Class 12

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The digestive system of a human being

The digestive system consists of digestive organ and their working mechanism or physiology of digestion.
In human digestive system involves the alimentary canal and associated digestive glands like the liver pancreas etc.

A. Alimentary canal

It is a long and coiled duct extending bounded from the mouth to the anus.
It measures about 8-10 m in length and is differentiated into the following parts.

a. Mouth and mouth cavity

The mouth is a transverse aperture bounded by fleshy lips.
The mouth opens into a wide cavity called the mouth cavity or oral cavity.
It is bounded by the upper lower jaw and sides by cheeks.
The roof of the mouth cavity is called the palate.
From the soft palate, a finger-like projection hangs down called the uvula.
It separates the mouth cavity from the pharynx.
The floor of the mouth cavity contains a highly muscular tongue and it helps in the mastication of food.
The sides of the mouth cavity contain teeth which also help in the mastication of food.

b. Pharynx

The mouth cavity opens into the short and narrow duct called the pharynx.
It is the common duct for food and air.
It is about 5 inches long, lined with the membrane. It is divided into three parts: nasopharynx, oropharynx, and laryngopharynx.

i. Nasopharynx: The Nasopharynx is the pharynx’s upper part that connects nasal passages to the Oropharynx.

ii. Oropharynx: The oropharynx is the middle chamber of the pharynx that passes food from the mouth into the laryngopharynx.

iii. Laryngopharynx: It is the lower part of the pharynx. It has two openings: air opening (glottis) and food opening (gullet). Both openings are separated by the flap of elastic cartilage, epiglottis which covers the glottis so that food cannot enter into the trachea.

c. Oesophagus

The oesophagus or food pipe is a long narrow muscular tube measuring about 25cm.
It is extended from the pharynx to the stomach.
It conducts food from the pharynx to the stomach by peristalsis.

d. Stomach

It is a large J-shaped bag. It also measured 25 cm in length. It is situated on the left side of the upper abdomen.
The stomach is divided into three parts. The upper part is called the fundus part, the middle and broad part is called the body part and the lower and narrow part nearest to the duodenum is called the pyloric part.
The wall of the stomach contains gastric glands which secrete gastric juice in the stomach.
Food is stored for a long time and digested with the help of churning movement and gastric juice.
The stomach has two openings the entrance (cardiac sphincter) and exit (pyloric sphincter).

e. Small Intestine

The stomach opens into the small intestine through the pyloric sphincter (constriction).
The small intestine is the longest and narrowest part of the alimentary canal.
It measures about 6.5m in length and is differentiated into three parts: duodenum, jejunum, and ileum.

i. Duodenum

It is the first c-shaped curved tube that measures about 25 cm in length.
It receives the bile from the gall bladder and pancreatic juice from the pancreas. In the duodenum, the maximum amount of food is digested with the help of both bile and pancreatic juice.
Hence it is called the principal part of digestion.

ii. Jejunum

It is about a 2.5m long coiled duct. Its internal wall contains thick-walled villi not for absorption. Jejunum conduct food from duodenum to ileum.

iii. Ileum

It is the longest and most highly coiled part of the small intestine. It is about 3.5m in length. Its inner wall contains a thick-walled finger-like projection and the maximum amount of food absorbed in this part.

f. Large intestine

It is shorter but wider than the small intestine. It measures about 1.5m in length. It is divided into three parts:
Caecum
Colon
Rectum

i. Caecum

It is the first pouch-like structure which is about 6cm in diameter.
It is connected to the ileum by the ileocaecal junction.
In humans, the caecum contains 5-10cm long blind tube called a vermiform appendix.
In humans, it is vestigial but functional only in herbivores.

ii. Colon

It is an inverted U-shaped tube that measures about 1.2m.
It is differentiated into four parts: i.e. ascending transverse, descending and sigmoid colon.
The wall of the colon is constricted at different regions to form pouch-like haustra.
The colon helps in the formation of stool by the absorption of water.

iii. Rectum

The colon finally opens into the rectum. It is a muscular and straight terminal part measuring 15cm.
It leads outside through the anus. The terminal anal opening is guarded anal sphincters.

B. Digestive glands

These glans are associated with digestion of food and pour their secretion into the alimentary canal. These are:

1.Salivary glands

2.Pancreas

3.Liver

4.Gastric glands

5.Intestinal glands

1. Salivary glands

There are three pairs of salivary glands (Parotid gland, Submandibular gland and sublingual gland) which release saliva in the oral cavity.
A normal person secrete 1-1.5 lt. saliva per day.

Functions of saliva

It contains salivary amylase and breaks down the starch into maltose.
It keeps the mouth moist and cleans the teeth
It lubricates and makes moist the food which helps for easy swallowing.
It kills microorganisms associated with food.

2. Pancreas

The pancreas is a pale grey leaf-like gland weighing 60-75 grams.
It is the main digestive gland and is present in between the stomach and duodenum.
It secret pancreatic juice.

Functions of pancreatic juice

The pancreas performs two main functions: Exocrine function and Endocrine function.

i. Exocrine pancreas: The pancreas secretes all types of enzymes and digests all types of food. These enzymes and their functions are as follows:

Trypsin: Protein breakdown Amino acid
Amylase: Starch Maltose
Lipase: Fat Fatty acid

ii. Endocrine functions: Endocrine pancreas secrete two types of hormones: Insulin and Glucagon

a.Insulin: It is secreted by B-cell. It helps to convert excess glucose into glycogen and reduce blood sugar levels.

b.Glucagon: It is secreted by ἁ – cell. It increases the sugar level by the conversion of glycogen into glycogen into glucose. It is secreted only when sugar is decreased.

3. Liver

It is the largest gland in the body weighing between 1 to 2.3 kg. It is situated just above the stomach, below the diaphragm on the upper abdominal cavity. It is dark red in color.
Liver of human is divided into four lobes i.e. right, left, quadrate and caudate lobe.
Each lobe of liver consists of a large number of lobules formed by hepatic cells. The lobules are branches of the hepatic artery, hepatic portal vein and bile duct.
The hepatic vein connected to the hepatic artery and hepatic portal vein by sinusoids- blood spaces.
The sinusoids alternate with bile canaliculi.
Kupffer cells are found attached to the walls of the sinusoids they are phagocytic in nature and are involved in the breakdown of old RBC and ingestion of harmful bacteria.

Functions of Liver

a. Functions of bile juice

It secrete bile juice. It is an alkaline juice and neutralize the acidity of chyme.
It emulsify the fat into fat droplets.

b. Metabolic function

i.Glycogenesis: Conversion of excess glucose into glycogen with the help of insulin is known as glycogenesis

ii.Glycogenolysis: It is the conversion of glycogen into glucose by the liver cells with the help of glucagon hormone released by pancreas.

iii. Gluconeogenesis: It is the formation of glucose and glycogen from non-carbohydrate sources such as amino acids, fatty acids, fatty acids, etc.

iv.Fat metabolism: The liver cells convert the excess glucose and amino acids into fats.

c. Protective function

The Kupffer cells of liver engulf microorganisms and other foreign materials.
Liver converts the toxic substances into harmless substances
Liver cells convert the highly toxic ammonia into less toxic urea.

d. Hematological functions

Synthesis of RBC and blood proteins
b. Secretion of heparin which is an anticoagulant
Destruction of old RBC

4. Gastric glands

It is present in the wall of the stomach. It is a branched tubular gland. It secretes gastric juice. A normal person secretes about 2-3 lt. gastric juice daily.

Function of gastric juice

It makes the food acidic.
It activates the inactive pepsinogen to pepsin and prorennin to rennin.
It destroys bacteria and microbes present in the food.
It destroys hard substances like bone.
It controls the opening of the pyloric aperture.

2. pepsin: It breaks down the protein into peptones and proteases.

3. Rennin: It converts the milk into curd. It is only secreted in children.

4. Gastric lipase: It breaks down the fat into fatty acids.

5. Mucus: It protects the stomach wall from the acidic effect of HCL and also makes food slippery.

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