E.4. Stomach



A: Introduction.
1.
The stomach fulfills several roles at the same time.
2.
In the first place, it receives and ‘stores’ all the food that we eat during a meal, which can be just a little byte or quite a large plate!
3.
This storage is important to give time to the rest of the GI-system to gradually process all the food that we have eaten during this meal.
4.
But the stomach also starts to work on the ‘food’ (chime) that we have swallowed, to make it ready for further processing in the small intestine.

B. Anatomy of the Stomach: top?
1.
Although the stomach looks like one big ‘sack’, it can be defined into several regions:
a. cardia
b. fundus
c. corpus
d. antrum
2.
The cardia is the area that connects the stomach to the esophagus and is the first to receive the swallowed food. It is not very important.
3.
The fundus is the area where the food is stored. This area is crucial for a proper functioning and can adapt (as we will see in panel C) to the increasing amount of food that is being swallowed during a meal.



Diagram of the Stomach
4.
The corpus is the major area of the stomach and this is where the food is digested into small bits using contractions and gastric juices.
5.
The antrum is where the food is gradually pushed, by contractions of the stomach wall, in small bits, into the duodenum.
6.
Both at the beginning and at the end of the stomach, there are sphincters that regulate the food input and output: the LES (see previous page about the esophagus (link) and the pylorus (see later).
7.
Finally, two borders are often used in the anatomy of the stomach; the lesser (or small) curvature and the major curvature.
8.
As indicated in the diagram, the major curvature is the long, lower, border of the stomach, running from the LES to the pylorus along the fundus, corpus and antrum.
9.
The lesser curvature, on the other hand, is a much shorter border and runs along the upper part of the stomach wall.

C. The Fundus: top?
1.
The major function of the fundus is to store the swallowed food. It really functions like a reservoir or a storage and can expand to accommodate increasing amount of food, as shown in the diagram.
2.
So, when the food enters the stomach, the fundus relaxes (= receptive relaxation), which avoid increase in pressure in the whole stomach (and therefore possible reflux into the esophagus). This receptive relaxation is also called “gastric accommodation”.

The expension of the fundus (receptive relaxation) in the stomach

D. The Corpus and the Antrum: top?
1.
The corpus is the central part of the stomach and this is where the actual digestion takes place. This is performed by phasic contractions of the wall, induced by a pacemaker located at the border between the fundus and the corpus, along the greater curvature.
2.
The antrum is the final part of the stomach. It is the ‘pump’ that eventually will push the chime into the small intestine.

Propagating contractions in the stomach
3.
In these contractions, the pylorus plays an important role. It is often closed so that the food is pushed back and forth in the antrum while being mixed with the secreted gastric juices. Very much like a ‘cocktail shaker’, although at a much slower rate (approx. 3/min).
4.
When the chime is small enough and ‘ready’ to be processed by the small intestine, then the pylorus will open a little bit to allow that portion of chime to drip into the duodenum.
An open and a closed pylorus in the stomach
5.
In general, after a normal meal, it will take quite some time for the stomach to digest and to empty its contents. It obviously depends on what and how much was swallowed.
6.
For a drink, it takes 10-20 minutes to leave the stomach but for a good meal, it may take 3-5 hours!
7.
By the way, during this whole process of digestion, the LES remains tightly closed of course! Unless you have to burp (due to gasses accumulating in the stomach) Oops. Sorry!

E. Electrical propagation and contraction: top?
1.
The electrical activity in the stomach, which determines if the stomach contract is more complicated than in the heart.
2.
First of all, there is a pacemaker located along the greater curvature at the ‘beginning’ of the corpus.
3.
From this location, as in the heart, an action potential, called a ‘slow wave’ propagates from the corpus all the way to the antrum and finally reaches the pylorus.
4.
This propagation is very slow (2-4 cm/sec; about 20-50x slower than in the heart hence its name ‘slooowave’).
The slow waves in the stomach
5.
But the stomach does not always need to contract. Especially when the stomach is empty there is no point in contracting!
6.
Therefore, although the slow wave is always initiated and propagating in the stomach, whether or not this induces a contraction depends on the activity of the nervous system.
7.
If the stomach needs to contract, then the parasympathetic system, stimulated by the food in the stomach, will increase the amplitude of the slow wave that in turn initiates contractions and therefore mixing.

F. Gastric Juices: top?
1.
The stomach secretes a lot of juices. In fact, during 24 hours, the total amount of gastric juices secreted amounts to 4-5 liters/day!
2.
There are many different types of gastric juices secreted, with many different functions.
3.
The most common juice is mucus, secreted by mucous cells located throughout the stomach, very much the same as in the mouth and the esophagus.
4.
This mucus creates a mucous layer, 1-2 mm thick, on the surface of the gastric mucosa. This has two beneficial effects:
a) lubrication for the food transport
b) protecting the mucosa from the acid juices inside the stomach.
5.
Then there are two types of major glands:
a) the gastric glands
b) the pyloric glands.
6.
The gastric glands are located in the fundus and the corpus of the stomach while the pyloric glands are located in the antrum and the pylorus.

G. The Gastric Glands:: top?
1.
The gastric glands consists of several secretory cells that, together, secrete:
a) mucus
b) hydrochloric acid
c) pepsinogen
d) intrinsic factor
2.
To do this, the glands contains several secretory cells:
a) mucous cells (secretes mucus)
b) parietal cells (secretes HCl acid + intrinsic factor).
c) chief cells (secretes pepsinogen)
3.
The pepsinogen, secreted by the chief cells, is activated by HCl acid into pepsin inside the stomach lumen. Pepsin is used to break down proteins in the chime (= proteolytic). This proteolysis produces smaller polypeptides and, ultimately, amino acids.
4.
But note that HCL itself, as an acid, also has a proteolytic effect. So, both HCl and pepsinogen work together to break down proteins.

Propagating contractions in the stomach
5.
The intrinsic factor, produced by the parietal cells, is necessary to ‘capture’ the vitamin B12 in our food. This is necessary to avoid that the vitamin B12 is broken down by the acid in the stomach.
6.
The intrinsic factor is therefore a kind of a protector and also a carrier as it transports the vitamin to the ileum (at the end of the small intestine) where it is absorbed. This vitamin is essential in the production of red blood cells (erythrocytes; see link).

H. The Pyloric Glands:: top?
1.
The pyloric glands secrete:
a) mucus
b) gastrin (an hormone)
2.
The mucus secreted by the pyloric glands is as similar and as essential as the mucus secreted by the gastric glands.
3.
The gastrin however is actually a hormone which makes the stomach contract more strongly.
4.
Its secretion is stimulated by several factors, especially by the presence of rich proteins (meat!) in the ingested food.

I. Regulation of Gastric Secretion: top?
1.
Obviously, this gastric secretion has to be regulated (or else we would be secreting HCL the whole day!).
2.
So, we need more secretion when we eat and less when we do not eat. Simple!
3.
In fact, we can distinguish several phases in this regulation:
a) a cephalic phase
b) a gastric phase
c) an intestinal phase
d) an inter-digestive phase
4.
The cephalic (= head) phase actually starts before the food enters the stomach. It is stimulated by looking at the food and by smelling and tasting it when it enters the mouth.
The four digestive states in the GI-system
5.
All this stimulates the cerebral cortex in our brain and the appetite centers in other brain centers such as the hypothalamus.
6.
The end result of this is that the vagus nerve, which runs amongst others to the stomach, is activated. This in turn stimulates the gastric glands to secrete HCL etc.
7.
The gastric phase starts when the food enters the stomach. This stimulates both the gastric glands and the pyloric glands to secrete their products.
8.
This also includes the hormone gastrin (from the pyloric glands) that in turn, will secrete even more HCL.
9.
Finally, there is the inter-digestive phase. As the name implies, this is the phase between two meals, which can take several hours.
10.
In that phase, the stomach is essentially empty, becomes quiet, although occasionally, some contractile activity occurs, mainly to get rid, in the stomach of indigestible material (“cleaning up”).

Page Menu:

Image Gallery

Page PDF

(hover to peek!)

E.4. Stomach

© BasicPhysiology.com 2017-2020 mail to: info@BasicPhysiology.com