Digestive system of human being

• The digestive system consists of digestive organ and their working mechanism or physiology of digestion.
• In human digestive system involves the alimentary canal and associated digestive glands like the liver pancreas etc.

A. Alimentary canal

• It is a long and coiled duct extending bounded from mouth to the anus.
• It measures about 8-10 m in length and differentiated into the following parts.

a. Mouth and mouth cavity

• Mouth is a transverse aperture bounded by fleshy lips.
• Mouth opens into a wide cavity called the mouth cavity or oral cavity.
• It is bounded by the upper lower jaw and sides by cheeks.
• The roof of the mouth cavity is called the palate.
• From the soft palate, finger-like projection hangs down called uvula.
• It separates the mouth cavity from pharynx.
• The floor of the mouth cavity contains a highly muscular tongue and it helps in the mastication of food.
• The sides of the mouth cavity contain teeth which also help in the mastication of food.

b. Pharynx

• Mouth cavity open into the short and narrow duct called pharynx.
• It is the common duct for food and air.
• It is about 5 inches long, lined with the membrane. It is divided into three parts: nasopharynx, oropharynx and laryngopharynx.

i. Nasopharynx: The Nasopharynx is the upper part of the pharynx which connects nasal passages to the Oropharynx.

ii. Oropharynx: The oropharynx is the middle chamber of the pharynx that passes food from the mouth into the laryngopharynx.

iii.   Laryngopharynx: It is the lower part of the pharynx. It consists of two openings: air opening (glottis) and food opening (gullet). Both openings are separated by the flap of elastic cartilage, epiglottis which covers the glottis so that food cannot enter into the trachea.

c. Oesophagus

• The oesophagus or food pipe is a long narrow muscular tube measuring about 25cm.
• It is extended from pharynx to the stomach.
• It conducts food from the pharynx to the stomach by peristalsis.

d. Stomach

• It is a large J-shaped bag. It also measured 25 cm in length. It is situated on the left side of the upper abdomen.
• The stomach is divided into three parts. The upper part is called the fundus part, the middle and broad part is called the body part and the lower and narrow part nearest to the duodenum is called the pyloric part.
• The wall of stomach contains gastric glands which secrete gastric juice in the stomach.
• Food is stored for a long time and digested with the help of churning movement and gastric juice.
• The stomach has two openings the entrance (cardiac sphincter) and exist (pyloric sphincter).

e. Small Intestine

• The stomach open into small intestine through pyloric sphincter (constriction).
• Small intestine is the longest and narrow part of alimentary canal.
• It measures about 6.5m length and differentiated into three parts: duodenum, jejunum and ileum.

i. Duodenum

• It is the first c-shaped curved tube measures about 25 cm length.
• It receive the bile from the gall bladder and pancreatic juice from the pancreas. In the duodenum, maximum amount of food is digested by the help of both bile and pancreatic juice.
• Hence it is called principle part of digestion.

ii. Jejunum

• It is about 2.5m long coiled duct. It’s internal wall contain thick walled villi not for absorption. Jejunum conduct food from duodenum to ileum.

iii. Ileum

• It is the longest and highly coiled part of the small intestine. It is about 3.5m in length. It’s inner wall contain thick walled finger like projection and maximum amount of food absorbed in this part.

f. Large intestine

• It is shorter but wider than small intestine. It measures about 1.5m in length. It is divided into three parts:
• Caecum
• Colon
• rectum

i. Caecum

• It is the first pouch-like structure which is about 6cm in diameter.
• It is connected to the ileum by the ileocaecal junction.
• In human, caecum contains 5-10cm long blind tube called vermiform appendix.
• In human, it is vestigial but functional only in herbivores.

ii. Colon

• It is an inverted U-shaped tube that measures about 1.2m.
• It is differentiated into four parts: i.e. ascending transverse, descending and sigmoid colon.
• The wall of the colon is constricted at different regions to form pouch-like haustra.
• Colon helps in the formation of stool by the absorption of water.

iii. Rectum

• The colon finally opens into the rectum. It is a muscular and straight terminal part measuring 15cm.
• It leads outside through the anus. The terminal anal opening is guarded anal sphincters.

B. Digestive glands

• These glans are associated with digestion of food and pour their secretion into the alimentary canal. These are:

1.Salivary glands

2.Pancreas

3.Liver

4.Gastric glands

5. Intestinal glands

1. Salivary glands

• There are three pairs of salivary glands (Parotid gland, Submandibular gland and sublingual gland) which release saliva in the oral cavity.
• A normal person secrete 1-1.5 lt. saliva per day.

Functions of saliva

• It contains salivary amylase and break down the starch into maltose.
• It keeps the mouth moist and cleans the teeth
• It lubricate and make moist the food which help for easy swallowing.
• It kills micro-organism associated with food.

2. Pancreas

• Pancreas is a pale grey leaf like gland weighing 60-75 grams.
• It is the main digestive gland and present in between stomach and duodenum.
• It secrets pancreatic juice.