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Digestive system

Digestive system

  • The digestive system compares the alimentary canal and accessory digestive glands, which pay an important role in digestion.
  • The alimentary canal is a long coiled tube having a muscular wall and glandular epithelium extending from mouth to anus.

              Figure: Human digestive system

1.1 Teeth

  • Teeth are hard structures which are used for cutting, crushing, tearing, and holding food.
  • Human teeth are embedded in the alveoli  called thecodont, heterodont (presence of different types of teeth), and diphyodont (appearance of two sets of teeth in the lifespan).
  • Milk, deciduous, or temporary teeth begin to appear at the age of 6 – 11 months. Teeth are completed by the age of two. There are 20 milk teeth.
  • Permanent teeth begin to appear between the ages of 6 – 12 years. Milk teeth are lost. Last molars come out after 18 years of age. They are called wisdom teeth. Permanent teeth of man are of four types – incisors (8), canines (4), premolars (8), and molars (12).

                             Figure: Arrangement of teeth in the jaw

 

  • The dental formula of permanent teeth is-

 

The dental formula is the number of teeth in one half of the upper jaw divided by teeth of one half of the lower jaw.

1.2 Pharynx

  • The pharynx is a posterior part of the buccopharyngeal cavity that lies between the soft palate and the 6th cervical vertebra.

1.3 Oesophagus

  • The oesophagus is a long narrow, muscular tubular structure that connects the pharynx with the stomach. The oesophagus helps the food to pass by peristalsis (a series of waves of contraction that passes from one end to another and is meant for pushing the food) from the pharynx to the stomach.

1.4 Stomach

  • The stomach is the widest and distensible J-shaped part of the alimentary canal. It is differentiated into three parts: cardiac, fundic, and pyloric.

Figure: Anatomical regions of the human stomach

1.5 Intestine

  • Intestinal is divided into large and small intestines.
  • The small intestine is divided into three parts –

 proximal duodenum (shortest and wider part)

middle part jejunum (thicker and vascular)

 distal ileum (thinner than jejunum and less vascular).

  • The small intestine is the most longest part of the alimentary canal — large intestines formed by three parts: caecum, colon, and rectum. Caecum in humans is a pouch-like, a small junction between ileum and colon. The colon is the largest part, which has four segments: ascending, transverse, descending and sigmoid.
  • The alimentary canal is consists of four basic layers. From the outer surface to the lumen, these layers are Visceral peritoneum(serosa), muscularis, submucosa, mucosa.

    

                        Figure: Layers alimentary canal

1.6 Salivary gland

  • The human digestive glands include gastric glands, salivary glands,liver, pancreas, and intestinal glands. The salivary gland is of 3 types- parotid, submaxillary, and sublingual.

1.7 Gastric gland

  • The gastric gland is numerous simple, branched or unbranched tubular glands that are formed by the infolding of the epithelium. Three types of the gastric gland are – cardiac gland (secrete alkaline mucus), pyloric gland (secrete alkaline mucus), and fundic gland.
  • Fundic gland possesses four types of enzyme secreting cells. Chief or peptic (zymogen) cells, oxyntic cells, goblet cells, and argentaffin cells. The secretion of the gastric gland is called gastric juices.

 

4.5.8 Liver

  • The liver is the largest and multilobulated gland of about 1.5Kg weight and nearly 1/40 of total body weight. The liver is differentiated into a small left lobe and right lobe separated by the falciform ligament. Glisson’s capsule, characteristics of the liver, is a thin layer of dense connective tissue that surrounds the liver lobule enclosing branches of the portal vein, the hepatic artery, the bile duct, and lymphatic capsules.
  • Kupffer cells of liver sinusoidal act as phagocytes which eat up the dead cells and bacteria by phagocytosis. Lover produces anticoagulant heteropolysaccharides called heparin, which prevents blood clotting inside the blood vessels. The liver produces two protein- prothrombin and fibrinogen, which helps in clot formation.

 

4.5.9 Gall bladder

  • The gall bladder is a pear-shaped yellow-green sac-like structure lies on the inferior surface of the right lobe — gall bladder stores bile, which is secreted by liver cells and collected by bile capillaries. Bile produced liver makes the media alkaline having a pH of 7.8 – 8.6.
  • Bile contains 92% of water, 6% of bile salts, 0.3% bile pigments, 0.3% to1.2% fatty acids, 0.3%-0.9% of cholesterol and 0.3% of lecithin. Lecithin decreases surface tension and helps in fat emulsification.
  • Bile slats contain NaCl, sodium bicarbonate and sodium glycholate, and sodium taurocholate. Bile duct (choledocus duct) is formed by joining of the cystic duct and common hepatic duct from different liver lobes. In man, the bile duct first opens into Ampulla of Vater, and the latter is surrounded by sphincter of Oddi's muscle. Choleresis is the process of bile secretion.
  • The pancreas is an elongated compound organ situated in the limbs of the U shaped duodenum.

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