Digestion of food -Biological processes in human body - science lessons for life


Sunday, December 18, 2016

Digestion of food -Biological processes in human body

Many different biological processes take place in the human body. We will discuss about those processes and the systems specialized to perform them.

Digestion of food
Energy is required for different biological processes that take place in human body. Energy is obtained through food that we take into the body. These food contain nutrients such as carbohydrates, lipids and proteins. Carbohydrates, lipids and proteins are complex organic molecules that do not dissolve in water. These compounds cannot be absorbed into the human body. Threfore they should be broken down into small particles.

The process by which the complex organic compounds are converted into simple organic products to be absorbed into the human body is called digestion of food.

Food digestion takes place in two prosesses namely mechanical and chemical processes
During mechanical process the physical nature of the food is altered,
E.g. :- Breaking down of food into small pieces by teeth inside mouth.
During chemical process, the insoluble complex compounds are broken down into simple molecules by the action of enzymes.
E.g. :- Starch is converted into maltose by ptyalin (salivary amylase) enzyme inside mouth.
There are some nutrients, that can be used by the body without any digestion, such as mineral salts, some vitamins, glucose, fructose and galactose.
The organs involved in food digestion, are collectively called as digestive system.

Human digestive system
Human digestive system is a single tube, that runs from mouth to anus. According to the requirement, the structure has changed at different places, and the glands (salivary glands, pancreas, liver) that supply enzymes and other substances (bile) connect at different sites. The functions take place in the digestive system are food digestion, absorption of digested end products and removal of undigested
materials from the body.

Let us see the structures that belong to the digestive tract.
Human digestive system
Let us observe the changes that occur in food at first part of the digestive tract, the buccal cavity.

Digestion in the buccal cavity
Mouth opens the buccal cavity to the environment. It is surrounded by muscular lips at the bottom and top. The buccal cavity is made up of upper and lower jaws. Only the lower jaw can be moved. Teeth are present in both jaws. Buccal cavity is surrounded by cheeks. The tongue is attached to the floor of the buccal cavity. Three salivary glands are present and these secrete saliva and the tongue helps in identification of taste, mixing of food with saliva and swallowing.

A sweet taste is sensed when chewing rice or bread for sometime. Let's discuss why it is sweet?

The ptyalin enzyme (salivary amylase), acts on starch in digestion of food. Starch will be partially digested into maltose. Digestion of food is start in the mouth.
Starch----------------->  Maltose

When rice or bread is chewed for sometime starch is digested into maltose. As maltose is sweet, we sense the sweet taste.

Initially digested food is formed into a bolus and pushed to the posterior part of the buccal cavity. Next food is pushed into the pharynx.

Pharynx is a common area to both respiratory and digestive systems.

There is a movable organ called epiglottis found just above the opening of trachea. When bolus is swallowed the epiglottis moves down to close the opening of trachea. Then bolus enters into oesophagus without entering into trachea.

Trachea is closed with epiglottis when food enter into oesophagus

Epiglottis helps to prevent entering food into the trachea. When food enters to pharynx, respiratory track is blocked by epiglottis. This prolong blockcage of trachea may cause death. If the food is not removed instantly, the person may die due to blockage of respiratory tract.

Oesophagus is a constricted tube. How is food moved along a constricted tubule?
How the food pass through oesophagus
Then food is moved into stomach by peristaltic movements.

The stomach is a dilated sac like organ. Due to the peristaltic activity of muscles in the stomach wall the bolus is broken down and mixed well into a chyme. Several secretions ooze out into the stomach. It is collectively called the gastric juice.

The gastric juice contains mainly hydrochloric acid (HCl) and pepsin enyme. HCl activates pepsin and pepsin starts the protein digestion to produce polypeptides. Renin present in infants causes coagulation of milk.

Chyme containing partially digested proteins, digested and undigested carbohydrates, undigested lipids, water, minerals and vitamins are released into the proximal part of small intestine, duodenum part by part.

When the stomach is empty, it continues to contract. When the stomach is empty for a longer time period, the rate of contraction is also high. So it causes a pain. It gives a sense about hunger. Hunger is a signal that indicate the need of food.

Digestion in the small intestine
The chemical digestion of food mainly takes place in the small intestine. Pancreatic enzymes as well as intestinal enzymes involve in this digestion.

The small intestine is about 7 m in length. The proximal part of the small intestine is C shaped and known as duodenum.The duct of the pancreas and the gall bladder opens into the duodenum via a single pore. Pancreatic juice is secreted into the duodenum through pancreatic duct. It contains three main enzymes. They are trypsin, amylase and lipase. The bile carried through the bile duct is added to it. Bile is produced in the liver and stored in the gall bladder.

Bile contains bile pigments, bile salts, bicarbonate ions and water.

Due to mixing of bile with food at duodenum, the lipids in food are broken down into small droplets by the process called emulsification. Due to this action, enzymes get a greater surface area to act on lipid food.

Intestinal juice secreted by the wall of the intestine contains, maltose, sucrose, lactase, peptidase and mucous. Mucous lubricates food and then helps to move along the digestive tract.

Let us summarize food digestion takes place in small intestine
Enzymes in food digestion in small intestine
These are the end products of digetion

Proteins in wall of stomach and intestine is protected by the protein digestive enzymes as there is a layer of mucous on the wall.

What happens to the end products of food digestion?
The absorption of digested end products into body takes place mainly in the small intestine. The small intestine is adapted to increase its efficiency of absorption in different ways.

  • Being a long tube
  • Presence of circular folds in the inner wall
  • Presence of finger like projections called villi in the circular folds
  • Presence of microvilli in the epithelial cells of villi
  • Thin epithelial lining on villi
  • Villi are highly vascularised
Structure of villi in small intestine

The digestive end products given below are absorbed into the blood capillaries of villi.

  • Amino acids
  • Vitamins
  • Mineral salts
  • Monosaccharides (Glucose/ Galactose/ Fructose)

Fatty acids and glycerol formed by digestion of lipids are absorbed into lacteals. Finally they enter into blood circulatory system when there is high amount of glucose in blood, they are converted into glycogen and stored in liver. In the same way when the concentration of glucose is dicreased, glycogen breaks down to form glucose and is added to blood. The unabsorbed materials are sent to the large intestine.

Processes in the large intestine
Length of the large intestine is about 1.5 m. It starts with caecum and ends up at anus. The dialated part of the large intestine is the rectum. The opening of it, is the anus. The materials entering into the large intestine contain a very small amount of nutrients. Mainly it contains undigested cellulose and water. A small blind ended tubular structure starts at the end of the caecum. It is known as the appendix. It is very small in humans and it may be infected and become swollen. This disease is
known as appendicitis.

The main function is to absorb water from received matter by ileum. Thereby making it into semi solid.
When large intestine fills with faecal matter it passes out from the rectum.

The chance of getting infections to the digestive tract is high as materials are entered into it from outside frequently. Therefore digestive tract catches many diseases and disorders.
Engage in the following assigment to get knowledge about the diseases and disorders associated with digestive system.

Inflammation of inner lining of mucosa is known as gastritis. It is a common disease among people. Generally known as acidity. The symptoms are, regurgitation of acid to mouth, burning feeling and pain in stomach. When the condition becomes worse, ulcers appear in stomach or duodenal wall. Bleeding can take place. The reasons for this disorder are as follows,
  • Skipping of meals
  • Consumption of acidic and spicy food
  • Excessive smoking and alcohol consumption
  • Mental stress
By following healthy food diets and good habits one can avoid the above disease.

Difficulty in defaecation due to hardening of faecal matter is known as constipation. Faeces remain in the large intestine for a longer time period and absorption of water takes place excessively, Thereby this condition may occur. 
Reasons for constipation are as follows,
  • Consumption of food with low dietary fibres
  • Not taking required volume of water
  • Postponing of defaecation
By avoiding above bad habits one can avoid this disease. Some medicine may cause constipation. Due to forceful defaecation, the anal canal may damage and bleeding can occur.

Typoid is caused by a bacterium. The pathogen is transmitted through food. The bacterium can enter into the body through mouth while swimming and bathing in contaminated water. Pain in arms and legs, headache and fever are main symptoms. It is a disease which gradually becomes worse. Constipation can occur at initial stages. Tongue is covered by a plaque. After sometime stomachache and diarrhoea can occur. Ulcers can form in the small intestine and cause bleeding. Therefore blood is released with faecal matter. Due to ulcers, the wall can be damaged. Disease can be identified by a blood test or stool test.

Diarrhoea occurs when the intestines are infected with a virus, bacteria or a parasite. This disease is transmitted by the faeces of an infected person. The main symptom is release of faecal matter in liquid state. Absorption of water in the large intestine will not occur properly. Dehydration may occur due to loss of fluid. due to the difeare spready contaminated food or water.

The above two diseases can be avoided by taking preventive measures given below
  • Consumption of boiled water
  • Removing fly breeding places and cover is food to prevent entering of flies to food
  • Avoid consumption of food which are sold in open places
  • Use of water seal latrines
  • Wash hands well with soap after using the toilet
  • Get a vaccine for typhoid.
If dehydration becomes worse due to diarrhoea, it may be fatal. So it is better to consume more water and consult a doctor.

source by internet and books