The Great Physiology Of The Colon

Today we will talk about the physiology of the colon. However, we will only cover the last part of the digestive tract. This part includes the following sections: cecum, colon, rectum and the anal canal. Together they make up the widest and shortest part of the digestive tract.

The first part of the large intestine is responsible for absorbing water and electrolytes. Then the last part of the colon stores stool until its removal.

The aforementioned functions do not require that the movements of the colon be very intense. At least they are not as intense as in the earlier parts of the digestive tract. In fact, the movements are slow and gentle. The movements of the large intestine are therefore similar to the movements of the small intestine.

peristalsis

This term describes the contraction movements of the digestive tract. This squeezing movement aims to move the stool to the anus. In other words, these movements are propulsive movements of the intestines.

The physiology of the colon: bowel movements

Identical to what happens in the small intestine, the movement of bowel movements can be broken down into mixed and propulsive movements.

• The mixed movements are a combined contraction of the circular muscle and longitudinal muscle layer of the colon. This pushes out the unstimulated portion of the colon in a bag-like shape called a “haustra.”

A few minutes later, the process repeats a little further on, pushing the stool further into the colon. The faeces are, as it were, ‘milked’. Pushing in this way keeps all the stool touching the intestinal wall, facilitating the absorption of electrolytes.

• The propulsion movements depend on the ‘mass movements’. These movements are a modified form of peristalsis that unite the movements of the colon. They push the stool forward. Mass movements take place three times a day and last thirty minutes each time.

How do these movements start?

The mass movements respond in response to the stretching of the stomach and duodenum (gastrocolic and duodenal reflex). At other times, they may respond to irritation. This is the case, for example, in patients with ulcerative colitis.

The role of the ileocaecal valve

The ileocecal valve prevents chyme (food mash) from flowing back into the ileum (the ileum) once it has reached the colon. The contractions of the ileocaecal sphincter control the chyme. In addition, the cecum reflexes control peristalsis in the ileum to prevent backflow. When the walls of the cecum expand, it sends out signals that increase sphincter contraction and inhibit intestinal peristalsis.

What happens if these processes change?

Generally:

• excess intestinal motility leads to a reduction in substance absorption. The result is diarrhea, or soft stools.

• a defect in gut motility causes an increase in absorption. As a result, hard stools form, causing constipation later on.

The physiology of the colon: defecation reflex

The defecation reflex causes the expulsion of stool:

• Intrinsic reflex, powered by the enteric nervous system (although it itself is too weak).
• Parasympathetic reflex, propelled by the pelvic nerve fibers and acting as a reinforcement.

How does it happen?

When feces reach the rectum, its walls expand, sending afferent signals through the myenteric plexus. In response to the signals, peristaltic waves flow from the colon downward toward the rectum. By doing this, they push stool out of the anus.

The myenteric plexus transmits inhibitor signals that relax the internal anal sphincter. As a result, the stool continues the moment the peristaltic wave reaches the anus. The external anal sphincter is consciously relaxed.

When stimulation hits the nerve fibers of the anus, they send afferent signals to the spinal cord. The signals are transmitted through the parasympathetic nerve fibers of the pelvic nerves, enhancing peristalsis and relaxing the internal anal sphincter.

Physiology of the colon: secretion

Which substances are secreted?

In the colon, only one type of mucus forms the secretion containing controlled amounts of bicarbonate ions (pH > 8). The mucous cells in the digestive wall and crypts of Lieberkühn (intestinal glands) secrete this mucus. Different epithelial cells secrete the bicarbonate. The secretion itself is responsible for the alkaline pH of the mucus.

How is the slime made?

The mucus secretion is formed by the direct stimulation of the mucous cells. However, this also increases the response to a stimulation of the pelvic nerves (parasympathetic innervation).

What functions does mucous secretion have?

The partition has three functions:

• It protects the intestinal wall from possible damage and from fecal acids (mucus pH is > 8 due to the bicarbonate ions).
• Holds the stool together.
• Protects the gut from bacterial activity.

The physiology of the colon: absorption

The large intestine receives about 1500 ml of chyme each day. Most of the water and electrolytes contained in this chyme are absorbed in the first part of the large intestine. As a result, the eliminated stool contains only about 100 ml of water and somewhere between 1 and 5 mEq of sodium and chlorine ions.

How does the large intestine absorb substances?

The physiology of the colon allows for the absorption of sodium through active transport through the sodium-hydrogen exchange. Due to the electrical gradient, some chlorine ions move passively inside the cells. The rest of the chlorine ions are absorbed in exchange for bicarbonate ions.

The gut also uses active transport to absorb potassium, as well as other ions such as calcium or magnesium.

The connections between the cells of the colon are much narrower than those in other parts of the digestive tract. As a result, they prevent back diffusion of the ions and thus allow more sodium absorption.

Aldosterone (a steroid hormone) is an important aid in the absorption of sodium. The resulting concentration gradient allows water to be absorbed by osmosis.

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