Bowel obstruction

Bowel obstruction is when the normal flow of contents moving through the intestines is interrupted.

The causes of bowel obstruction can be either mechanical or functional, also called ileus.

Mechanical obstruction is caused by actual blockages in the large or small intestine, and it can be defined as partial or complete.

Partial obstruction is when gas or liquid stool can pass through the point of narrowing, while complete obstruction is when nothing can pass.

Functional causes disrupt peristalsis, which are the waves of contraction that move through the smooth muscles of the bowel wall that pushes food through the intestines.

The small and large intestines are tube-shaped structures through which chyme, or food that has been partly digested by the stomach, and stools pass until they’re excreted.

Now if we zoom into a cross-section of the intestinal wall, it’s lined by four layers of tissue: First, there’s the adventitia, or serosa; which is the outermost layer that faces the abdominal or peritoneal cavity. This is the space between the abdominal wall and the abdominal organs, and it’s lined by peritoneal membranes that contains a thin film of serous fluid.

Moving on, there’s the muscularis externa, which is smooth muscle that contract to move food through the bowel.

Deep beneath this layer is the submucosa, which has connective tissue as well as glands, blood and lymph vessels that supply the intestinal wall.

And finally, the innermost layer is the mucosa and it’s composed of a few of its own layers: the muscularis mucosae, which has smooth muscle, the lamina propria, which is rich with blood and lymph vessels, and the innermost layer which is the epithelial lining that faces the lumen.

Okay, so let’s go over some mechanical causes for bowel obstruction.

The most common cause in the small intestine is postoperative adhesions.

After a surgery, the scar tissue that forms during the healing process can form fibrous bands that cause organs to attach to the surgical site or to other organs, causing the lumen of the bowel to get kinked or pinched tight in certain spots.

Another cause of small intestinal obstruction is hernias, and they can occur when a portion of the bowel protrudes out of the abdominal cavity and can get trapped or tightly pinched at the point where it pokes out.

Mechanical causes for large bowel obstructions, on the other hand, are most often due to a volvulus, which is when a loop of intestine twists upon itself, kinking off the lumen.

Sometimes the volvulus can occur around a mass like in colorectal cancer.

Some mechanical causes of both small and large bowel obstruction include inflammatory bowel disease which can cause strictures and adhesions; ingestion of a foreign body, which can get lodged along the gastrointestinal tract; and intussusception, which is where a part of the intestine folds into the lumen of an adjacent section of bowel, kind of like retracting a telescope. This is the most common cause of bowel obstruction in children under the age of 2.

Now, regarding functional obstruction, causes include anything that decreases smooth muscle contractility.

The most common one is postoperative ileus, which is transient paralysis of the smooth muscles in the intestinal wall, and it’s usually caused by trauma during surgery.

Other causes include infection or inflammation, such as appendicitis or peritonitis, hypothyroidism, meaning the thyroid gland does not produce enough thyroid hormones, and electrolyte abnormalities like hypokalemia, meaning low potassium in the blood, or hypercalcemia which is high calcium in the blood, or various medications such as opioids.

So, when there's a bowel obstruction, whatever the cause, the bowel contents distal to the obstruction get passed; but after that happens, proximal to the obstruction, gas and stool start to accumulate, causing the bowel to dilate, and therefore, the overall abdominal cavity to distend.

Over time, all this gas and stool causes pressure inside the bowel lumen to increase, so the intestinal contents push towards the intestinal wall, compressing the mucosal blood and lymphatic vessels.

Since the walls of veins and lymphatics are weaker and easier to compress compared to arteries, venous and lymphatic drainage are the first ones to get blocked.

The pressure pushes the water in these vessels into the surrounding tissue, leading to mucosal edema.

If pressure inside the lumen gets even higher, it also compresses mucosal arteries, leading to ischemia or reduced blood flow to the intestinal wall.

In turn, ischemia causes hypoxia, or low oxygen supply.

At the cellular level, this is accompanied by the production of reactive oxygen species; which can damage DNA, RNA, and proteins of the cells in the epithelial layer and lamina propria of the mucosa, leading to cell death, or mucosal infarction.

So, when the mucosa becomes damaged and capillary blood vessels in the lamina propria rupture, blood enters the bowel lumen.

All this stool and blood in the lumen becomes a nutritious feast for bacteria that normally reside in the intestines, and they start growing out of control.

These bacteria can then get into the intestinal wall, where they get attacked by macrophages rushing into the mucosa.

These macrophages then release inflammatory cytokines like tumor necrosis factor-alpha, which cause blood vessels to become more permeable to fluid and to more immune cells, further increasing mucosal edema, inflammation, and damage.

The overall result is the compromised ability of the mucosa to absorb food and water, which may lead to dehydration and loss of electrolytes, like sodium, potassium and chloride.

Now, as all these lumen contents continues to build up, intraluminal pressure rises even higher, making the problem even worse if not corrected.