A hot bath reduces high blood pressure by reducing peripheral vascular resistance through two mechanisms - immersion in water and heat. To maintain cardiac output, the body's homeostatic response is to increase heart rate (quickening of the pulse). Whilst high blood pressure is not specifically related to a faster pulse, there are two possible associations. If you are anxious about known hypertension and symptomatic, you may have a faster heart rate. In addition, some older anti-hypertensive drugs such as betablockers (propanolol) also slow the heart rate.

Dr Chris Silvester

Mentone, Melbourne, Australia

Blood pressure has 3 basic (quick acting) methods of regulation: peripheral vascular resistance, heart rate and heart contractility (the force of heart contraction). By immersing yourself in a warm bath, you are causing your peripheral vasculature to dilate as the peripheries cannot determine whether the warming is caused from within the body or from external sources. The aim of the vasodilation is to lose heat but in this case actually results in you getting warmer which can lead to further peripheral vasodilation. The result of this is a drop in blood pressure as the blood is trying to fill a larger space within the vascular system. It also leads to the blood returning to the heart doing so at a lower pressure. This in turn has the effect of reducing heart contractility, further reducing blood pressure (this is governed by Starling’s Law in a person with a normal heart – the force of contraction is related to how much the heart muscle is stretched which is itself dependant on the filling pressure). The body tries to compensate for this by using an increase in heart rate to pump blood more quickly round the body and so restore working blood pressure to the major organs. One of the reasons some people feel faint in hot conditions is due to these mechanisms and blood pooling in the lower limbs – this is less likely to happen when you’re lying in the bath.

An increase in heart rate leading to an increase in blood pressure is just part of the body’s regulatory mechanisms; when exercising, the body needs more oxygen and nutrients delivered to the tissues and more waste and heat taking away. However, this is not part of the increase in blood pressure often associated with obesity or old age which can have many influences. In overweight people, a higher blood pressure is required to pump the blood round the bigger body; in people who smoke, a higher blood pressure is required to push the blood through narrowed arteries. A high blood pressure can be associated with a slow heart rate as well.

There are other regulatory mechanisms but these tend to act locally (e.g. for the brain) or more slowly (e.g. fluid regulation). It is a very complex and dynamic system.

David Farnsworth

Lancashire, UK