Not merely the nails and the lips, but the whole surface of the skin is underlaid with a thick mat, or network, of blood vessels.

These vessels are all quite small, so that a cut has to go down completely through the skin, and generally well down into the muscles, before it will reach any blood vessel which will bleed at a dangerous rate.

But there are so many of them, and they cover such a wide surface throughout the body, that they are actually capable of holding, at one time, nearly one-tenth of all the blood in the body.
This "water-jacket" coat of tiny blood vessels all over our body has some very important uses: It allows the heart to pump large amounts of blood out to the surface to be purified by the sweat glands, and to breathe out a little of its carbon dioxid and other gas-poisons.
The Skin as a Heat Regulator.

Heat, as well as waste, is given off by the blood when it is poured out to the surface; so another most important use of the skin is as a heat regulator.

As we have already seen, every movement which we make with our muscles, whether of arms and limbs, heart, or food tube, causes heat to be given off.