Human digestive system consists of alimentary canal and associated digestive glands. Alimentary canal is a long tubular structure of varying diameter starting from mouth and ending with anus. The associated digestive glands include salivary, glands, liver and pancreas.
Alimentary canal :
Alimentary canal is about 8-10 meters long and it consists of mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine and anus. Histologically its wall shows four layers which from outside inward are-serosa, muscularis, sub-mucosa and mucosa.
1. Mouth :
This is the outermost transverse-slit-like opening of the alimentary canal. It is bound by an upper lip and a lower lip. It leads to a large cavity called buccal cavity or oral cavity. Mouth is used to ingest the food.
2. Buccal cavity :
It is lined internally by a mucous membrane. Tongue, teeth and salivary glands are also present in the buccal cavity.
Tongue :
It is a muscular fleshy organ which lies along the floor of the buccal cavity. It is roughly triangular in shape. The upper surface of the tongue bears numerous projections called papillae. These papillae contain sensory receptors called taste buds which are used to detect different tastes of the food. Besides, the tongue is used to mix saliva with the food. It helps in swallowing and in speech.
Teeth :
There are total 32 teeth in the buccal cavity of an adult human being. Each tooth in made up of two parts, the root embedded in the jaw bone (thecodent) and the Crown is covered by a white coating material called enamel which is the hardest substance of the body. The body of the tooth is made up of dentine which is bone like substance. Beneath the dentine is the central cavity which contains blood vessels, nerves and pulp. The study of teeth with respect to their number, arrangement, development etc. is known as dentition. In human beings, like in other mammals, dentition is of the heterodont type as there are four different types of teeth namely, incisors (I), canines (C), premolars (PM) and molars (M). human dentition in adult can be represented in the form of a. formula called dental formula as follows : (Each half of a jaw shows)
Thus the total number of teeth in the upper jaw is 16 and that in the lower jaw is also 16. Therefore the total number of teeth in the buccal cavity is 32.
As human being shows two sets of teeth in life, the dentition is called diphyodont. The teeth masticate the food into small particles and help to mix food with saliva.
3. Pharynx :
The buccal cavity leads to pharynx which is the meeting point of the digestive and respiratory systems. The opening of the respiratory system in pharynx is called glottis. It is guarded by a cartilagenous flap called epiglottis which prevents entry of food into the respiratory system. It closes during the swallowing action. Pharynx passes the food towards the oesophagus.
4. Oesophagus :
It is long, thin and narrow tube connecting pharynx to the stomach. It is about 25 cms. long. It passes through the thoracic cavity, pierces the diaphragm and leads to the stomach. The food moves through the oesophagus due to rhythmic contraction of longitudinal and circular muscles. This wave of contraction and relaxation is called peristalsis. Mucous secreted by the epithelial cells in the inner lining helps in the smooth passage of food.
5. Stomach :
It is a sac-like, 'J' shaped 25-30cms. long organ. It can be divided into three parts namely: a) Cardiac b) Fundus and c) Pyloric stomach. Cardiac stomach is the first parts in which oesophagus opens. The middle part of called funds and the remaining part is the pyloric stomach, which opens into the duodenum.
Sphincters are present at both the ends of stomach. The anterior sphincter present at the opening of the oesophagus into the stomach is called cardiac sphincter while the posterior one present at the opening of the stomach into the duodenum is called pyloric sphincter. The cardiac sphincter prevents the regurgitation of food i.e. the passage of food back into the buccal cavity. The pyloric sphincter regulates the passage of food into the duodenum of intestine. The stomach stores the food and churns it. The churning breaks up of food into smaller pieces and also facilitates the mixing of the food with gastric juice secreted by the inner wall of the stomach.
Small intestine :-
It is about 6 meters long and 2.5 cms broad tube, compactly coiled within abdominal cavity. The coils are held together by connective tissue membrane called mesenteries supporting the blood vesseles, lymph vessels and nerves. It is divided into three parts:-
a. Duodenum - It is 'U' shaped and is about 25 cms long. The bile duct and the pancreatic duct open into the duodenum.
b. Jejunum - It is about 2.5 meters long and narrower than the duodenum.
c. Ileum - It is about 3.5 meters long. It is a little bit broader than the jejunum.
Large Intestine :-
It is broader than the small intestine. It is about 1.5 meters long. It can be divided into two parts : a. colon and b. rectum.
a. Colon : It is the anterior region and is differentiated into three parts : the ascending colon, the transverse colon and the descending colon. The colon is lined internally by mucosal cells secreting mucous that makes the passage of undigested material easy.
b. Rectum : It is the posterior region of large intestine. Undigested material called faecal matter is stored in the rectum temporarily before it is expelled through the anus.
At the junction of the ileum and colon there is a blind pouch called caecum. An elongated worm like vermiform appendix arises from the caecum. It is a vestigial organ in human beings. It is functional in herbivorous animals for the digestion of cellulose.
Anus -
The rectum opens to the outside by the opening called anus which is guarded by a sphincter. It removes undigested matter outside by the process known as defaecation of egestion.
Digestive glands - These include the salivary glands, liver and pancreas.
1. Salivary glands - These are multicellular glands which open into the buccal cavity. These are three pairs of salivary glands: a) Sub-lingual glands below tongue. b) Sub-mandibular glands below lower jaw. c) Parotid glands in front of the ear. The salivary glands have two types of secretory cells: serous cells and mucous cells. The serous cells produce a watery fluid which contains the digestive enzyme salivary amylase or ptyalin. The mucous cells produce mucous. It binds the food and makes it slippery so that it can be easily swallowed. The secretion of the salivary glands is called saliva.
2. Liver - It is the largest gland of the body. It is dark reddish brown in colour. It weighs about 1.5kg. in am adult. It is located on the right side, just below the diaphragm. It is made up of two lobes. Cells of the liver produce the bile juice which is stored in a small bag-like structure called gall bladder.
Liver secretes bile which emulsifies fats and makes the food alkaline. It stores excess of glucose in the form of glycogen which is used during starvation. In liver, excess amino acids are converted into ammonia by the process called deamination. It synthesizes vitamin A, D, K and B12. Blood proteins like prothrombin and fibrinogen are formed in the liver. It produces red blood cells during early development thus working as a haemopoietic organ. Kupffer cells of liver digest old R.B.C.s.
3. Pancreas -
It is a leaf-shaped gland lying in the gap between the duodenum and stomach. It is a mixed gland because some part of it is exocrine in function i.e. its secretion is carried by ducts while the other part is endocrine i.e. its secretion is poured directly into the blood stream. The exocrine part is made up of pancreatic lobules called acini. They are flask-shaped and formed of a single layer of large glandular pyramid-shaped cells. These cells secrete the pancreatic juice. The endocrine part is made up of groups of cells present in the connective tissue between the acini. These are called islets of Langerhans. They are made up of alpha (α) cells that secrete glucagon and (β) cells that secrete insulin. Both these hormones together control the blood sugar level. In addition, the delta cells secrete somatostatin hormone, which inhibits glucagon and insulin secretion.