Basics of Human Digestion

We all have a very good idea about digestion. Let's revisit the basics. Digestion is the process of separating out the nutrients from the food and absorbing them into our body.
The human body is designed to keep up its normal functioning. In other words, the body tries to fight off sickness. Lots of under the hood activities happen inside the body to fight off sickness. Nutrients are needed for such maintenance. The body gets nutrition through the process of digestion.
Digestion happens along the digestive tract. The human digestive tract is very much a long passageway. Digestion is a two-step process.
First step is to crush the food into smallest possible particles. The aim is to expose the largest possible surface area of the food. This way the most amount of nutrition is available.
Second step is to mix crushed food with digestive enzymes. These enzymes break down the complex nutrient molecules into simpler molecules. That way nutrient molecules are easily absorbed within the digestive tract wall. Later on, nutrients are transported into the bloodstream.
The digestive tract has four main components. Mouth and throat are the first part of the digestive system. Esophagus joins the throat with the stomach. Esophagus along with stomach is the next part of the digestive tract. Small intestine, liver, pancreas and gall bladder form the hub of the digestive system.
Main digestive activities take place in the small intestine with the help of enzymes from liver, pancreas and gall bladder. Last part of the digestive tract is the large intestine. Bacteria ferment the remaining food in the large intestine.
Food is crushed and ground in the mouth with the help of teeth. Teeth are the hardest substance in the body. Strong jaw bones help crushing and grinding the food. The stomach has strong muscles, which vigorously churn and mix the food. Rhythmic muscular action - referred to as peristalsis - ensures that food keeps traveling along the digestive passageway.
As you can imagine, it is easy to crush soft and moist food compared to hard and dry food. There is either saliva or mucus present all along the digestive tract. This keeps food soft and moist.
There are different types of glands secreting different types of enzymes all along the digestive tract. Enzymes help the breakdown of protein, carbohydrates and fats in the food.
One unique feature of digestive tract is that within the stomach and small intestine the internal lining has a fold like structure which provides for extra surface for nutrient absorption.
On top of this fold like internal linings, there is hair like structure called villi. They give an extra absorption area in addition to the folds. This way digestive tract ensures that there is plenty of surface area and opportunities for nutrient absorption along the passageway.
Muscular action keeps the food moving along the tract at the proper rate. Food should not get stuck along the path. As blockage would prevent nutrient absorption. Similarly if food passes too fast, the body would miss the chance at separating most amount of nutrition.
There are control mechanisms for regulating the food flow. For example stomach can stop the pyloric valve (a valve between the stomach and small intestine) and act as a storage unit if it finds out that the small intestine is very busy with the digestive process.
You might wonder what controls various processes, such as muscle movement and enzyme release. Autonomic nervous system controls most of the muscular movements and some of the enzyme release mechanisms. This means they are pretty much involuntary. One can not wish and stop the food moving through esophagus or intestine!
The nervous system will decide what is the correct action for muscular movement. Presence or appearance of undigested food at various points along the digestive tract controls much of the enzymatic release. For example, the undigested protein in the stomach would trigger release of protein digesting enzymes in the stomach and small intestine.
Enzymes also play a key role in conveying messages to the nervous system about when stomach is full and one should stop eating more. We should eat slowly and give enough time for this messaging to take place. That way we could receive natural stomach 'full' signals and voluntarily stop eating!
As food passes along the digestive tract, it has different names. Chewed food in the mouth is called bolus. Bolus also travels through esophagus and reaches stomach. It is called chyme as it leaves the stomach. At this stage it is thoroughly mixed, partially digested and mixed with stomach acids and enzymes. By the time it reaches the end of the large intestine, it becomes feces.
Our digestive tract is a very busy system. It is essential for our survival.