Digestive juices

 Digestive juices

Introduction

Digestive juices are the secretions from various glands found in Gastro intestinal canal and accessory organs. Digestive fluids aid digestion by reducing food to simple nutrients. The body uses these nutrients for a variety of purposes after that. Enzymes found in digestive juices accelerate bodily chemical processes.

Types of digestive juices

There are five digestive juices-

·        Saliva

·        Gastric Juice

·        Pancreatic Juice

·        Succus Entericus (Intestinal Juice) 

·        Bile

The necessity for so many digestive juices is as follows-

(1) One juice does not contain all the enzymes necessary for digesting all the different types of foodstuffs. For instance, saliva contains only carbohydrate-splitting enzymes; whereas gastric juice contains both fat- and protein-splitting enzymes but none acting on carbohydrates.

(2) One particular digestive juice cannot digest a particular type of food up to completion. It will digest only up to a certain stage and then, the products will be handed over to the next digestive juice for further digestion. In this way digestion is completed.

For example, gastric juice digests protein up to the stage of peptone, pancreatic juice carries the digestion of peptone further up to lower peptide. The latter is digested completely up to amino acids by succus entericus.

(3) Their reactions are not all same. Saliva is slightly acid, gastric juice is strongly acid, but pancreatic juice is strongly alkaline. This alternate acid and alkaline reaction prevents any serious alteration of blood reaction.

Saliva

 Characteristics- Human saliva has the following characteristics-

1. Total amount- 1,200-1,500 ml in 24 hours. A large proportion of this 24-hour volume is secreted at meal time, when secretory rate is highest.

2. Consistency- slightly cloudy, due to the presence of cells and mucin.

3. Reaction- usually slightly acid (pH 6.02-7.05). On standing or boiling it loses CO₂ and becomes alkaline. This alkaline reaction causes precipitation of salivary constituents, as tartar on the teeth or calculus in salivary duct.

4. Specific gravity--1002-1012.

5. Freezing point--0.07-0.34°C.

Composition

·        Water -99.5%

·        Solids- 0.5%

1. Cellular constituents- Yeast cells, bacteria, protozoa, polymorphonuclear leucocytes, desquamated epithelial cells, etc.

2. Inorganic salts-about 0.2% which consists of Sodium Chloride, Potassium chloride, acid and alkaline sodium phosphate, Calcium carbonate, calcium phosphate, potassium thiocyanate. Smoker's saliva is rich in thiocyanate. In case of poisoning with metals like lead, mercury etc., they are secreted in saliva.

3. Organic- 0.2%

(a) Enzymes-ptyalin (salivary amylase), salivary lipase, carbonic anhydrase, phosphatase and a bacteriolytic enzyme, lysozyme,

(b) Mucin

(c) Urea

(d) amino acids

(e) cholesterol  

(f) vitamins (in small amounts)

4. Soluble specific blood group substances- these form part of organic constituents of saliva and their concentration in saliva is from 10 20 mg/ litre.

5. Gases- Saliva contains about 1ml O₂, 2.5 ml of N₂ and 50ml of CO_2.

6. Other substances- Bicarbonates, phosphates and the proteins act as buffers. Chlorides activate amylase. The thiocyanate (KCNS) is product excretion. Forms in the body from cyanogen radicle (-CN) derived proteins. Its formation is the process of detoxication of the poisonous cyanides and hence it is a protective synthesis.

7. Kallikrein- An enzyme kallikrein is also present in saliva which acts upon plasma protein to form a substance called kallidin or bradykinin which produces vasodilatation of salivary glands during secretion of saliva

Functions

I. Mechanical functions

1. It keeps the mouth moist and helps speech. Decrease in salivary secretion as after nervousness, impairment of speech.

2. It helps in the process of mastication of the foodstuff and preparing it into a bolus, for deglutition. Here, saliva also acts as a lubricant.

3. It dilutes hot and irritant substances and thus prevents injury to the mucous

4. Constant flow of saliva washes down the food debris and thereby does not allow the bacteria to grow. In acute fevers, where the salivary secretion is inhibited, the food debris are not properly washed away and the multiply. These collect as the sordes at the root of the teeth and upon the tongue.

The mechanical functions of saliva are its chief functions in human beings, and are mainly contributed by mucin.

II. Digestive functions- Saliva contains three enzymes-

(a) ptyalin- which splits starch up to maltose

(b) Maltase- (in traces) converts maltose into glucose

(c) Salivary lipase- which digests lipids  

III. Excretory functions- Saliva excretes urea, heavy metals (Hg. Pb, Bi, As, etc.), thiocyanates, certain drugs like iodide, Alkaloids, such as morphine, antibiotics, such as penicillin, streptomycin as also ethyl alcohol. It also excretes certain virulent micro-organisms, such as the virus of hydrophobia.

IV. Sensation of taste- Helps in the sensation of taste. Taste is a chemical sensation. Unless the substance is in solution, the taste buds cannot be stimulated. Saliva acts as a solvent and is thus essential for taste.

V. Water balance- Saliva keeps the mouth moist. When moisture is reduced in the mouth, certain nerve endings at the back of the tongue are stimulated and the sensation of thirst arises. When body water is lost (sweating, vomiting, diarrhea, etc.) saliva is reduced and thirst is felt. The subject feels the necessity of drinking water and thus water balance is restored.

VI. Buffering action- Some substances mainly bicarbonate and to a lesser extent phosphate and mucin present in saliva act as buffers. There is an increase in bicarbonate concentration during food intake.

VII. Bacteriolytic action- Cell membrane of different varieties of bacteria contains polysaccharides. Lysozyme, the enzyme present in the saliva is a polysaccharidase, thus it dissolves the cell wall of many bacteria and may kill them.

Gastric juice

Characteristics-  

Reaction- strongly acid due to free HC1.04-0.5%

Total acidity-0.45-0.6%, it includes free HCl, as well as HCl combined with proteins. It also includes other acids, such as lactic acid. Ordinarily the gastric contents show a lower acidity (0-15% to 0-25% HCI), because the HCI is partly neutralized by mucin and other substances.

pH-- 0.9-1.5

Specific gravity--1002-1004

Freezing point-- 0.59°C

Composition

Total quantity- about 500-1,000 ml per meal i.e. 1,200 ml-1,500 ml per day

·        Water	99.45%
·        Total solids	0.55%

 

A. Inorganic- 0.15% (NaCl, KCL CaCl,, calcium phosphate, Magnesium phosphate, bicarbonate, etc.).

B. Organic- 0.40%.

(a) Mucin.

(b) Intrinsic factor.

(c) Enzymes-

(i) Pepsin

(ii) Other proteolytic enzymes of the gastric juice are- Cathepsin, Gastricin, Parapepsin I and II

(iii) Gastric rennin

(iv) Gastric lipase

(v) Other gastric enzymes present in minute amounts - Lysozyme, Gelatinase, Urease, Carbonic anhydrase.

Functions

1. The enzyme pepsin, with HCl, digests proteins up to the stage of peptone.

2. Rennin coagulates caseinogen of milk.

3. Gastric lipase digests fat to some degree.

4. HCI acts as an antiseptic and causes some hydrolysis of foodstuffs.

5. Excretion-toxins, heavy metals, certain alkaloids excreted through gastric juice.

Pancreatic juice

Characteristics

Total quantity-- about 500 ml per meal. About 1,500 ml

Reaction-- alkaline; pH- 8.0-8.3

Specific gravity--1010 to 1030

Composition

A. Inorganic constituents- It has high levels of bicarbonates of sodium and potassium. Calcium and magnesium are also present in small amounts.

B. Organic constituents-

Enzymes- The enzymes found in pancreatic juice are-

1.     Trypsinogen,

2.     Chymotrypsinogen,

3.     procarboxypeptidase,

4.     nucleotidases (Ribonuclease and Desoxyribonuclease),

5.     elastase,

6.     collagenase,

7.     pancreatic lipase,

8.     lecithinase,

9.     cholesterol esterase  

10.            amylase

Functions

1. Digestive action- Digestion of Carbohydrates, fats and proteins

2. Neutralizing action- Due to its alkaline reaction, it neutralizes almost equal volume of gastric juice acid.

Succus entericus (intestinal juice)

Characteristics

Total quantity- roughly about 1-2 liter per 24 hours

Specific gravity-1010

Reaction- faintly acid to faintly alkaline

pH- 6.3-9.0, average 8.3

Composition

·        Water- 98.5%

·        Solids-1.5%

A. Inorganic-08%, chloride, bicarbonate and phosphate of sodium, potassium, calcium and magnesium.

B. Organic-07%

I. Activators- Enteropeptidase which was previously known as enterokinase, activates trypsingen into trypsin.

II. Enzymes-

1. Proteolytic enzymes

(a) Erepsin- a mixture of enzymes containing dipeptidases (break down dipeptides into amino acids) and aminopeptidases (remove terminal amino acid containing free NH₂ group from polypeptides).

(b) Several enzymes acting on different fractions of nucleic acid, such as nclease, nucleotidase and nucleosidase.

(c) Arginase- acts upon arginine producing urea and ornithin.

2. Carbohydrate splitting enzymes

(a) Amylase- found in traces, acts on starch and dextrin.

(b) Sucrase or Invertase- digests cane sugar.

(c) Maltase- acts on maltose to convert it to glucose.

(d) Isomaltase- acts on Isomaltose to convert it to glucose.

(e) Lactase-breaks down lactose into glucose.

3. Fat splitting enzyme -Lipase.

4. Other enzymes- Alkaline phosphatase, cholesterol esterase, lecithinase, etc.

C. Mucin

Note

The small intestine does not secrete enzymes in the sense that secretion occurs in the gastric mucosa or in the pancreas. Most of these digestive enzymes are actually intracellular and are present in the juice only because cells desquamate. Enteropeptidase and amylase are highly soluble and diffusible and are present in the succus entericus.

Other enzymes are mostly present in the epithelial cells. Peptidases (erepsin), lactase, maltase, sucrase (Invertase) and lipase are found in intestinal the epithelium as well as in the shed cells present in the juice. Proteases, nuclease, phosphatase and arginase are present in the scrapings of the mucous membrane only. These scrapings also show the presence of all the enzymes mentioned above.

Mode of action of digestive juices

From the above description it can be concluded that these digestive enzymes digest the foodstuffs in three ways:

1. The soluble enzymes-Enteropeptidase and amylase, freely exert their action on trypsinogen and starch respectively.

2. The shed cells break down in the succus entericus, set free their insoluble enzymes which digest polypeptides, disaccharides and fats.

3. Those insoluble enzymes which remain in the intestinal mucosa and found only in the scrapings, exert their actions of the corresponding substrates during their transit through the epithelium, in the course of absorption.

Functions- Digestion of Carbohydrates, Proteins and Fats

Bile

Introduction- Bile is essential for life. Though it does not contain any digestive enzyme, it still is a very important digestive juice. Bile is both a product of secretion as well as of excretion of the liver. Minute droplets of bile collect inside the tiny vacuoles of the liver cells and are discharged into the bile capillaries through the intracellular canaliculi.

The primary bile capillaries start between hepatic cells as blind tubules. They join together repeatedly and form bigger channels and ultimately me out of liver as the right and left hepatic ducts. The two ducts unite and form the common bile duct, which enter into the duodenum, through the ampulla of Vater. Through the same ampulla also the pancreatic duct opens.

From the upper part of the common bile duct commences the cystic duct, which ends in the gall-bladder. Formation of bile by the liver is an active process. But entry of bile into the duodenum is intermittent and takes place only after meal.

Characteristics

Total quantity- 500-1,000 ml daily, on average about 700 ml.

Specific gravity- normally 1010-1011, Gall-bladder bile-1026-1040.

Color- It is yellowish green.

Taste- bitter,

Consistency-It is a viscid, mucoid liquid.

Reaction- liver bile alkaline -pH 7.7, gall bladder bile acidic-pH 5.6

Composition-

·        Water	89-98%
·        Solids	2-11%

 (1) Inorganic salts- chlorides, carbonates phosphates Na, and Ca and NaHCO₃ the total base is equivalent to about 170ml of  (N/10) NaOH per of liver bile ( 300ml% in gall-bladder bile).

(2) Bile salts - sodium taurocholate and sodium glycocholate, these are the most important constituents and synthesized by the liver (secretion).

(3) Bile pigments- bilirubin and biliverdin (excretion)

(4) Cholesterol, lecithin and traces of fatty acids, etc.

Functions of bile- Bile serves the following functions:

1. Digestion- It is essential for complete digestion of fats and to some extent proteins and carbohydrates. This action is due to bile salts which act in the following ways-

(a) By reducing surface tension- It converts fats globules into an emulsion, thus larger area becomes available for Lipase to act. Due to this the process of digestion is quickened.

(b) Activating action- The bile salts, by virtue of the cholic acid radicle, act as a specific activator for different lipases.

(c) Solvent action- Bile acts as a good solvent. It serves as a good medium for the interacting fats and different lipases.

2. Absorption- Due to presence of bile salts bile helps in the absorption of various substances. The following things are absorbed with the help of bile-

 (a) Fats- Bile is essential for fat absorption. This is carried out in two ways-

(i) Hydrotropic action-By this property the insoluble fatty acids, cholesterol, calcium, fat soluble vitamins, etc are made readily soluble in the watery contents of intestinal canal. In this way they are made easily diffusible and thus suitable for absorption. This action is brought about by the combination of these substances with bile acids. Fatty acids, cholesterol and many such insoluble substances make loose compounds with desoxycholic acid. Such compounds are soluble in water and are called choleic acids.

(ii) By reducing surface tension- Bile salts reduce the surface tension of the absorbing epithelium, increase their permeability and thus facilitate absorption.

(b) Iron, calcium and probably other mineral constituents of diet.

(c) Vitamins- Bile salts help in the absorption of lipid-soluble vitamins A, D, E,K and provitamin carotene.

III. Excretion- certain substances are excreted through bile, for instance,

(a) Some metals like copper, zinc, mercury, etc.

(b) Toxins

(c) bacteria, etc.

(d) Bile pigments

(e) Cholesterol and lecithin are probably chiefly excretory products.

IV. Laxative action- Bile salts stimulate peristalsis.

V. Cholagogue action- Bile acts as its own stimulant. Bile salts are the strongest cholagogues. They are absorbed from intestine, carried to liver and stimulate further bile secretion. The taurocholate is stronger in this respect than the glycocholate.

VI. Maintenance of a suitable pH- Bile helps to maintain a suitable pH of the duodenal contents and thus helps the action of all the enzymes. Bile is an important source of alkali for neutralizing the hydrochloric acid entering the intestine from stomach.

VII. Lecithin and cholesterol, present in bile, also help in the action of bile. First, they are reabsorbed with digested food. Secondly, they act as adjuvants to bile salts in the process of emulsification of fats (but on the whole they are regarded as excreted products).

VIII. Buffer and a lubricant- Mucin of bile acts as a buffer and a lubricant. IX. Regurgitation of bile in the stomach helps to neutralize gastric acidity and thus prevents the injurious effect of acids on gastric mucosa.