Small intestine and large intestine: Functions of these two organs

Heather Campbell
 min read

Among all the surfaces of your body that somehow interact with the outside world, our small intestine and large intestine in the belly are the most important.

Small intestine and large intestine: Functions of these two organsDid you know that the small and large intestine really have a mind of their own?

As a whole, the small and large intestine have two separate functions which are completely complementary. The small intestine breaks down your food, while the large intestine solidifies what’s not needed for it to leave the body as a toxin.

Read on to learn more about how these two organs work together and whether it’s true that they work independently of the brain…

Small intestine and large intestine: Introduction

The small intestine and colon form the largest interface between the outside and inside of our body.

The side exposed to everything that enters our mouth and progresses through our digestive tract to the anus is called external. This is where the bacteria live.

On the other hand, the side in contact with the inside of our body is called internal and it is forbidden for bacteria to access it. So when bacteria get there anyway, it can create serious pathological situations.

So your abdomen and your belly have an anatomical peculiarity that deserves to be highlighted.

Indeed, it is the only space of your body whose front face, delimited by a membrane, called the peritoneum, and a musculature called abdominal, is not protected by any bone structure.

Small intestine and large intestine (colon)

To experience them from the inside, we know that these two organs, usually grouped under the generic term “intestine”, are very different.

Certainly they are both a welcoming habitat for bacteria, a protective barrier and an entry for food.

But they are also different partners in their participation in several physiological activities necessary for our body’s proper functioning and vitality.

The small intestine in detail

The small intestine is a long tube that forms multiple folds. Food bolus passes through it rather quickly and only stays there for a few hours.

The small intestine contains a relatively acidic medium (like vinegar), it is constantly filled with bile that emulsifies fats (to form a kind of mayonnaise-like product) and gluttonous enzymes that break down what you have eaten.

Lined with specialized cells, it has a high absorption capacity, which greatly reduces the volume of the food bolus as it passes through it.

With the oral cavity and the stomach, it forms the main sites of digestion. The small intestine is directly connected to several internal organs, including the liver (which provides bile) and the pancreas (which provides digestive enzymes).

Related postHow does the human digestive system work?

The colon in detail

Our colon, or large intestine, is an extension of our small intestine, but it is anatomically and physiologically different.

It is much larger and shorter and takes a straighter route.

The colon begins in the right side of the lower abdomen and is divided into 4 segments:

  • A straight or ascending segment that goes up to the waist
  • A transverse segment that fits horizontally
  • A segment descending to the left in the ventral cavity
  • The sigmoid, that ends in the rectum

Compared to the small intestine, its internal environment is different. It is less acidic, almost neutral, and its functionalities are more numerous, more varied and specific.

Its contribution to digestion is limited, which explains why its bordering cells have a low absorption capacity.

The favorite habitat of certain bacteria

Like all the other organs (skin, vagina, oral cavity, upper respiratory tract, etc.) that delimit our inner space, our small intestine and colon are covered, on their external surface, with a specific tissue called the epithelium, the major element of their mucous membrane.

Similar to a tile floor, the cells are aligned in a single layer and bonded to each other to ensure maximum watertightness while allowing selective and controlled permeability.

The mucosa is covered with a viscous secretion called mucus on its inner side. It is the favorite habitat of a number of bacteria. 

Mainly found in our small intestine, this covering is folded to form so-called villi and microvilli, greatly increasing its surface area for absorption. 

Beyond their palisade effect, these epithelia are, especially in the colon and thanks to the presence of specialized cells, the actors of multiple functionalities to which bacteria are proud to collaborate. 

This great functional diversity allows the intestines to participate, in many ways, in the general physiology of our body.

A submucosa composed of connective tissue under these epithelia houses our immune cells. Two layers of smooth muscle (a circular and a longitudinal layer) complete this tissue.

By successive contractions, they ensure the kneading, the mixing with the secretions, and the food bolus progression and the propulsion of the feces until their excretion.

Bacteria are more numerous in the colon

In our small intestine, bacteria have little time to interact with the food bolus because of the rapid transit.

On the other hand, in our colon a slow propulsion imposes a prolonged stay to what remains of the initial food bolus. 

The contact time of the bacteria with it is therefore longer, which offers more opportunities for the bacteria to act to transform it but also to feed on it.

This is why bacteria are much more numerous. Overall, it takes a few days for our food bowl to go from mouth to anus, but most of this time is spent in the terminal part of the digestive tract.

Although identified as external, these two organs are not permanently open to the outside world.

In the absence of food bolus, and outside the digestion phase, they are closed by muscles, called valve or sphincter.

Their contents and therefore the bacteria that inhabit them, are isolated from the outside world. 

Diaphragm and abdominal muscles provide a massage

Our small intestine and colon are located under the diaphragm. This muscle, shaped like a parachute, separates our body’s upper and lower compartments.

Together with the abdominal muscles, they give rhythm to the breathing in and out, they cradle and massage the small intestine and the colon continuously.

For too long, the colon has been considered as a bag with little function, a simple receptacle for the waste products of a noble but unfinished digestion in the upper part of the digestive tract.

For a long time, its role has been reduced to a few minor functions such as the capacity to recover water and some minerals as well as a small part of the energy of the predigested food bolus and then to eliminate the rest.

Such marginalization is not justified because it ignores the roles of bacteria in our physiology.

The colon is an important organ of our body and one of the most multifunctional. Bacteria play many roles that contribute to our vitality.

The enteric nervous system

Like all your organs and tissues, our intestine and colon are subject to nervous regulation (stimulation or inhibition).

They are connected to our brain through a nerve, called the vagus nerve.

This channel transmits information, mainly from the intestines to the brain and, occasionally, from the brain to the intestines.

Several molecular signals (hormonal or inflammatory) carried by the blood complete these interconnections.

In this reciprocal communication, a third partner intervenes, with which they are closely associated, a local nervous system, called intrinsic. 

This is called the enteric nervous system, it would be the survivor of a primitive nervous system, present in all your vertebrate predecessors. 

The production of neurotransmitters

Its cells produce the same neurotransmitters as all other neurons, among them, serotonin deserves a special mention. 

Indeed, +/- 85% of the total body quantity is produced there and then distributed everywhere, including in the brain.

It is also an essential component of the signaling system between the intestines and the brain.

Serotonin is known as the happiness hormone and plays an important role in changes in emotional state, control of sleep or appetite, and sensitivity to pain.

Enteric nervous system as a data processing center

This enteric nervous system always accompanies us, imposing itself as an essential component of our physiology to meet the specific needs of all the functions of our belly. 

Its reflex activities allow the propulsion of our food without the intervention from our cranial brain (via the spinal cord).

Therefore, its operation does not require permanent recourse to external instructions. It is not the slave of our brain but a rebel who has chosen and assumes its independence, even if the voice of our brain is heard.

Our enteric nervous system has the capacity to send several hundred messages to the brain, which sends only a few. It is a true local data processing center.

It can thus perform its important, and sometimes unpleasant, functions without mental effort.

For the great diversity of nutrients that we consume and feed our body, it is important that the digestive tract mixes them, kneads them, propels them, breaks them down, hydrolyzes them, absorbs them, eliminates the waste…

All these activities must also be controlled, coordinated and regulated to be optimized.

The stomach is endocrine

Our tens of millions of enteric neurons are not only the controllers/regulators of the activities of our belly.

They feel and exchange messages with immune cells and those that produce certain hormones and with certain organs such as our pancreas and gallbladder.

As the main link between the outside world and the inside of the body, our stomach must also protect and defend us against microbiological and chemical dangers.

To this end, the belly has developed a series of strategies that must also be followed, monitored, controlled and regulated.

Our belly is endocrine. The hormones it produces establish essential links with the rest of the body.

All this could not be done, in harmony, if there were not, in our belly, sensor relays that allow following, permanently, the progress of all these activities and evaluating, at any moment, their operational environment.

For all these reasons it is understandable that evolution has preserved, in the digestive tract, a largely autonomous regulatory center. The only form it could take in our body is this interconnected set of neurons.

Since no other organ in our body has to manage permanently and so quickly such a complexity of information and situations requiring such an impressive array of controls and regulations, it is logical that only our digestive tract is similarly equipped.

Bacteria are neither insensitive nor inactive. This is especially true for bacteria that live in close contact with the surface of our intestines.

Related postWhat can cause an imbalance in gut flora populations?

Small intestine and large intestine: Conclusion

Bacteria inhabit our small intestine and colon, two structurally and functionally different organs that together make up the stomach.

Located in the abdomen, this part of our body is not protected by any bone wall, making it sensitive to touch and even audible.

Like all our organs in contact with the outside world, our small intestine and colon are lined by an epithelium that forms a protective barrier and constitutes a selective absorption surface.

They are also the seat of physiological activities, particularly hormonal, which are essential to the proper functioning of our entire body.

Together with the underlying connective tissue, this epithelium forms a mucous membrane in which the majority of our immune cells as well as the cells of a nervous system are located.

The diaphragm, this muscle, which gives rhythm to the movements of the breath, cradles and massages the small intestine and the colon and therefore the bacteria that inhabit them.

Like all organs, those of our belly are connected to the brain through a nerve, called the vagus nerve. But this nervous control is not the only one.

It is completed by molecular signals produced by our intestines and carried by the blood and by a specific but largely autonomous nervous system, called the enteric nervous system.

It ensures the numerous controls and regulations without the intervention of the brain.

About Heather Campbell

As a dietitian, my field of specialization is science-based nutritional advice but more importantly, it is my goal to share capturing and inspiring stories, examples and solutions which can help plus-size individuals overcome their specific difficulties. Read More