Digestive system anatomy
Biomacromolecules which we consume in our food is not directly utilized by our body in its original form. Thus, they are subjected to a process called digestion and the system that helps in the 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 digestive system organs
Alimentary Canal- Human Digestive system
The Alimentary canal in human beings is a long tube (about 8 to 10 meters in length) which begins with an anterior opening i.e., mouth and ends posteriorly through the anus.
Parts associated with the alimentary canal are as follows
1. Mouth
It is 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.
2. Vestibule
Mouth mainly leads to vestibule which is a narrow space enclosed between the lips and cheeks externally and the gums and teeth internally. It’s lining contains mucous glands.
3. Buccal Cavity
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.
Teeth
These are hard structures present in the mouth and both the jaws (i.e., upper and lower jaw). Each tooth is embedded in a socket of jaw bone. A tooth consists of 3 major parts, namely crown (upper part), neck (middle part) and root (lower part).
In human beings, the number of teeth presents in 32, 16 teeth present in each jaw. The half teeth of the jaw are towards left while the rest half is towards right. The arrangement of these teeth includes two incisors, one canine, two premolars and three molars on each side (half).
Each of the teeth is specialized 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 helps 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.
4. Tongue
It is a highly muscular organ containing voluntary muscles attached to the floor of the buccal cavity with the help of a connective tissue (frenulum linguae).
There are taste buds present on the tongue to realize the nature of the food like sweet, salt and sour.
Functions The tongue helps in tasting and swallowing of food. It also helps in the process of speech and masticating the food by mixing saliva with it.
5. Pharynx
The pharynx 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 trachea (wind pipe).
6. Oesophagus
The oesophagus is a thin, long highly muscular and purely conducting (23 to 27 cm long) tube. It opens in the stomach. Its opening is called gullet and carries food to it. The secretion of fluid from the mucous glands present in the wall of oesophagus helps in the forward movement of food. A muscular gastro-oesophageal sphincter regulates the opening of oesophagus into the stomach.
Function It transfers food from the pharynx to the stomach.
7. Stomach
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 HCl).
The inner lining of stomach secretes various components like mucous, hydrochloric acid and digestive juices. The mucous lining of stomach protects it fromthe acidic environment and allow it to work firmly.
Stomach has three major parts as given below
- Cardiac Stomach, the upper portion into which the oesophagus opens.
- Fundic stomach, the middle portion
- Pyrolic stomach, the lower portion which open into the first part of small intestine, i.e., duodenum. The terminal pyrolus part of stomach (i.e., opening of stomach into duodenum) is guarded by a pyrolic sphincter.
Functions
- It acts as a short term reservoir of food
- Food become liquified in the stomach before being released into the small intestine.
8. Intestine
Small Intestine:
It is the longest part of the alimentary canal. It is about 6m long It is divisible into three main parts as
- Dudenum: It is U shaped, about 25cm 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.
- Ileum 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.
Functions:
Small Intestine completes digestion of components like proteins, carbohydrates, nucleic acid and fats. Thus, acts as major site in digestion of food. It absorbs nutrients into the blood and lymph and also helps in absorption of fats.
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
- Caecum 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.
- Colon The caecum part leads into the colon. The colon has three main parts as ascending, transverse and descending part.
- Rectum The descending portionof 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.
Functions
The absorption of water and nutrients and elimination of solid wastes takes place mainly in large intestine.
Digestive Glands
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+ Cl-, HCO3-) derived from blood plasma, mucous, serum fluids, and enzyme, i.e. salivary amylase or ptyalin and lysozyme (antibacterial agent)
Gastric Glands
Glands of stomach are called gastric glands. These are numerous microscopic, tubular glands formed by the epithelium of the stomach.
Gastric glands have three majro types of cells
- Chief cells or peptic cells which secrete inactive precursors of gastric enzymes.
- Oxyntic cells secrete hydrochloric acid.
- 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 gastric juice contains two proenzymes, i.e., pepsinogen (propepsin) and prorennin and the enzymes gastric lipase gastric amylase, mucous and hydrochloric acid.
In human body, about 2000-3000 mL of gastric juice is secreted per day.
iii) Liver
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 compared to females (about 1.2-1.5 kg).
Internally the structural and functional units of liver are the hepatic lobules (containing hepatic cells arranged in the form of cords). Each lobule is covered by a thin connective tissue sheat called the Glisson’s capsule. Fat storage cells are also present in liver.
iv) Pancreas
Pancreas is a soft, lobuled, greyish pink gland which weighs about 60 gm. 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 dudenum.
As it is mixed gland, the exocrine portion of pancreas seccretes 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.
v) Intestinal glands
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.
Next Mechanism of Digestive system or Functions of Digestive system