BIOLOGY DIGESTION AND ABSORPTION

BIOLOGY DIGESTION AND ABSORPTION

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Digestion and Absorption

Human Digestive System

The conversion of complex food substances into simple absorbable substances is called digestion.

The digestive system of humans consists of the alimentary canal and digestive glands.

Alimentary Canal

The alimentary canal extends from the mouth to the anus. It is 6 to 7 metre long.

Mouth

The mouth is the anterior end of the alimentary canal.

It opens into the buccal cavity or the oral cavity.

The oral cavity consists of teeth and the tongue.

The teeth are embedded in sockets and have well-developed roots. Such a type of arrangement is

called thecodont.



All mammals including humans have two sets of teeth during their lifespan. Hence, human teeth are

also diphyodont.

Humans exhibit heterodont dentition, i.e. four different types of teeth are found. They are incisors,

canines, premolars and molars.

The arrangement of teeth in each half of the upper jaw and the lower jaw in the order I, C, PM, M is

called the dental formula.

The dental formula in human beings is 2123

2123.

Incisors = 2/2, canines 1/1, premolars = 2/2, molars 3/3

Therefore,

2123X 2 32

2123

The tongue lies at the floor of the buccal cavity.

The posterior part of the tongue is attached to the floor of the buccal cavity by a soft fold called

frenulum.

The dorsal surface of the tongue bears numerous papillae.

Taste buds located near the tip of the tongue are for sweet taste.

The tongue helps in recognition of taste.

It also allows mixing of saliva with food.

Pharynx

The oral cavity continues into the pharynx.

The pharynx is a short, common passage for food and air.

The oesophagus and trachea open into the pharynx.

An epiglottis is a cartilaginous flap which prevents the entry of food into the trachea and entry of air

into the oesophagus.

Oesophagus

The oesophagus is a 25-cm long muscular tube.

It opens into the stomach.

The opening of the oesophagus into the stomach is surrounded by cardiac sphincters.



Stomach

The stomach is J-shaped and the widest part of the alimentary canal.

The stomach has three parts — cardiac region, fundic region and pyloric region.

The oesophagus opens into the cardiac region.

The cardiac region continues into the fundic region which further continues into the pyloric region.

The parietal cells of the stomach secrete HCl while the peptic cells secrete pepsinogen.

Small Intestine

The small intestine is the longest part of the alimentary canal.

It consists of duodenum, jejunum and ileum.

The opening of the stomach into the duodenum is guarded by the pyloric sphincter.

The mucosa of the small intestine bears folds called villi.

Each villus has numerous brush-like borders called microvilli.

Microvilli increase the area of absorption.

Complete digestion of food and its absorption take place in the small intestine.

Large Intestine

The small intestine opens into the large intestine.

It consists of three parts — caecum, colon and rectum.

The opening of the small intestine into the caecum is guarded by the ileocaecal valve.

The colon is divided into four parts—ascending colon, transverse colon, descending colon and

sigmoid colon.

The descending colon opens into rectum which opens out through the anus.

The large intestine is involved in the absorption of water.

Histology of Alimentary Canal

The wall of the alimentary canal from the oesophagus to the rectum consists of the following four

layers:

Serosa: It is the outermost layer made up of mesothelium.

Muscularis: It is made up of smooth muscles which are arranged in inner circular and outer

longitudinal layers.

Submucosa: It is formed of loose connective tissue. It contains nerves, blood and lymph vessels.

Mucosa: It is the innermost lining. In the stomach, it bears irregular folds called rugae. In the small

intestine, it bears numerous finger-like projections called villi. The cells lining the villi produce

numerous microscopic projections called microvilli.



Digestive Glands

There are three main digestive glands.

Salivary Glands

Three pairs of salivary glands are — parotid glands, sublingual glands and submaxillary glands.

The three glands are located outside the buccal cavity and they secrete saliva.

Liver

The liver is the largest gland of the human body.

It is situated in the abdominal cavity just below the diaphragm.

It consists of two lobes—right lobe and left lobe.

Each lobe is made of hepatic lobules which are the structural and functional units of the liver.

Each hepatic lobule is made of hepatic cells which are arranged in cords.

Each hepatic lobule is covered by a thin connective tissue sheath called Glisson’s capsule.

The gall bladder is a pyriform sac which lies just below the right lobe of the liver.

The cystic duct (the duct of the gall bladder) and the hepatic duct from the liver unite to form the

common bile duct.

The common bile duct and the pancreatic duct of the pancreas open together into the duodenum as

the common hepatopancreatic duct.

The opening of the hepatopancreatic duct into the duodenum is guarded by the sphincter of Oddi.



Pancreas

Pancreas is a compound gland.

Its exocrine part secretes digestive enzymes and its endocrine part secretes hormones.

Mechanism of Human Digestion

The six major steps in nutrition are

o Ingestion: Food is taken into the buccal cavity through the mouth.

o Deglutition: It the movement of the bolus from the buccal cavity to the pharynx to the oesophagus

and finally to the stomach.

o Digestion:

Mechanical digestion: It is the physical breaking of large food particles into smaller food

particles to prepare it for chemical digestion. It also includes the churning of food for proper

mixing of enzymes and its movement in the alimentary canal.

Chemical digestion: It starts in the buccal cavity and it continues in different parts of the

digestive system.

o Absorption: It is the process by which end-products of digestion pass through the intestinal

mucosa into the blood or lymph.

o Assimilation: Absorbed nutrients are used by tissues for their activities. This process is called

assimilation. o Egestion: The elimination of undigested material, i.e. faeces, from the alimentary canal is called

egestion or defaecation.



Summary of Chemical Digestion

Part of the

Alimentary Canal

Digestive

Juices/Enzymes

Source of

Secretion

Food Digested End-product

1. Mouth Saliva

Salivary amylase Salivary glands Polysaccharides

(Starch)

Disaccharides

(Maltose)

2. Oesophagus - - - -

3. Stomach Gastric juice

HCl Gastric glands - Converts

pepsinogen and

prorennin into

the active forms

pepsin and

rennin,

respectively.

Provides an

acidic

environment.

Kills bacteria in

food.

Pepsin Gastric glands Proteins Proteoses,

peptones

Rennin (found in

infants and young

children)

Gastric glands Casein Proteoses,

peptones

Gastric lipase Gastric glands Fats

(emulsification)

Fatty acids and

glycerol

4. Small Intestine

(Duodenum)

Bile

Bile (bilirubin +

biliverdin +

cholesterol +

phospholipids +

bile salts)

No enzymes

Liver (stored in gall

bladder)

Fats Emulsifies fats into

micelles

Pancreatic juice

Trypsin,

chymotrypsin,

carboxypeptidase

Pancreas Proteins, peptones,

proteoses

Dipeptides

Amylase Pancreas Polysaccharides Disaccharides

Lipase Pancreas Fats Diglycerides,

monoglycerides

Nucleases Pancreas Nucleic acids Nucleotides

Succus entericus

Disaccharidases Intestinal glands Disaccharides Monosaccharides



(Example: Maltase) (Example: Maltose) (Example:

Glucose)

Dipeptidases Intestinal glands Dipeptides Amino acids

Lipases Intestinal glands Monoglycerides Fatty acids and

glycerol

Nucleases Intestinal glands Nucleotides Nucleosides

Nucleases Intestinal glands Nucleosides Sugars and

nitrogenous bases

Disorders of Digestive System

• Liver is affected.

• Skin and eyes turn yellow because of the depositionof bile pigments.

Jaundice

• It is the ejection of the stomach contents through themouth.

• Vomiting is a reflex action controlled by the actioncentre of the medulla oblongata.

Vomiting

• It is caused by the abnormal frequency of the bowel movement.

• Diarrhoea affects the absorption of food.Diarrhoea

• Bowel movement takes place irregularly.

• Hence, the faecal matter is retained in the rectum for a long time.

Constipation

• Food is not digested properly.

• Indigestion gives the feeling of fullness.Indigestion

BIOLOGY DIGESTION AND ABSORPTION

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