Gastric secretions Part1

 Physiology of gastric secretions

this post will contain the following: 

  

1-Functional anatomy of stomach

2-Motor functions of stomach

3-Gastric secretions 

Starting with the Functional anatomy of the stomach, as we know the stomach has 3 main parts, the funds, the body the antrum, these parts have different types of cells and they are anatomically different than each other. First of all, we have something called the Orad region which contain 2/3 of the body of the stomach and the funds, this region of the stomach has 2 layers of muscles, which contribute to the action that this region does, which is Storage, the stomach can store 0.8-1.5L of food and what helps in this storage function is the VAGOVAGAL reflex, which is an inhibitory reflex to this region of the stomach, this reflex will cause relaxation of the muscles which will help the stomach to expand and accommodate MORE food. U can see the Region in this photo below: 

Now for the other region, which is the Caudad region, which is made up of the last 1/3 of the body of the stomach and the Antrum region, this layer differs than the Orad region in the number of muscle layers it has and it has thicker wall which contribute to its function of Mix and Grind the food, as we said it has 3 layers which are, The outer longitudinal layer which assists with peristalsis and stomach contractions, the middle circular layer forms the pyloric sphincter and helps with mixing food, and the inner oblique layer enhances churning and mixing of food with gastric juices . The muscle wall thickness increases from the proximal to the distal stomach to support stronger contractions for digestion.

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Now, to the motor functions of the stomach, 

1-Storage of large quantities of food.

stretching of stomach with food-vasovagal reflex initiated-reduces tone in muscle wall- this can accommodate large quantities of food-0.8 to 1.5 liters. this reflex reduces muscle wall tone allowing the stomach to expand without a significant increase in pressure, which again helps the stomach to accommodate larger amounts of food.

2-Mixing of food and propulsion.

When food reaches the distal stomach, slow waves from the stomach’s pacemaker (interstitial cells of Cajal) start the basic electrical rhythm (BER), which turns into peristaltic waves as food moves downward. These slow waves generate action potentials, with spike potentials that trigger full action potentials needed for peristalsis. Constrictor rings then push the food toward the antrum. When food reaches the pylorus, two key structures are involved: the pyloric pump, which regulates food flow into the duodenum, and the pyloric sphincter, which controls the passage of chyme. the food isn't properly mixed, it gets pushed back into the stomach Which is called as the retrograde movement for more mixing with gastric juices before passing through the pyloric sphincter.

3-Slow emptying of chyme

 

slow emptying of the chyme from the stomach into the small intestine-the appearance of chyme is semifluid or like a paste-the nature of chyme depends on food, water and secretions of stomach or on the degree of digestion, but did u ask u r self why the emptying of chyme from the stomach is slow?  cause it requires sufficient time to perform digestion and absorption efficiently to make this chyme.

Now we have the Gastric secretions which include: 

1- HCL (Hydrochloric acid) 

which helps in sterilize the meals that we eat, in other words it kills the pathogens that are swallowed with the meals we eat, it also activates an important enzyme called pepsinogen, and convert it to its active form pepsin, which aid in the breakdown of proteins.

2- Pepsinogen (Inactive form)

why do u think the stomach secrete it in an inactive form?  to avoid self-digestion of gastric cells, how? 

as we said what activate the pepsinogen is the HCL and HCL is being produced when we start eating, in this case the Active pepsin is working on the proteins in the meal we ate, but if we are not eating and there is pepsin that is always secreted this will cause damage to the Gastric cells, cause this is like making the body eating itself  in another words, If pepsin were active all the time, even when no food is present, it would start digesting the stomach lining itself. This could lead to ulcers or damage to gastric cells. So, by keeping pepsinogen inactive until food is present, the body ensures that digestion happens only when needed,

3- Intrinsic factors 

these Intrinsic factors are very important when it comes to the absorption of Vitamin B12, if the cells that secrete the intrinsic factors got damaged or in some cases when the patients do Partial gastrectomy or Total gastrectomy, in this case this will lead to malabsorption of Vitamin B12 causing one of the most distinguish types of anemia called pernicious anemia, it is a condition occurs when the body cannot properly absorb vitamin B12, which is essential for producing red blood cells.

4- Mucus 

secreted by the mucosal glands in the stomach, serves several important functions. It protects the gastric mucosa from the damaging effects of hydrochloric acid (HCl), provides lubrication for the smooth passage of gastric contents, and acts as a barrier to prevent pathogens from reaching the gastric lining. Clinically, any damage to the mucus layer or gastric mucosal barrier can lead to gastric secretions penetrating the mucosa, which may result in the formation of ulcers.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th edition (2020)- Chapter 65.

photo of the book: