Let’s talk shop! Anatomy and Physiology for beginners…
The Colon, Rectum and Anus are three separate entities. So when people come in and tell me they have something in their rectum, I usually assume they are talking about their anus. Why does it matter to know the difference? Well the rectum and the anus are made up of different cell types. These cell types make a huge difference when diagnosing patients with different conditions.
Colon: The colon is your large intestine. You’ve probably seen Dr. Oz on television with a cadaver colon scaring you into getting a colonoscopy. The colon is connected to your small bowel in the right lower quadrant of your abdomen. The first part is called the cecum and your appendix is a small finger-like projection that comes right off of it. The cecum becomes the ascending colon (like ascending towards the right upper part of the abdomen) then the transverse colon, descending colon and finally the sigmoid colon in the left lower quadrant of the abdomen.
Why do you need a colon? Well, the colon is the place where your poop gets packed up and processed for delivery into the rectum. Fluid and electrolytes are absorbed and some nutrients as well. You don’t need a colon to live, but without it, people have frequent, loose bowel movements which is obviously no good.
Rectum: When the sigmoid ends, the muscle fibers come together and form the rectum. It is about 15-20 cm in length and sits in the pelvis. The rectum acts like a large storage area for stool. When you are ready to have a bowel movement, the pelvic floor and rectum have a coordinated ‘dance” to ensure that poop is expelled. The rectal muscle fibers will eventually become part of the sphincter complex that controls continence.
Anus: The anus starts at a place called the Dentate line. If you take a look inside the rectum there are folds starting at about 3 cm from the edge of the opening of your anus. These folds look like teeth, hence the name “dentate.” The cell type changes at this point. Also, this is the point in your digestive system that pain is appreciated. The colon and rectum have stretch receptors, not pain receptors. So when distended with gas or stool, the stretch receptors are activated and the sensation of feeling “gassy” is appreciated. In the anus, however, there are pain sensors. When your doctor tells you hemorrhoid surgery is super painful, believe them!
The anus is the area where a number of disease processes occur, all of which will be discussed in upcoming posts.
Anal Sphincters: You have two sphincters. An internal sphincter that is a circular smooth muscle. At rest, the sphincter is contracted. This prevents you from stool leakage. The External sphincter is the one you can control. It is part of the pelvic floor and contracts when you have the urge to have a bowel movement but need to hold it. When the rectum fills with stool, your body will initiate the defecation reflex. Your rectum has the amazing ability to distinguish stool from gas! When its stool, the internal sphincter relaxes but you will squeeze the external so you can make it to the restroom.
Speaking of stool….
One of the best works of literature, in my opinion, is the book, “Everyone Poops” by Minna Unchi. Now this is a children’s book but it made talking about poop not so weird or embarrassing. But alas, many of my patients have not read the book. When I started in practice it was hard to get people to describe their poop. Some were embarrassed, others couldn’t put it into words. So one day I went online and ordered a giant poop chart. It is called the Bristol stool chart. I make everyone point to a number when I see them. Saves me time and makes people aware of what is normal and what is not and how I can help fix them. I put it in the book because a lot of these anorectal problems are treated by achieving the perfect poop (its number 4 on the chart); not too hard, not too soft… the “goldilocks” of shit, if you will.
Stool is basically food digested in the stomach and small intestine that is compacted in the large intestine for delivery to your rectum for expulsion. Stool quality changes depending on your state of hydration and diet. Bristol 1-3 usually happen in patients who aren’t hydrating well , have low fiber diets, or are on medication that can slow transit time through the colon. Bristol 5-7 are usually due to low fiber or irritants (spicy or acidic food) and even irritable bowel syndrome. Food is usually what alters stool quality. A healthy person should consume about 25-35 grams of fiber daily along with 64oz of water. Unfortunately, we in the western world don’t get enough fiber in our daily diet. I really think if everyone took fiber supplements daily, I would be out of a job.