DAY NINETEEN
Digestion and
Absorption
Learning & Revision for the Day
u Human Digestive System u Mechanism of Digestion u Disorders of the Digestive System
Digestion is a process which involves breakdown of complex food substances by hydrolysis
into smaller molecules that can be absorbed through the epithelium of the gastrointestinal
tract. Humans and other animals have holozoic nutrition, i.e. they intake solid or liquid form
of food.
Human Digestive System
The human digestive system consists of the alimentary canal and the digestive glands.
1. Alimentary Canal
The anterior opening of alimentary canal is mouth and posterior is the anus.
Alimentary canal consists of different structures which are described below
(i) Oral cavity is the cavity which opens through mouth, it contains teeth and muscular
tongue.
l
Teeth Each tooth is embedded in a socket of jaw bone. This condition is called as
thecodont.
n Humans and majority of mammals have two sets of teeth during their lifetime, a
set of temporary teeth and a set of permanent teeth. This type of dentition is
called diphyodont.
n Adult human has 32 teeth with heterodont dentition. The arrangement of teeth
in each half of the upper and lower jaw is represented by dental formula. The
arrangement order is, `Incisors (I), Canine (C), Premolar (Pm) and Molar (M).
The dental formula in human is
I-2, C-1, Pm-2, M-3 2123
= × 2 = 32
I-2, C-1, Pm-2, M-3 2123
n Elephant tusks are modified incisors, tusks of nabus are modified canines.
, l Tongue is the muscular organ which is attached to The bile secreted by the hepatic cells passes through
the floor of oral cavity by the frenulum. It has small the hepatic ducts and is stored in gall bladder.
projections called papillae on its upper surface, some l The bile duct and the pancreatic duct open together
of them contain taste buds. into the duodenum as the common hepatopancreatic
(ii) Pharynx the oesophagus and the trachea open into the duct which is guarded by a sphincter called sphincter
pharynx which is a common passage for food and air. of Oddi.
l The oesophagus is a thin, long tube which extends (iii) The pancreas is a compound organ (both exocrine and
posteriorly passing through the neck, thorax and endocrine) situated between the U-shaped duodenum.
diaphragm and leads to a J-shaped bag-like structure The exocrine part secretes an alkaline pancreatic juice
called stomach. containing enzymes and the endocrine portion secretes
l A muscular gastro-oesophageal sphincter regulates the hormones, insulin and glucagon.
opening of oesophagus into the stomach.
(iv) Brunner’s glands are branched, tubuloalveolar
(iii) Stomach is located in the upper left portion of the submucosal glands in the duodenum, whose, secretion is
abdominal cavity and has three major parts, i.e. a rich in bicarbonate ions and alkaline glucoproteins,
cardiac portion, a fundic portion and a pyloric portion. hence it neutralises the acidic chyme entering the
(iv) Small intestine It is distinguishable into three regions, duodenum from the stomach.
i.e. a U-shaped duodenum, a long coiled middle portion
jejunum and a highly coiled ileum. Duodenum is the
widest, shortest and most flexed part of small intestine.
Mechanism of Digestion
Digestion is the process by which large macromolecules of
Ileum opens into the large intestine.
food are broken down into smaller usable molecule with the
(v) Large intestine is shorter but, it is called large because it help of enzymes. It takes place in following four steps
is wider in diameter than small intestine. It consists of
following parts
l Caecum It hosts some symbiotic microorganisms.
1. Ingestion
The food is taken through buccal cavity. It is masticated by
l Colon It is divided into three parts, i.e. an ascending,
teeth and swallowed. Ingestion takes place in buccal cavity.
a transverse and a descending part.
Salivary glands lubricate the food and bind the food particles
l
Rectum The wall of alimentary canal from
together to form bolus. Salivary glands have starch digesting
oesophagus to rectum possesses four layers, i.e. enzyme ptyalin.
serosa, muscularis, submucosa and mucosa.
Histology of Alimentary Canal 2. Digestion
l
The innermost layer lining the lumen of the alimentary It is carried out in the following steps
canal is the mucosa. This layer forms irregular folds in the (i) Digestion in buccal cavity In buccal cavity, salivary
stomach and small finger-like foldings called villi in the amylase acts on starch.
small intestine. The cells lining the villi produce numerous
Ptyalin
microscopic projections called microvilli giving a brush Starch → Maltose
(Polysaccharide) pH 6− 8 (Disaccharide)
border appearance.
(ii) Digestion in stomach The food passes down through
l
Mucosa also forms glands in the stomach and crypts in
the oesophagus into stomach. Now, food is mixed with
between the bases of villi in the intestine (crypts of
gastric juice and hydrochloric acid, which disinfect the
Lieberkuhn).
food and create acidic medium. Pepsin digests proteins
and converts them into peptones and proteoses. Rennin
2. Digestive Glands converts milk to curd. Digested food now is called
The digestive glands are mentioned below chyme. Cellulose is not digested in humans.
(i) The salivary glands are found in three pairs, i.e. the (iii) Digestion in small intestine Chyme moves to
parotids (cheek), the sub-maxillary/sub-mandibular duodenum where it gets mixed with bile (liver) to
(lower jaw) and the sublingual (below the tongue) which breakdown fats into smaller globules. Trypsin acts upon
secrete salivary juice into the buccal cavity. proteins and breaks them into dipeptides. Amylase
converts starch into simple sugar. Lipase converts fats
(ii) Liver is the largest gland of the body weighing about
into fatty acids and glycerol. Food passes into ileum and
1.2-1.5 kg in an adult human.
mixes with intestinal juice. Maltase converts maltose
l
The hepatic lobules are the structural and functional into glucose. Lactase converts lactose into glucose and
units of liver containing hepatic cells arranged in the galactose. Sucrase converts sucrose into glucose and
form of cords. Each lobule is covered by a thin fructose. Dipeptidase digests the dipeptides into amino
connective tissue sheath called the Glisson’s capsule. acids.
Digestion and
Absorption
Learning & Revision for the Day
u Human Digestive System u Mechanism of Digestion u Disorders of the Digestive System
Digestion is a process which involves breakdown of complex food substances by hydrolysis
into smaller molecules that can be absorbed through the epithelium of the gastrointestinal
tract. Humans and other animals have holozoic nutrition, i.e. they intake solid or liquid form
of food.
Human Digestive System
The human digestive system consists of the alimentary canal and the digestive glands.
1. Alimentary Canal
The anterior opening of alimentary canal is mouth and posterior is the anus.
Alimentary canal consists of different structures which are described below
(i) Oral cavity is the cavity which opens through mouth, it contains teeth and muscular
tongue.
l
Teeth Each tooth is embedded in a socket of jaw bone. This condition is called as
thecodont.
n Humans and majority of mammals have two sets of teeth during their lifetime, a
set of temporary teeth and a set of permanent teeth. This type of dentition is
called diphyodont.
n Adult human has 32 teeth with heterodont dentition. The arrangement of teeth
in each half of the upper and lower jaw is represented by dental formula. The
arrangement order is, `Incisors (I), Canine (C), Premolar (Pm) and Molar (M).
The dental formula in human is
I-2, C-1, Pm-2, M-3 2123
= × 2 = 32
I-2, C-1, Pm-2, M-3 2123
n Elephant tusks are modified incisors, tusks of nabus are modified canines.
, l Tongue is the muscular organ which is attached to The bile secreted by the hepatic cells passes through
the floor of oral cavity by the frenulum. It has small the hepatic ducts and is stored in gall bladder.
projections called papillae on its upper surface, some l The bile duct and the pancreatic duct open together
of them contain taste buds. into the duodenum as the common hepatopancreatic
(ii) Pharynx the oesophagus and the trachea open into the duct which is guarded by a sphincter called sphincter
pharynx which is a common passage for food and air. of Oddi.
l The oesophagus is a thin, long tube which extends (iii) The pancreas is a compound organ (both exocrine and
posteriorly passing through the neck, thorax and endocrine) situated between the U-shaped duodenum.
diaphragm and leads to a J-shaped bag-like structure The exocrine part secretes an alkaline pancreatic juice
called stomach. containing enzymes and the endocrine portion secretes
l A muscular gastro-oesophageal sphincter regulates the hormones, insulin and glucagon.
opening of oesophagus into the stomach.
(iv) Brunner’s glands are branched, tubuloalveolar
(iii) Stomach is located in the upper left portion of the submucosal glands in the duodenum, whose, secretion is
abdominal cavity and has three major parts, i.e. a rich in bicarbonate ions and alkaline glucoproteins,
cardiac portion, a fundic portion and a pyloric portion. hence it neutralises the acidic chyme entering the
(iv) Small intestine It is distinguishable into three regions, duodenum from the stomach.
i.e. a U-shaped duodenum, a long coiled middle portion
jejunum and a highly coiled ileum. Duodenum is the
widest, shortest and most flexed part of small intestine.
Mechanism of Digestion
Digestion is the process by which large macromolecules of
Ileum opens into the large intestine.
food are broken down into smaller usable molecule with the
(v) Large intestine is shorter but, it is called large because it help of enzymes. It takes place in following four steps
is wider in diameter than small intestine. It consists of
following parts
l Caecum It hosts some symbiotic microorganisms.
1. Ingestion
The food is taken through buccal cavity. It is masticated by
l Colon It is divided into three parts, i.e. an ascending,
teeth and swallowed. Ingestion takes place in buccal cavity.
a transverse and a descending part.
Salivary glands lubricate the food and bind the food particles
l
Rectum The wall of alimentary canal from
together to form bolus. Salivary glands have starch digesting
oesophagus to rectum possesses four layers, i.e. enzyme ptyalin.
serosa, muscularis, submucosa and mucosa.
Histology of Alimentary Canal 2. Digestion
l
The innermost layer lining the lumen of the alimentary It is carried out in the following steps
canal is the mucosa. This layer forms irregular folds in the (i) Digestion in buccal cavity In buccal cavity, salivary
stomach and small finger-like foldings called villi in the amylase acts on starch.
small intestine. The cells lining the villi produce numerous
Ptyalin
microscopic projections called microvilli giving a brush Starch → Maltose
(Polysaccharide) pH 6− 8 (Disaccharide)
border appearance.
(ii) Digestion in stomach The food passes down through
l
Mucosa also forms glands in the stomach and crypts in
the oesophagus into stomach. Now, food is mixed with
between the bases of villi in the intestine (crypts of
gastric juice and hydrochloric acid, which disinfect the
Lieberkuhn).
food and create acidic medium. Pepsin digests proteins
and converts them into peptones and proteoses. Rennin
2. Digestive Glands converts milk to curd. Digested food now is called
The digestive glands are mentioned below chyme. Cellulose is not digested in humans.
(i) The salivary glands are found in three pairs, i.e. the (iii) Digestion in small intestine Chyme moves to
parotids (cheek), the sub-maxillary/sub-mandibular duodenum where it gets mixed with bile (liver) to
(lower jaw) and the sublingual (below the tongue) which breakdown fats into smaller globules. Trypsin acts upon
secrete salivary juice into the buccal cavity. proteins and breaks them into dipeptides. Amylase
converts starch into simple sugar. Lipase converts fats
(ii) Liver is the largest gland of the body weighing about
into fatty acids and glycerol. Food passes into ileum and
1.2-1.5 kg in an adult human.
mixes with intestinal juice. Maltase converts maltose
l
The hepatic lobules are the structural and functional into glucose. Lactase converts lactose into glucose and
units of liver containing hepatic cells arranged in the galactose. Sucrase converts sucrose into glucose and
form of cords. Each lobule is covered by a thin fructose. Dipeptidase digests the dipeptides into amino
connective tissue sheath called the Glisson’s capsule. acids.
