DENTITION, FEEDING AND DIGESTION IN MAMMALS - THE DIGESTIVE SYSTEM. PART 3

Digestive system is a network of organs and enzymes which converts the complex food (i.e. carbohydrates, proteins, lipids etc) eaten into small and absorbable forms for the use of the body.

Or

Digestion is the breakdown of complex food substances into simple, soluble and absorbable forms.

Structure and functions of the

digestive system of mammals

The digestive system of mammals is made up of the gut or alimentary canal and other organs and glands. 

The alimentary canal is

a muscular tube made up of the following parts:

Mouth

Digestion starts from the mouth, where food is chewed with the teeth. Chewing (or mastication) breaks down solid food into smaller particles. 

Salivary glands produce

saliva which:

 contains the an enzyme called saliva amylase that converts starch into maltose.

 is slightly alkaline and keeps the pH of the mouth approximately neutral which help in the action of saliva amylase.

 mixes with the food during chewing, making it soft and easier to swallow.

Oesophagus

A muscular tube that connects the mouth to the stomach. Food passes down the oesophagus through contraction and relaxation of the circular muscles of the gut. This action is called peristalsis.

The stomach

The stomach is a sac-like organ that

produces gastric juice which contains:

 hydrochloric acid (sterilizes food, ends action of saliva amylase, provide acidic pH for action of the enzyme

 pepsin which converts proteins to

polypeptides,

 mucus which forms a protective inner lining to the animal wall and

 rennin which solidifies liquid proteins so that they will remain in the stomach for pepsin to work on.

The contractions of the stomach wall

muscles churn the food into chyme (a creamy-white mixture of food substance).

At this stage glucose and alcohol are

absorbed from the food into the blood stream. 

Duodenum

The chyme from the stomach passes the pyrolic sphincter into the duodenum where the hormone secretin is released into the bloodstream. 

Secretin then stimulates the gall bladder to release bile and the pancreas to release pancreatic juice into the duodenum.

Liver

The liver is one important organ which is responsible for most chemical activities in the body.

Functions of the lever

1. Production of bile which emulsifies or breaks down fats, converts glucose to glycogen, produces urea (the main substance of urine), makes certain amino acids (the building blocks of proteins).

2. Filtration of harmful substances from the blood (such as alcohol).

3. Storage of vitamins and minerals

(vitamins A, D, K and B12)

4. Maintaining the proper level or glucose in the blood.

5. Production of cholesterol; it produces about 80% of the cholesterol in the body

6. Production of heat to the body through chemical activities.

7. Production of some proteins such as fibrinogen in the blood plasma.

8. Destruction and stopping of hormones from unlimited action.

9. Conversion of excess fats into glycerol and fatty acids.

10. Excretion of excess cholesterol and old red blood cells from the body.

Bile

Bile is alkaline and raises the pH of the chyme. It emulsifies fat by changing large fat globules into small fat droplets.

Emulsification increases the surface area of the fat making the digestion of fat by enzymes more efficient.

Pancreas

The pancreas secrets pancreatic juice which is alkaline and therefore raises the pH of the chyme. 

The pancreatic juice contains

 amylase which converts starch to

maltose,

 trypsin which converts protein to

peptides and polypeptides and

 lipase which converts fat to fatty acids and glycerol.

All these enzymes digest food as is passes though the duodenum into the ileum. Ileum (small intestines)

The digestion process ends in the ileum.

The walls of ileum secrets:

 lipase which converts all remaining fats into fatty acids and glycerol;

 maltase which converts maltose to

glucose;

lactase which converts lactose to

glucose and galactose,

 sucrase which converts sucrose to

glucose and fructose,

 erepsin which converts peptides to

amino acids and

 enterokinase

which converts

trypsinogen to trypsin.

The small molecules of glucose, amino

acids, fatty acids, and glycerol are the end

products of digestion in mammals and are

absorbed into the bloodstream by a process

known as diffusion.

The ileum contains thousands of

microscopic projections called

villi

(singular, villus), which increases the

surface area of the ileum and facilitates the

absorption process. Undigested food passes

into the colon.

Colon (large intestines)

The colon is responsible for the absorption

of water from the undigested food. It

contains some bacteria which digest the

cellulose and produce vitamin B and K

which are absorbed into the body. The

remains of the undigested food pass on to

become faeces.

Organ/ gland

Secreted enzyme

Enzyme action

Mouth (salivary glands)

 Salivary amylase

 Starch to maltose

Oesophagus

 Passage of food to

stomach

Stomach

 Pepsin

 Rennin

 Hydrochloric acid

 Polypeptides

 Solidifies protein

 sterilizes food; provide acidic pH

for action of the enzyme

Duodenum

 Secretin

 Stimulates gall bladder to release

bile and pancreas to release

pancreatic juice

Liver

 Bile

 Emulsifies starch

Pancreas (pancreatic juice)

 Amylase

 Trypsin

 Lipase

 Starch to maltose

 Protein to peptides and

polypeptides

 Fatty acids to glycerol

Ileum

 Lipase

 Maltase

 Lactase

 Sucrose

 Erepsin

 Fats into fatty acids and glycerol

 Maltose to glucose

 Lactose to glucose

 Sucrose to glucose

 Peptides to amino acids

 Enterokinase

 Trypsinogen to trypsin

Colon

Absorption of water from

undigested food

Rectum

Temporary storage for faeces

Rectum

The faeces move into the rectum, where

they are stored and released periodically

through the anus. The removal of food

from the rectum is known as egestion. Not

to be confused with ingestion, which is the

act of putting food into the mouth?