Absorption Of Digested Products

After digestion, nutrients from food are absorbed into the bloodstream or lymphatic system through the small intestine. This process ensures that essential molecules like glucose, amino acids, fatty acids, and vitamins reach body cells for energy production and growth.

1.0Introduction

• 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 and 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 facilitated transport. The transport of water depends upon the osmotic gradient. 
• Active transport occurs against the concentration gradient and hence requires energy. This mechanism absorbs various nutrients, such as amino acids, monosaccharides like glucose, and electrolytes like Na+, into the blood. 
• Fatty acids and glycerol are insoluble and cannot be absorbed into the blood. They are first incorporated into small droplets called micelles, which move into the intestinal mucosa. 
• They are reformed into very small protein-coated fat globules called chylomicrons, which are transported into the lymph vessels (lacteals) in the villi. 
• These lymph vessels ultimately release the absorbed substances into the bloodstream. Substances are absorbed in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. 
• However, maximum absorption occurs in the small intestine. A summary of absorption (sites of absorption and substances absorbed) is given in the table. 

2.0Absorption of Different Components of Food

3.0Absorption in Different Organs

Absorption in Duodenum : In duodenum iron and calcium ions are absorbed. 

Absorption in Jejunum : Maximum absorption of food takes place in jejunum. 

4.0Calorific Value 

• The energy requirements of animals and the energy content of food are expressed in terms of heat energy because heat is the ultimate form of all energies. 
• This is often measured as calorie (cal) or joule (J), the amount of heat energy required to raise the temperature of 1 g of water by 1 °C. 
• Since this value is a tiny amount of energy, physiologists commonly use kilocalories (kcal) or kilojoules (kJ). One kilocalorie 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 capitalized). 
• The heat liberated from the complete combustion of 1 g of 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.

5.0Assimilation of Food

Assimilation is the metabolic process that occurs after digestion and absorption, where the absorbed nutrients are incorporated into the body's tissues and fluids for energy production, growth, and repair. It involves the transport of these nutrients to the body's cells and their subsequent utilization in various metabolic pathways. In humans, absorbed nutrients are transported to cells and tissues primarily via the bloodstream and the lymphatic system.

(a) Proteins and amino acids 

• Amino acids are not stored in the body as they are highly reactive so converted into proteins which are building blocks of the body.

(b) Carbohydrates  

• Glucose is an instant source of energy.  
• Glucose  →  Glycogen in liver (Glycogenesis) (Glycogen is stored food material)  
• Whenever required, Glycogen →   Glucose (Glycogenolysis)  
• Amino acids and fats →  Glucose in liver (Gluconeogenesis)   
• Lactic acid formed in muscles →  Glycogen in liver by Gluconeogenesis 

(c) Fats

• Stored in the body, helps in insulation and thermoregulation.  
• Rich source of energy.

(d) Minerals, water and vitamins play vital roles in many vital processes of the body.

6.0Egestion

• 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. 
• Faecal matter is yellowish brown in colour due to the presence of pigments (Stercobilin, Urobilin). 
• These two are formed due to degradation of bile pigments. Foul smell in excreta is due to the presence of CH₄, NH₃, indole (scatole) and H₂S.