The body treats cold as a threat to its core temperature. To protect it, the nervous system makes a series of quick adjustments, and many of them run straight through the heart and blood vessels.
Some of these changes start within seconds of stepping outside. Others build slowly over the season and linger long after you've come back inside.
Here's what happens, and how each change affects the cardiovascular system.
1. Blood Vessels Near the Skin Tighten
Blood vessels near the skin tighten to hold in heat, which forces the heart to pump against more resistance.
When skin temperature drops, sensors in the skin signal the nervous system to narrow the small vessels near the surface. This keeps warm blood closer to the core, but it can raise the top blood pressure number by 5 to 30 points and the bottom number by 5 to 15. [1]
With less room inside the vessels, the heart has to beat harder to move the same amount of blood. That raises its workload and its need for oxygen at the same time. [2]
2. Blood Gets Thicker and Stickier
Cold doesn't only affect the vessels. It changes the blood flowing through them.
In a study published in the BMJ, six hours of mild cooling in moving air at about 75°F made whole blood 21% thicker. Platelets, the cells that help form clots, took up 15% more of the blood's volume, and blood pressure rose from an average of 126/69 to 138/87. [3]
Thicker blood with more platelets moves more slowly and clots more easily. Researchers in Wales also found that fibrinogen, a protein the body uses to build clots, runs higher in the coldest month of the year than in the warmest. [4]
3. Breathing Cold Air Raises the Heart's Workload
The heart responds to the air going into the lungs, not just the temperature on the skin.
Researchers at Penn State found that breathing cold air increases how much oxygen the heart needs while slightly limiting how much reaches it. That creates a gap between supply and demand, and the gap grew when people also did a squeezing effort similar to shoveling snow or carrying heavy bags. [5]
A follow-up study compared adults in their 20s with adults in their 60s. The older group's hearts were less able to increase blood flow to match the extra effort, which means the same cold-weather task asks more of an older heart. [6]
4. Blood Pressure Runs Higher All Season
The pressure spike from stepping outside doesn't fully reset once you're back indoors. Over the winter, the baseline itself shifts.
A French study of 8,801 adults ages 65 and older found that the top blood pressure number averaged 5 points higher in winter than in summer. High blood pressure showed up in 33.4% of participants in winter compared with 23.8% in summer, and the swings were even larger in people 80 and older. [7]
The researchers pointed to two likely reasons. The nervous system's cold response keeps nudging the vessels tighter, and the body's built-in pressure sensors become less responsive with age, so they're slower to bring pressure back down. [8]
5. Cholesterol Tends to Climb
Cholesterol levels move with the seasons, peaking in winter and dipping in summer.
In a study of 517 healthy adults, 22% more people had high cholesterol in winter than in summer, even though their diets and calorie intake stayed about the same. The swings were larger in women, whose levels shifted by about 5.4 points compared with 3.9 in men. [9]
The main reason is blood volume. The liquid part of the blood expands in warmer months and shrinks in colder ones, so the same amount of cholesterol ends up more concentrated in winter. [10]
6. Colds and Flu Put Extra Strain on the Heart
Colder months are also respiratory virus season, and those infections reach well beyond the lungs.
Researchers in Ontario looked at nearly 20,000 lab-confirmed flu cases and found the risk of a heart attack was six times higher in the first week after diagnosis. Other common respiratory viruses raised the risk too, though to a lesser degree. [11]
An infection can lower the amount of oxygen in the blood, so the heart beats faster to make up for it. At the same time, the inflammation the virus triggers makes clots more likely to form in the vessels that feed the heart. [12]
Conclusion
Cold weather doesn't change the cardiovascular system in just one way. Tighter vessels, thicker blood, harder-working lungs and heart, higher pressure, more concentrated cholesterol, and seasonal viruses all stack up during the same few months.
Each change on its own is the body doing its job to stay warm. Together, they explain why the heart carries a heavier load from late fall through early spring.
