Heat causes vasodilation, meaning it widens your blood vessels and increases blood flow. This is true for both local heat (like a heating pad on your knee) and whole-body heat exposure (like sitting in a sauna). Cold does the opposite, causing vasoconstriction. The distinction matters for understanding everything from injury treatment to how your body handles a hot summer day.
How Heat Opens Blood Vessels
When heat reaches your skin, the blood vessels beneath it relax and widen. This response happens in two phases. First, within seconds to minutes, local nerve fibers trigger a rapid initial increase in blood flow. Then a longer, sustained vasodilation kicks in, driven by the cells lining your blood vessels releasing nitric oxide, a signaling molecule that tells the smooth muscle around arteries to relax. In studies where researchers blocked nitric oxide production in the skin, blood flow during sustained local warming only reached about 43% of its maximum, compared to 84% in untreated skin. That means nitric oxide is responsible for roughly half of the total vasodilatory response to local heat.
Heat also appears to activate a protein called heat shock protein 90, which binds to the enzyme that produces nitric oxide and amplifies its output. So the warmer the tissue gets, the more nitric oxide is generated, and the wider the vessels open.
What Happens Inside Your Body During Heat Stress
Local heat application and whole-body heating share the same end result (more blood flow to the skin) but work through partially different mechanisms. When your entire body heats up, such as during a hot bath or in extreme environmental heat, the cardiovascular response is dramatic. Cardiac output can jump from a resting level of about 5 liters per minute to 12.5 liters per minute, a 2.5-fold increase. Skin blood flow increases by 7 to 8 liters per minute to carry heat from your core to the surface, where it can dissipate.
Your body pays for this by redirecting blood away from other organs. Blood flow to the digestive organs drops by about 40%, and kidney blood flow decreases by 15 to 30%. Heart rate rises above 100 beats per minute. Blood pressure typically drops slightly because so much of your blood volume is being routed to dilated vessels near the skin surface. To compensate, blood vessels in non-skin areas constrict and sympathetic nervous system activity ramps up. This is why prolonged heat exposure can cause dizziness or fainting: your body’s ability to regulate blood pressure is compromised by roughly 35% under heat stress, making you more vulnerable to lightheadedness when standing up quickly.
Cold Does the Opposite
While heat dilates vessels, cold constricts them. Applying ice or a cold pack narrows blood vessels, reducing blood flow to the area. This is why cold therapy is the standard recommendation for acute injuries like sprains or muscle tears: less blood flow means less swelling and inflammation in the first hours and days after injury.
Some therapeutic approaches alternate between heat and cold to create a pumping effect, with vessels repeatedly opening and closing to promote circulation and oxygen delivery to fatigued or healing tissue.
Therapeutic Heat: Temperatures and Timing
Clinical heat therapy uses temperatures between 36°C and 45°C (about 97°F to 113°F). The sweet spot for deeper tissues like muscles, tendons, and ligaments is around 42°C (108°F). Sustained temperatures at or above 45°C risk burns and tissue damage, so hotter is not better.
The vasodilatory response builds over time. The initial nerve-driven phase happens quickly, but peak vasodilation from nitric oxide production requires sustained warmth. This is why most heat therapy guidelines recommend applying heat for 15 to 20 minutes rather than a quick touch.
When Heat Is the Wrong Choice
Because heat increases blood flow, it can make certain conditions worse. Applying heat to a fresh injury (within the first 48 to 72 hours) can increase swelling and inflammation. For acute sprains, meniscal tears, or muscle strains, cold is the better option in the first week. Heat becomes useful in the subacute phase, typically after about seven days, when reducing stiffness and promoting healing blood flow is the goal.
Heat is also contraindicated over open wounds, healing skin, burn areas, and during active inflammatory flare-ups such as a swollen, reactive joint in rheumatoid arthritis. In chronic phases of joint disease, when pain is present without active swelling, heat can help relieve symptoms. The key distinction is whether inflammation is currently active: if a joint is hot, red, and swollen on its own, adding more heat will only amplify the problem.