What are hemorrhoids? 

A precise definition of hemorrhoids does not exist, but they can be described as masses or clumps ("cushions") of tissue within the anal canal that contain blood vessels and the surrounding, supporting tissue made up  of muscle  and elastic fibers. The anal canal is  the last four centimeters through which stool passes as it goes from  the rectum  to the outside world. The anus is the opening  of the anal canal to the outside world. 

Although most people think hemorrhoids are abnormal, they are present in everyone. It is only when  the hemorrhoidal cushions enlarge that  hemorrhoids can cause problems  and be considered abnormal or a disease. 

Prevalence of hemorrhoids 

Although hemorrhoids occur in everyone, they become large and cause problems in only 4% of  the general population. Hemorrhoids that cause problems are found equally in men and women, and their prevalence peaks between 45 and 65 years of age. 

Anatomy of hemorrhoids 

The arteries supplying blood to  the anal canal descend into  the canal from the rectum above  and form a rich network  of arteries that communicate  with each other around the anal canal. Because of this rich network of arteries, hemorrhoidal blood vessels have a ready supply of arterial blood. This explains why bleeding  from hemorrhoids is bright red (arterial blood) rather than dark red (venous blood), and why bleeding from hemorrhoids occasionally  can be severe. The blood vessels that supply the hemorrhoidal vessels pass through the supporting tissue of the hemorrhoidal cushions. 

The anal veins drain blood away from the anal canal and the hemorrhoids. These veins drain  in two directions. The first direction is upwards into the rectum, and the second is downwards beneath the skin surrounding the anus. The dentate line is  a line within the anal canal that denotes the transition  from anal skin (anoderm) to the lining of the rectum. 

What causes hemorrhoids? 

It is not known why hemorrhoids enlarge. There  are several theories about the cause, including inadequate intake of fiber, prolonged sitting on the toilet,  and chronic straining to have a bowel movement (constipation). None of these theories has strong experimental support. Pregnancy is a clear cause of enlarged  hemorrhoids though, again, the reason is not clear. Tumors  in the pelvis also cause enlargement  of hemorrhoids  by pressing on veins draining upwards from the anal canal. 

One theory proposes that it is the shearing (pulling) force  of stool, particularly hard stool, passing through  the anal canal that drags the hemorrhoidal cushions downward. Another theory suggests that with age or an aggravating condition, the supporting tissue that is responsible for anchoring the  hemorrhoids to the underlying muscle of the anal canal deteriorates. With time, the hemorrhoidal tissue loses its mooring and slides down into the anal canal. 

One physiological fact that is known about enlarged hemorrhoids that may be relevant to understanding why they form is that the pressure is elevated  in the anal sphincter, the muscle that surrounds the anal canal and the hemorrhoids. The anal sphincter is the muscle that allows us  to control our bowel movements. It is not known, however, if this elevated pressure precedes the development of enlarged  hemorrhoids or is the result of the hemorrhoids. Perhaps during  bowel movements, increased force is required to force stool through  the tighter sphincter. The increased shearing force applied to the hemorrhoids by the passing stool may drag the hemorrhoids downward  and enlarge them. 

What are the symptoms of hemorrhoids? 

There are two types of nerves in the anal canal, visceral nerves (above the dentate line) and somatic nerves (below the dentate line).  The somatic (skin) nerves are like the nerves of the skin  and are capable of sensing pain. The visceral nerves are like  the nerves of the intestines and do not sense pain, only pressure. Therefore, internal hemorrhoids, which are above the dentate line, usually are painless. 

As the anal cushion of an internal hemorrhoid continues to enlarge, it bulges into the anal canal. It may even pull down a portion of the lining of the rectum above, lose its normal anchoring,  and protrude from the anus. This condition is referred to as a prolapsing internal hemorrhoid. In the anal canal, the hemorrhoid is exposed to the trauma  of passing stool, particularly hard stools associated with constipation. The trauma can cause bleeding and sometimes pain when stool passes. The rectal lining that has been pulled down secretes mucus and moistens  the anus and the surrounding skin. Stool also can leak onto the anal skin.  The presence of stool  and constant moisture can lead to anal itchiness (pruritus ani), though itchiness is not a common symptom of hemorrhoids. The prolapsing hemorrhoid usually returns into the anal canal or rectum on its own or can be pushed back inside with a finger, but it prolapses again with the next bowel movement. 

Less commonly, the hemorrhoid protrudes  from the anus and cannot be pushed back inside, a condition referred to  as incarceration of the hemorrhoid. Incarcerated hemorrhoids can have their supply of blood shut off by the squeezing pressure of the anal sphincter,  and the blood vessels and cushions can die, a condition referred to as gangrene. Gangrene requires medical treatment. 

For convenience  in describing the severity of internal hemorrhoids, many physicians use a grading system: 

• First-degree hemorrhoids: Hemorrhoids that bleed but do not prolapse. 
• Second-degree hemorrhoids: Hemorrhoids that prolapse and retract on their own (with or without bleeding). 
• Third-degree hemorrhoids: Hemorrhoids that prolapse but must be pushed back in by a finger. 
• Fourth-degree hemorrhoids: Hemorrhoids that prolapse and cannot be pushed back in. 
• Fourth-degree hemorrhoids also include hemorrhoids that are thrombosed (containing blood clots) or that pull much of the lining of the rectum through the anus.