3. Parietal cells secrete hydrogen ions (H) and chloride ions (Cl) separately into the
stomach lumen
The net effect is secretion by HCL (Hydrochloride acid).
Proton pumps powered by H/K ATPases actively transport H into the lumen while
bringing potassium ions (K) into the cell.
At the same time, Cl and K diffuse out into the lumen through Cl and K channels in the
apical membrane.
The enzyme carbonic anhydrase, which is especially plentiful in parietal cells,
catalyzes the formation of carbonic acid (H2CO3) from water (H2O) and carbon dioxide
(CO2).
As carbonic acid dissociates, it provides a ready source of H for the proton pumps but
also generates bicarbonate ions (HCO3).
As HCO3 builds up in the cytosol, it exits the parietal cell in exchange for Cl via
Cl/HCO3 antiporters in the basolateral membrane. HCO3 diffuses into nearby blood
capillaries.
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4. Regulation of acid production through
parasympathetic nervous system
HCl secretion by parietal cells can be stimulated by several sources:
acetylcholine (ACh) released by parasympathetic neurons, gastrin secreted by
G cells, and histamine. G cells: Antrum of Pylorus.
Histamine: Group of amine (ECL Enterochromaffin-like cells). H2 Receptor
Ach: M3 Muscarinic receptor (vagus nerve)
These hormones increase the secretion of HCL. It is regulated by the
parasympathetic nervous system.
Acetylcholine and gastrin stimulate parietal cells to secrete more HCl in the
presence of histamine.
Receptors for all three substances are present in the plasma membrane of
parietal cells.
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5. The strongly acidic fluid of the stomach kills many microbes in food.
HCl partially denatures (unfolds) proteins in food and stimulates the secretion of
hormones that promote the flow of bile and pancreatic juice.
Enzymatic digestion of proteins also begins in the stomach.
Pepsin’s role in protein digestion
The only proteolytic (protein-digesting) enzyme in the stomach is pepsin, which is
secreted by chief cells.
Pepsin severs certain peptide bonds between amino acids, breaking down a protein
chain of many amino acids into smaller peptide fragments.
Pepsin is most effective in the very acidic environment of the stomach (pH 2); it
becomes inactive at a higher pH.
What keeps pepsin from digesting the protein in stomach cells along with the food?
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6. First, pepsin is secreted in an inactive form called pepsinogen.
In this form, it cannot digest the proteins in the chief cells that produce it.
Pepsinogen is not converted into active pepsin until it comes in contact with
hydrochloric acid secreted by parietal cells or active pepsin molecules.
Another enzyme of the stomach is gastric lipase, which splits the short-chain
triglycerides in fat molecules (such as those found in milk) into fatty acids and
monoglycerides.
A mono-glyceride consists of a glycerol molecule that is attached to one fatty
acid molecule. This enzyme, which has a limited role in the adult stomach,
operates best at a pH of 5–6.
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7. More important than either lingual lipase or gastric lipase is pancreatic lipase Bile
juice, an enzyme secreted by the pancreas into the small intestine.
Only a small amount of nutrients are absorbed in the stomach because its
epithelial cells are impermeable to most materials.
However, mucous cells of the stomach absorb some water, ions, and short-
chain fatty acids, as well as certain drugs (especially aspirin) and alcohol.
Foods rich in carbohydrates spend the least time in the stomach.
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8. Summary of Digestive Activities in the Stomach
STRUCTURE ACTIVITY RESULT
Chief cells • Secrete pepsinogen.
• Secrete gastric lipase.
Pepsin, the activated form, breaks down proteins into
peptides.
Splits triglycerides into fatty acids and monoglycerides.
Parietal cells • Secrete hydrochloric acid
• Secrete intrinsic factor
Kills microbes in food, denatures proteins; converts
pepsinogen into pepsin.
Needed for the absorption of vitamin B12 which is used
in RBC formation.
Surface mucous cells and
mucous neck cells
• Secrete mucus
• Absorption
Forms a protective barrier that prevents digestion of
stomach walls.
Small quantity of water, ions, short-chain fatty acids, and
some drugs enter the bloodstream.
G-cells • Secrete gastrin Stimulate parietal cells to secrete HCl and chief cells to
secrete pepsinogen; contracts lower esophageal sphincter
increases motility of the stomach, and relaxes pyloric
sphincter.
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9. Small Intestine
Most digestion and absorption of nutrients occur in a
long tube called the small intestine.
A Long, narrow, coiled tube extending from the stomach
to the large intestine. It is about 6 to 7 meters long.
Its length alone provides a large surface area for
digestion and absorption, and that area is further
increased by circular folds, villi, and microvilli.
It lies in the abdominal cavity surrounded by the large
intestine.
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10. Anatomy of Small Intestine
The small intestine is divided into three regions:
• Duodenum: This is the most proximal portion of the
small intestine and 25cm long curves around the head of
the pancreas.
• The secretions from the gall bladder and pancreas
merge in a common structure the hepatopancreatic
ampulla or duct which is guarded by the
hepatopancreatic sphincter.
• Jejunum: This is the middle section and it is about 2
meters long.
• Ileum: This is the terminal section and is about 3 meters
long and ends at the ileocecal valve, which controls the
flow of material from the ileum to the caecum (First part
of the Large intestine), and prevents backflow.
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11. Structure of small intestine
The walls of the small intestine are composed of 4 layers of tissue:
1. Peritoneum (Outer layer) (Serosa): The mesentery, a double
layer of peritoneum, attaches the jejunum and ileum to the posterior
abdominal wall.
2. Muscle layer: 1. Longitudinal muscle layer. 2. Circular muscle
layer (Helps in peristalsis).
3. Submucosal layer: Areolar tissue (It helps bind the skin together)
4. Mucosa: It contains tiny finger-like projections of the mucosal
layer called villi (1 mm) made up of enterocytes with tiny microvilli
on their free border. Food absorption takes place in enterocytes.
Goblet cells that secrete mucus are preset between the enterocytes.
Numerous lymph nodes are present to help in defense (Foreign
particles, microbes).
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12. Intestinal Juice & Functions
About 1500 ml of intestinal juice secreted by the intestinal glands
present between the villi.
Clear to pale yellow, watery secretion composed of Digestive
enzymes, Mucus, and Mineral salts.
The function of the small intestine
Onward movement of food by peristalsis.
Secretion of intestinal juice.
Completion of chemical digestion of carbohydrates, protein, and fats.
Protection against infection by microbes by lymph follicles.
Secretion of the hormone cholecystokinin (CCK) and secretin (these
hormones stimulate and further stimulate pancreatic juice)
Absorption of nutrients.
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13. Chemical digestion in small intestine
When acid chyme (Comes from stomach HCL and acidic) passes into the
small intestine it is mixed with pancreatic juice, bile, and intestinal juice and
comes in contact with the enterocytes of the villi. The digestion of all nutrients
is completed.
Carbohydrates: Monosaccharides
Proteins: Amino acids
Fats: Fatty acids
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14. Large Intestine
It is about 1.5 meters long tube.
It starts from the caecum and ends at the rectum and
anal canal.
Its lumen is big and it surrounds the small intestine.
Parts of large Intestine:-
1. Caecum
2. Ascending colon
3. Transverse colon
4. Descending colon
5. Sigmoid colon
6. Rectum
7. Anal canal (Anus)
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15. Caecum: This is a 6 cm long dilated region and is the 1st part of the
colon. The vermiform appendix tube is connected to the caecum.
The colon has 4 parts:
Ascending colon: Passes upwards from the caecum and is 10 to
20 cm long. At the level of the liver, it curves to the left to become a
transverse colon.
Transverse colon: This is located in the front of the duodenum,
stomach, and spleen where it forms splenic flexure and curves
downwards to become the descending colon.
Descending colon: This passes down the left side of the
abdominal cavity and is 25 to 30 cm long at the level of the iliac
crest it is known as the sigmoid colon.
Sigmoid colon: This is S shaped tube 40 cm long and continues
downwards to become the rectum.
Rectum: It is a 13 cm long tube it leads from the sigmoid colon and
terminates at the anal canal.
Anal canal: It is a 3.8 cm long tube. It helps in excretion of waste
material.
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16. Anus controlled by the 2 sphincters:
1. Internal sphincter: It is made up of smooth muscular fibers
and it is controlled by the autonomic nervous system.
2. External sphincter: It is made up of skeletal muscles and
humans can control it voluntarily.
Structure of large intestine:
1. Serosa: Outer layer
2. Muscle layer: Longitudinal muscle fiber collected into
three bands, called taeniae coli extended from the caecum to
the rectum.
3. Submucosa: More lymphoid tissue than any other part
which helps in defense.
4. Mucosa: A large number of mucus-secreting goblet cells are
present.
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17. Functions of large intestine
Absorption: Absorption of water, mineral salts, vitamins, and some drugs continues until
semisolid consistency of feces is achieved.
Bacteria present in the large intestine that are harmless and synthesize vitamin K and folic acid
are E coli and Enterobacter aerogenes.
The gases that are produced in the intestine due to food and drink consumption are excreted out
by the large intestine in the form of flatus (wind).
Food moves in the large intestine by peristaltic movement.
Food moves from the stomach to the large intestine by the food mass movement which is called
the gastrocolic effect.
Defecation:- In defecation, mass movement transfers to the rectum to excrete the waste product.
In infants, defecation is an involuntary action.
Defecation reflex developed at the age of 2 or 3 years.
Defecation starts when contraction of abdominal muscle and intra-abdominal pressure
increases.
In adults defecation is a voluntary control.
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18. Accessory organs
An organ that helps with digestion but is not part of the digestive system.
Accessory organs add secretions and enzymes that break down food into
nutrients.
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Accessory organs
Salivary
Glands
Liver Pancreas Gall Bladder
19. Glands
Glands produce and release
substances that perform certain
functions:
ENDOCRINE GLANDS:- Make
hormones and release directly into your
bloodstream.
EXOCRINE GLANDS:- Produce
other substances, not hormones that are
released through ducts.
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20. Salivary Glands
Exocrine glands release their secretion of saliva into ducts that lead to the mouth.
A salivary gland is a gland that releases a secretion called saliva into the oral
cavity. Ordinarily, just enough saliva is secreted to keep the mucous
membranes of the mouth and pharynx moist and to cleanse the mouth and teeth.
When food enters the mouth, however, secretion of saliva increases, and it
lubricates, dissolves, and begins the chemical breakdown of the food.
The mucous membrane of the mouth and tongue contains many small salivary
glands that open directly, or indirectly via short ducts, to the oral cavity.
There are three pairs of major salivary glands: the parotid, submandibular, and
sublingual glands.
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21. The parotid glands are located inferior and
anterior to the ears, between the skin. Each
secretes saliva into the oral cavity via a parotid
duct.
Parotid duct opening into the mouth at the level of
second upper molar teeth.
The submandibular glands are found in the floor
of the mouth; the submandibular ducts, run under
the mucosa on either side of the midline of the
floor of the mouth and enter the oral cavity.
Lie, one on each side of the face under the angle of
the jaw.
The sub-lingual glands are beneath the tongue
and superior to the submandibular glands. Their
ducts, the lesser sublingual ducts, open into the
floor of the mouth in the oral cavity proper. 04-03-2024 21
22. Composition and functions
Chemically, saliva is 99.5% water and 0.5% solutes. Among the solutes are ions, including
sodium, potassium, chloride, bicarbonate, and phosphate.
The bacteriolytic enzyme, salivary amylase, a digestive enzyme that acts on starch.
Chloride ions in the saliva activate salivary amylase, an enzyme that starts the breakdown of
starch. Bicarbonate and phosphate ions buffer acidic foods that enter the mouth, so saliva
is only slightly acidic (pH 6.35–6.85).
Mucus lubricates food so it can be moved around easily in the mouth, formed into a ball,
and swallowed.
Immunoglobulin A (IgA) prevents attachment of microbes so they cannot penetrate the
epithelium, and the enzyme lysozyme kills bacteria; however, these substances are not
present in large enough quantities to eliminate all oral bacteria
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23. The secretion of saliva, called salivation (sal-i-VA¯ -shun), is controlled by the
autonomic nervous system. Amounts of saliva secreted daily vary considerably but
average 1000–1500 mL (1–1.6 qt).
Parasympathetic stimulation promotes continuous secretion of a moderate
amount of saliva, which keeps the mucous membranes moist and lubricates the
movements of the tongue and lips during speech.
Eventually, most components of saliva are reabsorbed, which prevents fluid loss.
Sympathetic stimulation dominates during stress, resulting in dryness of the
mouth.
The feel and taste of food also are potent stimulators of salivary gland secretions.
Chemicals in the food stimulate receptors in taste buds on the tongue, and
impulses are conveyed from the taste buds to two salivary nuclei in the brain
stem (superior and inferior salivatory nuclei). Returning parasympathetic
impulses in fibers of the facial (VII) and glossopharyngeal (IX) nerves stimulate
the secretion of saliva.
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