Digestion process involves two processes namely, mechanical and chemical.
When the food is taken in the mouth, it passes through the alimentary tract. As it passes down, it is treated mechanically and chemically.
It refers to treatment of food materials with enzymes. The process can be studied by the digestion occurring in the various regions of alimentary canal. It is grouped in to three types, that is, (1).Buccal digestion or salivary digestion, (2) Gastric digestion (3) Instestinal digestion.
(1).Buccal digestion or salivary digestion:
Digestion of the food material occurs inside the Buccal cavity as salivary enzymes play the major role. Hence it is also called salivary digestion.
The food in the mouth is masticated during the mastication.
During such process food is crushed into smaller particles.
After it is crushed, it mixes with the secretions of salivary gland.
Salivary Gland of Man
It is present in the Buccal cavity. Vertebrates contains four pairs of salivary glands, but for man it has three pairs of salivary glands, namely,
(1) Parotid: These glands are present just below and infront of the ears
(2) Submaxillary or Submandibular: These glands are present in the
angles of the lower jaw and
(3) Sublingual: These glands are present below the tongue.
Saliva: The secretion of salivary gland is Saliva. The saliva of the human has the following salient features:
Colour: It is colourless. It is slightly cloudy because of the presence of cells and mucin.
Quantity: A man secretes 1 to 1.5 litres of saliva per day.
The nature of Saliva is slightly acidic with a pH of 6.02 to 7.05.
The water content is 99.5%.
The Solid content is 0.5%
Cellular components of saliva:yeast, bacteria, protozoa, leucocytes, etc,.
Inorganic salts include NaCl, KCl, CaCo3, Calcium Phosphate, Sodium Phosphate and Potassium thiocyanate.
Enzymes include ptyalin (amylase), lipase, carbonic anhydrase and a bacteriolytic enzyme called lysozyme.
Organic Substances like urea, amino acid, cholesterol and vitamins are present.
Gases such as Oxygen, N2 and Co2 are present in saliva.
Functions of Saliva:
It moisturizes the mouth and helps in speech.
Lubricates the mouth cavity and avoids thirsty.
It moisturizes the food and helps mastication and deglutition.
It cools the hot substances.
It is essential for the appreciation of taste.
It dilutes the irritant substances and prevents injury to the mucus membrane.
Saliva excretes toxic heavy metals, thiocynate, viruses of rabbies and mums, urea, etc,.
It washes down the food debris and thereby bacteria do not grow.
The bicarbonates, phosphates and mucin present in the saliva act as buffers.
Saliva is an important digestive agent and contains two digestive enzymes and they are ptyalin (amylase) and maltase.
Control of salivary secretion:
The secretion of saliva happens because of the reflex action. The physiological stimulus is the presence of food in the mouth. The food induces the mucous membrane of the mouth which in turn stimulates the salivary centre of brain.
(2) Gastric digestion:
Gastric juices are the secretion of gastric glands and it is present inside the stomach. When the food is swallowed, it reaches the stomach and it is treated both mechanically and chemically. The chemical changes are made due to gastric juices. About 3,50,00,000 gastric glands are present in man and it is present below the surface of epithelium. Gastric glands are simple tubular glands arranged as parallel tubes opening on the surface. Main tubular portion of the gland is called body. The inner end extends into the muscular mucosa where it terminates in a bulbar end called fundus of the gland. The neck of the gland connects the body of the isthmus and it communicates with the gastric crypt.
Gastric glands are formed of four types of cells namely:
Mucous neck cells
Chief cells or zymogenic cells or peptic cells
Oxyntic or parietal cells and
(3) Instestinal digestion:
Small intestine especially duodenum is significant because it receives pancreatic juice from the pancreas, bile juice from liver and intestinal juice from intestinal glands. Digestion occurs in the small intestine because of these events.
Pancreas: It is a combination of exocrine and endocrine gland. Exocrine component of pancreas is a tubular gland and is highly branches and the terminal portion of the glands is known as acni and alveoli. The alveoli resemble the alveoli of salivary glands and the alveoli are the secretary units. Main duct of pancreas is called duct of wirsung. The endocrine component of pancreas is islets of langerhans and it is present between the acni.
Pancreatic juice is secreted by the acini cells and the major enzymes present in the pancreatic juice are trypsin, chymotrypsin, carboxypeptidases, pancreopeptidase or elastase, amylopsin or amylase, steapsin or lipase, sucrose, maltase, lactase and nuclease.
It secretes bile juice. Liver of man contains two lobes and is formed of hepatic cells. It is composed of numerous capillaries and capillaries are called bile capillaries. Capillaries join together repeatedly to form bigger vessel and in each lobes they join together to form a hepatic duct. The two hepatic ducts come out and unite together to form the common bile duct. It opens into the duodenum along with the pancreatic duct through the ampulla of vater. The upper end of the bile duct gives out a branch called cystic duct. It ends in an oval sac called gall bladder. It is a reservoir of the bile secreted by the liver.
Bile: Bile is a product of secretion and excretion of liver. It is produced by the parenchymel cells of the liver. It is stored in the gall bladder. Bile is formed continuously in the liver but it flows into the duodenum from the gall bladder.