The human body needs to break down food items into much smaller molecules for absorption and use by various metabolic processes, and it is done through digestion. Smaller molecules have benefits like they can be combined by the body in different combinations to create whatever it needs, from building various tissues to hormones, and so on.
So, when people eat carbs, like bread, potatoes, they contain large molecules called polysaccharides. Digestion helps break them into much smaller monosaccharides compounds, like glucose. Similarly, proteins are broken down to simpler amino acids, fats to more manageable molecules. Digestion also helps food items to release minerals and vitamins.
Technically, digestion starts much earlier than even the food is ingested. Cooking can be regarded as the first step in digestion, as it breaks down complex molecules, and makes the further digestive process more manageable.
Five main activities involved in the digestion:
- Ingestion- of food to the alimentary canal.
- Propulsion- or movement of food items through the digestive tract.
- Digestion – which is both a mechanical and chemical process.
- Absorption- of the nutrients.
- Elimination- of waste materials.
Mechanical Digestion
Starts from the mouth, where front teeth are made for biting, cutting food items, and molar teeth located in the back of the mouth are suitable for grinding. Chewing is especially important when eating raw food items like fruits, salads. Many fruits and vegetables are covered by cellulose, which is indigestible; however, chewing breaks that barrier.
Chewing breaks food items into small particles, which considerably increases their surface area. Since digestive enzymes can only act on the surface, chewing improves the digestive processes a lot. Further, the grinding of food items makes them easy to swallow; chewed food items can also move more quickly through the digestive tract.
Saliva secretion helps make food moistened and thus easier to swallow.
Swallowing is a very complex task, in which pharynx is temporarily converted into the digestive tract. Once the food items go down the throat, they mostly move through the gastrointestinal tract with the help of peristalsis.
Another organ that is highly active and plays an important role in mixing food items with digestive juices is the stomach. It is a place where churning happens. A bolus of food is converted into a more readily digestible liquid called chime.
Chemical Digestion
Involves breaking down of food items into smaller organic molecules with the help of digestive enzymes like carbs to monosaccharides, proteins to amino acids, and lipids to fatty acids and glycerol.
Digestion of Carbohydrates
Grains like wheat, rice, and root vegetables like potato are the primary source of complex carbs (polysaccharides) along with table sugar (disaccharides), and milk (lactose). Some fruits may have monosaccharides like glucose or fructose. During digestion polysaccharides and disaccharides must be broken down to monosaccharides for absorption.
Digestion of carbs starts in the mouth, as saliva contains a potent digestive enzyme called amylase. It is estimated that almost 30% of complex carbs are converted to a disaccharide (maltose) due to amylase. Further, saliva also contains lysozyme, an enzyme that kills infective bacteria.
Most of the digestion of carbs happen in the stomach and intestines. A small amount of carbs is digested by gastric amylase. However, the majority of them are digested in a small intestine with the help of pancreatic amylase.
Carbs digested with the help of amylase cannot be absorbed. Thus, they go through the final breakdown from disaccharides to monosaccharides with the help of various enzymes released by the intestinal epithelium.
Digestion of Proteins
Protein digestion begins in the stomach when they interact with gastric pepsin and are broken down to smaller polypeptide chains. About 10-20% of proteins are digested in the stomach.
Stomach juices have many other functions; it secretes mucosa, proenzyme pepsinogen (chief cells), and parietal cells secrete lots of HCL and intrinsic factor required for the absorption of vitamin B12.
Food spends quite a lot of time in the stomach, about 4 to 5 hours, where food items are dissolved into HCL to form chyme. HCL also converts pepsinogen to pepsin. HCL provides a pH of 1.8, which is necessary for the working of pepsin.
Proteins are further broken down to amino acids with the help of a number of enzymes produced in liver and pancreas and released via the hepato-pancreatic duct. Pancreatic proteinases become active only in the intestine, breaking polypeptides to amino acids for further absorption.
Digestion of Lipids
The human body must digest so many kinds of nutrients. One of the problems with lipids (triglycerides, cholesterol, steroids, phospholipids, fat-soluble vitamins) is that they are almost insoluble in the water. Therefore, the first step of fat digestion is emulsification of fats with the help of bile juices produced by the liver and stored in the gall bladder.
Emulsification results in the breakdown of fats into small particles that can be readily digested with the help of lipase. About 10% of lipase comes with saliva. However, most of it comes from pancreatic juices. The stomach also produces gastric lipase, which plays its part in lipid digestion.
Final products of lipid digestion are free fatty acids, glycerol, phospholipids, and some cholesterol.
Vitamins and Minerals
It is important to understand that they do not require much digestion. Nonetheless, the digestion process makes them readily absorbable. Further, some vitamins and minerals may require active transportation, that is they must combine with a specific molecule in the intestine to get transported into the body.
Absorption of Nutrients
Digested nutrients must pass through the intestinal wall into the blood and lymph. However, it is not a straightforward process. Only a small amount of nutrients is absorbed passively, that is they diffuse through the cell membrane due to the concentration gradient. Some glucose, amino acids, and electrolytes may get absorbed passively.
Most nutrients will still need active transportation. Thus, the majority of glucose and amino acids are absorbed with the help of carrier proteins.
Fatty acids and glycerol have even more complex mechanism of absorption. They are first incorporated into micelles, which are absorbed into the intestinal wall (intestinal villi, finger-like structures, play an important role in it). Inside the intestinal wall, they are coated with protein to form globules called chylomicrons. These Chylomicrons are ultimately transported to lymph vessels and then released to blood for further processing by various organs and tissues.
Defecation
Solidification of waste material mainly occurs in the large intestine; from where they are pushed to the rectum. The final process of digestion that is excretion is a voluntary process.
The Role of Digestive Organs – Step-by-Step
Mouth. Is mainly involved in mastication, but saliva has some enzymes helping in digestion. Further, it ensures that food it moistened and infectious agents are removed.
Next, food goes to the stomach via a long pipe called esophagus. Food does not stay there, and its only role is the transfer of food from the mouth to the stomach.
Stomach. It a place where food stays for several hours and serious task of digestion start happening here. Gastric juices start digesting carbs, lipids, and proteins, and they also play a role in the absorption of micronutrients. The stomach can be regarded as an organ of pre-processing — a place where nutrients are broken for further digestion in the intestine. The stomach also acts as a place of temporary storage of food, and it slowly empties its content into the intestine, thus promoting adequate digestion.
Small intestine. It is the place where the final stages of digestion happen. It gets most enzymes from the pancreas, and another essential source of digestive juices is bile (produced in the liver). However, some essential enzymes are also secreted by the intestinal wall. These three sources of digestive enzymes ensure that everything gets digested to small molecules ready for absorption.
Most of the absorption of nutrient occurs in the small intestine, especially the distal part of it.
Colon or large intestine. It is mainly responsible for the absorption of the water and certain micronutrients. The colon is rich in the intestinal flora; these bacteria produce some vitamins and other essential chemicals.
Once absorbed, most of the nutrients sent to the liver via portal vein for pre-processing (except fats). The liver is rightly called the chemical factory of the body. It removes all the unnecessary things absorbed from the intestine and ensures that fully processed and safe nutrients enter the bloodstream.
Role of Other Organs in Digestion
In the human body, nothing happens in isolation. Every organ and tissue try to fulfill its function and also help other organs to work better.
Heart ensures that there are adequate blood supply and optimal transportation of nutrients through the blood movement.
Endocrine organs help regulate gut activity; they also ensure that various compounds, enzymes, are available when they are required most.
Skin ensures that it produces enough of vitamin D, which is necessary for the absorption of calcium.
Lymphatic tissues transport lipids, and they also defend against the attack of various pathogens that try to enter the body through the digestive tract, and not only.
Skeletal muscles help support the digestive tract and play a role in defecation.
The nervous system regulates the movement of gut and secretion of digestive juices.
Similarly, respiratory system, skeletal system, and even urinary tract play its role. This explains why if one organ is affected by some disease condition, the whole body suffers.
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