Physiology Of Digestive System

OPTOMETRY · SEMESTER 1

Human Anatomy and Physiology

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PHYSIOLOGY OF DIGESTIVE SYSTEM

LEARNING OUTCOMES

BY THE END OF THIS SESSION STUDENTS MUST ABLE TO:

Describe propulsive and mixing movements of the alimentary canal
Describe the mechanism of swallowing(deglutition)
Describe composition, functions and regulation of secretion of saliva
Describe composition, functions and regulation of secretion of gastric juice
Describe composition, functions and regulation of secretion of pancreatic juice
Describe composition, functions and regulation of secretion of bile
Describe composition, functions and regulation of secretion of the small intestines

Describe absorption in the small and large intestines

PART I

INTRODUCTION

Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrients by the body.

Digestive process is accomplished by mechanical and enzymatic breakdown of food into simpler chemical compounds.

While the terms Alimentary Canal and Gastrointestinal (GI) Tract are often used interchangeably to refer to the Digestive System, it's important to understand the distinction: the alimentary canal and GI tract are actually components (building blocks) of the broader Digestive System.

FUNCTIONAL ANATOMY OF DIGESTIVE SYSTEM

Digestive system is made up of gastrointestinal tract (GI tract) or alimentary canal and accessory organs, which help in the process of digestion and absorption GI tract is a tubular structure extending from the mouth, pass through the thorax, abdomen and pelvis and ends at the anus, with a length of about 30 feet. It opens to the external environment on both ends. GI tract is formed by two types of organs:

Primary digestive organs.

Accessory digestive organs

FUNCTIONAL ANATOMY OF DIGESTIVE SYSTEM CONT …

Primary Digestive Organs :are the organs where actual digestion takes place. i. Mouth ii. Pharynx iii. Esophagus iv. Stomach v. Small intestine vi. Large intestine 2. Accessory Digestive Organs: are those which help primary digestive organs in the process of digestion. Accessory digestive organs are: i. Teeth ii. Tongue iii. Salivary glands iv. Exocrine part of pancreas v. Liver vi. Gallbladder.

FUNCTIONAL ANATOMY OF DIGESTIVE SYSTEM CONT …

WALL OF DIGESTIVE SYSTEM

In general, wall of the GI tract is formed by four layers which are from inside out:
1. Mucosa layer (Mucus Layer)
2. Submucus layer
3. Muscular layer

4. Serosa (Serous) or fibrous layer

WALL OF DIGESTIVE SYSTEM

Mucosa is the innermost layer made up of three layers
The inner epithelium
Lamina propria

Muscularis mucosae

Submucosa,this layer is composed of connective tissue, it contains numerous small glands, blood vessels, and parasympathetic nerves that form the submucosal plexus (Meissner plexus)

WALL OF DIGESTIVE SYSTEM CONT ..

Muscular is thick layer of smooth muscle tissue which consists of inner circular layer and outer longitudinal layer Between the two layers of muscles there is a nervous plexus called myenteric plexus (Auerbach plexus) Serosa is the outermost layer of made up loose connective tissue (visceral peritoneum)

FOOD INTAKE REGULATION

Food intake and energy expenditure are key regulators of body weight In humans, food intake is an essential function that is coordinated by the gastrointestinal, endocrine and nervous systems to maintain energy homeostasis The food intake cycle stages are hunger, satiation and satiety Hunger is the desire to eat and it is described as an uncomfortable emptiness in the abdomen.

Satiation refers to the sensation of fullness that results in meal termination.

Satiety is the term given to the postprandial events that determine the timing for the next meal

FUNCTIONS OF DIGESTIVE SYSTEM

Digestive system plays the major role in the digestion and absorption of food substances. Thus, the functions of digestive system include:

Ingestion or consumption of food substances

Breaking them into small particles by Mastication.

Transport of small particles to different areas of the digestive tract through Propulsion.

Secretion of necessary enzymes and other substances for digestion
Digestion of the food particles
Absorption of the digestive products (nutrients)

Removal of unwanted substances from the body.

FUNCTIONS OF DIGESTIVE SYSTEM CONT …

Ingestion; taking in food into the stomach through the oral cavity, although liquid can be introduced directly into the stomach by nasogastric tube.

Mastication; is the process by which food taken into the mouth is chewed by the teeth and broken down into small particles.

Digestive enzymes cannot easily penetrate solid food particles and can only work effectively on the surfaces of the particles.

FUNCTIONS OF DIGESTIVE SYSTEM CONT …

Propulsion; moves the contents along the alimentary tract.

Followed by a wave of strong contraction of the circular muscles behind the bolus, which forces the bolus along the digestive tube Each segment of the digestive tract is specialised to assist in moving its contents from the oral to the anal end.

Mixing some contractions don’t propel food from one end of the digestive tract to the other but rather move the food back and forth within the digestive tract to mix it with digestive secretion and help break into smaller pieces.

Segmental contraction is mixing contractions that occur in the small intestine.

FUNCTIONS OF DIGESTIVE SYSTEM CONT …

Secretion; as food moves through the digestive tract, secretion are added to lubricate, liquefy, and digest the food.

Mucus secreted along the entire digestive tract lubricates the food and the lining of the tract.

The secretions contain large amount of water which liquefies the food, thereby making it easier to digest and absorb.

Liver secretions break large fat droplets, which makes possible the digestion and absorption of fats.

Enzymes secreted by the oral cavity, stomach, intestine and pancreas break large food molecules into smaller molecules that can be absorbed by the intestinal wall.

FUNCTIONS OF DIGESTIVE SYSTEM CONT …

Digestion; is the breakdown of large organic molecules into their components parts; carbohydrates into monosaccharides, protein into amino acids, and triglyceride into fatty acids and glycerol.

Digestion consists of
Mechanical digestion, which involve mastication and mixing of food.

Chemical which prepares food for absorption and utilization by all the cells of the body.

FUNCTIONS OF DIGESTIVE SYSTEM CONT …

Absorption; is the movement of molecules out of the digestive tract and into the circulation or into the lymphatic system.

Elimination; is the process by which the waste products of digestion are removed from the body.

Food material not absorbed becomes faeces that are eliminated by the process of defecation.

SWALLOWING/DEGLUTITION

MECHANISM OF SWALLOWING/DEGLUTITION

Swallowing or deglutition, is a complex reflex mechanism by which food is pushed from oral cavity into the oesophagus and then pushed to the stomach. This movement of food from the oral cavity on to oesophagus and stomach by pushing is called propulsion and it is an important part of the digestive process.

THERE ARE FOUR STAGES OF SWALLOWING.

Oral Stage (Pre-Oral & Oral Phase)

Pharyngeal Stage

Esophageal Stage

MECHANISM OF SWALLOWING/DEGLUTITION CONT …

1.a)THE PRE-ORAL PHASE.

Starts with the anticipation of food being introduced into the mouth-Salivation is triggered by the sight and smell of food(as well as hunger).

MECHANISM OF SWALLOWING/DEGLUTITION CONT …

1.b)THE ORAL PHASE.

The lips close and form a seal, chewing/mastication of food begins, the food is mixed with saliva to form a bolus, the bolus is then transferred to the back of the mouth.

Deglutition or swallowing moves food and liquids called bolus from the oral cavity into the oesophagus which is enhanced by:

Peristalsis is responsible moving material through most of the digestive tract.

Muscular contractions occur in peristaltic waves, consisting of a wave of relaxation of the circular muscles, which forms a leading wave of distension in front of the bolus.

MECHANISM OF SWALLOWING/DEGLUTITION CONT …

2. THE PHARYNGEAL STAGE.

Reflexive stage (reflex response i.e. Not planned ).initiation of the swallow. The nasal cavity is sealed when the soft palate raises(that prevents food /fluid coming out of the nose).-the larynx ( voice box) moves upwards and forward, the vocal folds close, and then the epiglottis closes over the air ways( breathing stops momentarily).Pharynx pushes the bolus down by contracting in a stripping motion.

MECHANISM OF SWALLOWING/DEGLUTITION CONT …

3. THE ESOPHAGEAL STAGE.

The bolus is transported through the esophagus by wave like muscular contraction (peristalsis) into the stomach-the lower esophageal sphincter opens to allow the bolus into the stomach- the lower esophageal sphincter closes after the bolus has passed through(to prevent reflux/regurgitation).

SALIVA

Saliva is the combined secretions from the salivary glands and the small mucus-secreting glands of the lining of the oral cavity.

About 1.5 litres of saliva is produced daily and it consists of:

Water

Mineral salts/Electrolytes: Sodium (Na⁺)- Potassium (K⁺)- Chloride (Cl⁻)- Bicarbonate (HCO₃⁻) Enzyme: Salivary amylase, Lingual lipase Mucus

Lysozyme: Antibacterial enzyme
Immunoglobulins: Immunoglobulin A (IgA)

Blood-Clotting Factors

FUNCTIONS OF SALIVA

1. PREPARATION OF FOOD FOR SWALLOWING : When food is taken into the mouth, it is moistened and dissolved by saliva. It facilitates chewing. By the movement of tongue, the moistened and masticated food is rolled into a bolus. Mucin of saliva lubricates the bolus and facilitates swallowing 2. APPRECIATION OF TASTE: Taste is a chemical sensation. By its solvent action, saliva dissolves the solid food substances, so that the dissolved substances can stimulate the taste buds. The stimulated taste buds recognize the taste.

3. DIGESTIVE FUNCTION: Saliva has three digestive enzymes, namely salivary amylase, maltase and lingual lipase

FUNCTIONS OF SALIVA CONT …

4.CLEANSING AND PROTECTIVE FUNCTIONS: Due to the constant secretion of saliva, the mouth and teeth are rinsed and kept free off food debris, shed epithelial cells and foreign particles. In this way, saliva prevents bacterial growth by removing materials, which may serve as culture media for the bacterial growth. Enzyme lysozyme of saliva kills some bacteria such as staphylococcus, streptococcus and brucella 5. ROLE IN SPEECH: By moistening and lubricating soft parts of mouth and lips, saliva helps in speech. If the mouth becomes dry, articulation and pronunciation becomes difficult.

FUNCTIONS OF SALIVA CONT …

6.EXCRETORY FUNCTION: Many substances, both organic and inorganic, are excreted in saliva. It excretes substances like mercury, potassium iodide, lead, and thiocyanate. Saliva also excretes some viruses such as those causing rabies and mumps.

7. REGULATION OF WATER BALANCE: When the body water content decreases, salivary secretion also decreases. This causes dryness of the mouth and induces thirst. When water is taken, it quenches the thirst and restores the body water content.

REGULATION OF SALIVARY SECRETION

Salivary secretion is regulated by;
Reflex mechanisms control the secretion of saliva
Chemical and mechanical stimuli come from presence of food in the mouth
Olfactory and visual stimuli come from smell and sight of food

Autonomic nervous control

REGULATION OF SALIVARY SECRETION CONT …

Reflex secretion occurs when there is food in the mouth and the reflex can easily become conditioned so that the sight, smell and even the thought of food stimulates the flow of saliva.

Autonomic Nervous System Control

Function of Parasympathetic Fibers: Parasympathetic stimulation causes vasodilatation and profuse secretion of watery saliva with a relatively low content of enzymes and other organic substances.

Function of Sympathetic Fibers: Sympathetic stimulation causes vasoconstriction and secretion of small amounts of saliva rich in organic material, especially from the submandibular glands.

STOMACH

THE STOMACH AND GASTRIC JUICE

Stomach is a hollow organ situated just below the diaphragm on the left side in the abdominal cavity. Volume of empty stomach is 50 mL. Under normal conditions, it can expand to accommodate 1 L to 1.5 L of solids and liquids. However, it is capable of expanding still further up to 4 L.

The stomach has a J-like shape
PARTS OF STOMACH: In humans, stomach has four parts:
Cardia or Cardiac region
Fundus
Body or corpus

Pyloric region.

THE STOMACH AND GASTRIC JUICE

Stomach wall is formed by four layers of structures:

Outer serous layer: Formed by peritoneum Muscular layer: Made up of three layers of smooth muscle fibers, namely inner oblique, middle circular and outer longitudinal layers Submucus layer: Formed by areolar tissue, blood vessels, lymph vessels and Meissner nerve plexus.

Inner mucus layer: Lined by mucus secreting columnar epithelial cells. The gastric glands are situated in this layer. Under resting conditions, the mucosa of the stomach is thrown into many folds. These folds are called rugae. The rugae disappear when the stomach is distended after meals. Throughout the inner mucus layer, small depressions called gastric pits are present. Glands of the stomach open into these pits.

THE STOMACH AND GASTRIC JUICE

The stomach acid itself does not break down food molecules; rather it provides an optimum pH for the reaction of the enzyme pepsin and kills many microorganisms that are ingested with the food.

Other small molecules such as alcohol are absorbed in the stomach, passing through the membrane of the stomach and entering the circulatory system directly

CELLS LINING STOMACH

The stomach is lined with simple columnar epithelium The epithelium forms numerous tubelike gastric pits, which are the opening of the gastric glands The epithelial cells of the stomach are of five types The first type is surface mucous cells which produce mucus, is on the surface and lines the gastric pits

CELLS LINING STOMACH CONT …

The remaining four cell types are in the gastric glands:

mucous neck cells which produce mucus;

parietal (oxyntic) cells which secrete hydrochloric acid and intrinsic factor needed for vitamin B12 absorption Chief (zymogenic) cells which produce pepsinogen and Endocrine cells, which produce regulatory hormones

STRUCTRE

GASTRIC GLANDS

NERVE SUPPLY TO THE STOMACH

The sympathetic supply to the stomach is mainly from the coeliac plexus and the parasympathetic supply is from the vagus nerves.

Sympathetic stimulation reduces the motility of the stomach and the secretion of gastric juice WHILE

Vagal stimulation(Parasympathetic) has the opposite effect.

NOTE; In other systems the nerve stimulation works different from GI system, forexample in Cardiac System; Sympathetic stimulation results results in increase in cardiac activity while Parasympathetic stimulation result in the opposite effect.

FUNCTIONS OF THE STOMACH

Reservoir for food (Temporary Storage) until it is partially digested and moved further along the GI tract Secrete gastric juice to aid in digestion of food Breaks food into small particles and mixes them with gastric juice (Chyme) Secrete intrinsic factor (Intrinsic factor of Castle, secreted by parietal cells of gastric glands) Carries limited absorption (water, some water-soluble drugs like aspirin, and a small amount of alcohol) Produces gastrin hormone

Helps protect body from pathogenic bacteria swallowed with food

GASTRIC JUICE

Gastric juice is a digestive fluid secreted by the gastric glands in the stomach, essential for food breakdown.

About 2 litres of gastric juice are secreted daily by special secretory glands in the mucosa It consists of:

FUNCTIONS OF GASTRIC JUICE

Water further liquefies the food swallowed.
Function of Hydrochloric acid:
Acidifies the food and stops the action of salivary amylase kills ingested microbes

Provides the acid environment needed for effective digestion by pepsins.

Pepsinogens are activated to pepsins by hydrochloric acid and by pepsins already present in the stomach.

They begin the digestion of proteins, breaking them into smaller molecules.

Pepsins act most effectively at pH 1.5 to 3.5.

FUNCTIONS OF GASTRIC JUICE CONT …

3. Intrinsic factor secretion; (a protein) is necessary for the absorption of vitamin B12 from the ileum, which is important precursor in hematopoiesis.

4. Function of Mucus; prevents mechanical injury to the stomach wall by lubricating the contents.

It prevents chemical injury by acting as a barrier between the stomach wall and the corrosive gastric juice.

Hydrochloric acid is present in potentially damaging concentrations and pepsins digest protein

FUNCTIONS OF GASTRIC JUICE CONT …

3. PROTECTIVE FUNCTION – FUNCTION OF MUCUS: Mucus is a mucoprotein, secreted by mucus neck cells of the gastric glands and surface mucus cells in fundus, body and other parts of stomach. It protects the gastric wall by the following ways:

Mucus:

Protects the stomach wall from irritation or mechanical injury, by virtue of its high viscosity.

Prevents the digestive action of pepsin on the wall of the stomach, particularly gastric mucosa.

Protects the gastric mucosa from hydrochloric acid of gastric juice because of its alkaline nature

FUNCTIONS OF GASTRIC JUICE CONT …

4. FUNCTIONS OF HYDROCHLORIC ACID: Hydrochloric acid is present in the gastric juice:

i. Activates pepsinogen into pepsin

ii. Kills some of the bacteria entering the stomach along with food substances. This action is called bacteriolytic action iii. Provides acid medium, which is necessary for the action of hormones.

FOOD DIGESTION IN THE STOMACH

Mixing with the gastric juice takes place gradually and it may be some time before the food is sufficiently acidified to stop the action of salivary amylase.

Gastric muscle contraction consists of a churning movement that breaks down the bolus and mixes it with gastric juice, and peristaltic waves that propel the stomach contents towards the pylorus.

When the stomach is active the pyloric sphincter closes Strong peristaltic contraction of the pyloric antrum forces gastric contents, after they are sufficiently liquefied, through the pylorus into the duodenum in small spurts.

REGULATION OF GASTRIC JUICE SECRETION

Secretion of gastric juice is a continuous process. But the quantity varies, depending upon time and stimulus.

The nervous and endocrine systems collaborate to increase gastric secretion and motility when food is eaten and suppresses them as the stomach empties Gastric activity is divided into three stages called the cephalic, gastric, and intestinal phases, based on whether the stomach is rolled by the brain, by itself, or by the small intestine respectively These phases overlap and all can occur simultaneously

CEPHALIC PHASE

Secretion of gastric juice by the stimuli arising from head region (cephalus) is called cephalic phase.

This phase of gastric secretion is regulated by nervous mechanism.
The gastric juice secreted during this phase is called appetite juice.
During this phase, gastric secretion occurs even without the presence of food in stomach.
Sight, Taste, Smell and Thought stimulate the cerebral cortex
Taste and smell stimulus is sent to the hypothalamus and medulla oblongata
After this response is brought through the vagus nerve and release of acetylcholine
Gastric secretion at this phase rises to 40% of maximum rate

The quantity of the juice is less but it is rich in enzymes and hydrochloric acid.

GASTRIC PHASE

Secretion of gastric juice when food enters the stomach is called gastric phase. This phase is regulated by both nervous and hormonal control. Gastric juice secreted during this phase is rich in pepsinogen and hydrochloric acid. Mechanisms involved in gastric phase are:

1. Nervous mechanism through local myenteric reflex and vagovagal reflex
2. Hormonal mechanism through gastrin
Stimuli, which initiate these two mechanisms are:
Distention of stomach
Mechanical stimulation of gastric mucosa by bulk of food

Chemical stimulation of gastric mucosa by the food contents.

INTESTINAL PHASE

When the partially digested contents of the stomach (chyme) reach the small intestine two hormones Secretin and Cholecystokinin (CCK) are produced by endocrine cells in the intestinal mucosa They slow down secretion of Gastric juice and reduce Gastric motility

GASTRIC SECRETION INHIBITION BY GI HORMONES

Presence of chyme in the intestine stimulates the secretion of many GI hormones from intestinal mucosa and other structures. All these hormones inhibit the gastric secretion. Some of these hormones inhibit the gastric motility also.

GI hormones which inhibit gastric secretion:

i. Secretin: Secreted by the presence of acid chyme in the intestine ii. Cholecystokinin: Secreted by the presence of chyme containing fats and amino acids in intestine iii. Gastric inhibitory peptide (GIP): Secreted by the presence of chyme containing glucose and fats in the intestine iv. Vasoactive intestinal polypeptide (VIP): Secreted by the presence of acidic chyme in intestine v. Peptide YY: Secreted by the presence of fatty chyme in intestine.

In addition to these hormones, pancreas also secretes a hormone called somatostatin during intestinal phase. It also inhibits gastric secretion.

FACTORS INFLUENCING GASTRIC SECRETION

Gastric secretion is also influenced by some factors which increase the gastric secretion by stimulating gastric mucosa such as:

1. Alcohol

2. Caffeine

PART II

PANCREATIC JUICE

PANCREATIC EXOCRINE SECRETIONS

Pancreas is a dual organ having two functions, namely endocrine function and exocrine function.

Endocrine function is concerned with the production of hormones.

The exocrine function is concerned with the secretion of digestive juice called pancreatic juice Exocrine part of pancreas resembles salivary gland in structure

PANCREATIC EXOCRINE SECRETIONS

A small duct arises from lumen of each alveolus. Some of these ducts from neighboring alveoli unite to form intralobular duct. All the intralobular ducts unite to form the main duct of pancreas called Wirsung duct. Wirsung duct joins common bile duct to form ampulla of Vater, which opens into duodenum. In some persons, an accessory duct called duct of Santorini exists. It also opens into duodenum, proximal to the opening of ampulla of Vater.

COMPOSITION OF PANCREATIC JUICE

Pancreatic juice contains 99.5% of water and 0.5% of solids.

The solids are the organic and inorganic substances.

Bicarbonate content is very high in pancreatic juice. It is about 110 to 150 mEq/ L, against the plasma level of 24 mEq/L.

High bicarbonate content of pancreatic juice is important because of two reasons:

i. High bicarbonate content makes the pancreatic juice highly alkaline, so that it protects the intestinal mucosa from acid chyme by neutralizing it ii. Bicarbonate ions provide the required pH (7 to 9) for the activation of pancreatic enzymes.

FUNCTIONS OF PANCREATIC JUICE

Digestive function; digest carbohydrate, protein, fats and nucleic acids Neutralizing action; The bicarbonate in pancreatic juice is alkaline, neutralizing the acidic chyme that enters the small intestine from the stomach

REGULATION OF PANCREATIC JUICE SECRETION

HORMONAL REGULATION

Secretin: Released when the stomach acid (chyme) enters the duodenum. It stimulates the pancreas to secrete large amounts of bicarbonate-rich fluid to neutralize the acid.

Cholecystokinin (CCK): Released in response to fats in the small intestine. It stimulates the acinar cells to release digestive enzymes and also causes gallbladder contractions and relaxation of the hepatopancreatic sphincter to allow bile and pancreatic juice to flow into the duodenum.

Gastrin: Also stimulates pancreatic enzyme secretion.

Other hormones: Glucagon, vasoactive intestinal polypeptide (VIP), somatostatin, motilin, and bombesin may also play a role.

REGULATION OF PANCREATIC JUICE SECRETION

Secretion of pancreatic juice is regulated by both nervous and hormonal factors.

STAGES OF PANCREATIC SECRETION:

Pancreatic juice is secreted in three stages like the gastric juice:
Cephalic phase
Gastric phase

Intestinal phase.

PHASES OF PANCREATIC JUICE SECRETION

CEPHALIC PHASE: As in case of gastric secretion, cephalic phase is regulated by nervous mechanism through reflex action.

GASTRIC PHASE: Secretion of pancreatic juice when food enters the stomach is known as gastric phase. This phase of pancreatic secretion is under hormonal control. The hormone involved is gastrin. When food enters the stomach, gastrin is secreted from stomach. When gastrin is transported to pancreas through blood, it stimulates the pancreatic secretion. The pancreatic juice secreted during gastric phase is rich in enzymes INTESTINAL PHASE Intestinal phase is the secretion of pancreatic juice when the chyme enters the intestine. This phase is also under hormonal control. When chyme enters the intestine, many hormones are released. Some hormones stimulate the pancreatic secretion and some hormones inhibit the pancreatic secretion.

HORMONES THAT STIMULATE AND INHIBIT PANCREATIC SECRETION

Hormones Stimulating Pancreatic Secretion

i. Secretin
ii. Cholecystokinin

Hormones Inhibiting Pancreatic Secretion

i. Pancreatic polypeptide (PP) secreted by PP cells in islets of Langerhans of pancreas
ii. Somatostatin secreted by D cells in islets of Langerhans of pancreas
iii. Peptide YY secreted by intestinal mucosa

iv. Peptides like ghrelin and leptin

NEURAL REGULATION OF PANCREATIC JUICE SECRETION

The autonomic nervous system, particularly the vagus nerve, influences pancreatic secretion.

Acetylcholine, a neurotransmitter from the vagus nerve, stimulates the pancreatic acini to secrete their enzymes.

Neural input and hormonal signals, like CCK, work together in a synergistic manner to regulate secretion

BILE SALTS

Bile Salts are amphipathic compounds derived from cholesterol, essential for digestion and absorption of fats.

Amphipathic: Have both hydrophilic and hydrophobic regions- This allows them to interact with fats and water. Main components include;

Water constitute 97.0%, in the gall bladder the amount is reduced 5 folds
Bile pigments 0.2%

Cholesterol 0.06%

Inorganic salts 0.7% such as Na+, K+& Ca2+ salts Cl- HCO3- & Phosphorus

Fatty acids	0.15%
Lecithin	0.1%
Fat	0.1%

Alkaline phosphatase

Proteins

FUNCTIONS OF BILE SALTS

1. Emulsification of Fats: Emulsification is the process by which the fat globules are broken down into minute droplets and made in the form of a milky fluid called emulsion in small intestine, by the action of bile salts.

2. Absorption of Fats and Fat Soluble Vitamins (Vitamin A, D, E & K): Bile salts help in the absorption of digested fats from intestine into blood. Bile salts combine with fats and make complexes of fats called micelles. The fats in the form of micelles can be absorbed easily.

3. Choleretic Action: Bile salts stimulate the secretion of bile from liver. This action is called choleretic action.

FUNCTIONS OF BILE SALTS

4. Cholagogue Action: Cholagogue is an agent which causes contraction of gallbladder and release of bile into the intestine. Bile salts act as cholagogues indirectly by stimulating the secretion of hormone cholecystokinin. This hormone causes contraction of gallbladder, resulting in release of bile.

5. Laxative Action: Laxative is an agent which induces defecation. Bile salts act as laxatives by stimulating peristaltic movements of the intestine.

6. Prevention of Gallstone: Bile salts prevent the formation of gallstone by keeping the cholesterol and lecithin in solution. In the absence of bile salts, cholesterol precipitates along with lecithin and forms gallstone

REGULATION OF BILE SALTS SECRETION

Secretion of Bile Saltd is regulated also by hormonal and neural mechanisms, as follow;

HORMONAL

Cholecystokinin (CCK): – Released from the duodenum in response to fatty chyme – Causes gallbladder contraction and relaxation of the sphincter of Oddi – Releases bile into the duodenum Secretin: – Stimulates liver to increase bile secretion, especially its watery, bicarbonate-rich component

NEURAL

Neural Control:- Vagus nerve (parasympathetic): – Stimulates gallbladder contraction during the cephalic and gastric phases

SMALL INTESTINE

FUNCTIONAL ANATOMY OF SMALL INTESTINE

Small intestine is the part of gastrointestinal (GI) tract, extending between the pyloric sphincter of stomach and ileocecal valve, which opens into large intestine.

It is called small intestine because of its small diameter Important function of small intestine is absorption. Maximum absorption of digested food products takes place in small intestine.

Small intestine consists of three portions:
1. Proximal part known as duodenum
2. Middle part known as jejunum
3. Distal part known as ileum

Wall of the small intestine has all the four layers as in stomach

FUNCTIONS OF SMALL INTESTINE

1. Mechanical Function: Mixing movements of small intestine help in the thorough mixing of chyme with the digestive juices like succus entericus, pancreatic juice and bile.

2. Secretory Function: Small intestine secretes succus entericus, enterokinase and the GI hormones, also Small intestine secretes many GI hormones such as secretin, cholecystokinin, etc. These hormones regulate the movement of GI tract and secretory activities of small intestine and pancreas 3. Absorptive Function: Presence of villi and microvilli in small intestinal mucosa increases the surface area of mucosa. This facilitates the absorptive function of intestine. Digested products of foodstuffs, proteins, carbohydrates, fats and other nutritive substances such as vitamins, minerals and water are absorbed mostly in small intestine. From the lumen of intestine, these substances pass through lacteal of villi, cross the mucosa and enter the blood directly or through lymphatics

INTESTINAL VILLI AND GLANDS OF SMALL INTESTINE

INTESTINAL VILLI: Mucous membrane of small intestine is covered by minute projections called villi. Villi are lined by columnar cells, which are called enterocytes. Each enterocyte gives rise to hair-like projections called microvilli. Villi and microvilli increase the surface area of mucous membrane by many folds. Within each villus, there is a central channel called lacteal, which opens into lymphatic vessels and contains blood vessels too.

CRYPTS OF LIEBERKÜHN OR INTESTINAL GLANDS: Crypts of Lieberkühn or intestinal glands are simple tubular glands of intestine. Intestinal glands do not penetrate the muscularis mucosa of the intestinal wall, but open into the lumen of intestine between the villi. Intestinal glands are lined by columnar cells. Lining of each gland is continuous with epithelial lining of the villi. Epithelial cells lining the intestinal glands undergo division by mitosis at a faster rate. Newly formed cells push the older cells upward over the lining of villi. These cells which move to villi are called enterocytes. Enterocytes secrete the enzymes

INTESTINAL VILLI AND GLANDS OF SMALL INTESTINE

SMALL INTESTINE SECRETIONS (SUCCUS ENTERICUS)

PROPERTIES AND COMPOSITION OF SUCCUS ENTERICUS

Succus Entericus is the term for intestinal juice secreted by the glands of the small intestine, mainly from the crypts of Lieberkühn.

Composition of Succus Entericus:
Water
Mucus (from goblet cells)

Electrolytes (e.g., bicarbonate, Na⁺, Cl⁻) Enzymes (mostly bound to brush border, not free in juice): – Peptidases – break down peptides to amino acids, Disaccharidases – e.g., maltase, lactase, sucrase – Enterokinase – activates trypsinogen to trypsin – Nucleotidases & nucleosidases – digest nucleic acids

FUNCTIONS OF SUCCUS ENTERICUS

1. Digestive Function: Enzymes of succus entericus act on the partially digested food and convert them into final digestive products. Enzymes are produced and released into succus entericus by enterocytes of the villi 2. Protective Function:

i. Mucus present in the succus entericus protects the intestinal wall from the acid chyme, which enters the intestine from stomach; thereby it prevents the intestinal ulcer.

ii. Defensins secreted by paneth cells of intestinal glands are the antimicrobial peptides. These peptides are called natural peptide antibiotics because of their role in killing the phagocytosed bacteria.

3. Activator Function: Enterokinase present in intestinal juice activates trypsinogen into trypsin. Trypsin, in turn activates other enzymes.

4. Hemopoietic Function: Intrinsic factor of Castle present in the intestine plays an important role in erythropoiesis. It is necessary for the absorption of vitamin B12.

5. Hydrolytic Process: Intestinal juice helps in all the enzymatic reactions of digestion.

REGULATION OF SECRETION OF SUCCUS ENTERICUS

Secretion of succus entericus is regulated by both nervous and hormonal mechanisms.

Nervous Regulation: Stimulation of parasympathetic nerves causes vasodilatation and increases the secretion of succus entericus. Stimulation of sympathetic nerves causes vasoconstriction and decreases the secretion of succus entericus. But, the role of these nerves in the regulation of intestinal secretion in physiological conditions is uncertain. However, the local nervous reflexes play an important role in increasing the secretion of intestinal juice. When chyme enters the small intestine, the mucosa is stimulated by tactile stimuli or irritation. It causes the development of local nervous reflexes, which stimulate the glands of intestine.

Hormonal Regulation: When chyme enters the small intestine, intestinal mucosa secretes enterocrinin, secretin and cholecystokinin, which promote the secretion of succus entericus by stimulating the intestinal glands.

LARGE INTESTINE

FUNCTIONAL ANATOMY OF LARGE INTESTINE

Large intestine or colon extends from ileocecal valve up to anus

PARTS OF LARGE INTESTINE:

Large intestine is made up of the following parts:
1. Cecum with appendix
2. Ascending colon
3. Transverse colon
4. Descending colon
5. Sigmoid colon or pelvic colon
6. Rectum

7. Anal canal

STRUCTURE OF WALL OF LARGE INTESTINE

Wall of large intestine is formed by four layers of structures like any other part of the gut.

1. Serous layer: It is formed by peritoneum 2. Muscular layer: Smooth muscles of large intestine are distributed in two layers, namely the outer longitudinal layer and inner circular layer. The longitudinal muscle fibers of large intestine are arranged in the form of three long bands called tenia coli. The length of the tenia coli is less when compared to the length of large intestine. Because of this, the large intestine is made into series of pouches called haustra 3. Submucus layer: It is not well developed in large intestine 4. Mucus layer: The crypts of Leiberkühn are present in mucosa of large intestine. But the villi, which are present in mucus membrane of small intestine, are absent in the large intestine. Only mucus-secreting glands are present in the mucosa of large intestine.

SECRETIONS OF LARGE INTESTINE

Large intestinal juice is a watery fluid with pH of 8.0.

COMPOSITION OF LARGE INTESTINAL JUICE: Large intestinal juice contains 99.5% of water and 0.5% of solids. Digestive enzymes are absent and concentration of bicarbonate is high in large intestinal juice.

FUNCTIONS OF LARGE INTESTINAL JUICE:

Neutralization of Acids: Strong acids formed by bacterial action in large intestine are neutralized by the alkaline nature of large intestinal juice. The alkalinity of this juice is mainly due to the presence of large quantity of bicarbonate.

Lubrication Activity: Mucin present in the secretion of large intestine lubricates the mucosa of large intestine and the bowel contents, so that, the movement of bowel is facilitated. Mucin also protects the mucus membrane of large intestine by preventing the damage caused by mechanical injury or chemical substances.

FUNCTIONS OF LARGE INTESTINE

1. ABSORPTIVE FUNCTION: Large intestine plays an important role in the absorption of various substances such as: i. Water ii. Electrolytes iii. Organic substances like glucose iv. Alcohol v. Drugs like anesthetic agents, sedatives and steroids.

2. FORMATION OF FECES: After the absorption of nutrients, water and other substances, the unwanted substances in the large intestine form feces. This is excreted out.

3. EXCRETORY FUNCTION: Large intestine excretes heavy metals like mercury, lead, bismuth and arsenic through feces.

4. SECRETORY FUNCTION: Large intestine secretes mucin and inorganic substances like chlorides and bicarbonates.

5. SYNTHETIC FUNCTION: Bacterial flora of large intestine synthesizes folic acid, vitamin B12 and vitamin K. By this function, large intestine contributes in erythropoietic activity and blood clotting mechanism

ABSORPTION IN THE INTESTINES

ABSORPTION IN SMALL AND LARGE INTESTINE

Absorption in the small intestine and large intestine is fundamentally different, reflecting their specialized roles in the digestive process.

The small intestine is the primary site for nutrient absorption, while the large intestine focuses on water and electrolyte reclamation The small intestine is highly adapted for absorption due to its massive surface area, which is achieved by large folds, finger-like projections called villi, and microscopic folds on the villi called microvilli.

KEY ABSORBED SUBSTANCES IN SMALL INTESTINE

Carbohydrates: Absorbed as monosaccharides (glucose, galactose, fructose) mainly in the jejunum. Glucose and galactose use a sodium-dependent co-transporter, while fructose uses facilitated diffusion.

Proteins: Absorbed as single amino acids or small peptides (di- and tripeptides), primarily in the jejunum. Absorption of amino acids also often involves sodium co-transport.

Fats (Lipids): Absorbed as fatty acids and monoglycerides throughout the small intestine, but mostly in the jejunum and ileum. They are reformed into triglycerides inside the absorptive cells, packaged into chylomicrons, and enter the lymphatic system via specialized vessels called lacteals.

KEY ABSORBED SUBSTANCES IN SMALL INTESTINE CONT …

Water-Soluble Vitamins (B and C): Absorbed by diffusion or specific carrier-mediated transport.

Fat-Soluble Vitamins (A, D, E, K): Absorbed with dietary fats via micelles.

Minerals: Iron and Calcium are mostly absorbed in the duodenum (the first part).Vitamin B12 and Bile Salts are specifically absorbed in the terminal ileum (the last part).

LARGE INTESTINE: WATER AND ELECTROLYTE RECLAMATION

By the time the remaining contents reach the large intestine, about 90% of the water and almost all digestible nutrients have been absorbed by the small intestine. The large intestine's role is to convert the remaining liquid material (chyme) into solid feces.

LARGE INTESTINE: WATER AND ELECTROLYTE RECLAMATION CONT …

Water: The primary role; absorbed via osmosis, which is driven by the active absorption of sodium.

Electrolytes: Mainly sodium (Na + ) and chloride (Cl − ) ions.

Vitamins: The resident bacterial flora in the colon synthesize significant amounts of Vitamin K and some B vitamins (like biotin and folate), which can then be absorbed into the body.

Short-Chain Fatty Acids (SCFAs): These are produced when bacteria ferment undigested carbohydrates (like dietary fiber). SCFAs (such as acetate, propionate, and butyrate) are absorbed and used by the colon cells for energy or transported to the liver.

DEFECATION

Voiding of feces is known as defecation.

Feces is formed in the large intestine and stored in sigmoid colon. By the influence of an appropriate stimulus, it is expelled out through the anus. This is prevented by tonic constriction of anal sphincters, in the absence of the stimulus.

DEFECATION REFLEX: Mass movement drives the feces into sigmoid or pelvic colon. In the sigmoid colon, the feces is stored. The desire for defecation occurs when some feces enters rectum due to the mass movement. Usually, the desire for defecation is elicited by an increase in the intrarectal pressure to about 20 to 25 cm H2O. Usual stimulus for defecation is intake of liquid like coffee or tea or water. But it differs from person to person.

Physiology Of Digestive System

Act of Defecation: is preceded by voluntary efforts like assuming an appropriate posture, voluntary relaxation of external sphincter and the compression of abdominal contents by voluntary contraction of abdominal muscles.

Usually, the rectum is empty. During the development of mass movement, the feces is pushed into rectum and the defecation reflex is initiated.

The process of defecation involves the contraction of rectum and relaxation of internal and external anal sphincters.

Internal anal sphincter is made up of smooth muscle and it is innervated by parasympathetic nerve fibers via pelvic nerve.

External anal sphincter is composed of skeletal muscle and it is controlled by somatic nerve fibers, which pass through pudendal nerve.

Pudendal nerve always keeps the external sphincter constricted and the sphincter can relax only when the pudendal nerve is inhibited.

Gastrocolic Reflex: is the contraction of rectum, followed by the desire for defecation caused by distention of stomach by food. It is mediated by intrinsic nerve fibers of GI tract. This reflex causes only a weak contraction of rectum. But, it initiates defecation reflex.

PATHWAY FOR DEFECATION REFLEX

When rectum is distended due to the entry of feces by mass movement, sensory nerve endings are stimulated. Impulses from the nerve endings are transmitted via afferent fibers of pelvic nerve to the defecation center, situated in sacral segments (center) of spinal cord. The center in turn, sends motor impulses to the descending colon, sigmoid colon and rectum via efferent nerve fibers of pelvic nerve. Motor impulses cause strong contraction of descending colon, sigmoid colon and rectum and relaxation of internal sphincter. Simultaneously, voluntary relaxation of external sphincter occurs. It is due to the inhibition of pudendal nerve, by impulses arising from cerebral cortex