Isn't the body clever?

Every square millimeter of the body has a different sensitivity to pain so that a spec of dirt may cause excruciating pain in the vulnerable eye whereas it would go unreported on the tough extremities. Internal organs such as the bowels and kidneys have no receptors that warn against cutting or burning—dangers than they normally do not face—but show exquisite sensitivity to distention.

When organs such as the heart detect danger but lack receptors, they borrow other pain cells ("referred pain"), which is why heart attack victims often report pain in the shoulder or arm. The pain system automatically ramps up hypersensitivity to protect an injured part (explaining why a sore thumb always seems in the way) and turns down the volume in the face of emergencies (soldiers often report no pain from a wound in the course of battle, only afterwards).

Pain serves us subliminally as well: sensors make us blink several times a minute to lubricate our eyes and shift our legs and buttocks to prevent pressure sores. Pain is the most effective language the body can use to draw attention to something important. (Philip Yancey, “That Hurts,” in Books and Culture, May/June, 2008, 32-33)

(HT Desiring God)