Gastric Mixing and Emptying Physiology > Digestive > Digestive

GASTRIC MIXING AND EMPTYING: FINAL STAGES OF DIGESTION

SUMMARY

Key Functions of the Stomach (Review)

• Temporary storage to slow food transit to the small intestine and maximize nutrient absorption.

• Physical Breakdown (like in the mouth)

• Chemical Breakdown of proteins into their amino acids (at the same time that salivary amylase from the mouth continues to breakdown carbohydrates in the stomach).

Three Gastric Phases (Review) 1. Filling, in which food enters the stomach through the gastroesophageal sphincter. 2. Mixing, in which peristaltic contractions churn the food while the gastric lining secretes juices to produce chyme. 3. Emptying, in which peristaltic contractions propel chyme into the small intestine. Mixing Phase – In Depth

• Peristalsis – contractions of circular smooth muscle, move from fundus to antrum

– Pushes the stomach's contents towards the pyloric sphincter. – Facilitates physical breakdown of food

• Pyloric sphincter almost closed

– Forces the chyme to spill backwards into the antrum (stomach's body) and continues mixing. Exocrine Cells of Stomach

• Located in tubular gastric glands that comprise gastric pits

– Epithelial cells at entrance of gastric pits: secrete thick mucus – Mucous layer – Submucosa layer

• Secrete products into stomach lumen

• Secretions convert food to chyme

Exocrine Cell Types

• Mucous cells (mucous neck cells): secrete alkaline, bicarbonate mucus, which protects our stomach wall from erosion

1 / 7 in an acidic luminal environment.

• Chief cells: secrete pepsinogen, an inactive enzyme that, once activated, breaks down proteins.

• Pepsinogen is a zymogen

– An inactive enzyme that, once activated, breaks down proteins. – A substance must convert to its active form, pepsin

• Pepsin

– Breaks down peptide bonds to promote chemical breakdown.

• Parietal cells

– Secrete HCl which denature proteins.

• HCl functions:

– Converts pepsinogen ? its active form: pepsin. – Aids in the breakdown of food ? smaller particles. – Denatures proteins via its acidic environment. – Kills most of the microorganisms that we ingest with our food, thus, providing a protective function. (Tight junctions between the digestive tract epithelium and mucus production serve as a protective barrier). – Facilitates chemical breakdown; it denatures proteins but, unlike pepsin, it doesn't break peptide bonds. The layer of alkaline mucus in our illustration protects our gastric mucosa from HCl's acidic properties.

Stem cells also located in gastric pit

• Rapidly divide and mature into cells that produce gastric mucosa

– Replenishes gastric mucosal cells every 3 days due to constant exposure to the harsh, acidic environment in the stomach. Clinical correlation

• Peptic ulcers

– Erosions that penetrate our gastric mucosal layer.

• Pepsin and HCl access exposed regions and erode the stomach wall.

GASTRIC EMPTYING IN DEPTH

• Induced by strong antral contraction

– Antrum has thicker layer of smooth muscle, which allows more forceful contraction

• Antral contraction pushes chyme through the pyloric sphincter

• The volume of chyme that passes depends on the force of antral contraction.

– Despite the force of antral contraction, only a little chyme enters the duodenum, which is where nutrient absorption occurs. – Pyloric sphincter limits the flow of chyme to promote slow and efficient absorption in the duodenum.

FULL TEXT

2 / 7 • Here, we will learn the final two phases of gastric digestion: mixing and emptying.

• First, start a table to remind ourselves of the 5 key functions of the digestive tract:

- Motility

- Secretion

- Digestion

- Absorption

- Protection

- And the minor function: Elimination.

• Here we will discuss Secretion, Digestion, and Protective functions in greater depth.

• Now, let's summarize the key functions of the stomach, itself:

- Temporary storage to slow food transit to the small intestine and maximize nutrient absorption.

- Physical Breakdown (like in the mouth)

- Chemical Breakdown of proteins into their amino acids (at the same time that salivary amylase from the mouth continues to breakdown carbohydrates in the stomach).

• Denote that the stomach accomplishes these functions in three phases:

- Filling, in which food enters the stomach through the gastroesophageal sphincter.

- Mixing, in which peristaltic contractions churn the food while the gastric lining secretes juices to produce chyme.

- Emptying, in which peristaltic contractions propel chyme into the small intestine.

Let's start with the peristaltic contractions of the mixing phase.

• Draw the cross section of a stomach with wave like smooth muscle lining its walls, which represents the contractions of circular smooth muscle: the peristalsis.

• Label the following:

- Gastroesophageal sphincter

- Fundus

- Body

3 / 7 - Antrum

- Duodenum

• Show that it is filled with chyme.

The filling phase is complete.

• Show that peristaltic contractions move from the fundus to the antrum, which pushes the stomach's contents towards the pyloric sphincter.

- Indicate that the pyloric sphincter remains almost closed, thus it forces the chyme to spill backwards into the antrum.

• Show that the chyme returns to the stomach's body, where it continues mixing.

Let's step away from our anatomical stomach and create some imagery for the mixing phase.

• Draw a blender.

• Add some fruit to it to represent the solid contents of the stomach before they get broken down.

• Next draw blades and indicate that they produce the same result as gastric smooth muscle does via peristalsis

- They break down food into smaller particles.

• When you turn on the blender, the large solid ingredients break down to a liquid smoothie.

- Indicate that the liquid smoothie is analogous to chyme.

• Now, specifically denote that peristaltic contractions facilitate physical breakdown.

Next, let's zoom in on the exocrine cells of the stomach wall to learn how their secretions convert food to chyme.

• Denote that the stomach lining comprises the following exocrine cells:

- Mucous cells, which protect our stomach

- Chief cells, which break down proteins.

- Parietal cells, which denature proteins.

- These cells secrete their products directly into the stomach lumen.

4 / 7 • Draw a cuboidal cross section of the stomach wall, specifically from the body of the stomach.

• Draw a surface layer of mucus penetrated by gastric pits.

- Show that the gastric pits branch into tubular gastric glands, which also contain a number of secretory cells.

• Indicate that our cuboidal cross section comprises a mucous layer and a submucosa.

• Illustrate a layer of epithelial cells at the entrance of the gastric pits; these cells secrete a thick mucus.

• Show that a layer of mucous cells lies below these epithelial cells

- These are often referred to as "mucous neck cells" because they can be found in the neck-like region of gastric pits.

• Indicate that these cells secrete alkaline, bicarbonate mucus, which protects our stomach wall from erosion in an acidic luminal environment.

• Below the mucous cells, draw a layer of chief cells and sparsely distributed parietal cells.

- Indicate that the chief cells secrete pepsinogen, an inactive enzyme that, once activated, breaks down proteins.

- Denote that pepsinogen is a zymogen, which is an inactive enzyme precursor; a substance must convert the zymogen to its active form.

- Pharmaceuticals have a similar corollary: a pro-drug is an inactive form of a drug that requires conversion to its active state.

- Zymogens play an integral role in digestion throughout the GI tract.

• Denote that the active form of pepsionogen is pepsin, and that it promotes chemical breakdown by breaking peptide.

• Indicate that the sparsely distributed parietal cells secrete hydrochloric acid (HCl), which produces the acidic environment of the stomach lumen.

• Write that HCl functions in the following ways:

- It converts pepsinogen to its active form: pepsin.

- It aids in the breakdown of food into smaller particles.

5 / 7 - It denatures proteins via its acidic environment.

- It kills most of the microorganisms that we ingest with our food, thus, providing a protective function.

- Tight junctions between the digestive tract epithelium and mucus production serve as a protective barrier.

Now, return to our blender analogy.

• Add some orange juice to it, which functions much like the acidic exocrine secretions in the stomach.

- As peristalsis occurs, gastric cell secretions help dissolve and reduce our food to a liquid chyme.

- Gastric mucosal cells must be replenished about every three days because the stomach lining is constantly exposed to this harsh, acidic environment.

• So indicate that gastric pits also include stem cells, which rapidly divide and mature into the cells that populate the gastric mucosa.

- Although this cellular regeneration protects our stomach from everyday distress, we can still develop breaks in our mucosal layer.

- As a clinical correlation, denote that peptic ulcers are erosions that penetrate our gastric mucosal layer. Pepsin and HCl gain access to these exposed regions and erode the stomach wall.

Finally, let's illustrate gastric emptying.

• To begin, draw another stomach with wave-like smooth muscle, but here show a strong antral contraction, which is a key difference between these phases.

• In both cross-sections indicate that the antrum contains a thicker layer of smooth muscle, which allows it to contract more forcefully

• Illustrate that the pyloric sphincter still remains almost closed due to tonic contraction.

- This opening is too small for chyme to pass through.

• Use arrows to show that peristalsis moves from the fundus to the antrum, just like the mixing phase.

• Use arrows to show that the antral contraction pushes chyme through the pyloric sphincter; the volume of chyme that passes depends on the force of antral contraction.

- Indicate that despite the force of antral contraction, only a little chyme enters the duodenum.

• Write that nutrient absorption begins in the duodenum.

6 / 7 - The pyloric sphincter limits the flow of chyme to promote slow and efficient absorption in the duodenum.

- Thus, although the oral cavity, esophagus, and stomach all breakdown and transport food, they do not absorb any nutrients.