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Digestive System in Humans

The digestive system can be broadly divided into two parts:
  1. Alimentary canal and
  2. Digestive glands.

Alimentary Canal

The alimentary canal is a long coiled tube of varying diameter and thickness. It is attached to the body wall by mesenteries. It is open at both ends, starting from mouth anteriorly and ending at the anus, posteriorly. The alimentary canal can be subdivided into various parts.

These are:
  1. buccal cavity
  2. pharynx
  3. oesophagus
  4. stomach
  5. small intestine
  6. large intestine

The Digestive System of Man

Buccal Cavity
The mouth opens into a large space called the buccal cavity or mouth cavity. It is bounded by the palate dorsally, by the throat ventrally and by the cheeks laterally. The palate separates the buccal cavity from the nasal chambers. The anterior part of the palate is hard and bony (the hard palate) and the posterior fleshy part of the palate is called the soft palate.
  1. Tongue
    A fleshy muscular, protrusive organ called the tongue is attached to the floor of the buccal cavity. Its anterior end is free and the posterior end is fixed to the floor of the mouth cavity. The tongue can move in all directions and has the surface papillae which contain taste buds in them.
  2. Teeth
    Embedded in both the upper and the lower jaws are the teeth which help in tearing and chewing (mastication) the food. Teeth are of different types namely incisors, canines, premolars and molars. There are two pairs of incisors, one pair of canines, two pairs of premolars and three pairs of molars on the upper jaw of the human being. The same number of teeth are present on the lower jaw as well. Thus human beings have a total of 32 teeth. However, the teeth found during the early childhood are not the permanent teeth. They are called milk teeth which are shed and replaced by permanent teeth. The molars are not shed and they have no deciduous or milk predecessors.

    The teeth are epidermal structures arranged in the sockets of the ovular processes of the jaws. Each tooth has a crown (the exposed part), the neck (the middle part) and the root (the part embedded in the socket). Internally, the tooth contains a cavity filled with dental pulp formed of connective tissue, blood vessels and nerves.

Diagram of Vertical Section of Human Tooth

  1. Salivary Glands
    Dry food, when it enters the mouth cavity is mixed with saliva to moisten it. Saliva is produced by the salivary glands. There are three pairs of salivary glands situated around and opening into the buccal cavity. These are:
    1. Sublingual glands, lying below the tongue,
    2. Submaxillary glands, lying below the maxilla in the middle of the lower jaw, and
    3. Parotid glands, situated beneath the ears.
The buccal cavity opens posteriorly into the pharynx. The internal nasal apertures and a pair of eustachian tubes also open into the pharynx. Posteriorly the pharynx opens into the oesophagus and the larynx. The masticated and lubricated food always finds its way to the oesophagus. Any food material entering the laryngeal portion accidentally is thrown out by coughing.

It is a thin-walled, long, narrow, straight and muscular tube. It passes through the entire length of the neck and thorax and after piercing through the diaphragm, enters the stomach. The oesophagus is normally kept closed except when food materials are passing through.

The stomach is situated transversely in the abdomen slightly towards its left side, just behind the diaphragm. It could be subdivided into three parts:
  1. Cardiac Portion: A broad part of the stomach into which the oesophagus opens.
  2. Fundus Portion: The middle portion forming the main portion of the stomach.
  3. Pyloric Portion: A narrower part opening into the duodenum, the opening being guarded by the pyloric sphincter.

Different Regions of the Stomach

Small Intestine
Two different parts can be distinguished in the small intestine. These are:
  1. Duodenum
    It is a narrow, thin-walled tube forming the first part of the intestine. It runs backwards and then forwards to form a U-shaped loop. The inner lining of the duodenum is produced into transverse folds. The bile duct from the liver and the pancreatic duct open into the duodenum.
  2. Ileum
    It is a long (2-3 metres) thin walled coiled tube following the duodenal part of the intestine. Its inner lining is produced into numerous finger-like processes known as intestinal villi. The villi increase the absorptive surface of the intestine. The middle portion of the small intestine, just after the duodenum, is often distinguished as jejunum. However, there is no clear demarcation between the jejunum and the ileum.
Large Intestine
Large intestine includes two parts:
  1. Colon
    It is the proximal part of the large intestine and is about 1.5 meters long. Just after the junction of the small and large intestine is a finger-like organ attached to the large intestine. This is called the vermiform appendix.
  2. Rectum
    This is the terminal portion of the large intestine. It contains at intervals, the faecal pellets which give it a beaded appearance. The rectum opens outside through the anus.
Histology of the Digestive Organs
Four major layers of different types form the wall of the alimentary canal. The cavity is referred to as the lumen. The innermost layer is the mucosa and just outside the mucosa is the submucosa.

A transverse section showing the general plan of arrangement of tissues in the stomach and the intestine

A transverse section of the stomach examined under the microscope shows the following details:

The outermost layer called the serosa which is a connective tissue covering. The next layer is made up of smooth muscle fibres arranged longitudinally. This muscular layer is the muscularis externa. This is followed internally by a layer of circular muscles, the muscularis interna. The next layer, called the submucosa, has loose collagen fibres, blood, lymphatic vessels and nerve fibres. This layer is thrown into several folds. The innermost region, forming the mucous membrane is the mucosa. This is lined by simple columnar epithelium. The muscularis mucosa is a layer of smooth muscles lying in between the submucosa and the mucosa.

The mucous consists of simple columnar epithelium which is folded into many ridges. The columnar epithelium contains many mucous producing cells called goblet cells. The surface of the epithelium is produced into small irregular areas and grooves called gastric pits. At the base of the gastric pits are the gastric glands supported by delicate connective tissue fibres, the lamina propria. Gastric glands are of three types:
  1. fundic glands, distributed in the mucus,
  2. cardiac glands and
  3. pyloric glands found in the cardiac and pyloric portion of the stomach. There are three types of cells in these glands which play a major role in the digestion of food. These cells are:
    1. chief cells which produce digestive enzymes,
    2. parietal cells which produce dilute hydrochloric acid HCl, and
    3. mucous cells which produce mucus.

Section of a Portion of the Stomach Wall

Small Intestine
The basic histological structure of the small intestine is very similar to that of the stomach. However, the submucosa is produced into numerous finger-shaped folds called villi. Each villus consists of a lining of a single layer of a simple columnar epithelium which has absorbing cells and mucous cells called goblet cells. The villus encloses a capillary loop and a small lymphatic vessel (the lacteal). Lacteals are channels of absorption of glycerol and fatty acids which are products of the digestion of fat. From the lacteals these products find their way to the villi in the glandular pits called the Crypts of Lieberkuhn. These are simple glands containing goblet cells and parietal cells which are secretory in function. The parietal cells produce digestive enzymes. Certain isolated patches of lymphoid tissues called Peyer's patches are also found in the wall of the intestine. The intestinal submucosa bears a close resemblance to that of the stomach. There are two muscle layers in the muscularis externa, an outer longitudinal and an inner circular. The serosa forms the outer covering for the muscles.

A transverse section of the intestine showing two villi, one showing lymphatics and the other blood vessels. B. An enlarged view of the gland of Lieberkuhn with the paneth and goblet cells

The histological picture of the large intestine is similar to that of the small intestine. The mucous layer of the large intestine does not contain any villi. However, numerous mucus secreting cells are found in the mucosa of the large intestine.

Digestive Glands

The glands which help in the digestion of food and are associated with the alimentary canal are called digestive glands which are of two main types: (a) the liver, and (b) the pancreas.

  1. The Liver
    It is the largest gland of the body weighing about 1.5kg in the adult and is situated in the abdominal cavity touching the diaphragm. It produces bile and has also many other important functions.

A. Section of liver showing histological details. B. The relationship between bile canaliculi and liver cells

Section of liver (magnified) to show Kupffer cell and hepatic sinusoid
The liver is made up of small tubules called lobules, with a central vein. The space between the lobules is filled with connective tissue containing blood vessels, nerves, small ducts and lymph vessels. The liver cells or hepatic cells are arranged in cords or rows and are in close contact with blood capillaries and blood spaces or sinusoids. The cells are polyhedral or angular with a centrally located nucleus. Anchored to the wall of the sinusoids are certain cells called Kupffer cells which are phagocytic in function. In between the hepatic cells are tiny spaces, bile canaliculi which lead to small bile ducts. Smaller ducts join together to form larger hepatic ducts which join to form the common bile duct.

Diagram showing the joining of cystic duct, hepatic duct and pancreatic duct to the duodenum
This common bile duct opens into the intestine (duodenum). Bile is secreted by the secretory cells of the liver. When there is no food in the intestine, bile does not flow into the intestine. Instead, it flows through the cystic duct and remains stored in the gall bladder. When food arrives in the small intestine, the gall bladder contracts and the bile stored there is expelled into the small intestine. Bile contains bile salts which aid in digestion and absorption of food. Bile also contains bile pigments which are excretory waste products.
Bile pigments
Normally, in healthy individuals, the plasma contains only a small quantity of bile pigments. Bilirubin is such a bile pigment produced by the catabolism of haemoglobin from destroyed RBC. Bile pigment is removed from the blood by the liver and secreted into the bile. Through bile, these pigment reach the intestine and are excreted along with the faeces. The amount of bile pigments in blood some times reaches a high level. Then, the skin, the white of the eyes and mucous membranes are stained yellow, this condition is known as jaundice which sometimes results from a disease of the liver caused by a virus.
  1. Pancreas
    Pancreas is a pinkish irregular organ situated in the mesentery connecting the two limbs of the duodenum. It consists of a number of branching tubes ending in secretory sacs called acini. Functionally, the pancreas is a compound gland with two parts (i) an exocrine part, and (ii) an endocrine part.
    1. Exocrine Part of the Pancreas
      This part consists of the secretory cells which produce pancreatic juice containing many digestive enzymes. These secretory products are drained into the duodenum by the pancreatic duct. The secretory cells are arranged in groups called acini (singular acinus) with a central space called the lumen. In between the acini are connective tissues containing blood vessels and nerves.

Diagram of section of pancreas showing the exocrine and endocrine portions
  1. Endocrine Part of the Pancreas
    Between the pancreatic acini are groups of cells known as islets of Langerhans. These groups of cells actually consist of two types of cells. Of these two types the α cells produce a hormone called glucagon and the β-cells produce another important hormone, insulin. These hormones are directly secreted into the blood.

Process of Digestion

Mammals differ in their food habits. Groups of mammals, like the tiger, lion and dog are called carnivores, because they are meat eaters. Another group called the herbivores, which include the cows, buffaloes, horses, etc. eat only plant materials. Yet another group eats both plant and animal materials. These are omnivores e.g. human being. The digestive organs and teeth of different mammals are modified to suit the particular type of food. The carnivores have well-developed canine teeth. These help them to tear the meat. The typical herbivores have no canines and their digestive system is modified in such a way that they can take large quantities of plant material, at a time, and digest it leisurely. They have larger small intestine to digest the plant material.

Animals eat raw or uncooked food whereas human beings cook the food before consuming. Cooking is the process by which food substances are softened so that they can be consumed easily.

The food, whatever may be its source (animal or plant) consists of complex carbohydrates like starch and glycogen, lipids such as oil and fat, and complex protein. These complex substances cannot be utilised by the animals as such. The complex substances have to be broken down to simpler substances, starch to simple sugars, lipids to fatty acid and glycerol and protein to amino acids, before these can be absorbed into the blood and transported to and assimilated in the individual cells of the body. Digestion essentially is the process by which complex food materials are broken down to simpler compounds so that these can be absorbed and assimilated by the body.

The digestion of food involves a series of chemical reactions occurring inside the digestive system. These chemical reactions can take place at higher temperatures only, if outside the body. But in the body of the mammal these reactions take place at the body temperature of the animal, that is, at 37° C. This is because the reactions are catalysed by a group of biological catalysts called enzymes. All metabolic reactions in the body such as synthesis of various substances, breakdown of materials and also transformation of substances, are all catalysed, by various types of enzymes.


All enzymes are proteins and their function is to catalyse biological reactions. Enzymes are characterised by certain properties. They are specific; any one enzyme acts only on one kind of substance called substrate. An enzyme is usually identified according to the substrate it acts on or by the reaction it catalyses. Maltase, for example, is an enzyme that catalyses the conversion of maltose to glucose; sucrase acts on sucrose to convert it into glucose. However, the enzymes such as ptyalin, pepsin, trypsin etc., are exceptions to this rule.
The action of enzymes is influenced by a variety of factors. Of these, the most important are (i) temperature, and (ii) pH (acidity or alkalinity).

The enzyme action is temperature dependent. The optional temperature for an enzyme action is 37° C. At higher temperatures the activity of the enzyme decreases and at 45° C or above the enzyme gets completely inactivated. Being proteins they undergo structural changes which result in their loss of functional activity. Such loss of structural and functional integrity is called denaturation. At lower temperatures the enzymes show minimal activity.

pH (Degree of Acidity and Alkalinity)
The acidity of the medium in which the enzyme acts increases with an increase in the number of hydrogen ions (H+). The alkalinity of the medium, on the other hand, increases with an increase in the number of hydroxyl ions (OH-). The pH is a number that indicates the alkalinity or acidity of the medium. Neutral pH is 7; acidic solutions have pH below 7 and alkaline solutions have pH above 7 (that is between 7 and 14). Every enzyme acts optimally at a definite pH. The enzymes in the stomach act at acidic pH and the intestinal enzymes act at alkaline pH.

Digestion in the Mouth

The food material in the mouth is masticated (chewed) by the teeth and broken down to smaller bits. During this process the food is lubricated by saliva secreted by salivary glands. The saliva contains an enzyme called salivary amylase or ptyalin. This enzyme acts on starch (contained in the food) by fragmenting the large starch molecule into a series of smaller compounds called dextrins. This enzyme works best at a pH of 6.7 which is the pH of the saliva. The dextrins are complex sugars, the further digestion of which occurs in the intestine.

The act of mastication and lubrication of food in the mouth aids in swallowing. The act of swallowing is a reflex initiated by voluntary action of the tongue so that the food is pushed backwards into the pharynx. The contraction of the pharyngeal muscles in waves (peristalsis) pushes the food materials into the oesophagus.

Digestion in Stomach

The well masticated, lubricated and partly digested food enters the stomach through the oesophagus. The stomach apart from its digestive function also serves as a reservoir for food. In the stomach the food is mixed with gastric juice, secreted by the gastric glands. The gastric juice contains a protein-splitting (proteolysis) enzyme pepsin and hydrochloric acid (HCl). The concentration of the HCl in gastric juice is about 0.5%. The presence of HCl makes the gastric juice acidic and the pH is between 1 and 2. In this acidic pH the digestion of starch stops and the salivary amylase does not act. The HCl also prevents all bacterial growth in the stomach.

The enzyme, pepsin is secreted is secreted by the gastric glands. It acts upon proteins, converting them into a number of products such as proteases, polypeptides and peptones. A few amino acids are also released but not many. Pepsin is secreted in an inactive form called pepsinogen, which is converted into active pepsin by the action of hydrochloric acid.
Rennin is another enzyme found in the gastric juice. In infants, it acts on a soluble protein, casein, found in milk. Renin converts milk into curd (soluble casein into an insoluble form) so that it is retained in the stomach for a longer time. This in turn allows other proteolytic enzymes like pepsin to have a chance to act on it. In the adult, rennin is not a very important enzyme. Apart from the proteolytic enzymes, gastric juice also contains gastric lipase which acts on fats to convert them into fatty acid and glycerol.

The semi liquid, partially digested food in the stomach is called chyme which passes into the duodenum by the wavelike contractions of the stomach (peristalsis). The peristaltic movements of the stomach are under the control of both the vagus nerve (parasympathetic) and sympathetic nerves. The opening between the pyloric region of the stomach and the duodenum is surrounded by a ring of smooth muscle and connective tissue (pyloric sphincter). The pyloric sphincter regulates the entry of chyme from the pyloric portion of the stomach to the duodenum.

Digestion in the Duodenum and Small Intestine

In the duodenum food is mixed with the bile from the liver and the pancreatic juice from the pancreas. The bile and the pancreatic juice contain sodium bicarbonate (NaHCO3) which neutralizes the HCl. The pancreatic juice contains a number of enzymes. Two of these, trypsin and chymo-trypsin digest proteins. Of the two, trypsin is more active. It is secreted in an inactive form, called trypsinogen, which is activated by a substance produced in the small intestine, called enterokinase.

Table - Summary of the Action of Digestive Enzymes

Part of the Digestive system

Source of Enzymes


Substances Acted upon


Mouth Salivary Glands Amylase, Ptyalin


Complex Sugars
Stomach Gastric glands Pepsin Proteins Peptones & aminoacids
Pancreas Pancreatic Juice Trypsin, Amylase, Lipase Proteins, Carbohydrates Peptides, Maltose or sugars, Fatty acids and Glycerol
Small Intestine Intestinal glands Erypsin, Maltase, Lactase, sucrose Peptides, Maltose, Lactose, sucrose Amino Acids, Glucose And Other sugars

Trypsin acts on proteins and converts them into peptides at an alkaline pH of 8.6. As a result of the action of the proteolytic enzymes, considerable quantities of amino acids are released from the proteins. However, a great deal of the protein is still present as small fragments called polypeptides which require further digestion.

Another pancreatic enzyme, amylopsin (also called 'pancreatic amylase') acts on the dextrins produced by ptyalin and other polysaccharides like starch. All of these materials are broken down to maltose (disaccharide).

Finally, a fat-splitting enzyme, pancreatic lipase, present in the pancreatic juice splits fat into fatty acid and glycerol. The action of lipase is greatly aided by the presence of bile salts secreted by the liver and which enter the duodenum with the bile. Bile salts help to emulsify the fat so that lipase can act on them.

There are many glands present in the wall of the small intestine. These secrete what is called the succus entericus or intestinal juice which contains the enzymes necessary to complete the process of digestion. Erypsin completes the digestion of polypeptides remaining after the action of trypsin. The products of this digestion are the final products of protein digestion, the amino acids, which can now be absorbed. Sucrase, maltase and lactase are enzymes present in the intestinal juice that act on the disaccharides, sucrose, maltose and lactose respectively converting them into simpler sugars (monosaccharides) like glucose, fructose and galactose, which are easily absorbable.

Cellulose is a complex polysaccharide abundant in the food of herbivorous animals. Most mammals do not have the enzyme cellulase to digest cellulose. Herbivorous mammals such as cows have bacteria in the intestine which digest the cellulose by fermentation.

The intestinal enzymes act at an alkaline pH. Once the process of intestinal digestion is complete, the intestine pushes the food downward by peristaltic movements of the intestinal wall. The movement of the intestine is under the control of nerves. The sympathetic nervous system has inhibitory influences on the intestinal mobility, whereas the parasympathetic system enhances the motility by a stimulatory influence.

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