Small Intestine.

 

Small Intestine:

• It is present between the stomach and large intestine.

• It starts at duodenum and ends at ileocaecal junction.

• It lies in the abdominal cavity surrounded by large intestine.

• Length is  5 Meters and diameter is 1.5 inches.

• Provides Chemical as well as mechanical digestion.

• Main site for chemical digestion and absorption of food.

• Contains “Three” parts,

1. Duodenum

2. Jejunum.

3. Ileum.

1. Duodenum.

• First part.

• Starts near the pyloric sphincter of the stomach.

• “C” shaped.

• Smallest part is only 25 cm.

• Contains openings of bile duct and pancreatic duct.

• It has little acidic pH.

2. Jejunum.

• Middle part.

• Present between duodenum and ileum.

• 3 Feet in length.

3. Ileum.

• Last part.

• Starts after jejunum and ends at the large intestine at “Ileocaecal Junction”.

• Contains ileocaecal sphincter.

• 6 feets in length.

• Cells of Small Intestine:

1. Absorptive Cells:

• Modified columnar epithelium.

• Contains “Microvilli '' provides greater surface area.

2. Paneth Cells:

• Secrete antimicrobial substances.

• Protects from microorganisms survived from stomach acid.

3. Enteroendocrine Cells:

• Cells with endocrine functions.

• S cells : Secretin (Regulates secretions of Pancreas and Liver.

• ICells: Secretes Cholecystokinin (Regulates secretion of Bile.)

• K Cells: Secrete a glucose dependent peptide that stimulates secretion of insulin.

• G Cells: Secrete “Gastrin”. (Stimulates Gastric Acid Secretion).

4. Goblet Cells: Secrete Mucus.

• Functions of Small Intestine:

1. Forward movement of food by peristalsis governed by ANS.

2. Secretion of Intestinal Juice, (Governed by ANS).

3. Absorption of nutrients and Vitamins.

4. Secretion of Secretin and Gastrin.

5. Mechanical and Chemical Digestion.

Movements of Small Intestine:

• Small intestine shows two types of movements,

• Peristalsis.

• Segmentation.

• Peristalsis:

• It is a spontaneous contraction and relaxation of “Longitudinal Muscles”.

• Results in forward movement of food.

• It is in a single direction i.e. towards anus.

• Controlled by “ANS” (Ach increases while Adr decreases).

2. Segmentation: 

• It is a spontaneous contraction and relaxation of “Circular  Muscles”.

• Results in forward and backward movement of food.

• It is in two directions.

• Not controlled by ANS, it is Myogenic in nature.

“Watch This Video”.

Commonly Asked Questions:

1. Write a short note on Human Small Intestine.

2. Write a short note on the movements of smooth muscles in the small intestine.

3. Name different special cells of the small intestine with their function.

Acid Secretion in Stomach.

 

Acid Secretion in Stomach:

• Overview:

• Gastric acid (HCl) is secreted by parietal cells located within oxyntic glands of the stomach. 

• The main purpose of stomach acid is to make the pH of the gastric contents sufficiently acidic to allow activation of pepsinogen to pepsin. 

• The activated pepsin is a strong proteolytic enzyme.

• Stomach walls have protection against pepsin and HCl action while other parts of GIT are not protected.

• Stomach is the only part of GIT which has a lower pH.

• Mechanism of Gastric Acid formation and Secretion:

• Gastric Acid is formed by combining carbon dioxide and water to form carbonic acid. 

• The hydrogen ion of carbonic acid is transported into the stomach lumen while the resultant bicarbonate ion is transported into the bloodstream. 

• This bicarbonate ion is later transported into the lumen of the duodenum where it is used to neutralize the previously generated gastric acid.

• Molecular Mechanism:

• CO2 within parietal cells is converted, by carbonic anhydrase, to H2CO3, which immediately splits into H+ and HCO3-. 

• The generated H+ ion is actively transported into the lumen of the oxyntic gland using an HK ATPase on the cell membrane. 

• The generated HCO3- ion is passively transported into the blood in exchange for a chloride ion.

• Regulation

• Three stimuli directly affect stomach acid secretion by parietal cells.

• Acetylcholine: Acetylcholine is released by parasympathetic fibers in the vagus nerve through “Muscarinic Receptors” present on the parietal cell membrane.

• Histamine: Histamine is released by enterochromaffin-like cells (ECLs) which lie directly adjacent to parietal cells in the oxyntic gland

• Gastrin: Gastrin is released by G Cells present in the pyloric glands of the stomach.

• Potentiation of Acid Secretion: 

• The vagus nerve helps coordinate these three stimuli so that they act in group. 

• These three chemical stimuli acting on the parietal cells have synergistic effects on stomach acid production. 

• This means that more stomach acid is secreted by parietal cells when stimulated by all three molecules simultaneously. 

• Hence, pharmacological inhibition of just one stimulus can help reduce stomach acid secretion.

• Phases of Gastric Acid Secretion:

• Secretion of gastric acid takes place in three stages,

1. Cephalic Phase.

2. Gastric Phase.

3. Intestinal Phase.

1. Cephalic Phase:

• This phase is initiated by the sensory experience of seeing and eating food and contributes about 20% of total acid secretion. 

• stomach acid secretion in this phase is mainly induced by central activation of the vagus nerve.

2. Gastric Phase:

• This phase is initiated by entry of food into the stomach and contributes about 70% of total acid secretion. 

• During the gastric phase a vagal reflex initiated by distension of the stomach further increases vagus nerve stimulation. 

• In addition, local production of food metabolites, especially proteins and amino acids, directly induce gastrin release from G Cells.

3. Intestinal Phase:

• This phase is initiated by entry of food into the duodenum and contributes about 10% of total acid secretion. 

• It is not fully clear how this phase is regulated but may be due to small amounts of gastrin secretion by the duodenal small intestine mucosa.

• Inhibition of Gastric Acid Secretion:

• Several stimuli act to prevent over-secretion of stomach acid which can be damaging both to the stomach as well as the duodenum.. 

• Many of the inhibitory stimuli originate in duodenum and jejunum and act to inhibit secretion of stomach acid. 

• They include, 

1. Presence of nutrients.

2. Acid-mediated irritation of the small intestine mucosa. 

3. Gastrin Inhibitory Peptide.

4. Somatostatin.

Commonly Asked Questions:

1. Write a short note on “Acid Secretion in Stomach”.

2. Discuss different phases of Gastric Acid Secretion.

3. How gastric acid secretion in the stomach is regulated.