Why is it that in vaso dilation,human hairs rise up and in vaso constriction,the human hairs lies on each other
The phenomenon you're describing is called "goosebumps" or "piloerection." It is a reflex response that occurs when the smooth muscle attached to the hair follicles contracts or relaxes.
In vasoconstriction, the blood vessels near the skin's surface become narrower, reducing blood flow and heat loss. When vasoconstriction happens, the smooth muscle around the hair follicles contracts, causing the hairs to lie flat against the skin. This flattening of the hairs helps to decrease heat loss from the body.
On the other hand, during vasodilation, the blood vessels near the skin's surface widen, allowing more blood to flow and increasing heat loss. In response to vasodilation, the smooth muscle around the hair follicles relaxes, allowing the hairs to stand up or "rise." This phenomenon is believed to have been useful in our evolutionary past as it helped to trap a layer of air near the skin, providing some insulation and warmth.
To observe this phenomenon, you can try running your fingers along the skin or exposing your body to cold temperatures. You may notice the hairs on your arms or legs rising in response to the cold stimulus. Keep in mind that the degree of hair elevation can vary between individuals, and it may be more noticeable in some people than others.