Question: What happens to your blood when you're dehydrated?
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Viscosity is a measure of a liquid's resistance to flow. It can be thought of as "thickness" – for example, tomato sauce and honey are more viscous than water.
Blood has the strange property of a "non-Newtonian" fluid, of which there are two types. A shock (or "shear") thickening fluid becomes thicker when agitated. On the internet you can see a video where a man runs across the top of a tub of corn starch in water. His pounding feet thicken the fluid as he runs, and when he stops, it thins and he sinks.
You've experienced a shock thinning fluid at the beach. If you jiggle your foot in the sand, it becomes loose, and you sink. Blood is more like this, which means it gets thinner when it flows faster. Slow blood is more viscous, which means it's more likely to clot around an obstruction, or when you have a wound. This viscosity is a result of interactions between the fluid layers sliding past each other.
The effect is strong enough to show a measurable difference in blood viscosity each time the heart beats. As the heart pumps, blood is more viscous than when it is at low pressure, between pumps.
When blood flows through the small vessels of the body it slows down and becomes more viscous, and this can be a risk for the formation of a clot, which can be the cause of ischaemic (clot) strokes.
The "thickness" of blood is related to the percentage of water and red blood cells. Normal blood is somewhere between 38-44 per cent red cells, but when you're dehydrated, the concentration increases. If you don't drink enough after sport, you might be dehydrated by up to 2per cent, meaning the concentration of red blood cells can rise to 48-50per cent.
That would mean the viscosity of the blood also increases, but in healthy people it's not a problem. Even with moderate dehydration, the body adapts, ensuring that blood flow to the different parts of the body, including the brain, is maintained. You would need to be very dehydrated and/or have a medical condition that would make this clinically important.
As we age, we tend to be become more anaemic to some degree. This means the percentage of red blood cells decreases, so that older people have slightly thinner blood.
Response by: Professor Jacqueline Phillips, Macquarie University
Next week: Eight litres a day?
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