There are five digestive juices; saliva, gastric juice, pancreatic juice, succus entericus (intestinal juice) and bile.
The necessity for so many digestive juices is that -(a) 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, and (b) The second reason is that, 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 instance, 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.
One more interesting fact about the digestive juices is that, their reactions are not all same. Saliva is slightly acid, gastric juice is strongly acid, but pancreatic juice is strongly alkaline. This, more or less, alternate acid and alkaline reaction, prevents any serious alteration of blood reaction. This is a special device for maintaining blood reaction constant.
The composition and functions of various digestive juices is mentioned below:
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.
Consistency – Slightly cloudy, due to the presence of cells and mucin.
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.
Specific Gravity – 1.002 – 1.012.
Freezing Point – 0.07 – 0.34°C.
1. Water – 99.5%.
2. Solids – 0.5%.
i. Cellular Constituents:
Yeast cells, bacteria, protozoa, polymorphonuclear leucocytes, desquamated epithelial cells, etc.
ii. Inorganic Salts:
About 0.2% consists of NaCI, KCI, acid and alkaline sodium phosphate, CaCO3, 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.
iii. Orgainc – 0.3%.
a. Enzymes P tyalin (salivary amylase) lipase, carbonic anhydrase, phosphatase and a bacteriolytic enzyme, lysozyme.
c. Urea, amino acids, cholesterol and vitamins (in small amounts).
d. The soluble specific blood group substances also form part of the organic constituents of saliva and they have the same characteristics as the agglutinogen on the erythrocyte. In human beings, A, B, O and Lea substances have been demonstrated. Their concentration in saliva is from 10 to 20 mgm per litre.
Saliva contains about 1 ml oxygen, 2.5 ml nitrogen and 50 ml of CO2 per 100 ml. Bicarbonates, phosphates and the proteins act as buffers. Chlorides activate amylase. The thiocyanate (KCNS) is a product of excretion. It is formed in the body from the cyanogen radicle (-CN) derived from proteins. Its formation is a process of detoxication of the poisonous cyanides and hence is an example of protective synthesis. With ferric chloride it gives a rich brown colour.
An enzyme kallikrein is present in saliva which acts upon plasma protein to produce a substance known as kallidin or bradykinin. This produces vasodilatation of salivary gland during secretion and is also responsible for the sudden fall and consequent rise in blood pressure observed after injection of saliva in animals.
1. Mechanical Functions:
a. It keeps the mouth moist and helps speech. Decrease in salivary secretion as occurs after nervousness, causes impairment of speech.
b. It helps in the process of mastication of the foodstuff and in preparing it into a bolus, suitable for deglutition. Here, saliva also acts as a lubricant.
c. 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 is not properly washed away and the bacteria multiply. These collect as the sordes at the root of the teeth and upon the tongue.
It is to be noted that the mechanical functions of saliva are its chief functions in human beings, and is mainly contributed by mucin one of its main constituents.
2. Digestive Functions:
Saliva contains two enzymes:
Splits starch up to maltose in the following manner [Fig. 9.24]
Maltase (in traces) converts maltose into glucose.
3. Excretory Functions:
Saliva excretes urea, heavy metals (Hg, Pb, Bi, As, etc.), thiocyanates, certain drugs like iodide, etc. Alkaloids, such as morphine, antibiotics, such as penicillin, streptomycin, etc. are also excreted in the saliva. The excretion of ethyl alcohol by the salivary gland has prompted the recommendation that such a test should be used for medico-legal purpose. [It also excretes certain virulent micro-organisms, such as the virus of hydrophoboea, acute anterior poliomyelitis, mumps, etc.]
4. Helps in the Sensation of Taste:
Taste is a chemical sensation. Unless the substances are in solution, the taste buds cannot be stimulated. Saliva acts as a solvent and is thus essential for taste.
5. Helps 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, diarrhoea, etc.) – saliva is reduced and thirst is felt. The subject feels the necessity of drinking water and thus water balance is restored.
6. Helps Heat Loss:
This is mainly found in animals (dog, sheep, etc.). When they become hot or excited more saliva is secreted causing greater heat loss.
7. Buffering Action:
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.
8. Bacteriolytic Action:
Cell membrane of different varieties of bacteria contains polysaccharides, lysozyme, the enzyme present in the saliva is polysaccharides, and thus it dissolves the cell wall of many bacteria and finally kills them.
Digestive Juice # 2. Gastric Juice:
The average composition of human gastric juice is as follows:
1. Water – 99.45%.
2. Total Solids – 0.55%.
a. Inorganic – 0.15% (NaCI, KCI, CaCI,, calcium phosphate, Mag. Phosphate, bicarbonate, etc.).
b. Organic – 0.40%.
ii. Intrinsic factor,
i. Other proteolytic enzymes of the gastric juice are; cathepsin, gastricin, parapepsin I and II.
ii. Gastric rennin.
iii. Gastric lipase.
iv. Other gastric enzymes are present in minute amounts and are; lysozyme, gelatinase, urease, carbonic anhydrase.
i. Total Quantity – About 500 -1,000 ml per meal (1,200 ml -1,500 ml per day).
ii. Reaction – Strongly acid.
iii. Free HCI – -0.4 -0.5%
iv. Total Acidity – – 0.45 – 0.6%. It includes free HCI, as well as HCI combined with proteins. It also includes other acids, such as lactic acid. As ordinarily examined, the gastric contents show a lower acidity (0.15% to 0.25% HCI), because the HCI is partly neutralised by mucin and other substances.
v. pH – 0.9-1.5
vi. Specific Gravity – 1.002 -1.004.
vii. Freezing Point – 0.59°C.
i. Pepsin – The enzyme pepsin, with HCI, digests proteins up to the stage of peptone.
ii. Rennin – Rennin coagulates caseinogen of milk.
iii. Gastric Lipase – Gastric lipase digests fat to some degree.
iv. HCI Acts as an Antiseptic – HCI acts as an antiseptic and causes some hydrolysis of all the foodstuffs.
v. Excretion – Toxins, heavy metals, certain alkaloids, etc., are excreted through gastric juice.
Digestive Juice # 3. Pancreatic Juice:
i. Total Quantity – About 500 ml per meal. About 1,500 ml in 24 hours.
ii. Reaction – Alkaline.
iii. Specific Gravity – 1.010 to 1.030
iv. pH – 8.0 – 8.3 (in dog).
i. Inorganic Constituents:
The distinguishing chemical characteristic is its high bicarbonate content. The principal bases are sodium and potassium. Small amounts of calcium, magnesium and zinc are also present.
ii. Organic Constituents:
The enzymes of pancreatic juice are trypsinogen, chymotrypsinogen, procarboxypeptidase, nucleotidases (ribonuclease and deoxyribonuclease), elastase, collagenase, pancreatic lipase, lecithinase, cholesterol esterase and amylase.
Its composition varies according to the means used to cause Secretion.
Table 9.1 shows the difference between secretin juice and pilocarpine juice:
Digestive Juice # 4. Succus Entericus (Intestinal Juice):
Intestinal juice, in pure form, is difficult to collect because it is mixed up with bile and pancreatic juice. It can be collected from fistula preparations, such as Thiry fistula, Thiry-Vella modification, and Mann Bollman fistula.
i. Total Quantity – Roughly about 1-2 litres in 24 hours. [Accurate measurement is not possible due to the great length of the small intestine.]
ii. Specific Gravity – Specific gravity -1.010.
iii. Reaction – Faintly acid to faintly alkaline.
iv. pH – Varies from 6.3 – 9.0 average 8.3.
1. Water – 98.5%
2. Solids – 1.5%
i. Inorganic – 0.8% salts of sodium, potassium, calcium and magnesium with that of chloride, bicarbonate and phosphate. The bicarbonate concentration is higher than it is in the blood or interstitial fluids.
ii. Organic – 0.7%.
a. Activator- Enteropeptidase (previously known as enterokinase). It activates trypsinogen into trypsin.
Intestinal Juice Enzymes:
i. Erepsin – A mixture of enzymes containing dipeptidases (break down dipeptides into amino acids) and amino peptidases (remove terminal amino acid containing free NH2 group from polypeptides).
ii. Enzymes – Several enzymes acting on the different fractions of nucleic acid, such as nuclease, nucleotidase and nucleosidase.
iii. Arginase Acts on arginine producing urea and ornithine.
2. Carbohydrate Splitting:
i. Amylase – Found in traces, acts on starch and dextrin.
ii. Sucrase (Invertase) – Digests cane sugar.
iii. Maltase – Acts on maltose.
v. Lactase – Breaks down lactose.
3. Fat-Splitting – Lipase.
4. Other Enzymes:
Alkaline phosphatase, cholesterol esterase, lecithinase, etc.
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.
As regards other enzymes they are mostly present in the epithelial cells. Peptidases (erepsin), lactase, maltase, sucrose (invertase) and lipase are found in the intestinal 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.
From this it can be concluded that the enzymes discussed above digest the foodstuffs in three ways:
1. 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.
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 come out of liver as the right and left hepatic ducts. The two ducts unite and form the common bile duct, which enter into the ducodenum, 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 (Fig. 9.25).
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. This necessarily indicates that bile must be stored somewhere. Gall-bladder acts as the chief storehouse. The common bile duct also stores some bile.
Composition of Bile:
Bile is a complex fluid containing various substances, some of which are merely waste products undergoing excretion, whereas others are products of secretion serving important physiological functions. In the gall-bladder bile is concentrated five to ten times and its alkalinity is reduced (vide ‘Gall-bladder’). The composition, as estimated by different observers, varies widely.
The usual composition and the characters are as follows:
i. Total Quantity – 500 -1,000 ml daily. On the average about 700 ml.
ii. Sp. Gravity – 1.010 -1.011 (Gall-bladder bile-1.026 -1.040).
iii. Colour – Human bile is yellowish-green. [In the carnivore, it is golden-yellow, due to the presence of more bilirubin. In herbivora, the colour is green, due to more biliverdin.]
iv. Taste – Bitter.
v. Consistency – Viscid, mucoid liquid.
vi. Reaction – Liver bile is definitely alkaline, pH 7.7. [Some hold pH 8.0 – 8.6] Gall-bladder bile is neutral or slightly alkaline (pH 7.0 – 7.6) or slightly acid (pH 6.8). That of dog and cat, definitely acid (pH 5.6).
Total solids, 2 -11%.
The chief constituents are:
i. Inorganic Salts:
Chlorides, carbonates and phosphates of Na, K and Ca and NaHCO3. The total base is equivalent to about 170 ml of (N/10) NaOH per 100 ml of liver bile (300 ml % in gall-bladder bile).
ii. Bile Salts:
Sodium taurocholate and sodium glycocholate. These are the most important constituents of bile and are synthesised by the liver (secretion).
iii. Bile Pigments:
Of which bilirubin and biliverdin are the chief .
iv. Cholesterol, Lecithin:
Cholesterol, lecithin and traces of fatty acids, soaps, etc. Cholesterol is probably an excretory product, because its amount in bile varies with its level in blood. It is kept in solution by the hydrotropic action of bile salts.
Average composition of human bile is given in Table 9.2.