The reaction is closely confined to the area where the pressure was applied, so only capillaries are involved, not arterioles. The white reaction is due to constriction of precapillary sphincters, possibly combined with changes in the endothelial cells, so that red blood cells do not flow through the capillaries. Around the red reaction is a white reaction because the structure of the skin spreads the load and the surrounding area is exposed to a low pressure. If the nerve supply to an area of skin is cut and the triple response is evoked some days later, the response will be normal except that there will be no flare, so the flare must be produced by nerve fibres, which will die after the nerves have been cut, but the red reaction and weal do not involve nerve fibres. Lymph Overall, slightly more fluid is filtered from capillaries than is reabsorbed and the difference forms the lymph.
Some people dispute whether there is any reabsorption at all.
Lymph returns to the major veins along lymphatics against a pressure gradient so work must be done upon it. The lymphatics are thin walled vessels with valves that allow fluid to flow only from the tissues to the veins so that intermittent compression by surrounding muscles or the pulsations of arteries can act as a pump. There is now considerable evidence that the smooth muscle in the lymphatics' walls alternately contracts and relaxes and contributes to the pumping. An important function of the lymphatics is to remove such protein as does leak out of capillaries so that the osmotic pressure of the interstitial fluid is kept low. Blockage of lymphatics can produce oedema.