an unnoticed delicacy

Circulatory system is one of the first meals that any medicine student get to chew on. After-all, it is what stitches our physiological systems together. The same thing happened to me. When I started to study for the biology olympiad, circulation was the fourth physiology subject that I encountered. Quite early, isn’t it? I believe that is the reason why most biology olympiad/medical students don’t get to digest this delicate meal. They don’t even have enough time to chew it. I recently found time to re-read the Guyton Medical Physiology Circulation section. This time I was reading it to teach it in a new style, and I had time. So I sat down and spent it. I finally could taste the delicacies that I missed as a student. And the delicate taste that I missed wasn’t just a subtle spice gone unnoticed, it was the whole meal.

Blood velocity doesn’t matter - the first delicacy

We can’t perceive the world as it really is. We have to compensate somehow or the information overload would destroy our limited brains and eat up our limited time. Thus, we fabricate mental models of the world. You make mental models of your friends, of the kind neighbour lady next door, of the lady’s noisy baby, and of the scientific subjects you read about. All these models are wrong, yet some are useful. We should carefully consider to what extent these models deviate from reality. You may think the kind lady next doors wouldn’t mind if you nag about her noisy children to her, but when you actually do it she gets angry at you and stops being kind. This happened because you ignored the simple fact that she really cares about her children and loves them. Even though she has told you that at least a hundred times. They may seem little devils to you but they are little angels in her eyes.

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We often commit the same mistake while we analyse creatures who reside in the science realm. We may ignore subtle facts. We might forget the distinction between two similar words. Our mental model won’t differentiate between these two words when we hear and use them casually. A simple indifference in meaning and perception, a simple lack of nuance can distort our mental model to a disastrous degree.

Go in your house’s kitchen. Turn the faucet on just a little. Water starts flowing. Turn the handle a little bit more. Water flows faster.

What if you increase the faucet’s diameter? Suppose the change in resistance is neglectful. Same volume of water comes out of the faucet at a given time. However its velocity has slowed down, a normal non-pedantic person would go on to say that the water is flowing slower. See the problem? The human mind thinks in terms of water velocity not the volumetric flow. When we compare a slow river with water rapidly coming out of faucet, we would describe the faucet as flowing faster. Wouldn’t you? I certainly would. The river volumetric flow is however obviously higher. This is counter-intuitive, isn’t it? Now here is where the problem begins, blood velocity doesn’t matter in most of our vessels. Yes I know, I know. Yes It does matter in our capillaries, yes it affects the shear stress our vessels experience which is important during placental and prenatal development, and yes blood velocity directly affects wether blood flow is turbulent or laminar. But a key aspect of our arteries and arterioles is the volumetric flow and not the blood velocity. A core responsibility of the arterial tree is to deliver sufficient volume of blood to the respiring tissue. Pay attention to the word volume. When blood reaches the capillaries it would slow down, due to high cross-sectional area of the capillary bed independent of its velocity before arrival. I am not telling you to ignore blood velocity altogether. In fact my purpose is quite the opposite. Blood velocity is a different aspect of circulatory system, whose significance may be even more than volumetric flow in certain scenarios. Ignoring such factor is reminiscent of non-systemic thinking and falling prey to tunnel vision. I am highlighting the nuance. These two characteristics are key components of the circulatory system and if we don’t emphasise the delicacy in their distinction, our perception will mislead us. Our mental models will be distorted beyond the point of usefulness.

This nuance is one of the many reasons why I’d always struggle while trying to predict what happens given a change in part of the circulatory system system. What would happen to the blood delivery of a piece of the respiring tissues if arteries or the arterioles constrict? What would happen if they are dilated? My struggle rooted in me implicitly thinking in terms of blood velocity and not the volumetric flow. Take this extremely simplistic scenario as an example. An arteriole gets dilated. My mental model would implicitly conclude that blood is slowing down because cross-sectional area just increased. I wouldn’t necessarily say out loud this prediction. It is my intuition. I would then proceed to use laws of physiology and fluid dynamics as explicit prediction of reality. Now I know that volumetric flow either stays the same or increases. It never decreases. So fortunately we are not doomed and we didn’t make a mistake. However the disagreement between intuition and reality induces discomfort. The problem is that both my intuition and the strict mathematical formula are correct¹. They are simply describing different events. Blood velocity decreases and volumetric flow changes in another direction. This was a simple scenario. A simple formula showed us everything². However if we were dealing with a much more complex scenario, the distinction between volumetric flow and blood velocity could fabricate implicit assumptions for us, that distorts our image of reality.

The other delicacies

I will continue the unnoticed delicacy series. At the time I am writing this, there are 2 other delicacies that I want to write about. So I will be hopefully read by you again while describing another delicacy gone unnoticed.

Footnotes

1. Well not entirely. The relationship between Volumetric Flow, Blood Velocity, Vessel Diameter is convoluted. Especially if this vessel is in a body and not isolated. An increase in cross-sectional area decreases blood velocity if volumetric flow is constant. This is what we intuitively conclude when we are thinking about this system. However a vessel diameter affects resistance. According to poiseuille law, resistance is inversely correlated to radius to the power of four. Thus volumetric flow is directly correlated to radius to the power of four. Blood velocity is inversely correlated to cross-sectional area and directly correlated to volumetric flow. Cross-sectional area is equal to $\pi r^2$. Thus Blood velocity can in fact increase when cross-sectional area increases. However poiseuille’s law has an assumption. It only holds in laminar flow conditions. As blood velocity increases there is a higher probability of turbulent flow which can decrease blood velocity. It’s convoluted isn’t it? And this the situation for an isolated tube outside of the body. Our circulatory system tries to compensate the alterations in volumetric flow through many mechanisms. This adds another layer of complexity. So it’s not bold of us to assume that volumetric flow stays constant in long-term if a minor modification in radius happens. ↩

2. As I said in footnote 1, even in this overly simplistic scenario, physiology doesn’t disappoint us in making everything utterly complex ↩