important in moistening food to make stomach easier. But once we
have saliva production, enzymes can be added to it that accomplish some digestion.
Specifically salivary glands produce secretions that can initiate break down of
starch. If you chew starch, eventually taste sweet since enz is breaking down starch
into sugar maltose.
- In stomach, have chemical digestion as well. Initiate digestion of proteins in
stomach then it continues in small intestine.
- In hindgut, there is considerable amount of chemical digestion
In fact most chemical digestion & absorption occur in small intestine.
Large intestine functions mainly in reabsorbing water + vitamins + to produce
residue in such a state that it can be removed from body
2 Specialization in stomach:
1) In birds, there are 2 parts of stomach:
= A proximal portion called “pro-ventriculus” – it’s the glandular part of bird’s
= Distal to it is, thick-walled muscular “gizzards” which accomplishes much of
mechanical digestion for further chemical digestion. In fact in many birds that eat
grains keep them in gizzards to help grind up tough outer coating of seeds. Birds
do not have teeth so they cannot mechanically break things down by chewing.
Instead then, use muscular gizzard to accomplish.
2) Advanced artiodactyls – ruminants which develop before chamber of stomach,
things like cows, sheep, goat, so on.
= In these groups, stomach becomes complex.
Digestive tract tube delivering + absorbing food things that are not absorbed will be removed from body. Visceral musculature = muscle surrounding the gut that does the motion of transporting. This muscle produces wave-like contractions from mouth to anus, pushing food down the tube (=peristalsis) Gi tract = produces mechanical digestion + chemical digestion where small bits of food are further broken down at the molecular level to be absorbed through wall of gut. Absorption = getting the stuff across through wall of gut + enter the circulatory cardiovascular stream. Until this happens, the food is not part of us. The inside of gi tract is just a tube and it goes right through us. So inside of it is not actually technically part of our body. For that reason you have to get absorption. Gut can be divided into various parts. Higher vertebrates has segmented along the gut, uniform across vertebrates.