The Digestive SystemThe Digestive System

Is the system responsible for:Is the system responsible for:

1. Ingestion of food 1. Ingestion of food

2. Digestion of food2. Digestion of food

3. Absorption of food3. Absorption of food

4. Egestion (Elimination) of food4. Egestion (Elimination) of food

The Digestive SystemThe Digestive System

The digestive organsThe digestive organs The accessory organsThe accessory organs

• The mouth The mouth • The pharynxThe pharynx• The esophagusThe esophagus• The stomachThe stomach• The small intestineThe small intestine• The large intestineThe large intestine

• The salivary glands The salivary glands • The liverThe liver• The pancreasThe pancreas

TTHHEE DDIIGGEESSTTIIVVEE

SSYYSSTTEEMM

Digestion:Digestion: is the breakdown of the complex food molecules is the breakdown of the complex food molecules

into simpler molecules that could be absorbed by the body.into simpler molecules that could be absorbed by the body.

DigestionDigestion

Mechanical (Physical) Chemical (enzymes)Mechanical (Physical) Chemical (enzymes)

Mechanical digestion:Mechanical digestion: includes the breakdown of the food includes the breakdown of the food

Without any change in its molecular structure i.e.: smaller Without any change in its molecular structure i.e.: smaller

particlesparticles

Chemical digestion:Chemical digestion: Bigger molecules are dismantled into Bigger molecules are dismantled into

smaller molecules which are different in structuresmaller molecules which are different in structure

The optimal functioning of digestive enzymes depends on:The optimal functioning of digestive enzymes depends on:

1. The temperature of the medium1. The temperature of the medium

2. The pH of the medium2. The pH of the medium

The diagram to the right shows the The diagram to the right shows the

optimal pH values for pepsin,optimal pH values for pepsin,

salivary amylase, and trypsinsalivary amylase, and trypsin

respectively.respectively.

* In case of an increase in the temperature above a certain limit, the enzyme * In case of an increase in the temperature above a certain limit, the enzyme

will be denatured and loses its function. The figure below explains this will be denatured and loses its function. The figure below explains this

concept.concept.

The Digestive System of Humans:The Digestive System of Humans:

The Mouth (Oral Cavity):The Mouth (Oral Cavity):

Oral CavityOral Cavity

The following processes The following processes

take place inside the mouth:take place inside the mouth:

Mastication (chewing) of Mastication (chewing) of foodfood

Mixing masticated food Mixing masticated food with salivawith saliva

Allowing for the sense of Allowing for the sense of tastetaste

Initiation of swallowing by Initiation of swallowing by the tonguethe tongue

2. Saliva:2. Saliva: secreted by the salivary glands, it is slightly alkaline and, includes secreted by the salivary glands, it is slightly alkaline and, includes salivary amylase as well as mucus. salivary amylase as well as mucus.

Functions of saliva:Functions of saliva:• Moistens the food for easy Moistens the food for easy

swallowingswallowing• Dissolves the food for discerning Dissolves the food for discerning

taste by the tonguetaste by the tongue• Contains salivary amylase which Contains salivary amylase which

digests starch into disaccharidesdigests starch into disaccharides• Act as a natural cleanser for the Act as a natural cleanser for the

teeth (isn’t a substitute for toothpasteteeth (isn’t a substitute for toothpaste

and a toothbrush though)and a toothbrush though)

Three pairs of the salivary glands Three pairs of the salivary glands

Parotid gl.Parotid gl.

Submaxillary gl.Submaxillary gl.

Sublingual gl.Sublingual gl.

3. The Tongue:3. The Tongue:

This diagram shows the This diagram shows the structure of a papilla as structure of a papilla as well as one of the taste well as one of the taste buds along the sides of buds along the sides of the papilla.the papilla.

The sensory nerve The sensory nerve endings end inendings end inbetween the sensorybetween the sensorytaste cells of the tastetaste cells of the tastebud.bud.

The dissolved food The dissolved food enters into the taste enters into the taste bud through the tastebud through the tastepore.pore.

The pharynx:The pharynx: The base of the tongue plus the muscular The base of the tongue plus the muscular pharynx result in the involuntary reflex of deglutition pharynx result in the involuntary reflex of deglutition (swallowing) of food towards the esophagus.(swallowing) of food towards the esophagus.

Process of deglutition using the base of the tongue and Pharynx (bolus in blue)Process of deglutition using the base of the tongue and Pharynx (bolus in blue)

The Esophagus:The Esophagus:

• Is a muscular tubeIs a muscular tube that conducts the that conducts the food to the stomach food to the stomach by by peristalsisperistalsis (slow (slow rythamic contractions) rythamic contractions)

• Its cells also secrete Its cells also secrete mucous that helps mucous that helps as a lubricant to as a lubricant to move the foodmove the food

The Stomach:The Stomach:

• Acts as a storage for foodActs as a storage for food

• Is made of three muscle Is made of three muscle layers, from the outside to layers, from the outside to the inside are the longitudinal the inside are the longitudinal muscle layer, the circular muscle layer, the circular muscle layer and the oblique muscle layer and the oblique muscle layermuscle layer

• The inner lining of the The inner lining of the stomach is called the mucosa stomach is called the mucosa it contains the glands of the it contains the glands of the stomach and is corrugated stomach and is corrugated forming the rugaeforming the rugae

• Has two sphincters, the Has two sphincters, the cardiac sphincter and the cardiac sphincter and the pyloric sphincterpyloric sphincter

• Secretes gastric juiceSecretes gastric juice

The Functions of the stomach:The Functions of the stomach:

1.1. Mechanical (physical) digestion: Mechanical (physical) digestion: through churning food into through churning food into ChymeChyme

2. 2. Controlling the passage of food:Controlling the passage of food: through the two sphincters, through the two sphincters, contraction of the cardiac sphincter closes the opening to contraction of the cardiac sphincter closes the opening to the stomach located near to the heart, while its relaxation the stomach located near to the heart, while its relaxation allows food to enter. A second sphincter, the pyloric allows food to enter. A second sphincter, the pyloric sphincter, regulates the movement of food and stomach sphincter, regulates the movement of food and stomach acid to the small intestine acid to the small intestine

3. 3. Production of gastric juice:Production of gastric juice: * * HClHCl → breaks down the food → breaks down the food → kills bacteria → kills bacteria

→ converts pepsinogen into pepsin → converts pepsinogen into pepsin

* * PepsinogenPepsinogen HClHCl Pepsin → incomplete digestion of proteinsPepsin → incomplete digestion of proteins

* * MucusMucus → protects the stomach inner walls from the acid → protects the stomach inner walls from the acid

The Small Intestine and Pancreas: The Small Intestine and Pancreas:

• Digestion and absorption take place in the small Digestion and absorption take place in the small intestineintestine

• The small intestine is a muscular tube extending The small intestine is a muscular tube extending form the pyloric sphincter to the ileocecal sphincter form the pyloric sphincter to the ileocecal sphincter about 6-7 metres longabout 6-7 metres long

• It is made of three segments namely, the It is made of three segments namely, the duodenum, jejunum, and the ileumduodenum, jejunum, and the ileum

• The duodenum is only 20-30 cm longThe duodenum is only 20-30 cm long

• The duodenum receives enzymes secreted from The duodenum receives enzymes secreted from the liver and pancreasthe liver and pancreas

Adjusting the small intestine’s pH (function of secretin):Adjusting the small intestine’s pH (function of secretin):

The Pancreas:The Pancreas:

Secretes the following Secretes the following

important enzymes:important enzymes:

1. Trypsinogen1. Trypsinogen

2. Erepsin2. Erepsin

3. Pancreatic amylase3. Pancreatic amylase

4. Pancreatic lipase4. Pancreatic lipase

Pancreas

LiverGall

bladder

Digestion of proteins:Digestion of proteins:

• Starts in the stomachStarts in the stomach

• Continues in the small intestineContinues in the small intestine

Proteins Pepsin

Digestion of Carbohydrates:Digestion of Carbohydrates:

• Starts in the mouthStarts in the mouth

• Continues in the small intestineContinues in the small intestine

CarbohydratesSalivary Amylase Disaccharides

DisaccharidesDisaccharidases

CarbohydratesPancreatic

Amylase Disaccharides

Monosaccharides

Digestion of Lipids:Digestion of Lipids:

• In the small intestineIn the small intestine

FatsBile Salts

Small fat droplets

Small fat dropletsPancreatic lipase

glycerol Fatty acids++

The liver and the gall bladder:The liver and the gall bladder:

• The liver is the largest gland of the bodyThe liver is the largest gland of the body• The gall bladder is a reservoir for storing Bile secreted by the liverThe gall bladder is a reservoir for storing Bile secreted by the liver• Bile comprises, bile salts, phospholipids, cholesterol and bilirubin (a Bile comprises, bile salts, phospholipids, cholesterol and bilirubin (a

pigment that results from the breakdown of RBCs)pigment that results from the breakdown of RBCs)• The liver has many functions, one of these functions is the emulsification of The liver has many functions, one of these functions is the emulsification of

fat (breaking large fat globules into smaller fat droplets)fat (breaking large fat globules into smaller fat droplets)

The hormone cholecystokinin and its role in releasing bile salts:The hormone cholecystokinin and its role in releasing bile salts:

Functions of the liverFunctions of the liver

Liver and Gall Bladder Problems:Liver and Gall Bladder Problems:

1. Gall bladder stones:1. Gall bladder stones: Cholesterol, an insoluble component Cholesterol, an insoluble component of bile, acts as a binding agent for the salt crystals found in of bile, acts as a binding agent for the salt crystals found in bile. The crystals precipitate and form larger crystals called bile. The crystals precipitate and form larger crystals called gallstonesgallstones. Gallstones can block the bile duct, impairing fat . Gallstones can block the bile duct, impairing fat digestion and causing considerable pain.digestion and causing considerable pain.

2.2. Jaundice (Icterus):Jaundice (Icterus): Bile contains a pigment called bilirubin Bile contains a pigment called bilirubin which results from the breakdown of haemoglobin, any which results from the breakdown of haemoglobin, any obstruction of the bile duct or accelerated destruction of red obstruction of the bile duct or accelerated destruction of red blood cells can cause blood cells can cause jaundicejaundice, turning skin and the eye , turning skin and the eye white yellow in color.white yellow in color.

3. Liver Cirrhosis:3. Liver Cirrhosis: Alcohol, like many other harmful agents, Alcohol, like many other harmful agents, can destroy liver tissue. Damaged liver cells are replaced by can destroy liver tissue. Damaged liver cells are replaced by connective tissue and fat, which are not able to carry out connective tissue and fat, which are not able to carry out normal liver duties. This condition, which can also result normal liver duties. This condition, which can also result from nutritional deprivation or infection, is referred to as from nutritional deprivation or infection, is referred to as cirrhosis cirrhosis of the liver.of the liver.

Digestion in the small intestine:Digestion in the small intestine:

Mechanical digestion in the small intestine:Mechanical digestion in the small intestine: includes what is called includes what is called segmentationsegmentation which is the movement of the food back and forth (different which is the movement of the food back and forth (different

than peristalsis which is mainly for the propulsion of the food forwards)than peristalsis which is mainly for the propulsion of the food forwards)

Chemical digestion in the small intestine:Chemical digestion in the small intestine: through the through the secretions received from the pancreas and the liver as well as secretions received from the pancreas and the liver as well as other secretions by the glands of the small intestine’s mucosa, other secretions by the glands of the small intestine’s mucosa, such as such as erepsin.erepsin.

Homeostatic control of digestion:Homeostatic control of digestion:

11. . Gastrin (stomach)Gastrin (stomach):: is produced when partially digested is produced when partially digested proteins are present in the stomach, gastrin stimulates the proteins are present in the stomach, gastrin stimulates the release of gastric juices to digest proteins.release of gastric juices to digest proteins.

2. Secretin (small intestine): 2. Secretin (small intestine): stimulates the pancreas to stimulates the pancreas to produce HCOproduce HCO33

-- and adjust theand adjust the pH from 2 to 9pH from 2 to 9

3. Cholecystokinin (small intestine): 3. Cholecystokinin (small intestine): stimulates the release stimulates the release of bile salts from the gall bladderof bile salts from the gall bladder

4. Enterogasterone (small intestine): 4. Enterogasterone (small intestine): If the meal is fatty, the If the meal is fatty, the small intestine secretes a digestive hormone small intestine secretes a digestive hormone (enterogasterone) that slows peristaltic movements, allowing (enterogasterone) that slows peristaltic movements, allowing time for fat digestion and absorption.time for fat digestion and absorption.

If the meal is If the meal is fatty, the small fatty, the small intestine secretes intestine secretes a digestive a digestive hormone hormone (Enterogasterone(Enterogasterone) that slows ) that slows peristaltic peristaltic movements, movements, allowing time for allowing time for fat digestion and fat digestion and absorption.absorption.

Absorption of Materials:Absorption of Materials:

The small intestine:The small intestine:

• The absorption of food takes place in the small intestineThe absorption of food takes place in the small intestine

• Long fingerlike tubes called Long fingerlike tubes called villivilli (singular: villus) greatly (singular: villus) greatly increase the surface area of the small intestine. The cells increase the surface area of the small intestine. The cells that make up the lining of each villus have that make up the lining of each villus have microvillimicrovilli, which , which are fine, threadlike extensions of the membrane that further are fine, threadlike extensions of the membrane that further increase the surface for absorption.increase the surface for absorption.

• VilliVilli: : small fingerlike projections that extend into the small small fingerlike projections that extend into the small intestine which increase surface area for absorptionintestine which increase surface area for absorption

• MicrovilliMicrovilli: : microscopic fingerlike outward projections of the microscopic fingerlike outward projections of the cell membranecell membrane

• Each villus is supplied with a capillary network which Each villus is supplied with a capillary network which intertwines with lymph vessels called intertwines with lymph vessels called lactealslacteals that transport that transport materials. Some nutrients are absorbed by diffusion, but materials. Some nutrients are absorbed by diffusion, but some nutrients are actively transported from the digestive some nutrients are actively transported from the digestive tract. Carbohydrates and amino acids are absorbed into the tract. Carbohydrates and amino acids are absorbed into the capillary networks; fats are absorbed into the lacteals.capillary networks; fats are absorbed into the lacteals.

The role of the large intestine in absorption and egestion:The role of the large intestine in absorption and egestion:

• The large intestine consists of the ascending colon, the transverse colon, The large intestine consists of the ascending colon, the transverse colon, the descending colon, the sigmoid colon and the rectumthe descending colon, the sigmoid colon and the rectum

• Chemical digestion is complete by the time food reaches the large Chemical digestion is complete by the time food reaches the large intestine.intestine.

• The colon stores wastes long enough to reabsorb waterThe colon stores wastes long enough to reabsorb water

• The large intestine houses bacteria (E.Coli) that use waste materials to The large intestine houses bacteria (E.Coli) that use waste materials to synthesize vitamins (B and K).synthesize vitamins (B and K).

• Most absorption takes place in the small intestine, however reabsorption Most absorption takes place in the small intestine, however reabsorption of water plus the absorption of minerals and vitamins takes place in the of water plus the absorption of minerals and vitamins takes place in the large intestinelarge intestine

• Defecation reflex takes place when wastes build up in the largeDefecation reflex takes place when wastes build up in the large

intestine, receptors in the wall of the intestine provide information to the intestine, receptors in the wall of the intestine provide information to the

central nervous system which, in turn, prompts a bowel movement. The central nervous system which, in turn, prompts a bowel movement. The

bowel movement ensures the removal of potentially toxic wastes from the bowel movement ensures the removal of potentially toxic wastes from the

body. Individuals who do not eat sufficient amounts of cellulose (roughage body. Individuals who do not eat sufficient amounts of cellulose (roughage or fibre), have fewer bowel movements.or fibre), have fewer bowel movements.

Essential NutrientsEssential Nutrients1. The essential amino acids are the eight amino acids that 1. The essential amino acids are the eight amino acids that

cannot be synthesized by your body and must be ingested.cannot be synthesized by your body and must be ingested.• • A common source of essential amino acids is animal products.A common source of essential amino acids is animal products.• • The essential amino acids can be obtained in vegetarian diets The essential amino acids can be obtained in vegetarian diets

by eating complementary foods.by eating complementary foods.2. Vitamins are either water soluble or fat soluble.2. Vitamins are either water soluble or fat soluble.• • Water-soluble vitamins dissolve in the blood and are excreted Water-soluble vitamins dissolve in the blood and are excreted

in urine. Thus they cannot be stored and must be consumed in urine. Thus they cannot be stored and must be consumed daily.daily.

• • Fat-soluble vitamins can be stored in fatty tissue. Excessive Fat-soluble vitamins can be stored in fatty tissue. Excessive amounts of fat-soluble vitamins can build up in the body and amounts of fat-soluble vitamins can build up in the body and cause problems.cause problems.

3. Antioxidants can make free radicals less toxic.3. Antioxidants can make free radicals less toxic.• • Free radicals are created in the body in response to things like Free radicals are created in the body in response to things like

sunlight, X-rays, pollution, etc.sunlight, X-rays, pollution, etc.• • Free radicals may contribute to cancer and other disorders.Free radicals may contribute to cancer and other disorders.