High blood pressure can be reduced by taking a hot bath, but why does one’s pulse quicken when relaxing this way? And is high blood pressure related to a faster pulse in any way?
• Your body does not contain enough blood to fill all your blood vessels at the same time so blood flow is regulated to different parts of the body as needed – to your gut after a meal, to your muscles when exercising or to your skin when you are hot. Your blood pressure (BP) is calculated by multiplying the volume of blood pumped out by the heart per minute (the cardiac output, or CO) and the total resistance to that flow in your blood vessels (the systemic vascular resistance, or SVR). Thus BP = CO × SVR.
When you step into a hot bath your blood vessels dilate to help you lose heat – you see this as a reddening of the skin – and your SVR falls, hence you experience an immediate fall in blood pressure. This reduction is detected by baroreceptors in your carotid body. These are a type of “stretch receptor” found in the blood vessels located on each side of your throat, and they control blood pressure by constantly reporting to your brain. Their aim is to keep BP at its “normal” level by increasing or decreasing CO. This, in turn, is a product of heart rate (HR) and stroke volume (SV), which is the volume of blood that your heart pumps out each beat, typically 55 to 100 millilitres. So CO = HR × SV. You will easily detect a rise in HR by your pulse quickening, and feel an increase in SV as a pounding in your chest, most commonly during exercise.
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“When you step into a hot bath your blood vessels dilate to help you lose heat and so your skin reddens”
A faster pulse is not necessarily related to high blood pressure. Combining the above equations shows that heart rate, stroke volume and vascular resistance are all variable factors in determining blood pressure. The “fight or flight” response, which creates an adrenalin rush in humans, raises all three variables, for example, whereas a hot bath or blood loss will reduce only one.
Longer-term control of blood pressure is mediated largely by the kidneys and I’m afraid to say that once you cool down from your relaxing hot bath your blood pressure will largely return to normal, unless your high blood pressure was stress-induced in the first place.
Roger McMorrow, Consultant anaesthetist, National Maternity & St Vincent’s University Hospitals, Dublin, Ireland