DIGESTION AND ABSORPTION PART-3: ABSORPTION OF DIGESTED FOOD AND DISEASES RELATED TO DIGESTION

            In the previous posts, we have discussed the anatomy of digestive system and the process of digestion. In this post we are going to discuss the process of absorption of digested food and diseases related to digestion. For the previous post (Part-1) click here and for the next post (Part-2) click here.

Absorption of Digested Food

•Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. 

•It is carried out by

passive, active or facilitated transport mechanisms. 

•Small amounts of

monosaccharides like glucose, amino acids and some electrolytes like

chloride ions are generally absorbed by simple diffusion.

•The passage of these substances into the blood depends upon the concentration gradients.

•However, some substances like glucose and amino acids are absorbed with

the help of carrier proteins. 

•This mechanism is called the facilitated transport.

•Transport of water depends upon the osmotic gradient.

•Active transport occurs against the concentration gradient and hence requires

energy. 

•Various nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+

 are absorbed into the blood by this mechanism.

•Fatty acids and glycerol being insoluble, cannot be absorbed into the blood.

•They are first incorporated into small droplets called micelles which move into the intestinal mucosa. 

•They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi.

•These lymph vessels ultimately

release the absorbed substances into the blood stream.

•Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. However, maximum absorption occurs in the small intestine.

•The absorbed substances finally reach the tissues which utilise them for their activities.

•This process is called assimilation.

•The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal.

•The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.

Overview of Absorption of Digested Food  by the Region of Digestive Tract:

Mouth:

Certain drugs coming in contact with the mucosa of mouth and lower side of the tongue are

absorbed into the blood capillaries lining them.

Stomach:

Absorption of water, simple sugars, and alcohol etc. takes place.

Small Intestine:

Principal organ for absorption

of nutrients. The digestion is

completed here and the final

products of digestion such as

glucose, fructose, fatty acids,

glycerol and amino acids are

absorbed through the mucosa

into the blood stream and lymph.

Large Intestine:

Absorption of water, some minerals and drugs takes place.

CALORIFIC VALUE OF PROTEIN, CARBOHYDRATE AND FAT

•The energy requirements of animals, and the energy content of food, are expressed in terms of measure of heat energy because heat is the ultimate form of all energies. 

•This is often measured to as calorie (cal) or joule (J), which is the amount of heat energy required to raise the temperature of 1 g of water by 1°C.

•Since this value is tiny amount of energy, physiologists commonly use kilocalorie

(kcal) or kilo joule (kJ). 

•One kilo calorie is the amount of energy required to raise the temperature of 1 kg of water by 1 °C. 

•Nutritionists, traditionally refer to kcal as the Calorie or Joule (always capitalised). 

•The amount of heat liberated from complete combustion of 1 g food in a bomb calorimeter (a closed metal chamber filled with O2) is its gross calorific or gross energy value.

•The actual amount of energy combustion of 1 g of food is the physiologic value of food.

•Gross calorific values of carbohydrates, proteins and fats are 4.1 kcal/g, 5.65 kcal/g and 9.45 kcal/g, respectively, whereas their physiologic values are 4.0 kcal/g, 4.0 kcal/g

and 9.0 kcal/g, respectively.

DISORDERS OF DIGESTIVE SYSTEM

The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections. The infections are also caused by the parasites of the intestine like tapeworm, roundworm, threadworm, hookworm, pinworm, etc.

•Jaundice: The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.

•Vomiting: It is the ejection of stomach contents through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling of

nausea precedes vomiting.

•Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the

absorption of food.

•Constipation: In constipation, the faeces are retained within the colon as the bowel movements occur irregularly.

•Indigestion: In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating, and spicy food.

PEM

•Dietary deficiencies of proteins and total food calories are widespread in many underdeveloped countries of South and South-east Asia, South America, and West and Central Africa.

•Protein-energy malnutrition

(PEM) may affect large sections of the population during drought, famine and political turmoil. 

•This happened in Bangladesh during the liberation war and in Ethiopia during the severe drought in mid-eighties. 

•PEM affects

infants and children to produce Marasmus and Kwashiorkar.

Marasmus:

•Marasmus is produced by a simultaneous deficiency of proteins and calories. 

•It is found in infants less than a year in age, if mother’s milk is

replaced too early by other foods which are poor in both proteins and caloric value. 

•This often happens if the mother has second pregnancy or childbirth when the older infant is still too young. 

•In Marasmus, protein deficiency impairs growth and replacement of tissue proteins; extreme emaciation of the body and thinning of limbs results, the skin becomes dry, thin and wrinkled. Growth rate and body weight decline considerably.

•Even growth and development of brain and mental faculties are impaired.

Kwashiorkar:

•Kwashiorkar is produced by protein deficiency unaccompanied by calorie

deficiency. 

•It results from the replacement of mother’s milk by a high calorie￾ low protein diet in a child more than one year in age.

•Like marasmus, kwashiorkor shows wasting of muscles, thinning of limbs, failure of growth and brain development.

•But unlike marasmus, some fat is still left under the skin; moreover, extensive oedema and swelling of body parts are seen.