Essay on the Digestive System of Mammals (4036 Words)



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Alimentary canal is continuous tube running through the ventral body cavity extending from the mouth to anus. The total length of alimentary canal in a man is about 30 feet which increases food digestion and absorption. The organs comprising the alimentary canal include buccopharyngeal cavity, oesophagous, stomach, and intestine.

Digestive glands secrete digestive juices containing digestive enzymes in addition to serous fluids for the process of digestion. Types of digestive glands are –

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Salivary gland, Gastric gland, Intestinal gland, Liver in association with gall bladder and pancreas

Alimentary Canal:

I. Buccopharyngeal cavity is a space between the jaw which is again divided into mouth & pharynx. Mouth – Mouth is a transverse slit like aperture which is bounded by two movable lips. Upper lip has a depression, called philtrum below the nose. Mouth consists of two parts – vestibule & buccal cavity

Vestibule:

Vestibule is a cavity/space which is bounded externally by lips & cheeks and internally by the gums & teeth.

Buccal cavity/oral cavity:

This is a large chamber just inside the mouth in which food is chewed. It is bounded by palate above (dorsally) throat with tongue below & jaws with teeth on the sides.

Tongue is a large thick muscular sensory organ which is most developed in mammals amongst the vertebrate. It not only helps in ingestion, mastication & deglutition of food but also help in voice production & tasting of food. It extends into a part of pharynx & is attached basically by a fold called frenulum.

The mucous membrane of tongue contains taste buds which contain receptors sensitive to sweet, salty, sour & bitter taste.

Receptors for taste buds are innervated by the branches of the facial, gloss pharyngeal & vagus nerves (cranial nerves VII, IX & X respectively).

Taste buds:

Taste buds (group of epithelial cells) are located on tongue/lingual papillae, hard & soft palate, epiglottis & in the pharynx. Each bud has two kinds of elongated columnar cells (neuroepithelial cells) & supporting cells.

Each neuroepithelial cells bears a apical microvilli at its free ends which project into a small pit like depression called taste pore and are associated with a nerve fibre that conducts sensory impulses to the CNS.

Taste pore is an opening in the lingual epithelium that allows the substances to reach the interior of taste buds. The posterior 3rd of the tongue is closely associated with paired lingual and palatine tonsils and doesn’t contain papillae.

Lingual papillae are of four types – filliform, fungiform, foliate & circumvallate.

Filliform:

i. Flame like structures, covered by irregular epithelium.

ii. Smallest, most numerous, white, conical in shape

iii. Without taste buds

iv. Located near centre & most of the upper surface of the tongue

v. Play a role in breaking up food particles

Fungiform:

i. Abundant and are scattered among the filliform papillae.

ii. Mushroom shaped structures with a constricted base and an expanded, flattened upper part.

iii. Large, sub-spherical & appeared as red dots

iv. Located at tip & margin of the tongue

v. Contains few (8-10) taste buds

Foliate:

i. Few, broad flattened & leaf like

ii. Located on the posterior part of tongue anterior to circumvallate papillae

iii. Contain numerous taste buds

iv. Absent in man, found in rabbit & other mammal

Circumvallate:

i. Few but largest knob like

ii. Located in single V or U shaped row upon the middle region of posterior part

iii. Contains approximately 200 taste buds

Teeth are hard structures which are meant for holding prey or cutting, grinding and crushing the food.

Cyclostomes, tadpoles of amphibians and adult Platypus possess horny teeth derived from skin epidermis while other vertebrates other than birds possess true teeth derived from dermis and associated with bone.

In true teeth there are two kinds of roots, open & closed. In open type the pulp cavity or root canal is wide open by addition of new layers of dentine from beneath; such teeth may continue to grow throughout life. E.g. tusk of elephant & incisor of rodents. In closed type the root canal is very small & serves only for the passage of nerves & blood vessels. These teeth fail to grow after they have attained their definite size.

Structure of tooth:

A tooth is formed of three parts – crown, neck & root.

Crown is the exposed or visible part as it projects out of the gum. It is covered by hardest bluish white, shiny translucent substances called enamel. Neck is usually covered by gums (fleshy skin) Root is embedded in the jaw bone & is fixed by bone like cement & peridontal membrane.

Enamel:

1. Covers the crown and represents the hardest substance of the body.

2. Consists of tall, tightly packed enamel prisms cemented together by a scarce interprismatic substance. Both contain 98% minerals (apatite crystals). Secreted by ameloblast.

Enamel is completely acellular and avascular, non-regenerable.

Mature enamels differ from bone in that it lacks ameloblast process. This accounts for the extreme hardness of enamel.

The element that hardens the tooth enamel is flourine.

Dentine:

1. Represents the skeleton of the tooth – substance similar to but harder than bone. It is regenerable living tissue Secreted by odontoblast.

Consists of dentinal tubules which run from the pulp cavity to the periphery. Tubules contain process of odontoblasts. Odontoblasts lie at dentine-pulp junction.

7. Intertubular substance (dentinal matrix) is composed of bundles of collagen fibers embedded into calcified ground matrix.

8. Dentine is similar to bone except that odontoblasts are not embedded into calcified matrix.

Cement:

1. Regularly covers dentine of the root.

2. Resembles bone with osteocytes however, the cells here are called cementocytes.

3. These are live cells.

Pulp cavity:

1. It is present internally and lined by odontoblast

2. Contains sensory ending of nerves and blood vessels which deliver nutrients to living tissue of tooth and remove their waste products.

Peridontal membrane:

1. Dense connective tissue, no mineral.

2. Contains fibroblasts which have both fiber-synthetic ability and osteogenic potential.

3. Fibers of peridontal ligament penetrate as Sharpey’s fibers into the cementum on one side and insert into the bony alveolus on the other side, thus fixing the tooth in situ.

Dental pulp

1. Jelly like substance containing vessels, nerves, and fine collagenous and reticular fibers.

2. At the periphery, a continuous layer of columnar odontoblasts send their processes into the dentinal tubules.

Human teeth are thecodont, heterodont, bunodont & diphyodont. These teeth are present in both jaws socket (called alveoli) i.e., the fixed upper jaw and movable lower jaw of maxilla & mandible bone respectively. They have two successive sets of teeth – milk or deciduous teeth and permanent teeth.

Types of permanent teeth

(i) Incisor:

i. 8 in number

ii. Chisel shaped & possesses sharp cutting edges

iii. Present in anterior part of buccal cavity

iv. Specialised for cutting & biting food

v. Has one root.

(ii) Canines:

i. 4 in numbers

ii. Dagger shaped/pointed

iii. Lie immediately behind the incisor

iv. Designed for piercing & killing prey & tearing flesh

v. Has one root which is large & deeply embedded

vi. Poorly developed in man

(iii) Premolars:

i. 8 in number

ii. Grinding teeth

iii. Also called cheek teeth

iv. Located in posterior part of buccal cavity

v. Specialised for crushing & grinding the food

vi. Upper premolar has 2 roots & lower premolar has 1 root

(iv) Molars:

i. 12 in number

ii. Grinding teeth

iii. Also called cheek teeth

iv. Specialised for crushing & grinding the food

v. Upper molar has 3 roots & lower molars has 2 roots

vi. Not present in deciduous dentition.

In man the deciduous set of teeth includes 8 incisors, 4 canines & 8 molars. The permanent dentition has 8 incisors, 4 canines, 8 premolars & 12 molars.

The permanent incisor & canines corresponds to deciduous incisor & canines but molars of deciduous set are replaced by premolars of permanent sets. Thus molar of permanent set are monophyodont.

The four, third molars which appear very late between the 18 to 24th year are called wisdom teeth. They are vestigial in human. Time when these teeth appear is extensively used in forensic medicine to figure out the age of the person. Jaw and the hip bone help to determine the sex of the skeleton.

Pharynx:

Pharynx is the posterior most part of buccopharyngeal cavity which lies between the soft palate and sixth cervical vertebra.

It opens through gullet into the oesophagus and through glottis into the larynx. An elastic cartilage plate, epiglottis, covers the glottis at the time of swallowing. Food mixed with saliva in buccal cavity is called bolus (slightly acidic).

II. Oesophagus is a narrow, muscular thin tube, 25 cm long that pierces the diaphragm and enters the abdominal cavity. No digestive glands are present. Mucous is secreted.

Oesophagus dilates only to convey fluids and foods through peristalsis. [Peristalsis is a series of wave of contraction that passes from one end to other and is meant for pushing the food.

III. Stomach is the most distensible and widest organ of the alimentary canal. It is thick muscular J-shaped organ which is placed obliquely behind the diaphragm on the left sides.

Its lower convex and upper concave surfaces are called greater and lesser curvature respectively. It helps in mechanical churning and chemical digestion of food.

It also acts as food reservoir. Empty stomach possesses folds called gastric rugae. Rugae disappear when the stomach is distended with food. Loss of the rugae is one of the earliest sign of stomach cancer. Stomach is differentiated into four parts –

1. Cardiac part (cardia):

It is so called because it is present near the heart. It is broad at upper part with cardiac sphincter (functional, lies in the opening between oesophagus and stomach) which prevents regurgitation of food.

2. Fundus:

It extends superiorly, from the cardiac part. The fundus is commonly filled with air and gases.

3. Body:

It is the main and middle dome shaped part of stomach.

4. Pyloric part:

It is the distal part of stomach which is differentiated into 3 parts – antrum, canal and sphincter. Pyloric canal opens into one duodenum and pyloric sphincter guards the opening between the stomach and duodenum. Periodically it permits partially digested food to leave the stomach and enters the duodenum.

IV. Intestine is responsible for the most of the digestion and absorption of food and (usually) formation of dry faeces. It is located in between the stomach and anus (cloaca).

Intestine is divided into two parts – small intestine and large intestine.

Small intestine:

It is the largest (6.25 m long) narrow and tubular part of alimentary canal that lies coiled in abdomen. It extends from the pylorus to the ileocaecal valve, where it forms the large intestine. It contains circular folds of submucosa, called plicae circulares (also called valves of Kerkring) and villi.

Plicae circulares are found in the lower part of duodenum and jejunum, but these are well developed in jejunum. Villi (containing connective tissue, lacteals and blood capillaries) are numerous highly vascular fingers like projections which come out from the wall into lumen.

Villi increase the internal surface area (for absorption) about 10 times in ileum and imparting a velvetty appearance to the surface.

Cells lining the villi have brush border or microvilli that means each villus has a surface (adjacent to inside of the small intestinal opening) covered in microvilli that form on top of an epithelial cell known as brush border. Microvilli further increase the surface area for absorption.

Villi are absent over the Peyer’s patches. Peyer’s patches are lymphoid aggregates present in the submucosa of ileum and involved in the production of (3-lymphocytes and protect the intestine from infection. Small intestine is the major site of digestion and absorption of nutrients.

Small intestine is divided into three parts – proximal duodenum, middle jejunum and distal ileum. Duodenum is the shortest and wider part. It is of 20-25 cm long. It is somewhat C-shaped.

It has ampulla of vater which receives both bile duct from liver and main pancreatic duct from pancreas and whose opening is guarded by sphincter of oddi.

Jejunum is thicker and more vascular. It is about 2.5 m long.

Ileum is thinner than jejunum and less vascular. It is the longest (3.5 m) part of the small intestine. Its distal end is dilated into a small bulb like sacculus rotundus.

Both the jejunum and ileum are greatly coiled and are suspended by mesentery.

Large Intestine:

It is about 1.5 m long. Its diameter varies from one region to another but it is always larger than that small intestine. Hence it is so named. It is divisible into 3 parts – caecum, colon and rectum.

Caecum is a pouch like small junction between ileum and colon. The junction of the ileum and colon is called ileocaecal junction which is guarded by ileocaecal valve.

The latter prevents the regurgitation (= backward flowing, as the casting up of undigested food) of food from caecum. Caecum is 6 cm long and externally it bears a blind tube having lymphoid tissue called vermiform appendix (as an outgrowth of caecum and considered as vestigial in human beings).

It is slightly coiled and thought to be vestigial as these are not involved in the cellulose digestion.

Inflammation of vermiform appendix due to decay of food or worm infection is called appendicitis and rupture of appendix leads to spilling of faecal contents onto the peritoneum leading to its infection and inflammation known as peritonitis. The peritoneum is known as ‘Policeman of stomach’ as it forms a protective membrane over the digestive organs.

Caecum is well developed in herbivores mammals like rabbit, ass, horse and involved in cellulose digestion.

Colon is the largest part of large intestine which is thicker than small intestine but thinner than caecum. Colon has longitudinal bands composed of longitudinal muscle fibre called taeniae coli.

The taeniae coli contracts and draw the remainder of the wall into small pouches called haustra. Large globules of adipose tissue called appendices epiploicae hang down from the colon.

These three components (taenia coli, haustra and appendices epiploicae) help to distinguish large intestine from small intestine especially in X-rays. Colon is divided into four parts –

1. Ascending colon:

(i) Extends upto liver on the right side.

(ii) Absorbs fluids and salts.

2. Transverse colon:

(i) Crosses the abdominal cavity below the pancreas.

(ii) Remove more water from the waste materials.

3. Descending colon:

(i) Running downwards on the left side

(ii) Hold resulting waste.

4. Sigmoid or pelvic colon:

S-shaped and enters the pelvis and joins the rectum.

[Hence colon is concerned with conservation of water, sodium and other minerals and formation of faeces].

Functions of Large intestine:

1. Secretes no enzymes and plays only a minor role in absorption of nutrients.

2. Serves to store unabsorbed food remanants temporarily.

3. Concentrates the content by absorbing water to form faeces.

4. Important for cellulose digestion (in herbivores).

5. Moderate quantities of vitamin K and B complex are manufactured by bacteria (E. coli) present in colon part of the intestine.

Rectum is the thinnest and last part of the alimentary canal. It is wider than colon and bear longitudinal folds. It terminates into a 3 cm long anal canal whose mucosa is folded into several vertical folds, called anal column supplied arteries and veins.

The opening of anal canal is called anus which is guarded by two anal sphincter muscles – internal anal sphincter composed of smooth muscle fibre and external anal sphincter composed.

Digestive Glands

I. Salivary gland:

Three pairs of salivary glands open into the mouth cavity. These glands secrete saliva containing digestive enzymes. Types of three glands are

Parotid gland:

1. Largest sized gland

2. Located below and in front of ears

3. Duct is called Stensons duct

Submaxillary gland:

1. Medium sized gland

2. Located at the angle of lower jaw

3. Duct is called Submandibular or Wharton’s duct

Sublingual gland:

1. Small sized gland

2. Located below the tongue.

3. Duct is called Bartholins ducts or ducts of Rivinus.

In man infra orbital gland (Present in rabbit, beneath eye orbits) are absent.

Secretion of salivary glands is called saliva. Daily secretion of saliva is 1.5 litres.

II. Gastric gland:

Gastric glands are numerous simple branched or unbranched tubular glands which are formed by infolding of the epithelium. These glands are lined with cells which secrete digestive juice containing digestive enzymes, acids (HCI), mucous, inorganic salts etc.

It is estimated that human stomach has about 25 million gastric glands. Gastric glands are of three types – cardiac, pyloric and fundic. Fundic gland has different types of secreting cells which secretes acids and digestive enzymes.

Chief (Zymogenic, Peptic, Serous) cells:

(a) Predominate in the body region of the gland

(b) Cuboidal or columnar cells with basophilic cytoplasm at the basal pole (granular ER) and zymogenic granules at the apical pole.

(c) Principal secretion is pepsinogen and prorennin as proenzymes and gastric lipase.

(d) Gastric lipase is active in infants and capable of hydrolysing triacylglycerols of short and longer chain length.

Parietal cells (Oxyntic cells):

(a) Large, eosinophilic, triangular or round

(b) Contain many mitochondria, agranular ER, and intracellular canaliculi

(c) Secrete HCI and intrinsic factor.

(d) Intrinsic factor (also called Castle intrinsic factor) helps in absorption of vitamin B12.

(e) HCI does not directly help in digestion. It converts proenzyme prorennin and pepsinogen into active rennin and pepsin respectively by lowering the pH of stomach between 1.5 to 2.5.

(f) HCI acidifies the food and stops the action of ptyalin.

(g) HCI kills microorganisms ingested with the food thereby preventing their entry into the intestine.

(h) Pepsin functions at acidic medium and changes proteins into peptones.

(i) Rennin causes the coagulation of milk.

Role of intrinsic factor in vit B12 absorption:

Intrinsic factor is a glycoprotein secreted by parietal (humans) or chief (rodents) cells of the gastric mucosa. In humans, it has an important role in the absorption of vitamin B12 (cobalamin) in the intestine, and failure to produce or utilize intrinsic factor results in the condition known as pernicious anaemia.

Dietary vitamin B12 is released from ingested proteins in the stomach through the action of pepsin and acid.

It is rapidly bound by one of two vitamin B12 binding proteins that are present in gastric juice; at acidic pH, these binding proteins have a greater affinity for the vitamin than doe’s intrinsic factor.

In the small intestine, pancreatic proteases digest the binding proteins, releasing vitamin B12 which then becomes bound to intrinsic factor.

Finally, there are receptors for intrinsic factor on the ileal mucosa which bind the complex, allowing vitamin B12 to be absorbed into portal blood.

Mucous neck cells (Goblet cells):

(a) Found in the upper portion (neck) of the gland

(b) Morphologically similar to the mucous surface epithelial cells but have fewer mucin droplets.

(c) Mucous is a glycoprotein and helps to neutralize the acid in stomach and protects stomach wall against HCI action and protein digestive enzyme.

Argentaffin cells:

(a) Less numerous, located between chief cells and the basal lamina in the body region. Beside stomach, they are also found in the intestinal epithelium.

(b) Best demonstrated with silver stain which reveals small granules in the cytoplasm.

(c) These are gastrointestinal endocrine cells since their secretions (gastrin, secretin, cholecystokinin, etc.) are directly released into the blood vessels and control the secretion of digestive enzymes.

III. Intestinal glands, formed by surface epithelium of small intestine, occur as crypts of Leiberkuhn and Brunner’s gland. Secretions of this gland are called succus entericus or intestinal juice. This juice is slightly alkaline and contains digestive enzymes, water, mineral salts (CI, HCO3 and PO4 etc) and mucous.

IV. Liver:

Largest digestive gland. In man it has four lobes. It is situated just below the diaphragm in the abdomen and makes upto 3.5% of the body weight. It is basically an organ of homeostasis.

It controls many metabolic activities essential for maintaining a constant blood composition. It is multilobulated organ and is differentiated into small left lobe and large right lobe joined together by a falciform ligament.

The latter is further divided into right lobe proper, quadrate lobe and caudate lobe. The liver is surrounded by Falciform ligamenttwo sheaths, an outer membrane is tough serous (fibrous) capsule containing visceral peritoneum and an inner membrane is a thin layer of dense connective tissue called Glisson’s capsule.

Its trabeculae divide liver lobes into hexagonal lobules (also called acini). Polyhedral hepatocytes (parenchymal cells) are arranged in cords around a central venule.

Portal triads contain hepatic artery, portal venule, bile ductule and lymphatics. Sinusoids (vascular spaces) are also present which are radiating from the portal vein and carry blood from the portal canal to the central vein.

These blood spaces are lined by discontinuous capillaries and whose lining contain phagocytic cells called kupffer cells. Kupffer cells are phagocytic and also involved in the breakdown of old red blood cells and ingestion of potentially harmful bacteria.

[Kupffer cells are also called macrophages stellate cells because they perform scavenging activities.] Gall bladder is situated on the inferior surface of right lobe. It is 8 cm long and 2 cm wide.

Bile is an alkaline viscous greenish yellow fluid. It is secreted by liver cells (called hepatocyte) and stored in gall bladder for concentration. It is released into the duodenum on CCK stimulation and involved in digestion and absorption of fats.

Without any digestive enzymes, it contains – bile salts (sodium taurocholate and sodium glycholate); inorganic salts; bile pigments (bilirubin, bilivirdin); fatty substances (cholesterol, lecithin and other phospholipids) and water.

Bile is drained from liver by bile duct or ductus choledochus through canaliculi (made by hepatocytes which line them). Bile duct is formed by the joining of cystic duct from the gall bladder and a common hepatic duct formed from different lobes of liver.

Just near the duodenum the bile and the pancreatic ducts join to form hepatopancreatic duct. This duct swells and form ampulla of vator before opening in theduodenum. Opening is guarded by sphincter of oddi. Daily secretion of bile – 700 to 1000 ml.

Function of bile:

1. Neutralization – Mineral salts neutralizes chyme from the stomach.

2. Emulsification – Sodium glycholate and sodium taurocholate breaks large fat droplets into smaller ones

3. Excretion – Bile pigments are excretory products

4. Stimulates peristalsis

5. Activation, of lipase for lactose digestion

6. Absorption of fats (fatty acids and glycerol) and fat soluble vitamins (A, D, E, K).

Functions of Liver:

1. Connections of the ducts of the gall bladder, liver and pancreas

2. Secretion of bile – Bile contains bile pigments-bilirubin and biliverdin, bile salts-sodium glycocholate, sodium taurocholate & sodium bicarbonate.

3. Storage of fat

4. Urea synthesis

5. Erythropoeisis (during embryonic period only)

6. Breakdown of RBCs (Though spleen acts as the main graveyard for old/deformed or dysfunctional RBC’s) Maximum iron storage in the body. Iron is stored as ferritin.

7. Produce anticoagulant heparin for preventing blood coagulation hence liver disease causes delayed blood coagulation for which heparin or vitamin K injections have to be given.

8. Synthesises vitamin A from carotenes and stores vitamin A, D, E, B12 and minerals.

9. Liver has maximum power of regeneration.

10. Performs endocrine function – It synthesizes and secretes blood proteins such as serum albumin and transferrin (but not antibodies) and it regulates blood sugar level by storing massive amount of glycogen.

11. Primary site for detoxification and elimination of body wastes and poisons.

V. Pancreas:

It is a racemosely branched gland situated between stomach and duodenum. Pancreas consists of acini (which secrete digestive enzymes) and islets of Langerhans (which secrete insulin and glucagon hormones). Pancreatic juice and bile are poured in the duodenum through a common bile duct.

The secretion of pancreas is called pancreatic juice. Pancreatic juice is also called complete digestive juice because it contains all the four digestive enzymes, involved in the digestion of starch, proteins, nucleic acid and fats. It also contains high contents of HCO3– and other cations in small amount.

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