Throughout the digestive process some organs play a more hands-on role than others. Though this doesn’t necessarily mean the others aren’t as important, it’s just these organs have a more direct role in the digestive process. And this section examines these various organs and discusses the roles they have.
Our journey stats at the mouth. The first break down occurs when we chew, and when we chew, several things happen simultaneously. The epiglottis flap (located at the back of the throat) closes and prevents food from entering the breathing pathway, the teeth physically breakdown the food, and the tongue mixes an enzyme containing saliva that initiates the nutrition extraction process.
The most abundant enzyme in saliva is amylase. This important enzyme is responsible for the breakdown of starches. Thus, the breakdown of starches is among the first processes to start and starts almost instantly upon ingestion.
From the mouth, the food now passes into the esophagus. The esophagus is about a 10 inch passage that connects the mouth to the stomach. In this passage, mucous is secreted to coat the food and prepare it for the stomach. A very important component of this part of the process is a constriction of the upper and lower esophageal sphincters. This ensues that gastric juices from the stomach do not enter the passage. If these juices do enter the esophagus, we experience heartburn (gastric reflux or GERD).
Total time that food spends in the esophagus is fairly short. Typically it takes food content only 5-6 seconds to travel through the passage and make it’s way into the stomach.
As the food enters the stomach (basically a bag of muscle), gastric juice is added to the mix and the stomach begins to churn and contract. This thoroughly mixes the food with the gastric juice and creates a mixture referred to as chyme.
Gastric juice is mainly made up of water, hydrochloric acid, and an enzyme by the name of pepsin. Hydrochloric acid serves the role of preparing the protein for digestion, neutralization of pathogens, and activation of the enzymes.While, pepsin is responsible for the breakdown of protein into amino acids, which can be used by the body.
The environment in the stomach is extremely acidic and this plays an important role in killing any bacteria which may be present. The actual pH of the stomach ranges from 1.5-2.5. Without this acidity, foreign bacteria may get the upper hand and make it further into the body.
Also, it must be noted that at this stage, carbohydrate and protein digestion is well on it’s way, while only minimal digestion of fats has occurred.
The total time that food spends in the stomach is typically somewhere from 2 to 4 hours. However, this will depend on a variety of factors including the total amount of food consumed, fat content, and the current acidity of the stomach.
From the stomach, the chyme now travels into the small intestine. Once the chyme arrives there, bicarbonate is used to neutralize its acidity.Additionally, more digestive enzymes are secreted and begin further breakdown of food and extraction of nutrients.
Even though it’s called the “small” intestine, this organ is actually quite large. To be specific, the small intestine of an adult is roughly 7 meters long. In addition to it’s sheer size, it consists of a unique structure which helps to further increase the amount of surface area that comes into contact with the chyme. This is achieved through the presence of bristle like extensions called villi which cover the small intestine; to further increase surface area, these villi are covered with even smaller bristles called micro-villi. This distinct structure allows for the maximum amount of surface area to come into contact with the chyme and allows for effective nutrient extraction.
The small intestine is actually so effective at its job, that the majority of nutrient absorption occurs here. For example, it absorbs 80-90% of fluids, and the majority of vitamins, minerals, carbohydrates, proteins and fats.
On average, the food (now chyme) spends about 5-6 hours in the small intestines. However, this number will vary depending on several factors (such as fat content, total amount, caloric availability, etc).
The large intestine is basically the last step in the digestive process before emptying (defecation). Even though it is called the “large” intestine, it is actually shorter than the “small” intestine. The total length of the large intestine in an adult is on average only about 1.5 meters. However, it gets its name from its relatively large width.
Instead of depending solely on acids and enzymes for nutrient extraction and breakdown, the large intestine makes use of bacteria to do a large portion of the work. It is through this process that the rest of the available nutrients should effectively be extracted and absorbed. This includes any remaining fats, carbohydrates, vitamins, minerals (sodium, potassium and chloride), and water content.
In addition to the absorption, some vitamins and fats are actually produced by the bacteria residing here. This includes things such as vitamin K, certain B vitamins and substantial amounts of short-chain fatty acids. Unfortunately, the large intestine lacks the ability to absorb amino acids and B vitamins, so the host will only benefit if these are fermented by the bacteria to short chain fatty acids or if they can support beneficial microbial growth.
Since such a large portion of the work inside the large intestine depends on the bacteria living there, it is essential to keep these communities healthy and well diversified. That may not be a simple task, especially when you consider that the microbial density here is thought to be around 100 billion microbial cells per milliliter of content.
Towards the end of the large intestine is a section called the rectum. This is the final stage of the journey, and food is stored here until it can be safely expelled from the body.
This section examined the key organs involved in the digestive process and provided an overview of the path the food we eat travels. Key points include:
- The journey starts at the mouth with physical chewing and the secretion of enzyme rich saliva which targets starches
- The esophagus connects the mouth to the stomach, coats the food with mucous and ensures gastric juices do not escape the stomach
- In the stomach, the food is thoroughly mixed with enzyme containing gastric juices (mixture is called chyme) and protein absorption begins
- Gastric juices contain hydrochloric acid and are highly acidic (pH of 1.5-2.5) and this ensure various pathogens are effectively neutralized
- Upon arrival at the small intestine, the chyme is neutralized using bicarbonate and further enzymes are secreted to start the absorption of fats
- An adult’s small intestine is roughly 7 meters long and the majority of vitamins, minerals, carbohydrates, proteins and fats are absorbed here
- The final step, is the large intestine which is only about 1.5 meters long, has a relatively large width and is home to a large bacterial population
- Bacteria in the large intestine ferments the chyme, releasing nutrients and squeezing out the remaining water content