Hemorrhoids result when blood moving through the muscles in your lower abdomen creates more pressure than the veins in those muscles can handle, causing the veins to stretch like a balloon under the pressure.
When muscles contract, blood-flow to those muscles increases to meet the additional energy requirements. Healthy veins will expand and contract as needed to support the additional blood-flow, but strong and extended contractions to the muscles of the lower abdomen, such as in heavy lifting, child birth, or straining to use the toilet, can force the veins in those muscles to expand too much. If the veins distend beyond their limits, they can lose their ability to contract again and return to their normal shape, resulting in hemorrhoids.
Sitting for long periods of time - at a desk or behind the wheel, for example - can sometimes be a culprit in hemorrhoids by cutting off circulation to the muscles in your lower abdomen. Impaired circulation over an extended period can result in weakening of the veins, causing them to distend and pool with blood. When this happens in your bottom, we call it Hemorrhoids. When it happens in your legs, we call it Varicose Veins.
The swelling and inflammation that occurs with hemorrhoids can be extremely painful and, in some cases, the vein walls can break completely, resulting in bleeding hemorrhoids. In other cases, hemorrhoids inside your anus (internal hemorrhoids) can become so distended they protrude outside the anal opening. This is called prolapsed hemorrhoids and can be extremely painful when the anal opening closes and pinches the hemorrhoid.