Human beings are complex organisms and they have various nutritional needs in order to maintain proper functioning of the body. To meet this need, they consume food. However, the food consumed by them cannot be directly assimilated into the cells. Prior to assimilation, these complex food molecules should be broken down into simpler substances by the process of digestion and absorbed into the bloodstream so that they can be carried to the tissues where they are needed.
Absorption is the process through which simpler digested nutrient molecules are taken up from the gastrointestinal(GI) tract into the lymph or blood. The task of absorption is performed in the specified regions of the GI tract. These regions have several anatomical adaptations that allows them to carry out this function efficiently.
The alimentary canal or the GI tract is divided into five major regions− the oral cavity, oesophagus, stomach, small intestine, and large intestine The absorption of nutrients occurs majorly in the small intestine. However, certain substances are absorbed in other parts of the GI tract like the oral cavity, stomach, and large intestine as well.
Absorption in the oral cavity − The oral cavity is mainly involved in chewing and mixing of food with saliva. However, certain drugs, alcohol, and simple sugars are absorbed in the oral cavity
Absorption in stomach − The stomach is a hollow muscular organ that facilitates the breakdown of nutrients in presence of hydrochloric acid and enzymes. Absorption of ethanol, lipid-soluble compounds like aspirin, non-steroidal anti-inflammatory drugs (NSAIDs), caffeine, and water (during dehydration) occurs in the stomach and certain vitamins occur in the stomach.
Absorption in the small intestine − The small intestine is the longest part of the alimentary canal and is about 6m in length. It is adapted to carry out absorption very efficiently. These adaptation include the presence of villi and microvilli , presence of specialized absorptive cells, and huge network of blood capillaries and lymphatic vessels. The small intestine is divided into three sections based on their feature and function. These are −
Duodenum − It is the initial short section that receives chyme from the stomach and secretions from the liver and pancreas. This region mainly performs digestion of food rather than absorption.
Jejunum − It is the middle coiled section of the small intestine which is highly vascularized. It serves as the initial site for the absorption of nutrients.
Leum − It is the last and longest section of the small intestine. Most of the nutrient absorption occurs in this region.
Large intestine − The large intestine is the continuation of the small intestine nut slightly broader than it. Its major function is to absorb vitamin B12, Vit K, water, and other ions.
The food we eat can be divided into different components- macronutrients and micronutrients.Those which are needed in large quantities by our body are called macronutrients and those which are needed in very feeble amount are called micronutrients.
Various enzymes are secreted by specialised cells and glands of the GI tract. The action of these enzymes on the nutrients is summarized in table 2.
| Macronutrients | Enzymes | End products |
|---|---|---|
| Carbohydrates | Salivary amylase, pancreatic amylase, maltase, sucrase, lactase. | Monosaccharides (like glucose, sucrose, fructose, maltose, lactose, and galactose) |
| Proteins | Hydrochloric acid, trypsin, chymotrypsin, elastase, carboxypeptidase, and aminopeptidase. | Amino acids |
| Fats | Lipase, bile salts | Fatty acids, glycerol |
| Nucleic acids | Nucleases | Nitrogenous bases, pentose sugars, phosphate ions |
Table: Summary of enzymes acting on different macronutrients.
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Water-soluble substances like vitamins, minerals, monosaccharides, and amino acids are directly absorbed by the capillaries into the bloodstream.
Fat-soluble vitamins like A, D, E, glycerol, and fatty acids are first absorbed by the lacteals of the lymphatic system which are later taken up by the blood vessels. Fat-soluble substances and derivatives of lipids are initially taken up by spherical, small, water-soluble molecules called micelles.
The electrolytes like calcium, sodium, potassium, phosphate, chloride, bicarbonates, and magnesium are absorbed in the duodenum. Absorption of bile salts occurs in the ileum region.
The simple molecules that are produced after digestion of macromolecules are absorbed in the digestive tract by three mechanisms- Simple diffusion, facilitated diffusion and active transport. Except some non-nutrient substances like alcohol and aspirin nothing is absorbed by the mucosa of the stomach. About 90 percent of nutrients are absorbed by the small intestine and the 10 percent is absorbed by the large intestine.
Simple diffusion.
Simple diffusion involves the movement of molecules along their concentration gradient without the use of energy. Substances like dietary lipids are absorbed by intestinal cells by simple diffusion.
Active transport.
Active transport involves the movement of molecules against the concentration gradient using energy. Proteins and carbohydrates are substances that are absorbed by active transport.
Facilitated Transport.
Facilitated transport involves the movement of molecules along their concentration gradient mediated by any other ion, carrier protein, or channels. Such transport does not use energy to take place. Glucose and amino acids are some of the substances that are absorbed by these mode.
Passive transport.
Simple diffusion, facilitated diffusion, and osmosis are together classified as passive transport. These modes do not require chemical energy and involve movement of molecules across the cell membrane along the concentration gradient.
Assimilation.
The simple molecules that have been absorbed from the GI tract that reach the blood circulation circulate throughout the body and is absorbed by the cells. Energy is generated from glucose by its utilization in respiration. Excess glucose is temporarily stored in the form of glycogen in the liver.Amino acids generated during protein catabolism undergo deamination and release urea, a type of nitrogenous waste.
Absorption is the process that occurs in the GI tract. The complex nutrients that are broken down into simpler substances are absorbed into the blood vessels and lymphatic vessels. Simple substances like monosaccharides, glycerol, amino acids, etc move from the small intestine and finally assimilate into cells to be used up for various processes to sustain the cellular activities.
Q1. Why are dietary fibres not absorbed in the human GI tract?
Ans. The GI tract of human beings does not secrete the enzymes required to digest the dietary fibres present in the food. The bacteria that secrete these enzymes are also absent in the GI tract. The enzyme cellulase breaks down cellulose present in the dietary fibres.
Q2. What happens when the nutrients are not absorbed properly?
Ans. Reduction in the ability to absorb nutrients from the GI tract is termed malabsorption. It affects the functioning of our bodies and leads to the deficiency of nutrients.
Q3. Enlist the factors that affect absorption in the small intestine.
Ans. Food allergies, infections, lack of movement, and long term use of certain drugs are a few factors that affect the absorption of nutrients in the small intestine.
Q4. Where are the excess lipids and glycerol stored in the body?
Ans. Excess lipids and glycerol is stored in fat cells that form the adipose tissue. This tissue is found all over the body- underneath the skin, internally around the organs, in the bone marrow, and between the muscles.
Q5. What happens if we do not consume enough dietary fibres?
Ans. If enough dietary fibres are not consumed, it will lead to fibre deficiency. This can further cause constipation, heart diseases, irritable bowel syndrome, and certain forms of cancers related to the GI tract.
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