The rectum is the final storage chamber of your digestive tract, holding stool until you’re ready to have a bowel movement. It sits between the sigmoid colon and the anal canal, measuring about 12 to 15 centimeters long. While that sounds simple, the rectum performs several coordinated jobs: it stores waste, senses what’s inside it, communicates with your brain, and controls when and how you release stool.
Temporary Storage
Most of digestion and water absorption happens higher up in the intestines. By the time waste reaches the rectum, very little absorption is left to do. The rectum’s primary role is acting as a holding area so you don’t need to find a bathroom the instant stool arrives from the colon.
The rectum can comfortably hold a moderate amount of material. Balloon distension studies show that most people first sense something in the rectum at around 78 milliliters of volume. The feeling of urgency, where you actively need to go, kicks in at roughly 150 milliliters. The walls of the rectum are flexible enough to stretch and accommodate stool for a period of time, which is what allows you to delay a bowel movement until it’s convenient.
A sphincter-like zone at the junction between the sigmoid colon and the rectum helps regulate when stool enters. This narrow, contractile segment acts as a gatekeeper, controlling the flow of feces downward so the rectum isn’t constantly filling.
Sensing What’s Inside
The rectum is surprisingly good at telling you what it contains. Nerve endings in the rectal wall detect stretch and pressure, which is how your body distinguishes between gas, liquid, and solid stool. This sensory ability is what lets you confidently pass gas without worrying about something else coming along with it.
Normal sensory thresholds follow a predictable range. You typically notice a first sensation at around 100 milliliters or less, feel a desire to defecate by about 200 milliliters, and reach maximum tolerance near 280 milliliters. When these thresholds are off, problems follow. People with rectal hyposensitivity (reduced feeling) need much larger volumes before they sense anything, which contributes to chronic constipation. On the other end, people with rectal hypersensitivity feel urgency at volumes below 65 milliliters, a common finding in irritable bowel syndrome.
The Defecation Reflex
When stool stretches the rectal wall enough to trigger the urge to go, a reflex kicks in automatically. The internal anal sphincter, a ring of smooth muscle you can’t consciously control, relaxes in response to rectal distension. This is called the rectoanal inhibitory reflex, and it happens without any input from your conscious mind. The relaxation allows a small sample of rectal contents to reach the upper anal canal, where additional nerve endings help confirm whether you’re dealing with gas or stool.
This reflex is triggered by surprisingly small volumes. In clinical testing, as little as 10 to 20 milliliters of air inflated in the rectum can set it off. The threshold is measured by looking for a drop in sphincter pressure of at least 5 mmHg or 15% from baseline.
Two Sphincters, Two Types of Control
The rectum works with two distinct sphincters to manage continence, and they operate on completely different systems. The internal anal sphincter is smooth muscle that stays contracted by default, creating a seal that prevents leakage around the clock, including while you sleep. It’s controlled by the autonomic nervous system, meaning it runs on autopilot. Sympathetic nerves keep it tight, while inhibitory nerves release nitric oxide and other signaling molecules to relax it when it’s time for a bowel movement.
The external anal sphincter is skeletal muscle, the same type of muscle as your biceps. You control it voluntarily. When the internal sphincter relaxes and you sense stool but aren’t near a bathroom, you squeeze the external sphincter to override the reflex and maintain continence. This voluntary squeeze generates pressures around 168 mmHg in healthy adults, roughly two and a half times the resting pressure of the internal sphincter alone (about 65 mmHg). That difference gives you a significant margin of safety for holding on when you need to.
How the Rectum Absorbs Medications
The rectum plays only a minor role in absorbing water and electrolytes from digestive contents. But it turns out to be a useful route for delivering medications, and the anatomy explains why.
The rectum has a dual blood supply. The lower portion drains into veins that flow directly into the general circulation, bypassing the liver entirely. The upper portion drains into the portal vein, which routes blood through the liver first. As a rough estimate, about 50% of a drug absorbed from the rectum avoids the liver’s filtering process. This matters because many oral medications lose a significant portion of their potency when the liver processes them before they reach the rest of the body. Rectal delivery sidesteps much of that loss.
Drug molecules cross the rectal lining mainly by diffusing directly through cells rather than squeezing between them. This makes the rectum effective for absorbing certain medications in situations where swallowing a pill isn’t possible, such as during nausea, seizures, or when a patient is unconscious.
The Rectal Wall Structure
The rectal wall has five distinct layers, starting from the inside: the mucosa (the inner lining that contacts stool), the deep mucosa, the submucosa (which contains blood vessels and nerves), the muscularis propria (the muscle layer responsible for contractions), and an outer covering called the serosa. The muscle layer is arranged in two sheets: an inner circular layer that narrows the tube when it contracts, and an outer longitudinal layer that shortens it. Together, these layers coordinate the squeezing motions that push stool toward the anal canal during a bowel movement.
This layered structure also gives the rectal wall its ability to stretch without tearing, which is essential for its storage function. The circular muscle layer gradually thickens toward the lower end of the rectum, eventually forming the internal anal sphincter where the rectum transitions into the anal canal.