The alimentary canal, also known as the gastrointestinal tract has two purposes
- break down food into its basic components, known as digestion and
- to absorb these basic components into the body
The process of digestion involves breaking down the food we eat in two main ways.
Mechanical breakdown. Food is broken up into smaller and smaller physical pieces. The main tool for doing so is our teeth, but the contraction of muscles along the alimentary tract also aids in mechanical breakdown.
Chemical breakdown sees the macronutrients of protein, carbohydrate and fat broken down from their original, complex molecular structures into their chemical basic components.
Chemical breakdown/digestion takes place in all major parts of the alimentary canal such as the mouth, small intestine and stomach, but not in the large intestine and colon.
However the only macronutrient that is significantly digested in the mouth is starch, which is a form of carbohydrate.
Protein is mainly digested in the stomach, which also has a minor role in fat digestion.
Most chemical digestion occurs in the small intestines, particularly the upper part. Here enzymes that are produced by the pancreas and intestinal cells go to work.
Any remaining undigested food will be broken down in the large intestine, especially dietary fibre. A large variety and quantity of bacteria reside in the lower intestine to assist with this.
A variety of cells along the alimentary canal produce digestive enzymes that assist with chemical breakdown. These digestive enzymes are known as catalysts.
What do catalysts do?
Simply put, for a chemical reaction to take place, it must reach a certain level of energy. Catalysts lower this energy barrier. Think of reducing the height of a wall to make it easier to jump over.
Reactions that take place using a catalyst comprise the process known as hydrolysis. In a hydrolysis reaction a chemical bond is broken via the addition of water to the reaction.
Proteins become amino acids, carbohydrates mainly become glucose and fats are broken down into fatty acids.
Once this process of digestion is complete, these components are absorbed into intestinal cells and then distributed throughout the body in a process called absorption.
The main role of the alimentary canal is to allow nutrients to be absorbed into the body, following the process of mechanical and chemical digestion, assisted by digestive enzymes.
Structure of the Alimentary Canal
Salivary glands in the mouth add moisture to the food. The primary role of this moisture is to lubricate the food and allow easy passage of it through the esophagus. However it also plays several other important roles. It starts the chemical digestion of food, helps us taste the food and even has anti bacterial/microbial properties.
The presence of food in the mouth causes saliva to be produced, but even just smelling or thinking about food can cause saliva production.
Salivary glands are located beneath the tongue, in the floor of the mouth and even up by the ears. The action of chewing with the mouth and teeth mixes saliva in with the food. Food is 99% water, and 1% enzymes, salts, mucus and anti bacterial compounds.
By swallowing, food is moved out of the mouth and into the esophagus. This long tube transports food from the mouth to the stomach. To ensure the esophagus stays closed when food is not being swallowed, a muscular ring called the upper sphincter contracts.
At the other end of the esophagus is the lower or esophageal sphincter, which prevents food from travelling back up from the stomach. The lower sphincter also relaxes and opens when we swallow to allow food to pass through it.
To ensure food only enters the esophagus and not the windpipe leading to the lungs, a leaf shaped flap called the epiglottis covers the entrance to the trachea, called the larynx.
Food does not just fall down the esophagus. The walls of the esophagus are muscular and they contract and relax to propel the food down, in a process called peristalsis.
As a result of this peristalsis, food arrives in the stomach. The stomach is essentially a hollow bag which in the average person has a volume of roughly two ounces when relaxed. However it can expand to over a litre after a large meal.
The main role of the stomach is protein digestion. A unique feature of the stomach is its high acidity. The stomach walls produce hydrochloric acid. Food remains in the stomach for a few minutes up to several hours. It is stopped from progressing any further by the pyloric sphincter, another muscular ring.
Once food leaves the stomach, it enters the small intestine. We can look at the small intestine in three parts.
The upper part of the small intestine is known as the duodenum. It’s a tube about 30 cm long and leads into the jejunum. The duodenum is where the digestive juices produced by the pancreas and bile from the liver enter the alimentary canal.
These digestive juices contain a lot of bicarbonate to neutralise the acidity in the food mass, which it picked up during its time in the stomach. This allows the process of digestion to continue.
The duodenum is in charge of regulating how quickly food exits the stomach and intro the small intestine. It does this by producing the hormones secretin and cholecystokinin. This hormone production is triggered by food entering the duodenum.
These two hormones have other functions too, including causing the pancreas to release bicarbonate and digestive enzymes. The pancreas is located behind the stomach and produces several digestive enzymes called lipases, amylases and proteases.
The duodenum leads into the second part of the small intestine, called the jejunum. It is the major site of nutrient digestion. Here food is broken down into the basic chemical components which can be absorbed by the body. The cells which make up the intestinal tract, known as enterocytes produce several different enzymes, such as disaccharidases and peptidases to aid in the break down of food into its components chemicals.
Most the nutrient absorption takes place in the third and final part of the small intestine, the ileum.
To maximise the effectiveness of the intestine, it has several key features to maximise its surface area. Firstly it has many folds and creases. Additionally, tiny finger like projections called villi stick out into the intestinal path. The fingers have their own blood supply and work to absorb desired nutrients. Microvilli and even smaller and stick out from the enterocyte cells lining the small intestine.
Food moves out of the ileum and small intestine, into the large intestine or colon. The colon exists to remove water from the remaining indigestible food mass. It also provides a way for the body to excrete unwanted faecal waste.
Large numbers of bacteria, known as gut microbiota are present in the colon. These ferment and further break down dietary fibre.
The composition of this microbiota, both in terms of variety and effectiveness, is highly dependant on the type and quality of a persons diet.
By now the body has extracted and absorbed any nutrients from the food. What’s left in the colon is waste faecal matter. This collects in the colon and rectum, staying here for one to two days, and is released during a bowel movement.