The digestive system includes the gastrointestinal tract which is from mouth to anus and the glandular organs. The digestive system serves to transfer organic molecules, salts, and water from the external environment to the body’s internal environment.
Most of the food taken into the mouth are large particles containing macromolecules such as polysaccharides and proteins. As such they cannot be absorbed by the intestinal wall.
These must be dissolved and broken down into smaller molecules. This process is known as digestion. Digestion is accomplished by substances called Enzymes produced from the digestive glands. The enzymes are biocatalysts in the food breakdown process.
The Process of Digestion
The digestion starts with chewing, in the mouth. Chewing breaks up large pieces of food into smaller particles that can be swallowed without choking. Chewing is done by teeth, tongue, jaws, and saliva.
Chewing is controlled by the somatic nerves to the skeletal muscles of the mouth and jaw.
Rhythmic chewing motions are reflex activated by the pressure of food against the gums, hard palate at the roof of the mouth and tongue.
The saliva is secreted by three pairs of exocrine glands, namely: Parotid, Submandibular, and Sublingual. The daily secretion of saliva ranges from 1000 to 1500ml and it contains Amylase and Mucin, which are organic substances.
The salivary amylase or ptyalin can act on starch and converts cooked starch into the disaccharide, maltose. Mucin is a glycoprotein that helps in the lubrication of food.
The lubricated, swallowable form of food is called a bolus. The salivary secretion is controlled by reflex activities.
Swallowing is complex reflex activity. Swallowing is controlled by the swallowing centre in the medulla oblongata. During swallowing the soft palate is elevated, the larynx gets raised.
The tongue forces the food back into the pharynx, the epiglottis closes the glottis and the food slowly passes into the oesophagus. The oesophageal phase begins with relaxation of the upper oesophageal sphincter.
In the oesophagus, the food is moved towards the stomach by a progressive wave of muscle contractions that proceeds down to the stomach. Such waves of contraction in the muscle layer surrounding a tube are known as Peristaltic waves.
In the oesophagus one peristaltic wave takes about 9 seconds to reach the stomach. Due to peristaltic waves, swallowing can occur even when the person is upside down.
The stomach is a wide chamber, located below the diaphragm. The size and shape of the stomach depend on the food inside it. The stomach volume during feeding may increase up to 1.5 litres.
The stomach’s primary contractile action will produce peristaltic waves. Each wave starts in the body of the stomach and proceeds towards the pyloric region. The initial wave influences the muscles to close the pyloric sphincter, a ring of smooth muscles between the stomach and the duodenum.
The inner wall of the stomach is lined with gastric glands. There are nearly 40 million glands engaged in producing gastric juice. The chief cells of the lining of the stomach secrete enzymes and the parietal cells (oxyntic cells) produce HCL to create an acidic medium for enzymes.
The enzymes of the stomach are pepsin and rennin. Pepsin is secreted in an inactive precursor form known as Pepsinogen. The activity due to Hcl converts pepsinogen into pepsin.
Pepsin hydrolysis the proteins into short polypeptide chains and peptones. It is most effective in an acidic environment. Rennin acts on soluble milk protein caesinogen and converts it into insoluble casein.
In the presence of calcium ions, casein is precipitated as an insoluble calcium-casein compound (curds). Repeated peristaltic waves in the stomach help to soften the food.
The frequency of contraction is determined by the basic electrical rhythm and remains essentially constant. It is also aided by neutral and hormonal influences.
The food leaves the stomach in the form of chyme and enters the upper small intestine at periodic intervals.
It is about 5-7 meters long. It is divided into three segments namely the initial short segment the duodenum, the jejunum, and the longest segment the ileum.
The food is propelled down into the duodenum due to the peristaltic action of the stomach wall.
The Pyloric sphincter located at the junction of the stomach and duodenum regulates the movement of the chyme. The food in the small intestine is mixed with three juices namely bile, juice, pancreatic juice, and intestinal juice.
Bile Juice is a brownish-green, alkaline secretion of the liver. It is stored in the gall bladder and poured into the duodenum via the bile duct. The bile contains water, mucus, inorganic salts, cholesterol, and bile salts.
The bile salts emulsify fats and help enzymes like lipase to act upon fats. During emulsification, the bile salts convert bigger fat particles into smaller fat globules.
Pancreatic Juice is an alkaline fluid of pH 7 to 8. It is transported to the duodenum through the pancreatic duct. It contains water, mineral salts, and a variety of enzymes like trypsin, Chymotrypsin which are secreted in the form of inactive precursors trypsinogen and chymotrypsinogen.
The precursors are activated by enterokinase of the intestinal juice, the amylytic enzyme amylase, the pancreatic lipase (steapesin), carboxypeptidase, and nuclease.
The enzyme trypsin hydrolysis proteins into polypeptides and peptones. Chymotrypsin hydrolysis peptide bonds are associated with specific amino acids like phenylalanine, tyrosine, or tryptophan. It results in large peptides.
Carboxypeptidase is an exopeptidase. It attacks the peptide bonds at the carboxyl end of the polypeptide chain resulting in di, tripeptides, and amino acids. The pancreatic amylase converts starch into maltose.
The lipase acts on emulsified fat (triglycerides) and hydrolysis them into free fatty acids and monoglycerides. Monoglycerides may be further hydrolyzed to fatty acid and glycerol.
Intestinal Juice (Succus Entericus)
Absorption and Assimilation
Due to digestion, all macromolecules of food are converted into their corresponding monomeric units. Carbohydrates are broken into monosaccharides such as glucose and fructose. Proteins are hydrolyzed into amino acids.
Lipids get broken into glycerol and fatty acids. The simpler organic molecules along with minerals, vitamins, and water enter into body fluids through the villi. The villi are small microscopic finger-like projections.
Each villus is an absorbing unit consisting of a lacteal duct in the middle surrounded by a fine network of blood capillaries. While the fatty acids are absorbed by the lymph duct, other materials are absorbed either actively or passively by the capillaries of the villi.
From the lumen of the alimentary canal absorbed food materials are carried to the liver through the hepatic portal vein. From the liver, materials are transported to all other regions of the body for utilization. This conversion of food into energy and cellular organization is called assimilation.