The blood that has filled the two erection chambers* must be trapped there, otherwise the erection wilts. This is tricky, as the chambers are equipped with drainage veins along their surface. What keeps the blood from leaking out via these veins? The miracle of passive venous occlusion. (Stay with me here.) These drainage veins lie outside the erection chambers but inside the stiff outer membrane (called the tunica) that protects the erectile tissue. When the chambers expand with blood, they slam up against the tunica—which also expands, but not as much—and this pressure squeezes shut the
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