Digestive System

• The system by which ingested food is acted upon by physical and chemical means to provide the body with absorbable nutrients and to excrete waste products.

• The enzymes used in digestion system, their source of secretion and their work.

Digestive Process

Human beings depend on other organisms for food, therefore are known as 

heterotrophic organisms —they need nutrients for various day to day activities.

The complete process of nutrition is divided into five stages:

1. Ingestion

2. Digestion

3. Absorption

4. Assimilation

5. Defecation

Ingestion

The process of taking food into the mouth is called ingestion.

Digestion

•  There is some food that is not directly absorbable, so the process of conversion of non-absorbable food into the absorbable form is known as digestion.

•  Digestion of food is started from the mouth.
• In the mouth there are Salivary glands that secrete Saliva.

Digestion in Stomach

•  As the food reaches pyloric cells in the stomach, the gastric glands secrete the gastric juice. This is a light yellow acidic acid.
•  Hydrochloric acid secreted from the Oxyntic cells of the stomach kills all the bacteria coming with food and accelerates the reaction of enzymes.

Digestion in Duodenum

• As the food reaches the duodenum bile juice from the liver combines with it.
• The main function of the bile juice is to convert the acidic food into alkaline, due to its alkaline nature.
•  Pancreatic juice from the pancreas combines with food and it contains the following enzymes.

Small Intestine

•  Here the process of digestion completed and absorption of digested foods start.
• In the small intestine, intestinal juices secrete and it is alkaline in nature and around 2 liters of intestinal juice secretes per day. 

Absorption

• The process of reaching the digested food into the blood is called absorption.
• The absorption of digested foods takes place through small intestine villi.

Assimilation

The use of absorbed food in the body is called assimilation.

Defecation

Undigested food reaches into the large intestine where bacteria convert it into feces 

which are excreted through the anus.

Digestive Organs

The main organs of the digestive system are: 

1) Mouth 

2) Oesophagus or Food Pipe 

3) Stomach 

4) Small Intestine 

5) Large Intestine 

6) Liver 

7) Gall bladder 

Mouth

• Food is ingested through the mouth.
• The buccal cavity or mouth consists of the tongue, teeth, and salivary glands.
• For Example:- Digestion of carbohydrate or starch begins in the mouth itself.
• But the digestion of food remains incomplete in the mouth

.                                                           mouth

Points to Remember

•  The dental formula of milk teeth is 2120/2120=10. 
•  It means 4 incisors, 2 canines, 4 premolars and 0 molars on each side of each jaw. 
•  The dental formula of milk teeth is 2123/2123=16. 
•  It means 4 incisors, 2 canines, 4 premolars and 6 molars on each side of each jaw. 

Oesophagus

• The slightly digested food goes to the stomach through oesophagus.
• With the help of peristaltic movement food is pushed inside the stomach

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Stomach

•  In the stomach, food pieces further break into smaller pieces and form a semi-solid paste.
•  Glands that are present in the walls of stomach secrete gastric juice and contain three substances hydrochloric acid, enzyme pepsin and mucus.
•  The protein digestion begins in the stomach only with the help of pepsin enzyme.

Chyme

•  The duodenum is a short section of the small intestine lies between the stomach and the rest of the small intestine.
•  The duodenum also produces the hormone secretin to stimulate the pancreatic secretion of large amounts of sodium bicarbonate, which then raises pH of the chyme to 7.
•  The chyme then moves through the jejunum and the ileum, where digestion progresses, and the non-useful portion continues onward into the large intestine.
• The duodenum is protected by a thick layer of mucus and the neutralizing actions of the sodium bicarbonate and bile.

Small Intestine

•  The small intestine is a long, thin tube, about 1 inch in diameter and about 20 feet long.
•  With the help of ‘sphincter muscle’ food from the stomach releases in small amounts into the small intestine.
•  Small intestine is the organ of human body where complete digestion of food like carbohydrates, proteins, and fats takes place.

Large Intestine

•  The large intestine is a long, thick tube about 2.5 inches in diameter and about 5 feet long. 

•  A part of the food which cannot be digested by our body or cannot be absorbed by small intestine passes into the large intestine. 

•  The wall of intestine absorbs most of the water from the food and makes it solid.
• Rectum is the last part of the large intestine where this solid food for some time is stored and egested from our body through the anus as faeces or stool. 

Points to Remember

There are three classes of carbohydrates: 

1. Monosaccharides - Glucose Fructose and Galactose 

2. Disaccharides- Sucrose and Lactose

3. Polysaccharides - cellulose and starch

•  Large Intestine diameter is larger than small intestine but length of small intestine is longer than large intestine. 

                                                            Pancreas, Liver and Gall Bladder

 Pancreas

•  It is known for blood sugar regulatory function with the production of insulin
•  Digestive enzymes are secreted by the pancreas, released into the small intestine and helps in the digestion of fats, proteins and carbohydrates.

Beta Cells

•  Beta Cells are a type of cell found in the pancreatic islets of the pancreas.
• The primary function of a beta cell is to store and release insulin.
•  Insulin is a hormone that brings about effects which decreases blood glucose concentration.
•  Beta cells can respond quickly to spikes in blood glucose concentrations by secreting some of their stored insulin while simultaneously producing more insulin.

Liver

•  Liver produces bile juice which secretes enzymes and help in the digestion of fat.
• A human liver normally weighs 1.44–1.66 kg (3.2–3.7 lb), and a width of about 15 cm.
•  Liver is both the heaviest internal organ and the largest gland in the human body.
•  Its other roles in metabolism include the regulation of glycogen storage, decomposition of red blood cells and the production of hormones.

Gall Bladder

 When fatty food enters the small intestine, the gallbladder contracts and releases 

bile which is stored in it.

Bile Juice

•  There are no enzymes present in bile juice.
•  Bile tends to be alkali on average.
•  The pH of the common duct bile (7.50 to 8.05) is higher than that of the correspond-ing gallbladder bile (6.80 to 7.65).
•  The composition of gallbladder bile is 97% water,0.2% bilirubin, 0.51% fats (cholesterol, fatty acids, and lecithin), 0.7% bile salts, and 200 meq/l inorganic salts.