Digestion

DIGESTION is a hydrolytic process involving the breakdown of biomolecules to simpler molecules, which are absorbed from the small intestine into the blood and distributed to various tissues where they are needed.

  1. Cooking – initiates the hydrolysis of food making them accessible in the action of enzymes. It also improves the taste, odor and appearance of food causing an increased stimulation of secretion of digestive juices.
  2. Mastication – breaks down the foods into smaller particles increasing the surface area upon which enzymes will have to act.
  3. Movements of the stomach and intestines – help by promoting better contact between the foods and the enzymes.
  4. Absorption from the intestines – removes the digested molecules and brings about by a mass law effect, an enzyme action which is almost complete
  5. Autodigestion – involves the ripening of fruits and vegetables, and the aging of many foods produce desirable chemical changes favorable for digestion
  6. Salivary digestion – hydrolysis of starch by the salivary amylase which is taking place in the buccal cavity and to a certain extent in the fundic end of the stomach
  7. Gastric digestion – mainly concerned with the digestion of proteins through the action of the enzyme pepsin and hydrochloric acid occurring in the stomach
  8. Intestinal digestion – involves three distinct secretions in the small intestines that contribute in the completion of the digestive process; pancreatic juice, intestinal juice and bile

SALIVA – colorless, slightly viscid and opalescent fluid secreted by the salivary glands (parotid, submaxillary and sublingual glands)
Parotid gland – secretion is watery and rich in ptyalin

Sublingual and Submaxillary glands – secretion is more viscid containing mucin and poorer in ptyalin

Psychic factor – a reflex stimulation due to the thought, sight or smell of food.

Chemical factor – due to the stimulation of the taste buds by such substances as sugar, salt, condiments and other dissolved chemical substances.

Mechanical factor – due to the mastication of substances in the mouth.

• SALIVARY AMYLASE – it is an endoamylase attacking the interior of the polysaccharide chain to be broken down into low molecular weight, non – fermentable, soluble dextrins
• SALIVARY AMYLASE – also known as ptyalin

• GASTRIC JUICE – is an acidic, watery, colorless fluid secreted in the stomach having a specific gravity of around
1.003.
Parietal cells – secrete HCl
Chief cells – secrete pepsin

   Mucous cells – secrete mucin

Psychic phase – stimulation of the gastric secretion due to the smell, sight, taste and even the thought of the food
Gastric phase – attributed to the stimulating effect of the food in the stomach
Secretagogue – present in certain food that stimulates the secretion of gastrin

Gastrin – hormone secreted by the pyloric mucosa and stimulates the gastric glands to secrete a juice rich in
HCl but poor in pepsin

Intestinal phase – occurs when the acid chyme (partially digested food mixed with HCl) passes from the stomach into the duodenum

Secretin – hormone secreted by the duodenal mucosa and stimulates the secretion of the gastric juice, pancreatic juice, intestinal juice and the bile

Enterogastrone – hormone produced due to the presence of fats in the inhibits gastric secretion and motility and is used for the treatment of peptic ulcer

Urogastrone – hormone similar to enterogastrone but is found in the urine and exerts also inhibitory effect on gastric secretion

Acid inhibition – when HCl is present at a concentration of 0.03 N, it slows down secretion. At 0.1 N, the acid secreting cells are completely inhibited.

  1. HYDROCHLORIC ACID - secreted by the parietal cells, about 0.17 N (170 mEq. per liter) and a pH of about 0.87
       Alkaline tide – the replacement of chloride ions in the plasma by bicarbonate ions making the plasma more alkaline at the height of digestion. The normally acidic urine becomes alkaline.
       Pathological variations in HCl production:

Achlorhydria – absence of free HCl. It is common in pernicious anemia.

Aclorhydria Gastrica – absence of free HCl is below normal. This is an indication of degeneration of the
gastric glands.Hypochlorhydria – the production of free HCl is below normal. It is suggestive of chronic gastritis, carcinoma of the stomach.

Hyperchlorhydria – the amount of free Cl- is above normal. It is suggestive of gastriculcer, cholecystitis.

  1. GASTRIC ENZYMES

a. PEPSIN
– secreted by the chief cells as pepsinogen then activated by HCl and autocatalytically by pepsin at pH 5 or lower
– the principal digestive constituent of the gastric juice that has an optimum pH range of 1.5 to 2.5
– an endopeptidase, it catalyzes the hydrolysis of inner peptide bonds of proteins to form into smaller fragments of polypeptides and peptides

b. GASTRIC LIPASE
– a fat-digesting enzyme but does not initiate fat digestion in the stomach because the low pH in the gastric juice makes it inactive. Its optimum pH is about 5.5 to 7.5. it is however stable in acid media and may exert its effect in the intestines whose pH is more favorable. c. RENNIN
– has a milk curdling effect, hydrolyzing casein into soluble paracasein. In the presence of calcium, paracasein is clotted into insoluble calcium paracaseinate
– is secreted as pro-rennin and is activated by HCl to rennin
– not found in the gastric juice of adults but is present in infants

  1. GASTRIC MUCIN
       – a glycoprotein or mucoprotein secreted by the mucous membrane of the stomach
       – not digested by pepsin, it therefore protects the walls of the stomach from pepsin digestion – carries the intrinsic anemia factor which promotes the absorption of Vit. B12.

 INTESTINAL DIGESTION

As the acid chime leaves the stomach and enters the intestines, three distinct secretions contribute in the completion of the digestive processes, namely the pancreatic juice, the intestinal juice and the bile.

A. PANCREATIC JUICE – is a clear watery solution secreted by the acinar cells of the pancreas having a specific gravity of about 1.007.
– Average secretion: 500 ml. in 24 hours
– Optimum pH: 7.5 to 8.2
– most alkaline of the normal body fluids due to the large amount of bicarbonate content
– contains all the enzymes necessary to digest the different organic foodstuffs

Vagal stimulation - produces a secretion which is rich in enzymes but comparatively less volume that involves the aid of 2 HORMONES

Secretin – stimulates the secretion of a fluid of high volume and bicarbonate content, but low in enzymes

Pancreozymin – is released from the duodenal mucosa upon contact with the acid chime and produces a secretion similar to vagal stimulation

Only one third is organic which consists of: The powerful proteolytic enzymes, trypsin and chymotrypsin etc…

About two thirds are inorganic components that are made up of sodium, potassium, calcium, bicarbonates, chlorides and phosphates

TRYPSIN - A proteolytic enzyme secreted as trypsinogen.

Trypsinogen - is activated by enterokinase from the mucous entericus

TRYPSIN - An endopeptidase, it attacks both denatured and partially digested proteins breaking them into peptides. It acts upon peptide linkages involving the carboxyl group of arginine and lysine

  Trypsin - The strongest protein digesting enzyme, if activated in the pancreas, it will cause acute hemorrhage –

Pancreatitis – the equivalent of idiopathic sleep death syndrome.

CHYMOTRYPSIN - An endopeptidase secreted by acinar cells of the pancreas as chymotrypsinogen.
Chymotrypsinogen - is activated by trypsin into chymotrypsin.
CHYMOTRYPSIN - It acts on inner peptide bonds of proteins breaking them into peptides. The proteolytic effect is similar to pepsin, acting on peptide bonds involving phenylalanine, tyrosine and tryptophan

CARBOXYPEPTIDASE & AMINOPEPTIDASE – exopeptidases that attack the outer peptide bonds of peptides to release amino acids.

STEAPSIN (Pancreatic lipase) - hydrolyze fats into fatty acids and glycerol

DIASTASE (Pancreatic amylase) - It attacks the α – 1,4 – glycosidic bonds of starch in a random fashion into maltose

NUCLEODEPOLYMERASES - involves ribonuclease and deoxyribonuclease

  NUCLEODEPOLYMERASES  - hydrolyze the corresponding nucleic acid into mononucleotides

INTESTINAL JUICE – It is the combined secretion of the intestinal glands in the different portions of the small intestines (duodenum, jejunum and ileum)

INTESTINAL JUICE - also known as the Succus Entericus

CARBOHYDRASES - enzymes that primarily acts on disaccharides namely:

Maltase – splits maltose into two molecules of glucose

Lactase – splits lactose into glucose and galactose

Sucrase (Invertase) – splits sucrose into glucose and fructose

  1. PEPTIDASES - enzymes that acts on amino acids and peptide linkages namely:

Amino peptidase – an exopeptidase which attacks the peptide linkage of the terminal amino acid containing the free amine group. This enzyme requires Mn++ or Mg++ for its action.

Tripeptidases and Dipeptidases – splits tripeptides and dipeptides into their constituent amino acids

NUCLEOTIDASES (Nucleophosphatases) - Non – specific enzymes that split nucleotides into phosphoric acid and nucleosides

NUCLEOSIDASES (Glucosidases or Phosphorylases) - Intracellular, phosphorylytic enzymes that split the corresponding nucleosides into their component ribose or deoxyribose ant purine and pyrimidine bases.

two varieties of NUCLEOSIDASES: purine nucleosidase and pyrimidine nucleosidase

  1. ENTEROKINASE enzyme from the duodenal mucosa that activates trypsinogen into trypsin. It has no direct digestive effect.

C. BILE – clear, golden yellow, slightly viscid fluid with a bitter taste and a pH of 7.0 to 8.5

Liver Cells – bile is secreted by the
Bile - stored in the gallbladder and discharged into the bowel during digestion
Cholagogues - are the substances that stimulate the flow of bile
Cholecystokinin - is a hormone formed from the intestinal mucosa which stimulates the flow of bile

Cholic acid - (3,7,12 – trihydroxycholanic acid) – 26 to 60 %

Chenodeoxycholic acid - (3,7 – dihydroxycholanic acid) – 30 to 50 %

Deoxycholic acid - (3,12 – dihydroxycholanic acid) – 5 to 25 %

FERMENTATION - It is the bacterial degradation of carbohydrates under anaerobic conditions.

II. PUTREFACTION - it is the bacterial decomposition of proteins into peptides or amino acids under anaerobic conditions

• TOXIN – a poisonous substance that is a specific product of the metabolic activities of a living organism and is usually very unstable, notably toxic when introduced into the tissues and typically capable of inducing antibody formation.

• AUTOINTOXICATION – a state of being poisoned by toxic substances produced within the body.

III. DETOXIFICATION - includes all processes by which noxious substances are rendered less harmful and more easily excreted

Oxidation – is the most common means and one of the most important means of detoxification

Hydrolysis – some drugs used for therapy are hydrolyzed in the body

Conjugation – is the process called upon whenever oxidation becomes ineffective.

Conjugation - It is brought about by the combination of the toxic substance or one of its metabolites with a compound normally occurring in the body
(conjugating agent)

Feces - After the water, vitamins and electrolytes are absorbed in the proximal half of the colon, the semi solid residue left is excreted as the _____.

CONSTIPATION – the slow movement of feces through the large intestine, and it is often associated with large quantities of dry, hard feces or scybala

MEGACOLON or HIRSCHSPRUNG’S DISEASE – a severe case of constipation that bowel movements occur only once every several weeks.

DIARRHEA – the rapid movement of fecal matter through the large intestine (loose bowel movement) due to:

ENTERITIS – means infection in the gastrointestinal tract by a virus, parasite or bacteria. Everywhere that the infection is present, the mucosa becomes irritated and its rate of secretion becomes greatly enhanced. In addition, the motility of the intestinal wall increases many folds.

PSYCHOGENIC DIARRHEA – diarrhea that accompanies periods of nervous tensions. This is caused by excessive stimulation of the parasympathetic nervous system which excites bot motility and secretion of mucus in the distal colon.