Digestive glands in human body include salivary glands, gastric glands, liver, pancreas and intestinal glands.
Table of Contents
1. Salivary gland
There are three pairs of salivary glands: -parotid, sublingual and submaxillary or submandibular
(a) Parotid gland
These are the largest salivary glands. They lie on the sides of the face, just below and in front of the ears. The parotid ducts, also called Stensen’s duct or Parotid duct., open into the vestibule opposite the upper second molar teeth. Viral infection of the parotid glands, causing swelling and pain, is the disease called mumps.
(b) Sublingual glands
These lie under the front part of the tongue. The sublingual ducts , also termed ducts of Rivinus, also open under the that tongue.
(c) Submaxillary glands
These lie at the angles of the lower jaw. The submaxillary ducts, also known as Wharton’s ducts , open under the tongue.
The salivary glands secrete a viscous fluid , called saliva. It contains water salts mucin and an enzyme salivary amylase or ptyalin .It’s pH is nearly neutral , being 6.7 . The saliva is carried into the buccal cavity by the salivary ducts mentioned above. About 1-1.5 liters of saliva and mucus are secreted daily in the buccal cavity.
Saliva lacks amylase in domestic herbivores, such as cow and buffalo, and in carnivores, such as dog, tiger and lion. The salivary glands are under neural control. Salivation (flow of saliva) is activated by the sight, smell, idea and talk of food and by the presence of food in the buccal cavity.
Functions of Saliva
Saliva has many functions.
(i) It moistens and lubricates the buccal mucosa, tongue and lips, thus making speech possible . Moistening must be carried on continuously because of evaporation and swallowing of saliva.
(ii) It also moistens food and changes it to a semisolid mass for easy swallowing.
(iii) Moistening food allows it to be tasted. The taste buds are stimulated by chemicals in solution.
(iv) Saliva washes mouth and tongue clear of cellular and food debris.
Note -: In fever, the salivary secretion is reduced and tongue becomes dirty.
The gastric glands are in numerable , microscopic, simple ( straight or branched ) , tubular glands in the wall of the stomach .
They have 3 more common types of gland cells :-
(i) Peptic (chief, zymogen) cells that are usually basal in location and produce enzymes,
(ii) large ,deep – seated oxyntic acid, parietal) cells that secrete HCI,
(iii) mucous cells which add mucus.
The three secretions together form an acidic gastric juice with pH 2. It is discharged into the stomach. It contains water, salts, mucin, hydrochloric acid, two proenzymes or zymogens : pepsinogen or propepsin and prorennin, and an enzyme gastric lipase. About 2 to 5 liters of gastric juice is secreted per day. Secretion gastric juice is controlled by nerves and hormones. Gastric the secretion is stimulated by thought of, smell of food, chewing of food, and contact food with stomach wall.
The parietal cells also secrete the Castle’s intrinsic gastric factor that helps in the absorption of vitamin B12. The gastric glands also have two less common types of cells : argentaffin cells and endocrine cells .
The liver is the largest gland in the body, weighing about 1.6 kg. and forms about 1 / 40th of the body weight. It is located in the upper right side of the abdominal cavity. It is dark – red and spongy. Its superior (upper) surface is convex and fits into the concavity of the diaphragm. It is attached to the diaphragm by a median vertical fold of peritoneum. The liver consists of two main lobes: large right lobe and much smaller left lobe . Two small lobes called quadrate lobe and caudate lobe are also present behind main lobes. Liver consists of small structural and functional units called lobules . It shaped like polygonal cylinders .
Functions of Liver
The main functions are given below -:
Digestion -The bile produced in liver helps in the digestion of food .
Regulation of blood sugar – The liver separates the excess of sugar from the blood and stores it in its cells as glycogen ( animal starch) .
Deamination – In the liver ,the amino acids coming from alimentary canal are sorted out . Those are necessary for proteins synthesis are distributed to the tissues.
Detoxification – Liver makes ineffective the toxic ( poisonous) substances formed in the body itself or introduce from outside. Ammonia resulting from deamination is immediately converted into harmless urea .
Excretion – Liver collects haemoglobin of the worn out red blood corpuscles and changes it into bile pigments. The bile pigments and cholesterol pass into the alimentary canal along with the bile for eliminating with faeces.
Blood clotting – Liver produces heparin , prothrombin, and fibrinogen . Heparin prevent clotting of blood inside the blood vessels. Prothrombin and fibrinogen help in the clotting of blood at the injured surface .
Formation of Red Blood Corpuscles – Liver produces red blood corpuscles in the embryo . The process of formation of Red Blood Corpuscles is called erythropoiesis.
Phagocytosis – Foreign matter , dead cells and bacteria are disposed of in the liver . These are engulfed and digested by large ,star- shaped cells called the macrophages or Kupffer’s cells [ First described by Karl Wilhelm von Kupffer in 1876 as “sternzellen” (star cells or stellate cells) ] , that occur in the lining of liver sinusoids.
Storage- Besides Glycogen, liver stores
(I) lipids such as fats,fatty acids and cholesterol.
(II) minerals like copper and iron
(III) vitamins namely, A,B12,D,E
(IV) bile in the gall bladder.
A thin – walled,pyriform sac, the gall bladder, lies in groove in the inferior (lower) surface of the right lobe. The gall bladder stores and concentrates bile secreted by the liver . Rat and horse lack gall bladder. It may removed in man without ill effects . The bile is a yellowish -green fluid and gives the same colour to the gall bladder. Bile is carried from the liver by the bile duct , also called ductus choledous . The bile duct passes downward and is joined by the pancreatic duct . A strong sphincter muscle of Boyden surrounds the bile duct before it is joined by the pancreatic duct. This muscle closes when there is no food in the duodenum .
Notes – Bile contains no enzymes.
-Liver can regenerate rapidly .
Functions of bile
The bile is a yellowish brown or greenish , alkaline fluid . It secreted more or less continuously by liver cells and stored in the gall bladder till required. It has no enzymes,no chemical action on food . It contains water ,mucin, lecithin, cholesterol,bile salts (sodium carbonate, sodium glycocholate and sodium taurocholate ) and bile pigments green biliverdin and yellow bilirubin.Bile serves the following functions — Neutralization of HCl, Emulsification, Activate of lipase , Prevention of decomposition , Excretion , Stimulation of peristalsis.
Sometimes cholesterol precipitation as crystals and combines with bile salts and pigments to form gall stones in the gall bladder and bile ducts. Large gall stones may block the bile duct, causing poor fat and pain occur . Bile may be absorbed into the blood and cause yellowing of eyes and skin, called obstructive jaundice .
The pancreas is an elongated yellowish gland lying marginally behind the stomach in the abdomen. It is about 16 cm long, 2.5cm wide and weighs about 60 gms. It consists of head, body and tail . The head is located in the curve of the duodenum and the tail meets the spleen. The pancreas is both an endocrine and an exocrine organ. The exocrine tissue of the pancreas consists of rounded lobules (acini) that secrete an alkaline pancreatic juice with pH 8.8. About a liter of pancreatic juice is secreted each day. The juice is carried by the pancreatic duct, also called duct of Wirsung into the duodenum through the hepatopancreatic ampulla. An accessory pancreatic duct, also named duct of Santorini, may sometimes lead directly into the duodenum. The pancreatic juice contains sodium bicarbonate, 3 proenzymes:-trypsinogen,chymotrypsinogen,and procarboxypeptidases ,and several enzymes – pancreatic amylase, pancreatic lipase , and nucleases.
The juice acts on all types of food: – Proteins , starches ,fats ,and nucleic acids. Its sodium bicarbonate neutralizes the acidity of chyme caused by HCl. The factors which stimulate gastric secretion also enlist pancreatic secretion. The endocrine part of pancreas.
5- Intestinal glands
The intestinal glands are numerous and microscopic . They lie in the wall of small intestine .They are of 2 types –
Crypts of Lieberkuhn and Brunner’s glands.
The crypts of Lieberkuhn are simple tubular glands and occur throughout the small intestine between the villi . They secrete enzymes and mucus. The Brunner’s glands are branched tubular glands and are confined to the duodenum. They secrete alkaline watery fluid, a little enzyme and mucus. They secrete alkaline watery fluid ,a little enzyme and mucus. They open into the crypts of Lieberkuhn. The mixture of secretions is called intestinal juice or succuse entericus .About 2 to 3 liters of intestinal juice is The secreted each day. It is alkaline (pH 8.3) and is erine poured into the intestine. The juice contains many enzymes, e.g. aminopeptidases ,dipeptidases , Intestinal amylase , maltase ,isomaltase ,limit dextrinash , sucrase ,lactase , Intestinal lipase , nucleotidases or nucleophosphatases , nucleotidases , and enteropeptidase ( enterokinase ) .
These enzymes act on all types of food.
Renewal of Intestinal cells :-
Epithelium of the crypts produces new cells which may differentiate into various cell types of the intestine.
Paneth and Argentaffin Cells :-
The crypts of Lieberkuhn have in their base (i) paneth cells, which secrete antibacterial lysozyme; and (ii) gentaffin cells, that are produce a potent vasoconstrictor serotonin.
Endocrine Cells :-
The intestinal epithelium also contains endocrine cells that secrete many hormones.
Mucous Glands :-
In addition to the digestive glands and other specialized cells described above the, practically entire digestive tract has mucous glands that produce mucus .
☆Movement of Alimentary canal
The alimentary canal performs three types of movements : peristaltic , segmenting and pendular to propel the food and to mix it with the digestive juices.
These are waves of alternate ring like contraction and relaxation of intestinal wall that pass analysed and gradually carry the food and secretions toward the anus . Peristalsis is controlled by the myoenteric nerve plexus present in the intestinal wall. In the stomach, the peristaltic waves are not strong enough to propel the food but are strong enough to mix the food with the secretions.
2. Segmenting Movement
These are a series of regularly-spaced, ring like constrictions dividing the small intestine into a series of segments . These constrictions soon disappear and new ones appear alternating with them. These movements serve to mix the food with the juices, and to bring the absorptive surface in close contact with the digested food.
In the large intestine, similar but much slower contractions, called haustrations, occur, these slowly roll the faecal matter over and over, allowing almost complete absorption of water and electrolytes.
3. Pendular Movement
These movements are ring-like contractions that pass forward and backward over short lengths of intestine. They cause loops if intestine to away slowly in a pendular manner. They serve to churn and mix the contents.