Question:- Explain Digestive system
of human beings?
Answer:- The digestive system in human
beings is made of alimentary canal and some external glands like liver,
pancreas and salivary glands which help in digestion of food. The alimentary
canal is a long tube having a length of 8-10 meters. The various organs of
human digestive system are: mouth, buccal cavity, oesophagus, stomach, small
intestine (it consists of two parts duodenum and ileum), large intestine (it
consists of colon and rectum) and anus.
Now, let’s discuss the various
processes of nutrition taking place in the digestive system of human beings.
- Ingestion
In human beings ingestion of food takes place through mouth,
which is bounded by two movable lips.
- Digestion
In human beings digestion starts in the mouth. The mouth
consists of buccal cavity, teeth, tongue and salivary glands. Buccal cavity is the space between two
jaws. Both the jaws have teeth which take part in cutting and grinding of food.
Three pairs of salivary glands open into buccal cavity. Their names are parotid (situated below the ears), submaxillary (situated at the angles of
lower jaw) and sublingual (situated
below the tongue). The salivary glands produce saliva. The
saliva contains enzymes ptyalin
which converts starch present in the food to simple sugars. Saliva also helps
in lubrication and softening of food.
Saliva is mixed
with food by tongue present at the base of buccal cavity. Tongue also performs
a number of other functions such as:
1) It helps in chewing of food
2) It helps in speech
3) It helps in perceiving taste of
food
4) It helps in swallowing the food
The partially digested food in
the mouth is swallowed by the tongue and goes into the stomach through
oesophagus or food pipe. When the partially digested food enters oesophagus the walls of
the oesophagus starts rhythmic contraction movements called peristalsis. By these peristaltic movements the
partially digested food is pushed into the stomach. Oesophagus does not possess
any digestive gland. However, it secretes mucus for further lubrication of
food.
From the oesophagus the
partially digested food enters the stomach which is the widest part of the
alimentary canal. It is a J shaped organ that lies on the left side just behind
the diaphragm. The food is further broken down in small pieces by churning
movements of stomach. The stomach contains a number of gastric glands which secrete gastric
juice.
The gastric juice consists of
three substances: proenzyme pepsinogen, hydrochloric acid (HCl) and mucus. The presence of HCl in
gastric juice makes the partially digested food acidic and prevents the growth
of bacteria in food. HCl also converts the proenzyme pepsinogen present in the
gastric juice into proteolytic enzyme pepsin.
The enzyme pepsin digests the protein present in the food and covert it into
small and simple molecules called peptones. In this way, digestion of
proteins present in the food starts in stomach. It is to be noted, that the
pepsin can digest proteins only in the presence of acidic medium. Thus,
production of HCl in gastric juice is necessary for the working of pepsin.
Walls of stomach are protected from the action of HCl by mucus. It also
lubricates the food. The acidic partially digested food present at the end of its
stay in stomach is called chyme. This partially digested food
then passes into duodenum part of small intestine.
The small intestine is a narrow
and coiled tube which constitutes about 75% of the whole length of alimentary
canal. It is divided into two parts: duodenum
and ileum. Duodenum is the first
part of small intestine which is connected to stomach. It is about 25 cm in
length. The remaining part of small intestine which is about 6.5 meters long in
an adult man is called ileum. Duodenum receives the secretions of two glands: liver and pancreas. The secretions of liver and pancreas are received
through a common duct. Liver is the largest gland present in the human body. The job of liver is to make a
green liquid called bile which is alkaline in nature. So, bile neutralizes the acidity
of food received from stomach. Bile also helps to break up the large molecules
of fats present in food. Bile secreted from liver is normally stored in the gall bladder unless needed. Bile also helps in the
absorption of fatty acids, fat soluble vitamins (vitamin A, D, E, and K),
prevents putrefaction of food and helps in excretion of toxic materials out of
body. Pancreas
lies beneath the stomach. Pancreatic juice which contains digestive enzymes
such as trypsin and pancreatic amylase are secreted by
pancreas. Trypsin
breaks up the proteins and peptones while the pancreatic amylase digests the
starch present in food. The alkaline and partially digested food present in the
duodenum of small intestine is called chyle.
From duodenum the partially
digested food reaches ileum. In ileum the partially digested food is mixed with
the colourless intestinal juice
secreted by wall of ileum. The enzymes present in intestinal juice completely
digest the partially digested food i.e. the intestinal juice breaks up the
carbohydrates into glucose, proteins into amino acids and fats into fatty acids
and glycerol. In this way, the digestion of food completes in the ileum part of
small intestine.
3. Absorption
Absorption of digested food
mainly takes place in small intestine. The wall of ileum contains lots of
finger like structures called villi.
The presence of villi on the inner wall of ileum increases its surface area to
a large extent. This enlarged surface area helps in rapid absorption of
digested food. Also, the walls of the villi are only one cell thick. The
digested food can easily pass through them into tiny blood vessels present in
villi, and mixes with blood. The diagram of villi is shown on next page.
4. Assimilation
The food absorbed by blood is then transported to all the parts
of body and is used to obtain energy as well as for growth and repair of the
body.
5. Egestion
The food not digested by human digestive
enzymes goes from ileum to colon. Colon is the first part of large intestine.
Here, water is absorbed from undigested food. Due to this, the undigested food
becomes solid. This undigested solid food is stored temporarily in the rectum.
Rectum is the last part of large intestine. From rectum the undigested solid
food is finally removed out of the body through anus.