External hemorrhoids form from veins that surround the anus. These veins, for reasons not fully understood, weaken in some people, causing a bulge to form. The bulge then pushes out and appears as a lump, either just outside or just inside the anal verge.
Once formed, external hemorrhoids, are sensitive to pressures, either externally applied (such as excessive sitting), or internally applied, such as straining during defecation, lifting heavy objects etc... Since the bulging portion of the vein is already weak, any such pressure tends to magnify its size and cause further tissue dam ...
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