Page 24 - Veterinary Laser Therapy in Small Animal Practice

CHAPTER 2

Blood and light

Don’t be afraid of hard work. Nothing worthwhile comes constant volume inside a living, moving body made
easily. Don’t let others discourage you or tell you that you of less-than-rigid tissue. And so it’s slightly less intui-
can’t do it. In my day I was told women didn’t go into tive how these two are related when it comes to the
chemistry. I saw no reason why we couldn’t. circulation.
— Gertrude Elion The first impulse may be to think that more heat
equals more pressure. That’s how a steam engine
2.1 Circulation basics works, right? And if you have a sore back, putting a
heat pad on it definitely loosens things up: the warmer
In a very simplified picture of things, the heart creates the pad, the faster the relief. But do you put a heat pad
a massive pressure wave of blood, which like all fluids, on for 4 hours? No. Why not? First, because the body
will flow through the path of least resistance. This simply saturates. It’s a very well-regulated machine that
means flowing first through the widest vessels (arter- will soon start to counteract the external (foreign) heat
ies). In places close to the heart, it will also pass through source in order to regulate its own temperature.
the smaller vessels (capillaries) pretty forcefully, simply But secondly, and perhaps more importantly, it is
because there is a lot of pressure behind it. As you move not the heat itself that causes the circulation of fresh
farther from the heart, the pressure decreases, but so blood into and waste-filled blood away from your
too does the number of available avenues for the blood, aching back. Instead, it is the change in heat that pro-
so it still gets to the capillaries. So, on the large scale, motes circulation.
the circulatory system is a nothing more than a high- What about cold? An ice pack is used for a sprained
pressure pump with a series of tubes running to and ankle, and it works very well to decrease the inflamma-
from it. tion and get all the excess extracapillary blood away
In the capillaries, however, the flow is much more from the area. So low pressure (cold) leads to circula-
variable, depending on many smaller, local pressure tion too. Sort of. Can you leave an ice pack on that ankle
sources. Whether these are caused by muscle contrac- for 4 hours? No, and for both the same reasons. The ice
tions (how do you think blood gets up your leg against pack creates a temporary increase in local circulation
gravity on the way back to the heart?) or blockages in not because it is cold, but because is it colder than the
adjacent pathways, very small pressure changes can tissue around it. When the tissue around it becomes
divert blood flow in a significant way. closer in temperature to the ice pack, the effect is
quashed.
2.2 Heat … again It is the creation of temperature differences (or
gradients) that causes changes in pressure that lead to
It is fairly intuitive that pressure and temperature are increased motion of fluid through vessels. Those differ-
related. We are taught this about gases in high school ences can be temporal (in time) or spatial (in space):
chemistry. But it’s harder to make the assumption of temporal being 20 minutes on, 20 minutes off with the

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