The reason cold water helps with pain is that it causes your blood vessels to constrict. This reduces blood flow to the area which helps reduce swelling and inflammation.The cold water causes your body to release chemicals known as cytokines, which can also help to boost the body’s ability to fight off harmful bacteria and viruses. Cold water triggers the autonomic nervous system, which controls involuntary functions, such as heartbeat and breathing. The autonomic responses strengthen with each exposure. This stabilizes blood pressure, improves circulation and balances other bodily functions, such as the sleep and wake cycle.