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Heated debate 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?
n 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. 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
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
“When you step into a hot bath your blood vessels dilate to help you lose heat and so your skin reddens”
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
Lines on the sky Can anyone explain what made these trails on a second-floor skylight (see photo, above)? The skylight is under a tall lime tree.
n The trails were made by grazing molluscs. Slugs and snails have a radula, or “tongue”, rather like a nail file, with which they rasp the surface of their food, in this case a film of algae growing on the skylight. The trail is made as the animal moves forward, swinging its head from side to side to
remove the algae from the glass. Close observation of such marks often reveals the imprint of individual spikes from the radula. Each trail ends when the animal stops feeding and creeps off the glass, which is why there is a lower concentration in the centre of the pane: the slugs start feeding at the edge – and one edge here is obviously more accessible than the others. I think the trails are likely to have been caused by slugs, as these tend to be much more active than snails (the consequence of not having a shell). I also think they are more likely to reach a second-floor surface. Tim Bolton-Maggs Edinburgh, UK n The marks on the glass are almost certainly the feeding trails of garden molluscs grazing on algae, which is growing on the upper surface of the glass. The glass, sheltered by the lime trees, will provide a nice growing surface for algae. It may be heated from below, and will be fertilised from above by honeydew or excrement from insects feeding on the lime leaves. Roger Harris Romsey, Hampshire, UK
This week’s question Hidden evidence
Why do cats usually bury their poo but dogs don’t, even though they are good at digging? Jessica Turner-Walkley (aged 9) Preston, Lancashire, UK
Why are orangutans orange? A new collection: the usual insight, ingenuity and wit – this time with full colour photographs Available from booksellers and at newscientist.com/orangutans
Last words past and present, plus questions, at last-word.com
Heated debate 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?
n 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. 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
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
“When you step into a hot bath your blood vessels dilate to help you lose heat and so your skin reddens”
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
Lines on the sky Can anyone explain what made these trails on a second-floor skylight (see photo, above)? The skylight is under a tall lime tree.
n The trails were made by grazing molluscs. Slugs and snails have a radula, or “tongue”, rather like a nail file, with which they rasp the surface of their food, in this case a film of algae growing on the skylight. The trail is made as the animal moves forward, swinging its head from side to side to
remove the algae from the glass. Close observation of such marks often reveals the imprint of individual spikes from the radula. Each trail ends when the animal stops feeding and creeps off the glass, which is why there is a lower concentration in the centre of the pane: the slugs start feeding at the edge – and one edge here is obviously more accessible than the others. I think the trails are likely to have been caused by slugs, as these tend to be much more active than snails (the consequence of not having a shell). I also think they are more likely to reach a second-floor surface. Tim Bolton-Maggs Edinburgh, UK n The marks on the glass are almost certainly the feeding trails of garden molluscs grazing on algae, which is growing on the upper surface of the glass. The glass, sheltered by the lime trees, will provide a nice growing surface for algae. It may be heated from below, and will be fertilised from above by honeydew or excrement from insects feeding on the lime leaves. Roger Harris Romsey, Hampshire, UK
This week’s question Hidden evidence
Why do cats usually bury their poo but dogs don’t, even though they are good at digging? Jessica Turner-Walkley (aged 9) Preston, Lancashire, UK
Why are orangutans orange? A new collection: the usual insight, ingenuity and wit – this time with full colour photographs Available from booksellers and at newscientist.com/orangutans