Chapter 1 — Applied Anatomy and Physiology of the Anorectum

Before we learn about diseases of the anus and rectum, we must first understand their normal structure and function. Applied anatomy is anatomy that is useful for clinical work. A surgeon who knows this anatomy well can examine a patient with confidence. He or she can also plan surgery safely. Knowing the anatomy well helps avoid injuring important structures nearby. This chapter explains the anorectum in simple terms. It focuses on what matters most in the clinic and in the operating room.

1.1 The Rectum

The rectum is the last part of the large intestine. It is about 12 to 15 centimetres long. It begins where the sigmoid colon ends, at the level of the third sacral vertebra. It ends where the anal canal begins, at the level of the pelvic floor.

The rectum is not straight. It follows the curve of the sacrum, the bone at the back of the pelvis. Inside the rectum, there are three folds called the valves of Houston. These folds help to support the weight of stool. They also stop the stool from moving down too quickly.

The rectum can stretch to hold stool. It holds the stool until the person is ready to pass it. This stretching ability is called rectal compliance. When the rectum stretches, it sends a signal to the brain. This signal creates the urge, or desire, to defecate (pass stool).

1.2 The Anal Canal

The anal canal is short. It is only about 3 to 4 centimetres long in an adult. It begins where the rectum ends, at the anorectal ring. It ends at the anal opening, also called the anal verge.

A very important line runs across the middle of the anal canal. This is called the dentate line, or pectinate line. It divides the anal canal into two parts. These two parts are very different from each other:

Above the dentate line: the lining is like the lining of the rectum. It has no pain-sensing nerve endings. A doctor can cut or inject this area without causing sharp pain to the patient. Below the dentate line: the lining is like the skin. It has many pain-sensing nerve endings. This area is very sensitive. A cut or injury here causes sharp pain.

This difference is very important in clinical practice. Internal piles, or haemorrhoids, arise above the dentate line. So they are usually painless. External piles arise below the dentate line. So they are usually painful. This single fact explains many symptoms we will discuss in later chapters.

Small pockets called anal crypts sit along the dentate line. Anal glands open into these crypts. A crypt or gland can get blocked and infected. This can lead to an abscess, and later, a fistula. We will study this in the chapters on abscess and fistula-in-ano.

1.3 The Anorectal Ring and the Sphincters

Around the anal canal, there are two ring-like muscles. These muscles are called sphincters. They keep the anal canal closed most of the time. They open only when the person wants to pass stool or gas.

Internal Anal Sphincter (IAS): This is a smooth muscle. It works automatically. The person cannot control it by will. It stays contracted most of the time. It provides most of the resting pressure in the anal canal. This is why cutting the internal sphincter, as in surgery for fissure-in-ano, can reduce spasm and pain. But this must be done carefully, to avoid incontinence. External Anal Sphincter (EAS): This is a skeletal muscle. The person can control it by will. For example, a person uses it when holding back a bowel motion. It is used for sudden, strong squeezing. But it becomes tired quickly if held for too long.

The anorectal ring is the point where three structures meet. These are the top of the internal sphincter, the top of the external sphincter, and the puborectalis muscle, described below. This ring is felt clearly on digital rectal examination. A surgeon must be very careful not to cut through this ring completely. Doing so can cause permanent incontinence.

1.4 The Anal Cushions

Inside the anal canal, just above the dentate line, there are three soft, cushion-like pads of tissue. These are called the anal cushions. They are usually found in three positions: left lateral, right anterior, and right posterior.

The anal cushions are full of small blood vessels. They help the anal canal to close completely. They give a fine, watertight seal. This is important for controlling small amounts of gas and liquid stool. It is a job the muscles alone cannot do perfectly.

These cushions can become enlarged, congested, or displaced downward. When this happens, they cause the condition we call piles, or haemorrhoids. Piles are enlarged normal cushions. They are not new, abnormal growths. This understanding helps explain why treatment often aims to reduce and reposition them, rather than remove all anal tissue.

1.5 Blood Supply

The rectum and anal canal receive blood from three main arteries:

Superior rectal artery — a branch of the inferior mesenteric artery. It supplies the upper rectum. Middle rectal artery — a branch of the internal iliac artery. It supplies the lower rectum. Inferior rectal artery — a branch of the internal pudendal artery. It supplies the anal canal.

Veins usually follow the same path as the arteries:

Superior rectal vein — drains the upper rectum. It carries blood into the portal venous system.

Middle rectal vein — drains the lower rectum. It carries blood into the systemic venous system. Inferior rectal vein — drains the anal canal. It carries blood into the systemic venous system.

Above the dentate line, drainage is mainly through the superior rectal vein, into the portal system. Below the dentate line, drainage is through the middle and inferior rectal veins, into the systemic system.

This dual drainage is clinically important. It explains why rectal cancer above the dentate line can spread to the liver, through the portal system. It also explains why cancer below the dentate line can spread to lymph nodes in the groin.

1.6 Lymphatic Drainage

Lymph is the fluid that helps fight infection and drains waste from tissues. In the anorectum, lymph drainage also follows the dentate line:

Above the dentate line: lymph drains upward, along the superior rectal vessels, toward lymph nodes near the inferior mesenteric artery inside the abdomen. Below the dentate line: lymph drains outward, toward the inguinal (groin) lymph nodes.

This is why a doctor must always examine the groin in cases of anal cancer. The doctor checks for enlarged lymph nodes. This check is not routinely needed in most cases of upper rectal cancer.

1.7 Nerve Supply

Above the dentate line, the rectum and upper anal canal are supplied by the autonomic nervous system. This is the same system that controls organs without our conscious control. It does not sense sharp pain. But it can sense stretching and pressure.

Below the dentate line, the anal canal is supplied by the inferior rectal nerve, a branch of the pudendal nerve. This is a somatic nerve, the same type of nerve that supplies our skin. It senses pain, touch, and temperature very well.

This is exactly why conditions below the dentate line cause sharp, severe pain. Examples include fissure-in-ano and external piles. Conditions above the dentate line, such as internal piles, are usually painless.

1.8 The Pelvic Floor and the Anorectal Angle

The pelvic floor is a group of muscles that forms a sling at the base of the pelvis. The most important muscle in this group, for our purposes, is the puborectalis muscle.

The puborectalis muscle wraps around the back of the anorectal junction, like a sling or a strap. When it contracts, it pulls the rectum forward. This creates a bend between the rectum and the anal canal, called the anorectal angle. In a resting, continent person, this angle is normally about 90 degrees.

This angle acts like a kink in a hose pipe. It helps to block the passage of stool. This is especially true when a person is standing or sitting. During defecation, the puborectalis muscle relaxes. The anorectal angle straightens out. Stool can then pass more easily.

1.9 The Mechanism of Continence

Continence means the ability to hold stool and gas until a suitable time and place. This is not the work of one muscle alone. It depends on several factors working together:

The internal anal sphincter, which gives constant resting tone (background pressure). The external anal sphincter, which gives strong, voluntary squeeze when needed. The puborectalis muscle and the anorectal angle, which act as a mechanical valve. The anal cushions, which give a fine seal for gas and liquid. Normal sensation in the anal canal, which allows a person to tell the difference between gas, liquid, and solid stool. Normal rectal compliance, which allows the rectum to store stool comfortably until it is convenient to pass it.

Damage to any one of these factors can lead to some degree of incontinence. For example, this can happen after a torn sphincter from childbirth. It can also happen after a stretched anorectal angle, from long-standing straining. We will study these conditions in Part II of this manual.

1.10 Why This Chapter Matters

Every disease chapter in this manual will refer back to the anatomy described here. When we discuss piles, we return to the anal cushions. When we discuss fissure-in-ano, we return to the internal sphincter. When we discuss incontinence, we return to the puborectalis muscle and the anorectal angle. Keep a clear picture of this anatomy firmly in mind. It will make every later chapter easier to understand, and easier to remember.