Digestion

The digestive system

The digestive system includes the gastro-intestinal tract (mouth to
anus) and the glandular organs. This system serves to transfer organic
molecules, salts and water from the external environment to the body’s
internal environment. Most of the food taken into the mouth are large
particles containing macromolecules such as polysaccharides and proteins.
As such they cannot be absorbed by the intestinal wall. They must be
dissolved and broken down into much smaller molecules. This process is
named as digestion. Digestion is accomplished by substances called enzymes
produced from the digestive glands. The enzymes are biocatalysts in the food
breakdown process.

The process of digestion

Mouth :-

In the mouth, digestion starts with chewing. It breaks up large
pieces of food into smaller particles that can be swallowed without choking.
It is accomplished by teeth, tongue, jaws and saliva.
Chewing is controlled by the somatic nerves to the skeletal muscles
of the mouth and jaw. Rhythmic chewing motions are reflexly activated by
the pressure of food against the gums, hard palate at the roof of the mouth
and tongue.

Saliva :- (digestive system)

The saliva is secreted by three pairs of exocrine glands, namely :
the parotid, the submandibular and the sublingual. The daily secretion of saliva
ranges from 1000 to 1500ml. It contains the organic substances amylase
and mucin. The salivary amylase or ptyalin can act on starch. It converts
cooked starch into the disaccharide, maltose. Mucin is a glycoprotein. It
helps in the lubrication of food. The lubricated, swallowable form of food is
called the bolus. The salivary secretion is controlled by reflex activities.

Swallowing :- (digestive system)

It is a complex reflex activity. It is controlled by the
swallowing center in the medulla oblongata. During swallowing the soft
palate is elevated, the larynx gets raised. The tongue forces the food back
into the pharynx, the epiglottis closes the glottis and the food slowly passes
into the oesophagus.

The oesophageal phase begins with relaxation of the upper
oesophageal sphincter. In the oesophagus the food is moved towards the
stomach by a progressive wave of muscle contractions that proceed
downward to the stomach. Such waves of contraction in the muscle layer
surrounding a tube is known as peristaltic waves. In the oesophagus one
peristaltic wave takes about 9 seconds to reach the stomach. Due to
peristaltic waves, swallowing can occur even while a person is upside down.

The stomach (digestive system)

It is a wide chamber, located below the diaphragm. The size and
shape of the stomach depends on the food inside it. The stomach volume
during feeding may increase upto 1.5 lit. Stomach’s primary contractile
action will produce peristaltic waves. Each wave begins in the body of the
stomach and proceeds towards the pyloric region. The initial wave
influences the muscles to close the pyloric sphincter, a ring of smooth muscles
between the stomach and the duodenum.
The inner wall of the stomach is lined with gastric glands. There are
nearly 40 million glands engaged in producing gastric juice. The chief cells of
the lining of stomach secrete enzymes and the parietal cells (Oxyntic cells)
produce HCl to create acidic medium for enzymes.
The enzymes of the stomach are the pepsin and rennin. Pepsin is
secreted in an inactive precursor form known as pepsinogen. The activity
due to HCl converts pepsinogen into pepsin. Pepsin hydrolyses the proteins
into short polypeptide chains and peptones. It is most effective in an acidic
environment.
Proteins
pepsin
polypeptides + peptones
Rennin acts on soluble milk protein caesinogen and converts it into
insoluble casein. In the presence of calcium ions casein is precipitated as
insoluble calcium-casein compound (curds).
Repeated peristaltic waves in the stomach help to soften the food.
The frequency of contraction is determined by the basic electrical rhythm
and remains essentially constant. It is also aided by neural and hormonal
influences. The food leaves the stomach in the form of chyme and enters the
upper small intestine at periodic intervals.

The small intestine :- (digestive system)

It is about 5-7 metres long. It is divided into three
segments namely the initial short segment the duodenum, the jejunum and
the longest segment the ileum.
The food is propelled down into the duodenum due to peristaltic
action of stomach wall. The pyloric sphincter located at the junction of
stomach and duodenum regulates movement of chyme.
The food in the small intestine is mixed with three juices namely bile
juice, pancreatic juice and intestinal juice.

Bile juice :-

It is a brownish green, alkaline secretion of the liver. It is stored
in the gall bladder and poured into the duodenum via the bile duct. The bile
contains water, mucus, inorganic salts, cholesterol and bile salts. The bile
salts emulsify fats and help enzymes like lipase to act upon fats. During
emulsification, the bile salts convert bigger fat particles into smaller fat globules.

Pancreatic juice :-

It is an alkaline fluid (pH 7 to 8). It is transported to the
duodenum through the pancreatic duct. It contains water, mineral salts and a
variety of enzymes like : trypsin, chymotrypsin which are secreted in the
form of inactive precursors trypsinogen and chymotrypsinogen. The
precursors are activated by enterokinase of the intestinal juice, the amylytic
enzyme amylase, the pancreatic lipase (steapesin), carboxypeptidase and
nuclease. The enzyme trypsin hydrolyses proteins into polypeptides and
peptones.

Proteins ___________® polypeptides + peptones

Chymotrypsin hydrolyses peptide bonds associated with specific amino
acids like phenylalanine, tyrosine or tryptophan. It results in large peptides.
Proteins
chymotrypsin
large peptides
Carboxypeptidase is an exopeptidase. It attacks the peptide bonds at the
carboxyl end of the polypeptide chain resulting in di-, tripeptides and amino
acids.The pancreatic amylase converts starch into maltose. The lipase acts
on emulsified fat (triglycerides) and hydrolyses them into free fatty acids and
monoglycerides. Monoglycerides may be further hydrolysed to fatty acid
and glycerol.

Intestinal juice: (digestive system)

(Succus entericus)
maltase
Maltose ________________® glucose + glucose
sucrase
Sucrose ________________® glucose + fructose
lactase
Lactose ________________® glucose + galactose
peptidase
Di, Tripeptides ________________® aminoacids
nucleotidase
Nucleotides ________________® nucleoside + phosphoric acid
nucleosidase
Nucleoside ________________® nitorgenous base + sugar

Absorption and assimilation

As a result of digestion, all macromolecules of food are converted
into their corresponding monomeric units. Carbohydrates are broken into
monosaccharides such as glucose and fructose. Proteins are hydrolysed into
amino acids. Lipids get broken into glycerol and fatty acids. The simpler
organic molecules along with minerals, vitamins and water enter into body
fluids through the villi.
The villi are small microscopic finger-like projections. Each villus is
an absorbing unit consisting of a lacteal duct in the middle surrounded by fine
network of blood capillaries. While the fatty acids are absorbed by the lymph
duct, other materials are absorbed either actively or passively by the capillaries
of the villi.
From the lumen of the alimentary canal absorbed food materials are
carried to the liver through hepatic portal vein. From the liver, materials are
transported to all other regions of the body for utilisation. This conversion of
food into energy and cellular organisation is called as assimilation.