Digestive System

The physico-chemical process of splitting complex ingested food into smallest water soluble absorbable unit is called Digestion.

The Organs taking part in digestion are called digestive organs which are continuously connected & forms a structure called Digestive System.

Based on the site of digestion, it is of 2 types:

1. Intercellular digestion
2. Extracellular digestion

The digestion which takes place within cell is called intercellular digestion. It is a feature of protozoa.

The digestion which takes place outside of cell in a lumen is called extracellular digestion, which is a feature of higher animals like humans.

Steps in digestion:

In human digestion has following events:

1. Ingestion: The process of taking the food into oral cavity is called Ingestion.
2. Mastication: The ingested food is chewed, grinded & smashed with the help of different kind of teeth is called mastication. It makes the food fine.
3. Deglutition (Swallowing): The grinded food is swallowed & passed behind. This process is called Deglutition.
4. Peristalsis: The wave-like movement in the walls of alimentary canal is called Peristalsis. It pushes the food behind to different region called digestion.
5. Secretion: At different region of alimentary canal the digestive enzymes are released. The enzymes help in digestion.

Human Digestive System:

Digestive System is well developed and is represented by alimentary canal and associated glands. These glands are called digestive glands.

(I) Alimentary Canal:

[Diagram of Alimentary Canal showing: pharynx, oral cavity, trachea, esophagus, cardiac, fundus, stomach, body, pylorus, liver, gall bladder, hepatopancreatic duct, pancreas, duodenum, colon, jejunum, ileum, caecum, vermiform appendix, rectum, anus]

A continuous tube with different diameter that extends between oral cavity & anus is called alimentary canal. It is about 9 meters in length & has following roles:

• It gives pathway to ingested food for passing behind.
• It receives secretion of several digestive glands.
• It provides space for chemical reactions of ingested food.
• It allows absorption of digested food.

Human alimentary canal has following parts:

1) Oral cavity:

Alimentary canal starts with a cavity called oral cavity, which is surrounded on upper side by palate, lower side by floor & lateral sides by cheek wall.

• The roof of oral cavity is called palate which separates oral cavity from nasal cavity.
• The floor of oral cavity contains tongue where taste buds are available.

a) Palate:

The roof of oral cavity is called palate which has 2 parts. The anterior part is called hard palate where many transverse ridges (palatine rugae) help in cutting & grinding. The posterior part of palate is smooth and slippery. This region is called soft palatewhich is raised during swallowing & prevents passes of food into nasal cavity.

b) Teeth:

The upper and lower jaws has teeth of different kind adopted from different food. In human 4 kinds of teeth are available. These condition is called heterodont.

In human 2 sets of teeth, milk teeth & permanent teeth are found. Milk teeth are replaced with permanent teeth and are never replaced. This form of teeth is called diphyodont.

The teeth remains embedded into jaws bone. This kind of teeth is stronger and is called thecodont.

Structure of tooth:

[Diagram of V.S. of tooth showing: enamel, dentine, crown, pulp, neck, cavity, root]

• A tooth in transverse section shows outer non-living part called enamel and is followed by dentine and pulp cavity.
• Based on location a tooth has following parts:

1. Crown: The exposed part of a tooth is called crown which is externally lined with a hardest layer called enamel. It is composed of salts of calcium and phosphorus. It makes the bone stronger.
2. Dentine: Inner to enamel a living layer called dentine which is composed of cells called odontoblasts. The dentine layer encloses a cavity called pulp cavity where an artery, a vein and nerves are located.
3. Neck: A short segment of tooth which remains embedded into gum is called neck.
4. Root: The part of a tooth which remains embedded into socket of jaw bone is called root.

c) Tongue:

[Diagram of Tongue showing location of papillae: Circumvallate, filiform, fungiform]

The floor of oral cavity has skeletal muscle called tongue which is posteriorly fixed. The dorsal surface has elevations called papillae which may or may not have taste buds.

Based on structure papillae are of following types:

1. Filiform papillae: It is small, most numerous papilla and doesn't have taste buds. It is sensitive to touch.
2. Fungiform papillae: Among the filiform are fungiform papillae. They have taste buds for sweet, salt and sour.
3. Circumvallate papillae: The larger posterior papillo on tongue is called circumvallate. It is also called vallate and has numerous taste buds.

(ii) Pharynx:

Posterior to oral cavity is a tubular chamber called pharynx which is a crossroads for both food & air. Functionally pharynx has following 3 regions:

1. Nasopharynx: The upper region of pharynx where nose internally opens is called Nasopharynx. It has two openings called internal nostrils.
2. Oropharynx: The region posterior to oral cavity is called Oropharynx. It receives food.
3. Laryngopharynx: The base of pharynx where two openings are available is called laryngopharynx. The ventral opening is called glottis which is guarded with epiglottis and runs behind as trachea. Dorsal to glottis is gullet which passes behind into tube called esophagus.

(iii) Esophagus:

Gullet runs behind into a wider thin tube called esophagus. It passage the food behind to stomach by peristalsis. The wave-like movement in the wall of alimentary canal is called peristalsis.

(iv) Stomach:

• In abdominal cavity esophagus enlarges and becomes stomach which is widest part of alimentary canal. J-shaped and lies at left side of the cavity.
• Structurally stomach has 4 parts: Cardiac, fundus, body and pylorus. At the junction of cardiac & esophagus is a pair of valves called cardiac sphincter which ensures unidirectional flow of food. Likewise pyloric sphincter lies betn pylorus and duodenum.
• The outer surface of stomach is smooth whereas inner surface is rough. The rough surface has opening for gastric glands which secretes gastric juice.

(v) Intestine:

Stomach posteriorly becomes tube like and is called intestine which is variously coiled depending on diameter. Intestine is of 2 types: small & large.

i) Small intestine:

The anterior narrow tubular part of intestine is called small intestine which is about 6.25m in length. Structurally it is divided into 3 regions:

1. Duodenum: The anterior part of small intestine which joins with stomach is called duodenum. It is 25 cm in length & receives a duct from liver and pancreas. This duct is called hepatopancreatic duct.
2. Jejunum: The middle coiled part of small intestine is called Jejunum which is about 2.5m in length & provides space for digestion of several components of food.
3. Ileum: The terminal segment of small intestine is called Ileum which is about 3.5 m in length. The inner surface of ileum has many thin walled projection called villi. These villi absorb smallest units available in the lumen.

ii) Large intestine:

Small intestine posteriorly widens and is called large intestine which is about 1.5m in length. Structurally large intestine has following parts:

1. Caecum: The pouch like anterior part of large intestine is called caecum. It gives out 8-10cm long tube called vermiform appendix which is vestigial in human. At the junction of ileum and caecum is valve called ileocaecal valve.
2. Colon: It runs around small intestine and name as ascending colon, transverse colon, descending colon and sigmoid colon. As the food passes through colon majority of water and electrolytes (salts) are absorbed.
3. Rectum: Sigmoid colon posteriorly enlarges and becomes tubular structure called rectum where undigested semi-solid residue is stored. Rectum opens out through an opening called anus.

Digestive Glands:

The glands which secrets chemicals for digestion of food components are called digestive glands which are associated to alimentary canal at different region. Each digestive gland secrets digestive juice which contains water & enzymes. The water provides medium for chemical reaction and enzymes react with specific food components to make smallest unit free.

i) Salivary Glands:

3 pairs of salivary glands opens at different region of oral cavity. They are a pair of each of parotid gland, submandibular gland and sublingual gland.

• Parotid gland is largest of salivary gland & open at second molar teeth of upper jaw through a duct called parotid duct or Stenson's duct.
• The inner surface of low jaw at second molar teeth submandibular gland is located. It opens at the surface through a duct called submandibular duct or Wharton's duct.
• Underneath of tongue is a pair gland called sublingual gland which opens at floor of oral cavity through duct of Rivinus.

Saliva:

An adult healthy person in a day secrets 1.5 liter saliva which contains water and organic & inorganic substances. 99.5% of saliva is water & remaining is organic & inorganic substances. The organic substances include salivary enzyme ptyalin (amylase) & lysozyme. Salivary amylase absorbs monosaccharides and splittings polysaccharides whereas lysozyme is bacteria-killing.

The function of saliva mentioned below:

• It keeps the oral cavity moist.
• It protects inner lining of oral cavity from abrasion.
• It makes ingested food moist & slippery for swallowing ease.
• Salivary amylase secreted monosaccharides & begin to splitted polysaccharides.
• Lysozyme kills bacteria & maintains oral hygiene.

(ii) Gastric glands:

The inner surface of stomach is rough & has many groups where small opening called gastric pit are available. The pit turns internally into a tubular gland called gastric gland which is composed of following 3 types of cell:

1. Parietal cell (Oxyntic cell): It is also called oxyntic cell which synthesis HCl with pH value 2. HCl kills the microorganism & also dissolve hard substance like bones, nails, hoofs, etc.
2. Chief cell (Peptic cell / Zymogen cell): It is also called peptic cell or zymogen cell which synthesis two proenzyme. They are pepsinogen & prorenin. In presence of HCl pepsinogen & prorenin become pepsin & rennin respectively.
3. Mucus cell: It produces mucin (mucus) which spread over surface stomach & protects inner wall from action of HCl.

Gastric Juice:

It contains HCl, pepsinogen, prorenin & mucus. In presence of HCl, pepsinogen becomes pepsin & prorenin becomes rennin.

Function of HCl:

1. It has pH value of 2 which dissolve microorganisms that come along with food.
2. It prepares acidic medium which starts digestion of proteins.
3. It activates pepsinogen & prorenin to form pepsin & rennin respectively.
4. It dissolve hard substances like bone, cartilage, nails, wings, etc. that come along with food.

Function of Pepsin & Rennin:

• Pepsin reacts with protein to form smaller proteins called peptones and proteoses.
• Rennin reacts with milk protein called casein. In presence of calcium to forms calcium paracaseinate which is curdling of milk.

(iii) Liver:

Liver is reddish brown in colour and is largest gland weighing about 1.5 kg. It has four lobes (Right & Left lobes, Caudate & Quadrate lobes).

• The ventral surface of right lobes has a sac called gall bladder where an alkaline fluid called bile is stored. The liver is located on anterior upper surface of stomach & remains attached to diaphragm with the help of falciform ligament.

Histology of liver:

[Diagram of T.S. of liver (Glisson's Capsule) showing: bile canaliculi, hepatic sinusoid, hepatic portal, hepatocyte, central canal, connective tissue, Kupffer's cell, hepatic strand]

• Liver is composed of polygonal structure called Glisson's Capsules. Each Glisson's Capsule has connective tissue. The capsule at peripheral region.
• The capsule at centre has a canal called central canal which is surrounded with oval cell shaped called hepatocytes.
• These cells are radically arranged & form radical strand. The cells of radical strand form bile which is drained into central canal by bile canaliculi.
• But between radial strand is a space called hepatic sinusoid where phagocytic cell called Kupffer's cells are distributed. These cells removed toxic substance from blood.

Bile:

An adult healthy person in a day secretes 800-1000 ml of bile which is alkaline & contains bile salts & bile pigment. Bile salts are sodium glycocholate & sodium taurocholate. These compound coat lipid & form it water soluble. These phenomenon is called emulsification of fat.

The bile pigment are formed from remaining of destroyed RBC. They are bilirubin & biliverdin. These pigments are collated in gall bladder & drained with bile into duodenum where they mixed with residue & give yellow colour to it.

Function of liver:

• It is centre for heat production.
• It is store house for many nutrients.
• It convert nitrogenous wastes like ammonia to urea.
• The Kupffer's cells of liver are phagocytic and detoxify blood.
• It synthesizes bile which is alkaline & prepares alkaline medium in intestine so that all the nutrients are digested.
• It synthesis heparin which prevents clotting of blood which it flows with in blood vessel.

(iv) Pancreas:

[Diagram of Pancreas showing: head, body, tail, hepatic duct, hepatopancreatic duct, duodenum, ampulla of vater, duct of Wirsung, duct of Santorini]

• Pancreas is called mixed gland or dual gland or heterocrine gland because it acts as both exocrine and endocrine gland.
• Structurally pancreas is leaf like in shape and slightly yellowish in colour. The terminal free part is called tail, middle wider part is body & basal part is head.
• The pancreas contains many smaller tubes called duct of Santorini. These duct unit to form a median duct called duct of Wirsung which at the base swells and form ampulla of vater. The base of vater is guarded with a pair valves called sphincter of Oddi.
• The exocrine part of pancreas secretes an alkaline fluid called pancreatic juice which is drained through duct of Santorini & duct of Wirsung & is stored in ampulla of vater.

Pancreatic Juice:

An adult healthy person in a day secretes 1.5 L pancreatic juice. It contains enzymes for digesting almost all types of nutrients. Following are the enzymes:

1. Trypsinogen: It is inactive enzymes & becomes active in the form of trypsin in the presence of enterokinase. It splits proteins and forms peptone and proteoses.
2. Chymotrypsinogen: It become active in presence of trypsin & is called chymotrypsin which reacts with calcium paracaseinate & form peptone and proteoses.
3. Carboxypeptidase: It reacts with peptone & proteoses & forms dipeptides.
4. Pancreatic amylase: It reacts with polysaccharides & form disaccharides like sucrose, maltose, lactose.
5. Pancreatic lipase: It reacts with emulsified fat & forms fatty acid & glycerol.
6. Nuclease: It reacts with nucleic acid & forms nucleotides.

(v) Intestinal glands:

The mucosa of small intestine contains the glands called intestinal gland which is of 2 types:

1. Crypts of Lieberkuhn
2. Brunner's glands

1. Crypts of Lieberkuhn: It is simple tubular gland that secrete the intestinal juice called Succus Entericus. This intestinal juice contains several enzymes.
2. Brunner's glands: In the submucosa of duodenum intestinal gland called Brunner's gland is distribute. It contains simple tubular structure and produces intestinal juice which lacks enzymes.

Intestinal juice:

It is also called succus entericus and is alkaline in nature. It has following enzymes:

1. Nucleosidase: It reacts with nucleotides and makes nitrogen base, phosphoric acid & pentose sugar free.
2. Sucrase: It reacts with sucrose and liberated one mole of each other glucose and fructose.
3. Maltase: It reacts with maltose and gives two molecules of glucose.
4. Erepsin: It reacts with dipeptide and makes amino acid free.
5. Enterokinase: It activates trypsinogen and from trypsin which reacts with protein.
6. Lipase: It reacts with emulsified fat & form fatty acid & glycerol.
7. Lactose: It reacts lactose & form glucose and galactose.

Physiology of digestion:

• Digestion is a biochemical process by which complex molecule are splitting into simplest water soluble absorbable unit with the help of enzymes.
• The digestion is of 2 types, mechanical & chemical digestion.
• The digestion with the help of teeth of different kind is called mechanical digestion. It grinds the food into final particle. The digestion which take place in presence of enzymes is called chemical digestion.
• The physiology of digestion can be explained as follow:

1. Ingestion: The process of taking the food into oral cavity is called injection. After ingestion mechanical digestion take place.
2. Peristalsis: After swallowing of food. The wall of alimentary canal shows wave like movement toward the anus. These movement is called peristalsis which pushes ingested food downward.
3. Digestion: It is the chemical process & takes place in following parts:
   1. Digestion in oral cavity: After ingestion the food is grinded, smashed & made final with the help of teeth. The saliva is mixed thoroughly with food with the help of rolling movement of tongue. It makes the food ball like & is called bolus. The monosaccharides like glucose are readily absorbed. The bolus is swallowed.
   2. Digestion in stomach: The bolus reaches in stomach through esophagus by peristalsis. In stomach the bolus receives gastric juice which has HCl & other enzymes. HCl kills microorganism and dissolved hard substances like bone, nail, hair, hoof etc. The gastric juice contains pepsinogen & prorenin which becomes pepsin and rennin respectively.
      • Prorenin + HCl → Rennin + Casein → Calcium Paracaseinate
      • Protein + Pepsin → Peptones + Proteoses
      Now the food becomes semi-digested, acidic & creamy fluid which is called chyme.
   3. Digestion in small intestine: The chyme periodically passes into duodenum of small intestine where bile from bladder is poured. It makes the chyme alkaline & is called chyle. Bile salts mixed with lipid and make it water soluble. Then phenomenon is called emulsification of fat. The chyle receives pancreatic juice where several enzymes are available. The reaction of the enzymes are as follows:
      • Polysaccharide + Pancreatic amylase → oligosaccharide (Disaccharide)
      • Protein + Trypsin → Peptones + Proteoses
      • Calcium paracaseinate + Chymotrypsin → Peptone + Proteoses
      • Peptone + Proteoses + Carboxypeptidase → Dipeptides
      • Emulsified fat + Lipase → Fatty acid + Glycerol
      • Nucleic acid + Nuclease → Nucleotides
      The chyme become alkaline fluidly semi-digested & passes behind to jejunum where further chemical reaction take place.
      • Sucrose + Sucrase → Glucose + Fructose
      • Maltose + Maltase → 2x Glucose
      • Lactose + Lactase → Glucose + Galactose
      • Emulsified fat + Lipase → Fatty acid + Glycerol
      • Dipeptides + Erepsin → Amino acids
      • Nucleotides + Nucleosidases → Nitrogen Base + Pentose Sugar + Phosphoric acid
      After about reaction the complex food is splitted & form smallest water soluble unit. They remain in high concentration in lumen of small intestine.
4. Absorption: The process of transferring chyme smallest unit like glucose, fructose, amino acid, lactose etc from the lumen of small intestine to the blood stream is called absorption. The smallest unit from blood stream are transported to the cells that need them.
5. Egestion: The undigested food is temporarily stored in rectum and then eliminated through anus after contraction in the muscles of rectum.

Absorption of digested unit:

The transfer of smallest unit from lumen of intestine to blood stream through the wall of villi is called absorption. It is of 2 types:

1. Active absorption
2. Passive absorption / other

When the molecule pass from lumen of intestine to blood stream without expenditure of energy is called passive absorption. Water, medicine & alcohol are absorbed by this mode.

The molecule like glucose, galactose, amino acids etc are passed through transporter which are available in the wall of villi. This kind of absorption is called active absorption & required energy.

Absorption of Carbohydrates: Glucose & Lactose from lumen are actively absorbed.