Venules refer to the smallest veins in the circulatory system through which blood flows immediately after it leaves the capillaries. There are a tremendous number of such venules in the body, and they are capable of storing a considerable amount of blood. Such tiny vessels have no protection as do the arteries. There are no muscular fibers within their walls to resist compression or collapse. Consequently, they are quite susceptible to such compression whenever fluid retention causes the tissues of the body to swell.
Doctors call such tissue swelling edema.
A great deal of edema must be present before a person is aware it is even there. The best example is in the swelling that occurs in our feet and ankles on long trips. Our shoes feel tighter and our feet swollen. Were it not for prolonged periods of dependency, we would never be aware that such fluid retention was taking place. The tightness we experience reflects the increase in pressure within the tissues.