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Seven litres a day
The back pages Almost the last word Why don’t blue highlighters look as bright as the other colours?
Seven litres a day How does water hydrate us? If we drink a lot of it we only pass it as excess waste.
Andrew Sanderson Spennymoor, County Durham, UK We lose water in four principal ways: in urine, sweat, breath and faeces. This doesn’t include minor losses such as in tears and spitting. We gain water by drinking, and by breaking down food during metabolism into carbon dioxide and water. Sweat, breath and faeces stay at the same concentration, so the main control of body fluid content is via our kidneys. Their activity is controlled by a molecule known as vasopressin or antidiuretic hormone, which is secreted by the pituitary gland. This is regulated by an area of the brain called the hypothalamus, which contains receptors sensitive to the blood’s concentration of sodium and other substances. The kidneys are the main way for us to excrete salts. If you eat a lot of salt, your kidneys will increase the concentration of the urine up to their maximum ability. Past this, the volume will increase. If you drink a lot of water, 54 | New Scientist | 31 August 2019
Why do dogs and horses roll on their backs when happy?
DOUGLAS SACHA/GETTY
Eleanor Horton, Canterbury, Kent, UK Breaking down food – and our metabolism in general – generates waste. This needs to be removed or it would damage the body. Water is the solvent for these waste products – it dissolves them and allows them to pass out of the body as urine, as well as in sweat. Urine is produced in the kidneys and consists of urea and other waste products dissolved in water. We need to drink more water to replenish the fluid that leaves the body this way. When you are dehydrated, your urine will be dark yellow. This is because there isn’t enough water in your system to dilute the urea sufficiently. Water itself is not a waste product – it is a mechanism by which the body removes waste.
Roll of honour
This week’s new questions In the ink Why do blue highlighter markers never seem to have the high luminosity of pink, yellow, orange and light green highlighters? Ana Beard, London, UK Once upon a time Why does my brain like fictional stories? Shvets Roman, Moscow, Russia Run the world If the world’s population all met in one place and all ran in the same direction, would this affect Earth’s rotation? Neil Edwards, Guildford, Surrey, UK
urine concentration falls and volume increases. The more you exercise and sweat, the more salts you lose, because sweat can’t be concentrated, hence the marketable value of sports drinks. The amount you need to drink is unique to you. Linked to the hydration sensors are the thirst parts of your brain. If you feel thirsty, then drink water. Otherwise, keep someone else happy and rich by buying their fluid replacement and by all means carry a bottle with you to prove that selective advertising has an effect on you. Brian Pollard North Hill, Cornwall, UK The human body is made up mainly of water, and our physiology operates to keep within about a litre of the 45 or so litres in an average-sized person. The way it does this is by regulating the feeling of thirst. When the fluid level starts to get low, your body makes you feel
thirsty. You drink, and the body’s regulatory system works out how much you should drink to restore the balance to be within required limits. When you have drunk enough, you feel sated and stop drinking. This system is remarkably efficient. It takes several hours for the fluid levels in the body to respond to the liquid you have drunk, but the regulatory system works well enough most of the time to keep the body’s fluid content within its typical parameters. If you lose a lot of water quickly, on a very hot day for example, you may lose too much, and then you become dehydrated, and it feels unpleasant. The unpleasant feeling is your body requesting an urgent ingestion of liquid.
David Muir Edinburgh, UK Horses roll and writhe on their backs not because they are happy but because they want to get rid of an itchy irritation. They could be trying to get rid of their winter coat, which makes them sweaty in the summer. If they are being bothered by biting insects, then rolling in mud, or even dust, affords some protection. Dogs are different. A very relaxed dog will lie on its back with its vulnerable abdomen exposed. On the other hand, a dog that is frightened may roll over as a sign of submission and thus avoid attack by another dog. Some dogs retain their evolutionary urge to roll in other animals’ excreta, such as fox faeces, to disguise their own scent. This seems to make dogs happy and their owners very unhappy. Tony Holkham Boncath, Pembrokeshire, UK Horses, and many other animals, roll to rid themselves of irritation or parasites that they can’t reach with their mouths or feet. It is necessary, but leaves the animal vulnerable for a short time. Dogs roll for this reason too, especially because many modern breeds are unable to groom themselves effectively. They also do it to submit to another member of the pack and, in domesticated dogs at least, because they love to have their belly rubbed. My Jack Russell Sparky would probably put this last reason at the top of the list. ❚
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms
Seven litres a day How does water hydrate us? If we drink a lot of it we only pass it as excess waste.
Andrew Sanderson Spennymoor, County Durham, UK We lose water in four principal ways: in urine, sweat, breath and faeces. This doesn’t include minor losses such as in tears and spitting. We gain water by drinking, and by breaking down food during metabolism into carbon dioxide and water. Sweat, breath and faeces stay at the same concentration, so the main control of body fluid content is via our kidneys. Their activity is controlled by a molecule known as vasopressin or antidiuretic hormone, which is secreted by the pituitary gland. This is regulated by an area of the brain called the hypothalamus, which contains receptors sensitive to the blood’s concentration of sodium and other substances. The kidneys are the main way for us to excrete salts. If you eat a lot of salt, your kidneys will increase the concentration of the urine up to their maximum ability. Past this, the volume will increase. If you drink a lot of water, 54 | New Scientist | 31 August 2019
Why do dogs and horses roll on their backs when happy?
DOUGLAS SACHA/GETTY
Eleanor Horton, Canterbury, Kent, UK Breaking down food – and our metabolism in general – generates waste. This needs to be removed or it would damage the body. Water is the solvent for these waste products – it dissolves them and allows them to pass out of the body as urine, as well as in sweat. Urine is produced in the kidneys and consists of urea and other waste products dissolved in water. We need to drink more water to replenish the fluid that leaves the body this way. When you are dehydrated, your urine will be dark yellow. This is because there isn’t enough water in your system to dilute the urea sufficiently. Water itself is not a waste product – it is a mechanism by which the body removes waste.
Roll of honour
This week’s new questions In the ink Why do blue highlighter markers never seem to have the high luminosity of pink, yellow, orange and light green highlighters? Ana Beard, London, UK Once upon a time Why does my brain like fictional stories? Shvets Roman, Moscow, Russia Run the world If the world’s population all met in one place and all ran in the same direction, would this affect Earth’s rotation? Neil Edwards, Guildford, Surrey, UK
urine concentration falls and volume increases. The more you exercise and sweat, the more salts you lose, because sweat can’t be concentrated, hence the marketable value of sports drinks. The amount you need to drink is unique to you. Linked to the hydration sensors are the thirst parts of your brain. If you feel thirsty, then drink water. Otherwise, keep someone else happy and rich by buying their fluid replacement and by all means carry a bottle with you to prove that selective advertising has an effect on you. Brian Pollard North Hill, Cornwall, UK The human body is made up mainly of water, and our physiology operates to keep within about a litre of the 45 or so litres in an average-sized person. The way it does this is by regulating the feeling of thirst. When the fluid level starts to get low, your body makes you feel
thirsty. You drink, and the body’s regulatory system works out how much you should drink to restore the balance to be within required limits. When you have drunk enough, you feel sated and stop drinking. This system is remarkably efficient. It takes several hours for the fluid levels in the body to respond to the liquid you have drunk, but the regulatory system works well enough most of the time to keep the body’s fluid content within its typical parameters. If you lose a lot of water quickly, on a very hot day for example, you may lose too much, and then you become dehydrated, and it feels unpleasant. The unpleasant feeling is your body requesting an urgent ingestion of liquid.
David Muir Edinburgh, UK Horses roll and writhe on their backs not because they are happy but because they want to get rid of an itchy irritation. They could be trying to get rid of their winter coat, which makes them sweaty in the summer. If they are being bothered by biting insects, then rolling in mud, or even dust, affords some protection. Dogs are different. A very relaxed dog will lie on its back with its vulnerable abdomen exposed. On the other hand, a dog that is frightened may roll over as a sign of submission and thus avoid attack by another dog. Some dogs retain their evolutionary urge to roll in other animals’ excreta, such as fox faeces, to disguise their own scent. This seems to make dogs happy and their owners very unhappy. Tony Holkham Boncath, Pembrokeshire, UK Horses, and many other animals, roll to rid themselves of irritation or parasites that they can’t reach with their mouths or feet. It is necessary, but leaves the animal vulnerable for a short time. Dogs roll for this reason too, especially because many modern breeds are unable to groom themselves effectively. They also do it to submit to another member of the pack and, in domesticated dogs at least, because they love to have their belly rubbed. My Jack Russell Sparky would probably put this last reason at the top of the list. ❚
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms