Digestion and Absorption

Alimentary canal and digestive glands

  • The alimentary canal is a long tube (8 - 10 cm long) which consist of several organs : mouth, vestibule, oral (or) buccal cavity, pharynx, Oesophagus, stomach, small intestine, large intestine and anus.
  • Mouth is a transverse slit which is bounded by two soft, morable lips : upper and lower. Vestibule is a narrow space enclosed between the lips.
  • Oral (buccal) cavity is large space bounded above by the palate, below by the throat and on the sides by the jaws.
  • Tongue is attached to the floor of the mouth by a fold called the lingual frenulum. Human tongue has four taste areas (sweet, salt, sour and bitter). Areas of sweet and salt can overlap
  • Arrangement of teeth in each half of upper and lower jaw in the order of incisor, canine, premolar and molar.
    That is represented by (In adult human) \tt \frac{21  \ 23}{21 \ 23} \times 2
    The milk teeth is \tt \frac{2102}{2102} \times 2.
  • In tooth, exposed portion above the gums are called as crown. crown is covered with the hardest substance called enamel that protects the crown. Beneath the enamel dentine is present which is made up of hard substance similar to bone.
  • The oral cavity leads through the pharynx into a tube like esophagus (food tube), which run downwards through the throat and thorax. The esophagus invades the diaphragm a muscular transverse partition that separate thorax from abdomen and leads into stomach.
  • During the oesophageal phase of swallowing food is pushed through the esophagus by involuntary muscular movements called peristalsis.
  • Stomach is the most distensible and widest organ of the alimentary canal. It has thick muscle. It is a J shaped organ placed behind the diaphragm on left side.
  • It is differentiated into three parts - cardiac, fundic and pyloric which opens into first part of small intestine.
  • Small intestine is the largest (6.25 m - length), narrow and tubular part of alimentary canal that lies coiled in abdomen. It is divided into three parts - proximal duodenum, middle jejunum, and distal ileum.
  • Large intestine is divisible into three parts, caecum, colon and rectum. caecum bears a blind tube having lymphoid tissue called vermiform appendix (vestigial in human beings).
  • colon is the largest part of large intestine which is thicker than small intestine but thinner than caecum. colon is divided into - ascending colon, transverse colon, descending colon, sigmoid (or) pelvic colon. Rectum is the thinnest and last part of alimentary canal.
  • The human digestive glands include salivary glands, gastric glands, liver, pancreas and intestinal glands.
  • Three pairs of salivary glands namely parotid, sublingual and submaxillary (or) submandibular glands. (l) Parotid ducts - stenson's ducts which opens into the vestibule opposite the upper second molar tooth. Viral infection of parotid glands causes the disease called mumps which results in swelling, pain.
  • Ducts of sublingual gland is ducts of rivinus. This gland lie under front part of tongue and the ducts open under the tongue.
  • submaxillary glands lie at the angle of lower jaw. The duct of this gland termed as Wharton's ducts, open under the tongue.
  • The salivary gland secrete a viscous fluid called saliva. It contains water, salts, mucin and an enzyme salivary amylase (or) ptyalin. Its pH is neutral, approx 6.7. salivary amylase is absent in herbivores.
  • saliva moistens and lubricates the buccal mucosa, tongue and lips thus making speech possible. It neutralizes acidity in the buccal cavity. Its enzyme help in digestion.
  • Gastric glands are three types which are (i) Fundic glands, (ii) pyloric glands and (iii) cardiac glands.
  • Fundic glands (oxyntic glands) - secrete HCl, pepsinogen and soluble mucin - acidic secretion. Pyloric glands - secretion is rich in mucin and doesnot contain HCl. Cardiac glands secrete mucin and very little pepsinogen.
  • The epithelium of the gastric glands (fundic glands) consists of various types of cells (i) peptic cells (chief / zymogenic / zymoger cells) are usually basal in location and secrete gastric digestive enzymes ad pro enzymes, pepsinogen and prorennin.
  • (ii) Oxyntic cells (parietal cells) are large and most numerous on the side walls of gastric glands. They strongly stain with eosin so they named so. They secrete HCl and castle's intrinsic factor that helps in the absorption of vitamin B12.
  • (iii) Mucous cells (Goblet cells) are present between other types of cells and secrete mucus.
  • endocrine cells usually present in the basal parts of the gastric   (@@@@ DOUBT Lines Missing). These are argentaffin cells and gastrin cells (G - cells). Argentaffin cells - produce serotonin, somatonin, is histamin. G - cells are present in the pyloric region and secrete and store the hormone gastrin.
  • Serotonin is a vasoconstrictor and stimulates the smooth muscles. Somatostatin supresses the release of hormones from digestive track. Histamine dilates the walls of blood vessels. Gastrin stimulate the gastric gland to release the gastric juice.
  • Stem cells are undifferentiated cells that are also present in the epithelium of gastric glands, which are also play an important role in healing.
  • Liver  - is the largest internal gland of the body which is reddish brown in colour. It is covered by two sheaths - an outer membraneous serous capsule consisting of visceral peritoneum and an inner Glisson's capsule consisting of thin layer, of dense connective tissue. (Glisson's capsule is the characteristic feature of mammalian liver).
  • A pear - shaped sac like structure called gall bladder is attached to the posterior surface of the liver on the right side by connective tissue. [Rat and horse do not have gall bladder). Its function is to store the bile.
  • Liver has high power of regeneration. Ducts of liver is known as hepatic ducts, and duct of gall bladder is known as cystic duct
  • The bile duct passes downwards pastoriorty to join the main pancreatic duct to form the hepatopancreatic ampulla (ampulla of vater). The ampulla opens into the duodenum. The opening is guarded by the sphincter of oddi.
  • The liner cell secrete bile (pH 8.6) via hepatic duct to the gall bladder for storage (gall bladder bile pH is 7.6).
  • Bile salts (sodium carbonate, sodium glycocholate, sodium taurocholate) and phospholipids  help in the digestion of fats in the small intestine by bringing about their emulsification.
  • Functions of bile include, neutralisation of HCl, Emulsification, Absorption of fats and fat soluble vitamin, excretion and activation of lipase.
  • Pancreas is soft, lobulated, greyish - pink coloured gland (weight - 60 grams) which is known as dual gland.
  • Pancreatic juice is carried by the main pancreatic duct into the duodenum through the hepatopancreatic ampulla.
  • The pancreatic juice contains sodium bicarbonate, three proenzyme namely trypsinogen, chymotrypsinogen and procarboxypeptidase and some enzymes such as pancreatic amylase, DNA ase, RNAase, and pancreatic lipase. The juice helps in digestion of starch, proteins, nucleic acids and fats.
  • Intestinal glands lie in the wall of small intestine. They are 2 types (i) Crypts of Lieberkuhn and Brunner's glands.
  • Crypts of Lieberkuhn are simple tabular glands and occur throughout the small intestine between the villi.
  • Paneth cells are argentaffin cells are two types of cell present in crypts of lieberkuhn.
    Paneth cells → found in duodenum → secrete lysozyme
    Argentraffin cells (enterochromaffin cells)→ Among epithelial cells lining the crypts of Lieberkuhn → secrete serotonin.
  • Brunner's glands are branched tubular glands which are confined to the duodenum. They secrete alkaline watery fluid, alistle enzyme and mucus. They open into the crypts of Lieberkuhn. The mixture of secretion is called intestinal juice (or) succus entericus.
  • Succus entericus contains many enzymes.
    eg : aminopeptidases, dipeptidases, intestinal amylase, maltase, isomaltase, limit dextrinase, sucrose, lactose, intestinal lipase, nucleotidases, nucleophosphatases, nucleosidases, and enteropeptidase (enterokinase). All these enzyme act on all types of food.

Watch this video for the topic Alimentary canal and digestive glands Part-1 from 1:14 to 52:31

Watch this video for the topic Alimentary canal and digestive glands Part-2 from 0:34 to 57:31

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