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The Gastric System (complete)
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The gastric system easily described and explained
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Lesson 32-33- Gastric system:● Gastrointestinal physiology will keep a mass balance b/w the intake that we are having and the output that we
will have in the gastrointestinal system
● Gastrointestinal system is considered as an external environment of the body, its already forming a hallow
tube in which it will enter all the nutrients and all the food that we will ingest and then we will have different
processes that happens inside.
● It starts in the mouth in which we will see how the mastication will help for the formation of the bolus, the
food bolus will pass through into the esophagus and it will continue until it enters into the stomach, in the
stomach it'll start mixing with more substances that will be already secreted by the body and then it will form
the chymus, the chymus will pass into the intestines which we have the small and the large intestine and more
secretions will join the chymus until it arrives into the anus and the rectum.
● There are fundamental processes which are first the motility of the gastrointestinal system that allows for the
food to pass from the mouth to the anus. The second, is all the secretion that will be added to the food in order
to facilitate the other processes that we already have which are the digestion which is the decomposition of
the food into the small nutrients that are the ones that can be absorbed. At the moment they are absorbed, they
will pass into the circulation and arrive eventually to any tissue and organ in our body in order to maintain the
normal metabolism in our body. Therefore, this mass balance in b/w what we are taking and what we are
ingesting and the mass balance which is what were expelling through the gastrointestinal system should
match.
● So pretty much the ingestion that we have is about 2 L of food and drinks that we're ingesting daily and will
pass through our gastrointestinal system, that will be at another secretions that will be produced by the main
glands of the gastrointestinal system that will start in the mouth which are the salivary glands that will add up
to 1.5 L of saliva. Then we have the bile up to 0.5 L. Then we have all the gastric secretions that are produced
in the stomach which is of about 2 L, then we have all the pancreatic secretions ( remember that the pancreas
is an exocrine gland that will secrete enzymes and other products that helps with the digestion) 1.5 L . Then
we have another intestinal secretion that will count for another 1.5 L. Most of these secretions that are already
secreted or expelled into the tube by the gastrointestinal system are already regulated by hormones.
● All of that will account for about 9 L , which will be the total input that will enter into the lumen of our
gastrointestinal system and therefore from that our body will have to maintain the homeostasis and therefore
we ill have pretty much everything that will be absorbed mainly in the small intestine but also in the large
intestine that most of the water will be absorbed there.
● Remember that the lumen of this digestive tract is already considered an external environment of our body ,
this is why we will keep this mass balance about the intake and the things that will be incorporated into our
circulation that we will use for our metabolism
● The major part of our digestive tract which are the intestines. The stomach has a little bit of a difference and the
esophagus is pretty much really similar to what's happening in the intestinal system.
● So we will have an epithelium cause its already open to a cavity to the external environment and that epithelium
also contains a lot of striations which is what we call the microvilli, the microvilli which will increase the surface
of contact b/w the food that were ingesting and the secretions and those epithelial cells. We have the basement
membrane and then we will find that all of this layer will form which is what we call the mucosa in which we will
also find a lot of goblet cells and another type of glands that will secrete into the external environment.
● Then we will have a submucosa that is highly vascularized that will provide this interchange b/w the absorbed
nutrients and the blood
● Then we will have also a really strong muscular layer, this layer is mainly composed of smooth muscle which is
dividing two layers, one layer that is perpendicular to the movement and to the digestive tract and the other layer
that is parallel to the digestive tract, so we can see we have one perpendicular and another one longitudinal. Those
two layers allows for the different type of movements that already happen in the digestive tract. The first
movement, is the one that will allow for the longitudinal movement through the digestive tract and the other one
will be transversal movements that will allow for the mixing of all the foods that we already have in the digestive
tract.
● Of course since it’s a hallow tube we will have a mesothelium which is another type of epithelium that will be
keeping and protecting the rest of the layers which we also call serosa.
● Therefore, we will have a control from the autonomic nervous system, which is mainly controlling all the
movements and some of the secretions that already will happen there in the intestinal system , this is what we call
the myenteric plexus which is all the innervation that the digestive system already have and it will mainly be
controlled by two processes . The first one is a sympathetic and parasympathetic innervation that composes the
, extrinsic nervous system, but there's another intrinsic nervous system that is known as the enteric nervous system
that is able to self control the different movement that already happened in the digestive system. So all of them and
some of this movement are already reflexes that are controlling the movement and they are controlled directly by
the spinal cord.
● There's a number of regulatory substances that will help for this integration of all the functions in the digestive
tract , therefore we will find three main types of these regulatory substances.
● First type are hormones. The hormones in most of the cases will be secreted by some endocrine cells that we
already have in the gastrointestinal tract. They can be in groups or just single cells.
● In the image ( on slide) , the endocrine cell in the gastrointestinal tract will make and produce the secretion of the
hormone , this hormone will pass into the portal circulation ( portal circulation are the ones that are connected
directly and pass from one to another, the portal circulation that is in b/w the hypothalamus and pituitary gland that
allows for the connection of those by the blood circulation or hormones from one organ to another ) that will arrive
to the liver and then it will enter to systemic circulation. When they enter into systemic circulation, they can
approach the target cells, that could be and in most of the cases are in the gastrointestinal system itself. This is how
we are controlling the production of most hormones.
● The most important hormone that will be secreted by the digestive tract is gastrin. Gastrin is already helping …?
● We have cholecystokinin ( CCK) that will increase the pancreatic secretion , both secretion of the enzymes that are
producing the exocrine part of the pancreas but also the secretion of bicarbonate which is a really important buffer
that will allow to increase the pH after the chymus leaves the stomach and enter into the small intestine, and
therefore will help for all of the processes that will happen in the intestines that don’t really need that amount of
decrease of the pH. It also stimulates the contraction of the gall bladder so will also help for the elimination of the
bile. Also will stimulate growth of some of the exocrine pancreas so will help for the stimulation of more secretion
of the pancreatic juice. Finally, it will inhibit the gastric emptying, therefore at the moment you start to secrete the
cholecystokinin, already we abandoned the stomach and enter into the second phase of the digestion.
● We have also secretin that increase also this pancreatic secretion but also the biliary secretion by increasing the
number of this buffers that will help to increase the pH after the chymus already left the stomach and it will
decrease the gastric secretion so we can imagine that these hormones already started to secrete after we pass the
gastric phase.
● We have also the glucose-dependent insulinotropic peptide (GIP) , this is a peptide that is really similar to the
endocrine pancreatic secretions and will increase the secretions of insulin as well as decrease the gastric secretion
of protons to also conserve this increase of pH when it already enters the intestine
● Second regulatory substance are those paracrine peptides that already secrete by the cells of the gastrointestinal
system and then just by a paracrine mechanism will enter into all the surrounding cells to control and make the
physiological action in the target cells that are already in the gastrointestinal system
● So they will never enter into systemic circulation and will only have an action in the cells that are surrounding the
ones that are producing the secretion. We have two main ones: somatostatin and histamine.
● Somatostatin that decrease the gastric secretion of protons and also will inhibit the secretion of other
gastrointestinal hormone which means that they are ending and controlling the entire process of digestion.
● Also they produce histamine which is involved in process of inflammation and will increase the gastric secretion
of protons in the stomach.
● Finally we have the neurocrines secretion. They are synthesized in the cell bodies of the neurons of the
gastrointestinal system and therefore they will transmit directly the action potential in the target cells of the
gastrointestinal system and therefore they will produce a physiological action.
● Some of them we already know like acetylcholine, norepinephrine and also these vasoactive intestinal peptide (
VIP) that will produce the contraction and dilation of blood vessels in the gastrointestinal system but also we have
the gastrin-releasing peptide (GRP) ,enkephalins , we have another different type of neuropeptides that will have
directly an action into the cells of the gastrointestinal system by direct contact with the cells that are surrounding
the cell bodies of the neurons.
● The motility includes all the movements that will happen to move the food from the mouth to the anus during the
digestion.
● So they will find this mechanically mix of all the food so they can be broken into smaller particles but in a way
which they can be done uniformly and homogenously through all the digestive food
● They are determined by all the properties about the smooth muscle that is already in the histological structure of
the gastrointestinal system and they can produce different movements in different directions with different type of
objectives
● They will modify and they can be modified during the entire process of digestion either cause of the action of the
hormones and all the secretions of the digestive tract by all the neurons and all the nerves that are innervating and
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