Your body’s reaction to heat loss is involuntary and driven by the hypothalamus, the tiny gland in the brain acting as a thermostat. The hypothalamus is very sensitive to any temperature variation in your body. Even a small drop brings the temperature regulation mechanisms into action: blood vessels in the skin constrict to prevent excessive heat loss and muscles shiver to create heat.
 
The hypothalamus is a cruel gland, though. Its only concern is to keep the vital organs at an acceptable temperature, and couldn’t care less if your toes or fingers became icicles. All of the thermoregulation mechanisms are designed to protect the core.
 
To guarantee wellbeing, the body’s core temperature needs to be maintained at around 36.9°C. What comprises the core? It includes all the vital organs such as heart, lungs, liver and kidneys central to your body. Because the brain is also important for your survival, it is part of the core despite being nowhere near the centre.
 
Surrounding the core is the periphery. It includes the skin, the muscles, arms and legs. Shake the hands of someone on a cold day and you’ll find out that the temperature of the periphery can be well below the core temperature icy-cold hands. However, a cold periphery is quite natural and, to a degree, an advantage. You constantly lose heat to a cooler environment until the periphery reaches the same temperature, and then the heat flow stops and you preserve heat.
 
This has its limits. The problems begin when the surroundings have a very low temperature. In this case, the temperature exchange between your body and the environment doesn’t stop and your core temperature starts to drop. Women have an advantage over men in that respect because their skin generally reaches a slightly lower temperature, so they have less heat to lose.
 
Every person reacts differently to thermal stress. Age, fitness level and underlying diseases play a major part in someones reaction. In addition, cold reactions are not Your teeth chatter, your body shivers and despite all efforts, you can’t prevent it. The tiny hairs on your body stand upright and try desperately to function as an insulating fur. Your skin looks bloodless and feels cold. All these reactions are an attempt by your body to preserve heat, although not a very successful one. Humans are well equipped to lose heat, but are less efficient in retaining it.

Necessarily associated with wintry conditions, as a sharp temperature drop in summer may have similar effects. Cold snaps bring relief from stifling heatwaves, but the cardiovascular system has not much time to adjust to the rapid temperature change and is, therefore, subjected to immense stress.
 
What are the effects on our cardiovascular system?
 
When blood vessels constrict to preserve heat, the heart has to work harder to squeeze blood through the narrow vessels. This may be too much for a sick heart. Surveys of blood donors revealed that blood pressure rose markedly after temperature drops. Published figures showed increases of between 12 and 18mmHg. While such an increase is not significant for a healthy person, it is sometimes a deciding factor for a person suffering already from high blood pressure.
 
Further, a heart under strain needs a high volume of oxygen-rich blood. However, narrow vessels do restrict the supply and symptoms of angina could develop. The British Heart Foundation defines angina as: Angina is an uncomfortable feeling or pain in the chest. If it is a cold day or if you are walking after a meal, the angina may be triggered more easily.
 
Lower temperatures also change the composition of the blood. The change is almost immediate and persists for up to two days. The number of particles in the blood, such as platelets, red blood cells, fibrinogen and cholesterol, increases in cold conditions and make the blood thicker (increased viscosity). Some tests showed an increase in viscosity by up to 21%. The risk of blood clotting increases and, together with the high viscosity, may lead to blocked blood vessels in the heart, brain or lungs.
 
If this is not enough to make you sick, add a little overexertion. Any physical activity is good for your body, but in moderation. Slow down if you suspect or know of problems with your heart or circulation. Shoveling snow off your footpath, or any other strenuous activity, together with the cold effect on your body, can be a lethal combination. Hospital emergency departments register a significant increase of patient numbers after heavy snowfall.
 
Data collected from patients fitted with heart rhythm regulators (defibrillators) gave some proof of the relationship between weather extremes and heart diseases. When scientists compared the result with climate, they found that heart rhythm abnormalities are more frequent on days with either very high or very low outside air temperatures. Some patients reacted immediately to temperature changes, others reactions were delayed by two to three days.
 
While there is no more doubt that cold weather increases the risk of heart or circulatory disease, the question remained whether the link is only present in cold climate regions. Not so; cold is relative. The mortality rate during Hawaii’s winter increases by 22 per cent, a figure similar to some cold climate regions.
 
The relationship between weather extremes and mortality rates is well documented. Extreme heat or cold increases the number of heart attacks dramatically. Winter is the season for heart attacks in the US and the UK, with around 50 per cent more cases than during summer. Two studies conducted in New Zealand put the winter CHD mortality peak at 33 to 35 per cent above the summer low. Based on an annual average of 10,500 heart disease deaths (New Zealand Heart Foundation), the difference between the best rate in summer and the worst rate in winter is 3,500 excess deaths most, if not all, are preventable.

Avoiding the cold is obviously the best prevention, but not always practical. Life has to go on and work has to be done but safely. The less time you are exposed to the cold the less chance you have of being injured or getting ill. 

Awareness is just as important as avoidance. Be aware of the weather and bioweather forecasts – be prepared.