The basic process of digestion comprises breaking down pieces of animals and vegetables so that they can be transformed into pieces of human body. This may seem facetious but that is the essence of digestion.
Simply put, the digestive system is a portal for nutrients from the environment to gain access to the circulatory system. Before the transfer can occur, foodstuff first has to be reduced to very simple molecules by a combination of mechanical and enzymatic degradation. The resulting sugars, amino acids, fatty acids and the like are then transported across the epithelium lining the intestine into the bloodstream. The focus of this section is to examine the ‘big picture’ of digestive physiology and to look at fundamental aspects of the digestive system.
Consider for a moment a plate of biryani. The purpose of eating biryani, other than simple hedonism, is to assimilate the nutrients it contains and to make them available to build, repair and maintain your own tissues, as well as provide energy for daily activities.
You may ask yourself what nutrients are present in a biryani that the body can assimilate? The body has to provide the means to carefully break them down into much smaller molecules that can be imported into blood. Luckily, your digestive system takes care of this very complex process so efficiently that most of the time you don’t even need to think about it.
At its simplest, the digestive system is a tube running from mouth to anus. This tube is like an assembly line, or more accurately, a disassembly line. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed through the walls of the tube, and into the circulatory system for dissemination.
The breakdown of food, like biryani, is accomplished through a combination of mechanical and enzymatic processes. To accomplish this breakdown, the digestive tube requires considerable assistance from accessory digestive organs such as the salivary glands, liver and pancreas, which dump their secretions into the tube. The name ‘accessory’ should not be taken to mean dispensable; indeed, without pancreatic enzymes you would find it very difficult to live.
In many ways, the digestive system can be thought of as a well-run factory in which a large number of complex tasks are performed.
Each part of the digestive tube performs at least some of these tasks, and different regions of the tube have unique and important specializations. Like any well-run factory, proper function of the digestive system requires robust control systems. These control systems must facilitate communication among different sections of the digestive tract (i.e., control on the factory floor), and between the digestive tract and the brain (i.e., between workers and management).
Control of digestive function is achieved through a combination of electrical and hormonal messages, which originate either within the digestive systems own nervous and endocrine systems, or from the central nervous system and endocrine organs such as the adrenal gland. Different parts of these systems are constantly talking to one another. The basic messages are along the lines of I’ve just received an extraordinary load of food; so I suggest you get prepared (stomach to large intestine) or For goodness sake, please slow down until I can catch up with what you’ve already given me (small intestine to stomach).
The breakdown of complex food matter into its simplest parts—amino acids, glucose, free fatty acids and glycerol—is mainly accomplished in the gastrointestinal tract by enzymes (chemicals that degrade complex molecules). These simple building blocks thus produced are absorbed into the bloodstream for delivery to whichever cell in the body needs them. Once this is done, the cells have the ability to use these building blocks to construct the proteins, fats and carbohydrates needed to stay in business. And just as importantly, the fatty acids and sugars can also be burned by the body to provide energy to do all the construction.
To give a simple analogy—it is rare these days for the wealthy to walk around with all their money in their pockets, or stuff their cash inside their matresses. Instead, surplus wealth is stored elsewhere, in forms more complex than cash—mutual funds, government bonds, and offshore bank accounts! In the same way, surplus energy is not kept in the body’s form of cash (amino acids, glucose, fatty acids) but stored in more complex forms.
Enzymes in the fat cells can combine fatty acids and glycerol to form triglycerides. Accumulate enough of these in the fat cells and you grow fat. Meanwhile, enzymes in cells throughout the body can cause succession of molecules of glucose to stick together. These long chains, sometimes thousands of glucose molecules, are called glycogen. Most glycogen formation occurs in our muscles and liver. Similarly, the enzymes in cells throughout the body can combine long strings of amino acids and turn them into proteins.
The hormone that stimulates the transport and storage of these building blocks into target cells is insulin. In a sense, insulin plans for our metabolic future. Eat a big lunch and insulin pours out of the pancreas into the bloodstream, stimulating the transport of fatty acid to fat cells, stimulating glycogen and protein synthesis. To continue with our finance analogy, insulin fills out the deposit slips at our fat banks. It turns out that we even secrete insulin when we are about to fill our bloodstream with all those nutritive building blocks. If you eat dinner each day at 8 p.m., by 7:45 p.m. the parasympathetic nervous system is already stimulating insulin secretion in anticipation.