Function of gastric glands
Biomacromolecules which we consume in our food are not directly utilised by our body in its original form. Thus, they are subjected to a process called digestion and the system that helps in complete process of digestion by mechanical and biochemical methods is called digestive system.
The human digestive system consists of various parts (organs and glands) that are concerned with the uptake, digestion and elimination of indigestible remains of food from the body.
Human alimentary canal and its associated glands are shown in the figure below
The alimentary canal in human beings is a long tube (about 8 to 10 metres in length ) which begins with an anterior opening i.e., mouth and ends posteriorly through the anus The various parts associated with the alimentary canal are as follows
It is a transverse slit bounded by two soft, movable lips which are covered with skin on the outer side and lined with mucous membrane on the inner side.
Mouth mainly leads to vestibule which is a narrow space enclosed between the lips and cheeks externally and the gums and teeth internally. Its lining contains mucous glands.
It is bounded by lips and cheeks. It contains teeth, tongue and salivary glands. Mouth (buccal) passes through both the jaws. The uppermost portion of the buccal (mouth) cavity is called palate. The upper and lower jaws of the buccal cavity consist of two separate sequences of teeth.
These are hard structures present in the mouth and both the jaws (i.e., upper anda jaw). Each tooth is embedded in a socket of jaw bone. A tooth consists of 3 maiori namely crown (upper part), neck (middle part) and root (lower part).
Types of Teeth
Homodont ::- All the teeth are same type, e.g., fishes.
Heterodont ::- Different types of teeth are present , eg., humans
On the basis of their attachment
- Acrodont ::- Attached to the crest of bone, e.g., snake.
- Pleurodont ::- Attached to the median side of the bone ,eg., Lizard.
- Thecodont ::- Attached to the bony socket, eg., alligator.
On the basic of their appearance
- Monophyodont ::- Appear once in lifetime, e.g., 3rd molar and all premolars of humans.
- Diphyodont ::- Appear twice in lifetime, e.g., incisors, camines, first and second molars of humans.
- Polyphyodont ::- Appear many times in lifetime, e.g., in most lower vertebrates.
In human beings, the number of teeth present is 32,16 teeth present in each jaw .The half teeth of the jaw is towards left while the rest half is towards right. Arrangement of these teeth include two incisors ,one canine , two premolars and three molars on each side (half)
- Each of the tooth is specialised to perform a particular function. The main function of teeth is to perform physical digestion.
- Incisors are outer mostly forward, flatten and extremely sharp, which help in biting or cutting the food.
- Canines are sharply pointed which crack and split or tear the food.
- Premolars and molars, crush, grind and chew food smoothly.
Enamel ::-The outermost,, shining layer in the crown region is called enamel. It is the hardest substance in the human body.
Cement Layer ::- The outermost layer in the neck and root region is called cement layer.
Pulp Cavity ::- A cavity in the centre of the tooth containing pulp (mass of cells, blood vessels, lymph vessels and nerves) is called pulp cavity.
Dentine ::- The layer made up of hard ivory like substance, which surrounds the pulp cavity is called dentine.
It is a highly muscular organ containing voluntary muscles attached to the floor of buccal cavity with the help of a connective tissue (frenulum linguae). There are taste buds present on the tongue to realise the nature of the food like sweet, bitter, salt and sour.
The tongue helps in tasting and sviallowing of food, It also ha process of speech and masticating the food by mixing saliva in it.
It is small (12 cm long) vertical canal beyond the soft palate of the oral cavity. It acts as a common passage for both air and food, i.e., it communicates with both oesophagus and treachea. During swallowing of food, trachea is covered by epiglottis (a cartilagenous flap or lid) to prevent the entry of food into treachea (wind pipe).
It is a thin, long highly muscular and purely conducting (23 to 27 cm long) tube. It opens.. stomach. Its opening is called gullet and carries food to it. The secretion of fluid from mucous glands present in the wall of oesophagus helps in the forward movemento, A muscular gastro-oesophageal sphincter regulates the opening of oesophagus, into the stomach .
It transfers food from the pharynx to the stomach.
It is the widest organ and the most dilated organ of the alimentary canal. The stomach is J-shaped organ which churns, breaks up food and mix the pieces with gastric juice (include enzymes like renin, pepsin and HCI). The inner lining of stomach secretes various components like mucous, hydrochloric acid and digestive juices. The mucous lining of stomach protects it from the acidic environment and allow it to work firmly.
- It acts as a short term reservoir of food.
- Food become liquified in the stomach before being released into the small intestine.
Small Intestine It is the longest part of the alimentary canal. It is about 6 metres long. It is divisible into three main parts as
Duodenum :::- It is U-shaped, about 25 cm long and is the widest part of the small intestine.
Jejunum ::- It has a diameter of about 4 cm. It is the middle part of the small intestine and is about 2.5 metres long.
lleum ::- It is the last part with diameter around 3.5 cm. Its wall is thinner than that of the jejunum. It is the longest part of small intestine. Ileum opens into the colon of large intestine.
Although it is shorter, but its diameter is larger than that of the small intestine thus, it is known as large intestine. It is about 1.5 metres long and is divisible into three main part as
It is a small pouch like structure of about 6 cm. It also has an outgrowth known as vermiform appendix, which is slightly coiled tube of about 8 cm long.
The caecum part leads into the colon. The colon has three main parts as ascending, transverse and descending part.
The descending portion of colon leads into the rectum which is the last part of the intestine. Rectum is of about 20 cm in length and opens into the anus.
To bring about the simplification of complex food molecules chemically, secretion of digestive juices take place by different glands. These are as follows
Salivary or Mouth Watering Glands
These are exocrine glands which discharge their secretion into the oral cavity. In man, there are three pairs of salivary glands; parotid, sublingual and submandibular glands.
The fluids secreted by the salivary glands constitute saliva, which is a slightly acidic fluid (pH 6.8). Saliva is mainly a mixture of water, electrolytes (Na+, K+, CI-. HCO–3 ) derived from blood plasma, mucous, serum fluids, and enzyme, i.e., salivary amylase or ptyalin and lysozyme (antibacterial agent).
Glands of stomach are called gastric glands. These are numerous microscopic, tubular glands formed by the epithelium of the stomach. Gastric glands have three major types of cells
- (a) Chief cells or peptic cells which secrete inactive precursors of gastric enzymes.
- (b) Oxyntic cells secrete hydrochloric acid.
- (c) Mucous cells or Goblet cells secrete alkaline null cells.
The secretions from these cells form gastric juice with pH 1.5-2.5 (very acidic). The gastro juice contains two proenzymes, i.e., pepsinogen (propepsin) and prorennin anu enzymes gastric lipase gastric amylase, mucous and hydrochloric acid. In human body, about 2000-3000 mL of gastric juice is secreted per day
It is the largest gland of the body, that lies in the upper right side of the abdominal cavity just below the diaphragm. It is heavier in males (i.e., about 1.4-1.8 kg) as females (about 1.2-1.5 kg). Internally, the structural and functional units of liver are the hepatic lobules (containus hepatic cells arranged in the form of cords). Each lobule is covered by tissue sheath called the Glisson’s capsule. Fat storage cells are also present in liver.
It is a soft, lobuled, greyish-pink gland which weighs about 60 grams. It is about 2-5 cm wide and 12-15 cm long, located posterior to the stomach in the abdominal cavity, i.e., between stomach and duodenum. As it is a mixed gland, the exocrine portion of pancreas secretes an alkaline pancreatic juice (containing enzymes and hormones) while, the endocrine part of the pancreas consists of group of cells known as Islets of Langerhans, which secrete hormones to be passed into the circulating blood, i.e., insulin and glycogen.
Apart from the above mentioned glands involved in the process of digestion, intestinal glands are also present in the walls of small intestine called intestinal glands which secretes intestinal juice (containing lipolytic, proteolytic and amylolytic enzymes) commonly called as succus entericus.