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DIGESTIVE SYSTEM

An Introduction to the Digestive System

Every cell of the body needs nourishment. The food we eat, however, is not in a form that our body cells can

use. The food we eat consists of complex proteins, fats, and carbohydrates. The function of the digestive

system is to change these complex organic nutrient molecules into simple organic and inorganic molecules

that can then be absorbed (i.e. a process known as digestion) into the blood or lymph to be transported to

cells, where they are used. The digestive system supplies both the fuel that keeps all the body’s cells running

and the building blocks needed for cell growth and repair. The undigested portion of the food is moved

through the digestive tract and eliminated through the anus.

The digestive system consists of the digestive tract, a tube extending from the mouth to the anus, plus

associated organs, which secrete fluids into the digestive tract. It consists of the oral cavity, pharynx,

esophagus, stomach, small intestine, and large intestine. The term gastrointestinal (GI) tract technically

refers only to the stomach and intestines but is often used as a synonym for digestive tract. Digestion takes

place within the oral cavity, stomach, and small intestine; most absorption of nutrients takes place in the

small intestine. Indigestible material, primarily cellulose, is eliminated by the large intestine (also called the

colon). The length of the GI tract is about 5–7 meters (16.5–23 ft) in a living person. It is longer in a cadaver

(about 7–9 meters or 23–29.5 ft) because the muscles along the wall of the GI tract organs are in a state of

tonus (sustained contraction).

The accessory organs of digestion are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Digestion does not take place within these organs, but each contributes something to the digestive process.

TYPES OF DIGESTION

The food we eat is broken down in two complementary processes: mechanical digestion and chemical

digestion. Mechanical digestion is the physical breaking up of food into smaller pieces. Chewing is an

example of this. As food is broken up, more of its surface area is exposed for the action of digestive

enzymes. The work of the digestive enzymes is the chemical digestion of broken- up food particles, in

which large molecules changes into their monomers occurs by means of hydrolysis reactions that the body

can utilize.

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Functions of the Digestive System

The digestion of the large molecules into their monomers occurs by means of hydrolysis reactions. The

monomers thus formed are transported across the wall of the small intestine into the blood and lymph in the

process of absorption. Digestion and absorption are the primary functions of the digestive system. The

functions of the digestive system are to

1. Take in food: Food and water are taken into the body through the mouth.

a. Ingestion: Taking food into the mouth.

b. Mastication: Chewing the food and mixing it with saliva.

c. Deglutition: Swallowing food.

d. Peristalsis: Rhythmic, wavelike contractions that move food through the gastrointestinal tract.

2. Break down the food: The food that is taken into the body is broken down during the process of digestion

from complex molecules to smaller molecules that can be absorbed. Digestion consists of mechanical

digestion, which involves the chewing of food and the mixing of food with digestive tract secretions, and

chemical digestion, which is accomplished by digestive enzymes that are secreted along the digestive tract.

These include both exocrine and endocrine secretions.

a. Exocrine secretions: Water, hydrochloric acid, bicarbonate, and many digestive enzymes are secreted into

the lumen of the gastrointestinal tract. The stomach alone, for example, secretes 2 to 3 liters of gastric juice a

day.

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b. Endocrine secretions: The stomach and small intestine secrete a number of hormones that help to regulate

the digestive system.

3. Absorb nutrients: The small molecules that result from digestion are absorbed through the walls of the

intestine for use in the body. Water, electrolytes, and other nutrients, such as vitamins and minerals, are also

absorbed.

4. Eliminate wastes: Undigested material, such as fiber from food, plus waste products excreted into the

digestive tract are eliminated in the feces.

Layers of the gastrointestinal tract:

Various parts of the digestive tract are specialized for different functions, but nearly all parts consist of four

layers or tunics, or layers. Each tunic contains a dominant tissue type that performs specific functions in the

digestive process. The four tunics of GI tract, form the inside out, are the mucosa, submucosa, muscularis,

and serosa or adventitia.

1. Mucosa: The mucosa, which lines the lumen of the GI tract, is the absorptive and major secretory

layer. It consists of a simple columnar epithelium supported by the lamina propia, a thin layer of

areolar connective tissue containing numerous lymph nodules, which are important in protecting

against disease. External to the lamina propia is a thin layer of smooth muscle called the muscularis

mucosa. This is the muscle layer responsible for the numerous small folds in certain portions of the

GI tract. These folds greatly increase the absorptive surface area. Specialized goblet cells in the

mucosa secrete mucus throughout most of the GI tract.

2. Submucosa: The relatively thick submucosa is a highly vascular layer of connective tissue that

serves the mucosa. Absorbed molecules that pass through the columnar epithelial cells of the mucosa

enter into blood and lymphatic vessels of the submucosa. In addition to blood vessels, the submucosa

contains glands and nerve plexuses. The submucosal plexus (Meissner’s plexus) provides an

autonomic nerve supply to the muscularis mucosae.

3. Muscularis: The muscularis (also called the muscularis externa) is responsible for segmental

contractions and peristaltic movement through the GI tract. The muscularis has an inner circular and

an outer longitudinal layer of smooth muscle. Contractions of these layers move the food through the

tract and physically pulverize and mix the food with digestive enzymes. The myenteric plexus

(Auerbach’s plexus), located between the two muscle layers, provides the major nerve supply to the

GI tract. It includes fibers and ganglia from both the sympathetic and parasympathetic divisions of

the autonomic nervous system.

4. Serosa: The outer serosa completes the wall of the GI tract. It is a binding and protective layer

consisting of areolar connective tissue covered with a layer of simple squamous epithelium.

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Enteric Nervous System:

Enteric nervous system (ENS) is the “brain of the gut”. It consists of about 100 million neurons that

extend from the esophagus to the anus. The neurons of the ENS are arranged into two plexuses:

The myenteric plexus

submucosal plexus

The myenteric plexus, or plexus of Auerbach, is located between the longitudinal and circular smooth

muscle layers of the muscularis. The submucosal plexus, or plexus of Meissner, is found within the

submucosa. The plexuses of the ENS consist of motor neurons, interneurons, and sensory neurons. Because

the motor neurons of the myenteric plexus supply the longitudinal and circular smooth muscle layers of

the muscularis, this plexus mostly controls GI tract motility (movement), particularly the frequency and

strength of contraction of the muscularis. The motor neurons of the submucosal plexus supply the

secretory cells of the mucosal epithelium, controlling the secretions of the organs of the GI tract.

Autonomic Nervous System

Although the neurons of the ENS can function independently, they are subject to regulation by the neurons

of the autonomic nervous system. The vagus nerve (Xth cranial nerve) supply parasympathetic fibers to most

parts of the GI tract, with the exception of the last half of the large intestine, which is supplied with

parasympathetic fibers from the sacral spinal cord. Sympathetic nerves that supply the GI tract arise from

the thoracic and upper lumbar regions of the spinal cord. Sympathetic postganglionic neurons synapse with

neurons located in the myenteric plexus and the submucosal plexus. In general, the sympathetic nerves that

supply the GI tract cause a decrease in GI secretion and motility by inhibiting the neurons of the ENS.

Oral Cavity

The oral cavity, or mouth, is the first part of the digestive

tract bounded by the lips anteriorly, the cheeks laterally, the

palate superiorly, Anteriorly palate is hard due to the

presence of palatine bone and maxilla, which is known as

hard palate. Posteriorly it consists of soft tissue, which is

known as soft palate. Inside the mouth at the centre, an arch

like structure appears is called uvula. It is hanging from the

middle of the soft palate. Pharyngeal glands (tonsils) are

present in either side of the uvula. This helps in the protection

against infection. It consists of large amount of lymphatic

tissue and called as palatine tonsil. The floor of the mouth

consists of tongue and other soft tissues. The oral cavity is

divided into two regions: Figure 1 oral cavity

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(1) The vestibule is the space between the lips or cheeks and the teeth or gums; and

(2) The oral cavity proper, which lies medial to the teeth.

The oral cavity is lined with moist stratified squamous epithelium, which protects against abrasion. The oral

cavity connects posteriorly to the oropharynx through a space called the throat.

Tongue:

It is a voluntary muscle structure occupies the floor of the

mouth. Base is attached to hyoid bone. Frenulum- mucosal

layer present at dorsal part. Superior surface contains

stratified squamous epithelium along with number of

projections called taste buds. There are three types of taste

buds are there mainly, those are:

› Circum vallate papillae

› Filiform papillae

› Fungiform papillae

These are innervated with nerve endings of the sense of

taste.

Functions:

› Gripping and repositioning food during chewing (mastication/chewing).

› It helps in detection of the taste of food.

› It helps in swallowing of food.

› It is very essential in speaking.

› Mixing food with saliva and forming the bolus.

Teeth: Teeth are embedded in alveolar socket of maxilla and mandible. Teeth are classified as

Temporary teeth

Permanent teeth.

Temporary teeth: Also known as deciduous teeth /dentitions. They are 20 in number and arranged in 10 in

each jaw. They start to grow at the age of 6months and should complete by the age of 24 months.

Molar Canine Incisor Incisor Canine Molar

Upper jaw 2 1 2 2 1 2

Lower jaw 2 1 2 2 1 2

Figure 2 Tongue

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Figure 3(a) Permanent teeth. (b) Deciduous teeth. Dental professionals have developed a “universal” numbering and lettering system for convenience in identifying individual

Permanent teeth: These replaces the temporary teeth after the of about 6 years and should be completed by

the age of 24 years. Permanent teeth are 32 in number, arranged 16 in each jaw.

Molar Pre molar Canine Incisor Incisor Canine Pre molar Molar

Upper jaw 3 2 1 2 2 1 2 3

Lower jaw 3 2 1 2 2 1 2 3

Structure of teeth: The shape of the teeth may be

varied but the structure is always same.

Structurally the tooth consists of:

Crown- is the part that protrudes from the

gum

The root- the part embedded in the bone

The neck- the slightly constricted part where

the crown merges with the root.

In the centre of the tooth there is the pulp cavity

containing blood vessels, lymph vessels and

nerves. Around this a hard ivory-like substance

called dentine. Outside the dentine a thin layer of

very hard substance, enamel. The root of the teeth

is covered with a substance resembling bone,

called cement. Figure 4 Tooth structure

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Functions of teeth:

Incisor & canine – having sharp & pointed end- useful for cutting and biting purpose.

Molar & premolar – having broad surface – useful for grinding and chewing.

Salivary Glands: Salivary glands produce and secrete saliva that cleanses the mouth, moistens and dissolves food chemicals,

aids in bolus formation and contains enzyme that breakdown starch. Most of the salivation occurs during the

intake of food. Normally, the basal salivary secretion is about 4ml/ hr in children and 15ml/hr in adults.

Saliva is opalescent, colourless fluid. It has slightly acidic pH of 6.75-7.0, but the pH may vary from 5.8-

7.0. There are three pairs of salivary glands, they are:

Parotid glands

Submandibular glands, and

Sublingual glands

Parotid glands present one on each side of the face just below the external acoustic meatus, anterior to the

ear. Parotid duct opens into the mouth at the level of second upper molar tooth.

Submandibular glands present one on each side of the face under the angle of the jaw. Its ducts open on the

floor of the mouth, one on each side of the lingual frenulum of the tongue.

Sublingual glands present anterior to the submandibular gland under the tongue. It opens via 10-12 ducts

into the floor of the mouth.

Composition of saliva:

It contains 99.5% water and 0.5% of solids. Out

of that 0.5%

› Inorganic substances (0.2%):

Sodium chloride

Potassium chloride

Sodium phosphate

Calcium carbonate

› Organic substance (0.3%):

Ptyalin/ salivary amylase enzyme.

Immunoglobulin IgA

Lysozyme, maltase.

Others like mucin etc..

Functions of saliva:

Saliva keeps the mouth moist and helps for speech.

It helps in mastication of food, preparing bolus & in deglutition.

Figure 5 Salivary glands

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Buffering action: phosphate ions, bicarbonate ions and mucin present in saliva acts as buffers. They

keep pH within the the range at which salivary amylase is active and helps in inhibititing bacterial

growth and keeps saliva saturated with calcium.

It acts as lubricant.

It acts as solvent & stimulates the taste buds to detect the taste.

Lysozyme, immunoglobulins provide non-specific defense.

Cleaning of mouth.

Amylase action continues after the bolus is swallowed, until it is finally inhibited by gastric juices-

PH 1.5-1.8.

Amylase converts starch into disaccharides.

Excretion: Salivary glands also help in the removal of organic wastes such as urea and uric acid from

the body.

Role in homeostasis of the body water: During dehydration, salivation is ceased in order to conserve

the body water. This results in drying up of the mouth and pharynx which contributes to sensation of

thirst. This inturn leads to drinking of excess water, due to which fluid balance gets restored.

Pharynx: It is situated between mouth and esophagus. Pharynx is divided into three parts:

nasopharynx,

oropharynx, and

laryngopharynx

Of these oropharynx & laryngopharynx are associated with the alimentary tract. Food passes from the oral

cavity to the pharynx then to the esophagus below.

Esophagus: First part of GIT, to which general plan of alimentary canal applies. It is about 25cm long &

about 2cm in diameter, present behind the trachea. It is continuous with the pharynx above and just below

the diaphragm it joins the stomach. The upper & lower ends of the esophagus are closed by sphincter

muscle. Upper- cricopharyngeal sphincter – prevents air passage. Lower- cardiac/ lower esophageal

sphincter- prevents the reflux of acid gastric contents into esophagus.

Stomach:

The stomach is an enlarged, “J” shaped dilated portion of the alimentary and it is located in the upper left

quadrant of the abdominal cavity, to the left of the liver and in front of the spleen, just below the diaphragm.

The capacity of stomach is about 1.2-1.7 liter. The opening from the esophagus into the stomach is the

gastroesophageal opening, or cardiac (located near the heart) opening. The lower esophageal sphincter,

also called the cardiac sphincter, surrounds the cardiac opening. The most superior part of the stomach is

the fundus. The largest part of the stomach is the body, which turns to the right, forming a greater

curvature on the left, and a lesser curvature on the right. The body narrows to form the funnel-shaped

pyloric part of the stomach, which joins the small intestine. The opening from the stomach into the small

intestine is the pyloric orifice, which is surrounded by a relatively thick ring of smooth muscle called the

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pyloric sphincter. The pyloric sphincter helps regulate the movement of gastric contents into the small

intestine. The muscular layer of the stomach is different from other regions of the digestive tract in that it

consists of three layers: an outer longitudinal layer, a middle circular layer, and an inner oblique layer.

These three layers are innervated by the myenteric plexuses of the enteric nervous system. Stimulatory

impulses are carried from the CNS by the vagus nerves (10th cranial) and provide for very efficient

mechanical digestion to change food into a thick liquid called chyme. When the stomach is empty, the

mucosa appears wrinkled or folded. These folds are called rugae; they flatten out as the stomach is filled.

The stomach is lined with simple columnar

epithelium. The epithelium forms numerous, tube like

gastric pits, which are the openings for the gastric

glands. The epithelial cells of the stomach are of four

types:

Surface mucous cells are on the inner surface

of the stomach and line the gastric pits. Those

cells produce mucus, which coats and protects

the stomach lining.

Parietal cells, which produce hydrochloric

acid and intrinsic factor;

Chief cells (Zymogenic cells), which produce

pepsinogen; and

Endocrine cells, which produce regulatory

hormones. There are several types of endocrine cells. For example, G cells secrete gastrin and

enterochromaffin-like cells produce histamine.

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HCl converts pepsinogen to pepsin, which then begins the digestion of proteins to polypeptides, and

maintains pH 1 to 2. This very acidic pH is necessary for pepsin to function and also kills most

microorganisms that enter the stomach. The parietal cells also secrete intrinsic factor, which is necessary for

the absorption of vitamin B12.

The presence of food in the stomach causes the G cells to secrete gastrin, a hormone that stimulates the

secretion of greater amounts of gastric juice.

Functions of gastric juice:

Water liquefies the food material swallowed.

HCl converts pepsinogen to pepsin, which then begins the digestion of proteins to polypeptides, and

also gives gastric juice its pH of 1 to 2. This very acidic pH is necessary for pepsin to function and

also kills most microorganisms that enter the stomach.

Intrinsic factor – aids the absorption of vitamin B12.

Mucus prevents mechanical injury to stomach walls by lubricating the contents.

It relaxes at intervals to permit small amounts of chyme to pass into the duodenum of small intestine. This

sphincter then contracts again to prevent the backup of intestinal contents into the stomach.

The only stomach function that appears to be essential for life is the secretion of intrinsic factor. This

polypeptide is needed for the absorption of vitamin B12 in the terminal portion of the ileum in the small

intestine, and vitamin B12 is required for maturation of red blood cells in the bone marrow. Following surgical

removal of the stomach (gastrectomy), a patient has to receive B12 injections or take B12 orally together with

intrinsic factor. Without vitamin B12, pernicious anemia will develop.

Regulation of Stomach Secretion

Approximately 2–3 L of gastric secretion (gastric juice) is produced each day. Nervous and hormonal

mechanisms regulate gastric secretions. The neural mechanisms involve central nervous system (CNS)

reflexes integrated within the medulla oblongata. Higher brain centers can influence these reflexes.

Hormones produced by the stomach and intestine help regulate stomach secretions. The regulation of

stomach secretion is divided into three phases: cephalic, gastric, and intestinal. The cephalic phase can be

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viewed as the “get started” phase, when stomach secretions are increased in anticipation of incoming food.

This is followed by the gastric “go for it” phase, when most of the stimulation of secretion occurs. Finally,

the intestinal phase is the “slow down” phase, during which stomach secretion decreases.

1. Cephalic phase. The cephalic phase prepares the stomach to receive food. The sensations of the taste

and smell of food, the stimulation of tactile receptors during the process of chewing and swallowing, and

pleasant thoughts of food stimulate the centers within the medulla oblongata that influence gastric

secretions. Action potentials are sent from the medulla along the vagus (X) nerves to the enteric plexus of

the stomach which stimulate secretory activity in the cells of the stomach mucosa, causing the release of

mucus, hydrochloric acid, pepsinogen, intrinsic factor, histamine, and gastrin. The gastrin enters the

circulation and is carried back to the stomach, where it stimulates additional secretory activity.

Acetylcholine, released by postganglionic cells, stimulates the secretion of hydrochloric acid from parietal

cells, gastrin from G cells, and histamine from enterochromaffin-like cells. The gastrin stimulates parietal

cells to release additional hydrochloric acid. In addition, gastrin stimulates enterochromaffin-like cells to

release histamine, which stimulates parietal cells to secrete hydrochloric acid.

2. Gastric phase. The greatest volume of gastric secretions is produced during the gastric phase. The

presence of food in the stomach initiates the gastric phase. The primary stimuli are distention of the stomach

and the presence of amino acids and peptides in the stomach.

Distention of the stomach wall results in the stimulation of mechanoreceptors. Action potentials generated

by these receptors initiate reflexes that involve both the CNS and the ENS. These reflexes result in

acetylcholine release and the cascade of events in the cephalic phase. Peptides, produced by the action of

pepsin on proteins, stimulate the secretion of gastrin, which in turn stimulates additional hydrochloric acid

secretion. When the pH of the stomach contents falls below 2.0, further gastric secretion by distention of the

stomach is blocked. This negative-feedback mechanism limits the secretion of gastric juice.

3. Intestinal phase. The entrance of chyme into the duodenum of the small intestine stimulates neural and

hormonal mechanisms that inhibit stomach secretions. This reduces the acidity of chyme, making it easier

for pancreatic and liver secretions to neutralize the chyme, which is required for the digestion of food by

pancreatic enzymes and for the prevention of peptic ulcer formation.

When the pH of the chyme entering the duodenum drops to 2.0 or below, the inhibitory influence of the

intestinal phase is greatest. Acidic chyme in the duodenum inhibits CNS stimulation and initiates local

reflexes that inhibit gastric secretion. Acidic chyme also stimulates the duodenum to release the hormone

secretin, which enters the blood and is carried to the stomach, where it inhibits gastric secretion. Fatty acids

and certain other lipids in the duodenum initiate the release of the hormone cholecystokinin, (CCK) which

also inhibits gastric secretion.

Small intestine:

It is that portion of the GIT between the pyloric sphincter of the stomach and the ileocecal valve opening

into the large intestine. It is called small because of its relatively small diameter compared to that of the

large intestine. It is approximately 3m (12 ft) long in a living person, but it will measure nearly twice this

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length in a cadaver when the muscle wall is relaxed. The first 20 to 30 cm extending from the pyloric

sphincter is the duodenum. The next two-fifths of the small intestine is the jejunum, and the last three-

fifths is the ileum. The ileum empties into the large intestine through the ileocecal valve.

Histology of small intestine:

Like all other GI components, the small

intestine is also made up of the typical four

layers of tissue.

Mucosa: The epithelial layer composed of

simple columnar epithelium, comprises

numerous cells each with a specialized

function.

1. Absorption cells: these cells subject

the chyme to digestion and also

absorb nutrients from it.

2. Crypts of Lieberkuhn: These are

responsible for secreting the

intestinal juice and therefore makeup

the intestinal glands.

3. Paneth cells: These are associated with secreting a bactericidal enzyme called the lysozyme and have

a role in modulating the intestinal flora.

The lamina propia is composed of areolar connective tissue. Moreover, it exhibits the presence

mucosa-associated lymphoid tissue (MALT). This lymphoid tissue is very abundant and is scattered in the

distal ileum. In the ileum these are found in groups called aggregated lymphatic follicles or peyer’s patches.

Muscularis mucosa comprises the smooth muscle.

Submucosa: the duodenal submucosa is characterized by the presence of mucous secreting glands called the

Brunner’s gland. Mucus secreted by these glands has alkaline pH and buffer the acidic nature of the

incoming chyme.

Muscularis: It consists of outer longitudinal and inner muscle fibers.

Special structural features:

The mucosa and submucosa form large folds, called circular folds or plicae circulares, which can be

observed with the unaided eye. The surface area is further increased by microscopic folds of mucosa, called

villi, and by foldings of the apical plasma membrane of epithelial cells (which can be seen only with an

electron microscope), called microvilli.

Villi: Each villus is a finger like fold of mucosa that projects into the intestinal lumen. The villi are covered

with columnar epithelial cells, among which are interspread mucus-secreting goblet cells. The lamina

propia, which forms the connective tissue core of each villus, contains numerous lymphocytes, blood

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capillaries, and a lymphatic vessel called the central lacteal. Absorbed monosaccharides and amino acids

enter the blood capillaries; absorbed fat enters the central lacteals.

Microvilli: the apical or free membrane of the epithelial cells of villi project into the intestinal lumen. Each

microvillus measures about 1µm, each sq mm of the intestine containing about 200 million microvilli.

Digestive enzymes of microvilli are known as brush border enzymes which facilitate in digestion.

Digestion in small intestine:

Mechanical digestion:

Mixing and propulsion of chyme are the primary mechanical events that occur in the small intestine.

Segmental contractions are propagated for only

short distances. They mix secretions from the

small intestine, pancreas, and liver with ingested

materials, which promotes their digestion and

absorption. Peristaltic contractions proceed along

the length of the intestine and cause the chyme to

move along the small intestine. Parasympathetic

stimulation increases contractions but is not as

important for the intestines as for the stomach.

The ileocecal valve allows chyme to move from

the ileum into the large intestine and prevent

movement from the large intestine back into the ileum.

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Chemical digestion:

The process of breakdown of food molecules by chemical substances like enzymes is known as chemical

digestion. The partially digested chyme entering the small intestine undergoes complete digestion in the

small intestine with the aid of pancreatic juice from pancreas, bile from liver, intestinal juice and their

associated enzymes.

Release of pancreatic and bile juice into small intestine:

Secretions of the Small Intestine

The mucosa of the small intestine produces secretions that primarily contain mucus, electrolytes, and water.

Intestinal secretions lubricate and protect the intestinal wall from the acidic chyme and the action of

digestive enzymes.

The microvilli of absorptive cells have enzymes bound to their free surfaces that play a significant role in the

final steps of digestion.

Disaccharidases break down disaccharides into monosaccharides.

Peptidases break the proteins to form amino acids. Small molecules, which are breakdown

products of digestion, are absorbed through the microvilli and enter the circulatory or lymphatic

system.

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Composition of Bile

Bile is a yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, triglycerides,

phospholipids (lecithin and others), and a variety of electrolytes. Of these, only bile salts and phospholipids

aid the digestive process. Bile salts, primarily cholic acid and chenodeoxycholic acids, are cholesterol

derivatives. Their role is to emulsify fats.

The chief bile pigment is bilirubin, a waste product of the heme of hemoglobin formed during the

breakdown of worn-out erythrocytes. The globin and iron parts of hemoglobin are saved and recycled, but

bilirubin is absorbed from the blood by the liver cells, excreted into bile, and metabolized in the small

intestine by resident bacteria. One of its breakdown products, stercobilin, gives feces a brown color. In the

absence of bile, feces are gray-white in color and have fatty streaks (because essentially no fats are digested

or absorbed). The liver produces 500 to 1000 ml of bile daily, and production increases when the GI tract

contains fatty chyme.

The gallbladder stores bile that is not immediately needed for digestion and concentrates it by absorbing

some of its water and ions. (In some cases, bile released from the gallbladder is ten times as concentrated as

that entering it). When its muscular

wall contracts, bile expelled into its

duct, the cystic duct, and then flows

into the bile duct.

Composition of Pancreatic Juice:

Approximately 1200 to 1500 ml of

clear pancreatic juice is produced

daily. It consists mainly of water, and

contains enzymes and electrolytes

(primarily bicarbonate ions). The

acinar cells produce the enzyme-rich

component of pancreatic juice. The

high pH of pancreatic fluid helps neutralize acid chyme entering the duodenum and provides the optimal

environment for activity of intestinal and pancreatic enzymes. Like pepsin of the stomach, pancreatic

proteases (protein-digesting enzymes) are produced and released in inactive forms, which are activated in

the duodenum, where they do their work. This prevents the pancreas from self-digestion. For example,

within the duodenum, trypsinogen is activated to trypsin by entero-kinase, an intestinal brush border

protease. Trypsin, in turn, activates two other pancreatic proteases (procarboxypeptidase and

chymotrypsinogen) to their active forms, carboxypeptidase and chymotrypsin, respectively. Other

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pancreatic enzymes—amylase, lipases, and nucleases—are secreted in active form, but require that ions or

bile be present in the intestinal lumen for optimal activity.

Digestion and absorption of carbohydrates:

Most carbohydrates are ingested as starch, which is a long polysaccharide. The most commonly ingested

sugars are sucrose and lactose. The digestion of starch begins in the mouth with the action of salivary

amylase. This enzyme cleaves some of the bonds between adjacent glucose molecules, but most people

don’t chew their food long enough for sufficient digestion to occur in the mouth. The digestive action of

salivary amylase stops some time after the swallowed bolus enters the stomach because this enzyme is

inactivated at the low pH of gastric juice.

The digestion of starch, therefore, occurs mainly in the duodenum as a result of the action of pancreatic

amylase. This enzyme cleaves the straight chains of starch to produce the disaccharide maltose and the

trisaccharide maltriose. Pancreatic amylase, however, cannot hydrolyze the bond between glucose molecules

at the branch points in the starch. As a result, short, branched chains of glucose molecules called

oligosaccharides are released together with maltose and maltriose by the activity of this enzyme.

Maltose, maltriose, and oligosaccharides are hydrolyzed to their monosaccharides by brush border enzymes

located on the microvilli of the epithelial cells in the small intestine. The brush border enzymes also

hydrolyze the disaccharides sucrose and lactose into their component monosaccharides. These

monosaccharides are secreted from the epithelial cells into blood capillaries within the intestinal villi.

Digestion and absorption of proteins:

Protein digestion begins in the stomach with the action of pepsin. Some amino acids are liberated in the

stomach, but the major products of pepsin digestion are short-chain polypeptides. Most protein digestion

occurs in the duodenum and jejunum. The pancreatic juice enzymes trypsin, chymotrypsin, and elastase

cleave peptide bonds in the interior of the polypeptide chains. These enzymes are thus grouped together as

endopeptidases. Enzymes that remove amino acids from the ends of polypeptide chains, by contrast, are

exopeptidases. These include the pancreatic juice enzyme carboxypeptidase, which removes amino acids

from the carboxyl-terminal end of polypeptide chains, and the brush border enzyme aminopeptidase.

Aminopeptidase cleaves amino acids from the amino-terminal end of polypeptide chains.

Newborn babies appear to be capable of absorbing a substantial amount of undigested proteins (hence they

can absorb some antibodies from their mother’s first milk).

Digestion and Absorption of Lipids:

The salivary glands and stomach of neonates (newborns) produce lipases. In adults, however, very little lipid

digestion occurs until the lipid globules in chyme arrive in the duodenum. The arrival of lipids (primarily

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triglyceride, or fat) in the duodenum serves as a stimulus for the secretion of bile. In a process called

emulsification, bile salt break up the fat droplets into tiny emulsification droplets of triglycerides. Fat

digestion occurs at the surface of the droplets through the enzymatic action of pancreatic lipase, Through

hydrolysis, lipase removes two of the three fatty acids from each triglyceride molecule and thus liberates

free fatty acids and monoglycerides. Triglycerides, phospholipids, and cholesterol are then combined with

protein inside the epithelial cells to form small particles called chylomicrons. These tiny lipid and protein

combinations are secreted into the central lacteals (lymphatic capillaries) of the intestinal villi.

Transport of Lipids in the Blood

Once the chylomicrons are in the blood, their triglyceride content is removed by the enzyme lipoprotein

lipase, which is attached to the endothelium of blood vessels. This enzyme hydrolyzes triglycerides and thus

provides free fatty acids and glycerol for use by the cells.

Flora:

There are large number of microbes in the colon which synthesize vit-K and folic acid. Most bacteria belong

to the genera Bacteroides, Clostridium, Fusobacterium, Eubacterium, Ruminococcus, Peptococcus,

Peptostreptococcus, and Bifidobacterium. Other genera, such as Escherichia and Lactobacillus, are present

to a lesser extent. They may become pathogenic if transferred to another parts. Hydrogen, CO2 & methane

are produced by bacterial fermentation of unabsorbed nutrients, especially carbohydrates. Gases pass out of

the bowl as flatus. Gut flora's primary benefit to the host is the production of energy from the fermentation

of undigested carbohydrates and the subsequent absorption of short chain fatty acids. Intestinal bacteria also

play a role in synthesizing vitamin B and vitamin K as well as metabolising bile acids, sterols and

xenobiotics.

The human body carries about 100 trillion microorganisms in its intestines, a number ten times greater than

the total number of human cells in the body. The microorganisms perform a host of useful functions, such as

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fermenting unused energy substrates, training the immune system,

preventing growth of harmful, pathogenic bacteria, regulating the

development of the gut, producing vitamins for the host . (such as

biotin and vitamin K), and producing hormones to direct the host

to store fats.

DISORDERS

1. Upon standing or boiling, saliva becomes alkaline due to the

loss of CO2. The alkalinity results in precipitation of saliva

constituents which form as calculus in salivary duct or as

tartar on teeth.

2. Oral candidiasis (thrush): It is a common opportunistic

mycosis (yeast infection) of Candida species on the mucous

membranes of the mouth.

3. Cleft palate and cleft lip: it is a congenital deformity caused

by abnormal facial development during gestation. A cleft is a

fissure or opening—a gap. It is the non-fusion of the body's

natural structures that form before birth. Approximately 1 in

700 children born have a cleft lip or a cleft palate or both.

4. Mumps: Painful swelling of the salivary glands – classically

the parotid gland – is the most typical presentation.[2] Painful

testicular swelling (orchitis) and rash may also occur.

5. Dental caries: Dental caries, also known as tooth decay or a

cavity, is an infection, bacterial in origin, that causes

demineralization and destruction of the hard tissues (enamel,

dentin and cementum), usually by production of acid by

bacterial fermentation of the food debris accumulated on the

tooth surface.

6. Esophagitis:It is inflammation of the esophagus.

7. Gastritis: Gastritis can be caused by irritation due to

excessive alcohol use, chronic vomiting, stress, or the use of

certain medications such as aspirin or other anti-

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inflammatory drugs.

8. Peptic ulcers: A peptic ulcer is a defect in the lining of the first part of the small intestine, an area

called the duodenum. Main risk factors are H. Pylori, NSAID’S, Excess secretion of gastric acid,

Stress conditions, Smoking, Diet, Psychological factors, Genetic factors.

9. Gastroesophageal reflux disease or Heart burn: The lower esophageal sphincter is not a true

sphincter muscle that can be identified histologically, and it permit the acidic contents of the stomach

i.e., acidic gastric juice into the esophagus, are often treated with specific drugs (including omeprazole,

pantaprazole, rabeprazole…) that inhibit the K+/H+ pumps in the gastric mucosa. Since gastric acid

secretion is stimulated by histamine released from the ECL cells, people with peptic ulcers can be

treated with drugs that block histamine action. Drugs in this category, such as Rantac and Zintac,

specifically block the H2 histamine receptors in the gastric mucosa. This is a different receptor subtype

than that blocked by antihistamines commonly used to treat cold and allergy symptoms.

10. Lactose intolerance: The ability to digest milk sugar, or lactose, depends on the presence of a brush

border enzyme called lactase. This enzyme is present in all children under the age of 4 but becomes

inactive to some degree in most adults. A deficiency of lactase can result in lactose intolerance, a

condition in which too much undigested lactose in the intestine causes diarrhea, gas, cramps, and other

unpleasant . symptoms.

Lactase enzymes similar to those produced in the small intestines of humans are produced industrially by

fungi of the genus Aspergillus. The enzyme, β-galactosidase, is available in tablet form in a variety of

doses, in many countries without a prescription. It functions well only in high-acid environments, such

as that found in the human gut due to the addition of gastric juices from the stomach.

11. The appendix is a short, thin outpouching of the cecum. It does not function in digestion, but like the

tonsils, it contains numerous lymphatic nodules and is subject to inflammation—a condition called

appendicitis. This is commonly detected in its later stages by pain in the lower right quadrant of the

abdomen. If the appendix ruptures, infectious material can spread throughout the surrounding body

cavity, causing inflammation of the peritoneum—peritonitis. This dangerous event may be prevented

by surgical removal of the inflamed appendix (appendectomy).