Chapter 2: DIGESTIVE SYSTEM

Digestive system is made of alimentary canal (gut), associated organs and

glands. It has two main functions: digestion and absorption

.

The alimentary canal is a long hollow tube that runs from the mouth to the anus. It is

also known as the gut or digestive tract. The main parts of alimentary canal are mouth,

pharynx, oesophagus, stomach, small intestine (duodenum and ileum) and large intestine

(caecum, colon, rectum and anal canal). The associated organs include the liver and

pancreas.

Figure 2.1: Digestive System of a Human Being

(i)

Mouth: The region where food is chewed by teeth into small particles and mixed with

saliva.

(ii) The Pharynx: The region which directs food and liquids from the mouth to the

oesophagus. In this region, there is a structure known as epiglottis which prevents

food from entering the wind pipe or trachea.

(iii) Oesophagus (Gullet): This the tube that passes food from the mouth to the stomach.

(iv) Stomach: This a sac like that receives food from oesophagus and empties it into the

duodenum. Only proteins are digested here.

(v) Small Intestine: This part is divided into the duodenum and the ileum. These are

sites for digestion and absorption of nutrients. The small intestine is long tube about

9m with a diameter of 2.5cm.

(vi) Large Intestine: This includes the following parts; Appendix, Colon, Caecum and

Rectum. Large intestine lies between the small intestine and anus. It is about 6cm

wide and 1.5m long. Its main function is reabsorption of water.

(vii) The Anus: This is the exit point of the alimentary canal. It is where waste products

(faeces) are passed out of the body.

THE DIGESTION PROCESS

Digestion is a process by which complex food substances are broken down into the smallest

particles that can be easily absorbed and used in the body. Or

Digestion is the process by which food is broken down mechanically and chemically to

smaller particles which are absorbed and assimilated in the body.

Types of digestion

There are two types of digestion, namely:

(a) Mechanical Digestion: It is the breakdown of large food particles into smaller

particles without chemical change to the food molecules.

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It takes place mostly in the mouth. It is performed by the teeth by the process

called chewing. The process continues in the stomach where the small particles

swallowed from the mouth are further broken down into smaller particles.

Role of mechanical digestion

(i)

To reduce large food particles into small particles that can be swallowed easily.

(ii) To breakdown large food particles into small pieces to increases the surface area

of the food for the action of digestive juice called enzymes.

(b) Chemical Digestion: It is the breakdown of large insoluble molecules into small

soluble molecules. It is achieved by digestive enzymes. Enzymes are produced by

the glands found in the mouth, stomach, small intestine and pancreas.

GENERAL STAGES IN DIGESTION

The process of digestion involves the following stages in human being;

(i)

Ingestion: This is the process of taking food into the mouth.

(ii) Mechanical or Physical Digestion: This is the process by which large food

particles are mixed with saliva, chewed and broken down and rolled into food bolus.

(iii) Chemical Digestion: This involves the use of chemicals, that is, enzymes produced

in the digestion system to break down the food (carbohydrates, proteins, fats) into

smaller molecules ready to be absorbed in the body.

(iv) Absorption: This is the process by which soluble end products of digestion diffuse

into the blood stream.

(v) Assimilation: This is the process by which the body uses up the absorbed products

of digestion.

(vi) Egestion (Defecation): This is the process of eliminating or removing of undigested

food materials from the body as faeces through the anus.

1. DIGESTION IN THE MOUTH

Digestion begins in the mouth whereby food particles are mechanically broken-down into

small particles. The process of breaking down large food particles into small particles

under mechanical digestion is called mastication or chewing. The process of chewing

(mastication) is done by the teeth.

▪

Chewing breaks down large food particles into smaller particles and mixes the food

particles with saliva making the food slippery for easy swallowing.

Saliva contains digestive enzyme called salivary amylase or ptyalin which

catalyzes the breakdown of cooked starch into maltose.

▪

Salivary amylase

Starch + H

Maltose

₂O

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The enzyme (salivary amylase) works best in an alkaline/basic medium which is

provided by the sodium bicarbonates (푁푎퐻퐶푂₃) present in the saliva.

Components of Saliva

(i)

Salivary amylase (ptyalin): An enzyme that convert starch into maltose.

(ii) Water: It moistens the food and provide a medium for the action of enzymes.

(iii) Sodium and calcium salts. They ensure an alkaline pH suitable for the

functioning of amylase.

(iv) Mucus. It lubricates the food and make it easier to chew and swallow.

Functions of Saliva in the Mouth

(i) It contains salivary amylase which catalyzes the breakdown of starch into maltose.

(ii) It has mucus which lubricates food so that it can move through the esophagus

easily.

(iii) It maintains the pH of the mouth at about 6.5 –7.5.

(iv) It contains mineral salts, which speed up the activity of the enzymes.

(v) It contains lysozyme; an enzyme that harmful bacteria present in the food eaten.

(vi) It moistens the mouth, tongue and lips for easy chewing and mixing food in the

mouth.

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After the food has been chewed, it is rolled by the tongue into small balls called boli

(Singular; bolus). The food in form of bolus is then swallowed through pharynx and

oesophagus.

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During swallowing, the epiglottis closes the trachea to prevent food from entering

the air passage.

Figure 2.2: Structure of Oral cavity

THE OESOPHAGUS

The oesophagus is a narrow muscular tube connecting the pharynx to the stomach. The

food in form of bolus (boli) passes through the oesophagus by a series of wave-like

muscular contractions and relaxations of the oesophagus walls. The series of muscular

contractions and relaxations is called peristalsis.

Therefore; Peristalsis is an involuntary contraction of circular muscles and relaxation of

longitudinal muscles of the oesophagus walls to move food bolus forward. Peristalsis is

also defined as a wave-like muscular contractions that push food along the oesophagus.

Peristalsis facilitates the movement of food from the mouth to the stomach.

Figure 2.3: Peristaltic Movement of Food

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Between the oesophagus and the stomach, there is cardiac sphincter muscle. When

the food reaches at this point, the muscle automatically relaxes and allows the food

to enter the stomach.

When the food is in the stomach, the cardiac sphincter muscle contracts and closes

the entrance into the stomach to prevent the food from flowing back into the

oesophagus.

2. DIGESTION IN THE STOMACH

The walls of the stomach have gastric glands which secrete gastric juice, a chemical

substance that contains hydrochloric acid and enzymes. The stomach is involved in both

mechanical and chemical digestion of the partially digested food from the mouth.

▪

Mechanical digestion occurs through a process called churning where the muscles

of the stomach wall contract and relax breaking down food into smaller particles

and mix them with gastric juice to form a substance called chyme (or acidic chyme).

Chyme is creamy semi-liquid of partially digested food substances in the stomach

produced when food mixes with gastric juice WHILE Churning is the mixing of food

substance with the gastric juice in the stomach as the result of contraction and

relaxation of the stomach muscles.

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Chemical digestion in stomach begins with the release of gastric juice which

contains the following substances;

(a) Hydrochloric acid. The acid in the stomach has the following functions:

(i) It provides acidic medium in the enzymes for enzymes (pepsin and rennin) to

work best.

(ii) It kills diseases causing germs such as bacteria which might have entered the

stomach together with food.

(iii) It converts pepsinogen to pepsin as well as prorenin to rennin

.

(iv) It begins the hydrolysis of complex sugars such as sucrose or maltose into simple

sugars such as glucose.

(b) Pepsin. This an active form of pepsinogen. This enzyme is responsible for breaking

down proteins into smaller molecules of peptides and some amino acids.

Pepsin

Protein + Water

Peptides

(c) Rennin. The enzyme coagulates/solidifies milk proteins. This allows milk proteins

to stay in the stomach long enough to be broken down to peptides. Rennin is an

enzyme but not a digestive enzyme. It is abundantly secreted in suckling mammals

(infants) but absent in adults.

Rennin

Soluble milk

Insoluble milk (semi-solid milk)

(d) Mucus. This protects the stomach walls against damage/corrosion by the digestive

enzymes and hydrochloric acid.

(e) Water. It provides the medium for enzyme activities during digestion.

From the stomach, the pylori sphincter muscles at the lower end of the stomach

relaxes to allow small quantities of acidic chyme from the stomach to the duodenum

which is the first part of the small intestine.

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NOTE: Pyloric sphincter is a muscular ring-like valve which regulates the passage of food

from the stomach into the duodenum (the first part of the small intestine).

GENERAL FUNCTIONS OF THE STOMACH

It stores the food temporarily for about 4–5 hours.

(i)

(ii)

Mechanical digestion of food by its churning action, hence mixing food with gastric

juice of the stomach to form chyme.

(iii) It also contains cells which secrete hydrochloric acid (HCl) which provides acidic

medium, kills germs and converts pepsinogen and prorenin into pepsin and

rennin respectively.

(iv) The wall of the stomach contains cells which secrete mucus. Mucus protects the

stomach walls against damage by the digestive enzymes and hydrochloric acid.

(v)

It is a first site of protein digestion in the digestive system.

(vi) The stomach wall absorbs vitamins, water and drugs such as alcohol.

3. DIGESTION IN THE DUODENUM

When chyme enters the duodenum, it stimulates the pancreases to produce pancreatic

juice which is transported into the duodenum through the pancreatic duct. The pancreatic

juice contains the following substances:

(a) Sodium bicarbonates. This helps to neutralize the acidic chyme from the stomach

and provide alkaline medium in the duodenum for enzymes to work best.

(b) Digestive enzymes for breaking down protein, carbohydrates and fats. Such

enzymes include:

(i) Pancreatic lipase. It catalyses the breakdown of fats or oils into fatty acids and

glycerol.

(ii) Pancreatic amylase. It catalyses the breakdown of starch which was not

digested in the mouth into maltose.

(iii) Trypsin. It catalyses the conversion of proteins which were not digested in the

stomach into peptides and some amino acids. Trypsin is produced in an inactive

form known as trypsinogen. Trypsinogen is activated by enterokinase enzyme

into active from, trypsin.

Bile is also made in the liver and stored in the gall bladder. It is a greenish-yellow juice

which contains water, salts, mucins, greenish-yellow pigments and other substances. The

greenish-yellow colour of bile is due to the breakdown of worn out red blood cells by the

liver. The gall bladder releases bile through the bile duct that joins the pancreatic duct

before opening to the duodenum.

The Role of Bile

(i)

It helps to convert fat-soluble vitamins (A, D, E and K) to make them water soluble

hence easily absorbed by the body cells.

(ii) The bile salts facilitate the conversion of fats or oils into tiny droplets through a

process called emulsification. This increases the surface area for fat or oil

digestive enzymes to act upon.

(iii) Bile contains sodium bicarbonate that neutralizes the hydrochloric acid present in

the chyme and hence providing an alkaline medium for digestive enzymes in the

duodenum to work best.

4. DIGESTION IN THE ILEUM

The ileum is the final part of small intestine. The process of digestion ends in the ileum.

It is about 6 meters long in human bring.

When the chyme enters the ileum, it stimulates the intestinal walls to secrete an intestinal

juice called succus entericus. Intestinal juice or succus entericus contains, mucus,

sodium bicarbonate and digestive enzymes such as;

(i)

Lipase: It catalyzes the conversion of the remaining fat and oil droplets to fatty

acid and Glycerol.

Lipase

Lipid (Fat or Oil) + Water

Fatty acid + Glycerol

(ii)

Maltase: It catalyzes the conversion of maltose into glucose.

Maltase

Maltose + Water

Glucose

(iii) Sucrase: It catalyzes the conversion of sucrose into Glucose and Fructose.

Sucrase

Sucrose + Water

Glucose + Fructose

(iv)

(v)

Lactase: It catalyzes the conversion of lactose into glucose and galactose.

Lactase

Lactose + Water

Glucose + Galactose

Erepsin/Peptidase: It catalyzes the conversion of the remaining peptides into

amino acids.

Peptidase

Peptides + Water

Amino acids

Table 2.1: Summary of the enzymes involved in digestion and the products of digestion

in each part of the human digestive system.

Part of digestive

system

Digestive

juice

Enzyme

secreted

Salivary

amylase

Substance

digested

Products of

digestion

Cooked starch

(Carbohydrates)

Mouth

Salivary juice

Maltose

Peptides and

amino acids

Pepsin

Rennin

Trypsin

Proteins

Stomach

Gastric juice

Soluble milk protein

(caseinogen)

Insoluble milk

protein (casein)

Peptides and

amino acids

Proteins

Pancreatic

amylase

Pancreatic

lipase

Starch

Pancreatic

juice

Duodenum

Maltose

(Carbohydrates)

Fatty acids and

glycerol

Lipids (fats or oils)

Maltose

Maltase

Sucrase

Glucose

(Carbohydrates)

Glucose and

fructose

Sucrose

Intestinal

juice

Lactose

(Carbohydrates)

Peptides

Glucose and

galactose

Ileum

Lactase

Peptidase

Lipase

Amino acids

Fatty acids and

glycerol

Lipids (fats or oils)

EXERCISE

1. Match the following item in column A against its corresponding item in column B.

Column A

Pancreas

Chemical break down

Column B

(i)

A. The chewed food is rolled into a ball-like

structure.

(ii)

(iii) Mechanical breakdown

(iv) Lactase

B. Movement of food along the oesophagus involving

series of muscle contractions and relaxations.

C. Part of the digestive system where the process of

digestion ends.

(v)

Sucrase

(vi) Oesophagus

(vii) Bolus

D. The muscle that controls the opening into the

stomach.

(viii) Peristalsis

(ix) Cardiac sphincter

E. Part of alimentary canal used for swallowing food.

(x)

Ileum

F. An enzyme that converts lactose to glucose and

galactose.

G. And organ that secretes pancreatic juice in the

duodenum.

H. The breakdown of food into tiny particles by using

enzymes.

I. An enzyme that converts sucrose to glucose and

fructose.

J. The breakdown of food into small particles during

chewing by teeth.

2. (a) Which specific stomach enzymes are most effective at digesting the proteins

found in fish and milk?

(b) How do these enzymes work to breakdown these foods?

3. Summarize the main processes that take place in the human digestive system.

4. Study the following diagram carefully the answer the questions that follows.

(i) Name the parts labeled A, B, C and D.

(ii) Which food substances are digested in part A and D?

(iii) Name the secretions caring from part B and C

ABSORPTION IN THE SMALL INTESTINE

The absorption of digested food takes place in the small intestine.

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The end product of digestion such as glucose, galactose, fructose, vitamins (A, B, C,

D, E and K), amino acids and mineral salts are absorbed into the blood stream

through blood capillaries of the villi. These products are then transported to the liver

through the hepatic portal vein.

▪

Fatty acids and glycerol enter into the blood stream through lacteals or lymphatic

vessels. Lacteals help to increase the absorption of foods into the blood stream.

After absorption of the end products of digestion into the blood stream, the body uses

them for various functions such as growth, body repair, respiration, excretion and

reproduction. this process is called assimilation.

Adaptations of the Ileum for Digestion and Absorption of Food

(i) It has secretory glands, which produce digestive enzymes for digestion of food

substances.

(ii) The ileum has secretory glands which secrete mucus that ensures a constant

lubrication of the intestinal wall for effective digestion of food substances.

(iii) It has a network of blood capillaries through which digested food passes to be taken

to the liver and then around the body. The liver removes harmful and toxic

substances (detoxification) and stores fat soluble vitamins as well as excess glucose

in form of glycogen.

(iv) It is very long, about six meters. Foods tale long time to pass through the ileum.

This ensures maximum absorption of the end products of digestion.

(v) The lining of the ileum is thin, that is, it is made up of a single layer of cells for easy

absorption and diffusion of nutrients into the body.

(vi) Its internal surface has fingerlike projections called villi (singular villus) which

increase the surface area for absorption of digested food into the blood stream.

(vii) It is highly coiled to increase surface area for digestion and absorption.

(viii) Cells of ileum have large number of mitochondria to release energy for active uptake

of digested food.

(ix) It has a lymphatic vessel called lacteal for absorption and transportation of fatty

acids and glycerol.

(x)

The internal lining of the ileum is folded into ridges so that movement of food is

slowed down to ensure maximum absorption.

VILLUS

Villus is a finger like projections found in the walls of the ileum, important for absorption

process. The main function of villus is to increase the surface are for absorption of digested

food.

Figure 2.4: Structure of Villi

ABSORPTION IN THE LARGE INTESTINE

Large intestine is the tube of alimentary canal which lies between the small intestine and

anus. It is about 6cm wide and 1.5m long. It consists of colon, rectum, caecum and

appendix.

(a) Colon. Most of the water carried in the digestive system is absorbed in

the colon. Absorption of water in the large intestine is necessary to

prevent water loss/dehydration of the body. The colon also, has some

bacteria that produce vitamin B₁, B₂, B₁₂and K, although these vitamins

are also obtained from the foods we eat. The colon contains no digestive

enzyme, thus there is no digestion. The undigested and indigestible food

materials are passed into the colon from the ileum by peristalsis.

(b) Rectum. The undigested and indigestible food materials fo rm the colon

are transported to rectum and finally eliminated from the body as waste

materials in the form of faeces. Therefore, rectum is the temporary storage

of faeces until defecation or egestion happens.

(c) Caecum and appendix. Both caecum and appendix in human have no

useful functions. It collects hard food particles inside it and once becomes

infected by bacteria, causes a disease known as appendicitis. Under this

situation, the appendix must be removed. But, in ruminants like cows,

the appendix contains bacteria and secretes cellulase enzyme which

catalyzes the digestion of cellulose.

DIGESTIVE SYSTEM OF RUMINANTS

The term ruminant comes from the Latin word “ruminare” which means “to chew

over again.” Therefore, ruminant is an animal that chews food, swallows it and

later returns it to the mouth for further chewing. The returning of food to the

mouth is known as regurgitation. The food returned to the mouth is called cud.

Also, ruminant can be defined as an animal which has a rumen. A rumen is an

extended portion of the oesophagus where food is temporarily stored. Examples

of the ruminants; goat, cow, sheep, antelopes and giraffes.

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The digestive system of a ruminant is different from that of a human being.

Ruminants have a more elaborate system to enable cellulose digestion

The digestive system of a ruminant comprises the following parts; Mouth,

pharynx, oesophagus, rumen, reticulum, omasum, abomasum (true

.

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stomach), duodenum, ileum, caecum, appendix, colon, rectum and anus.

The associated structures are salivary glands, liver and pancreas and gall

bladder.

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The stomach of a ruminant has four compartments/chambers which are;

(i)

Rumen. It is the largest of the four stomachs. It has fingerlike projections

on its inner surface which looks like a towel.

(ii)

Reticulum. It is smaller than the rumen. On one side it is connected to

the rumen and on the other side it is connected to the omasum. Externally,

the reticulum looks round and the inner surface looks like a honey comb.

(iii) Omasum. It is the smallest of all the chambers. It has many folds on its

inner surface which looks like the pages of a book.

(iv) Abomasum. It is a true stomach. It is only chamber which secretes gastric

juice and the inner lining has ridges.

The rumen, reticulum and omasum are known as force stomach because they

just the extension of the oesophagus and they do not secrete gastric juice.