So, we just got done talking about the baroreceptors. The baroreceptors monitor the stretch that exists in the aorta and carotid arteries, and then it delivers a message about the changes to the brain via nerves. Basically, then, we have described changes about the amount of blood present, or (more specifically) changes about the blood pressure.
Well, how about changes in blood flow (which we will just talk about the biggie, cardiac output)?
Before we get too far, just know, the system of detection concerning blood flow is much less developed than the system that detects changes in blood pressure.
Generally, blood flow changes are more so detected in individual tissues or organs. As the body is constantly changing to ensure that the most important organs are receiving blood, this is factor one. Next thing to think about, the body is vast! There are plenty of places that blood flow could be slowed. So, then, how does the body react to changes in blood flow?
Unfortunately, there are no receptors that are going to detect an increase or decrease in blood flow (per se). Instead, changes in blood flow are detected (and then adjusted) by an organ itself. So, for example, perhaps with have a thrombus (or blood clot) in the right kidney. Oh no! With there being a blood clot in the right kidney, that means that the right kidney is going to be receiving less than adequate flow of blood due to blockage. When less than adequate blood is delivered to an organ, the organ tissues become ischemic (a term that describes a lack of adequate blood to a tissue). In response to ischemia, a series of events will occur, but most importantly vasodilation will occur. We discussed vasodilation, do you remember what that means? Right! The blood vessels will dilate (or expand) to cause more blood to flow to an area. In our case, the blood vessels will be vasodilating to cause more blood flow to the right kidney.
In the grand scheme of things, vasodilation to just that one right kidney is going to have that much of an impact on blood flow to the heart or brain. But, how about in a case where thrombi have clogged the right kidney, the liver, and the large intestine. Holy cow! The vasodilation that results in the right kidney, liver and large intestine (with the liver and large intestine being much larger organs that the right kidney) is likely to make a much more profound impact at the heart and brain. Why? Well, if more blood is flowing to the right kidney, liver and large intestine, where did that blood come from? It had to have taken from the blood that is flowing to the heart and brain. So, when less blood than expected returns to the heart and brain, those organs are going to notice!
How do you suppose they know that there is less blood returning? That's right! If there is less blood returning to the heart, that means there will be less stretch in the baroreceptors (indicating that blood pressure has fallen).
So, there are two methods by which a change in blood flow is measured. The first is a direct measurement of change in blood flow. This occurs locally in organ tissues. Direct detection occurs via detection of oxygen, nutrient and waste levels, all managed via blood flow. Changes in these levels can cause a tissue to vasoconstrict or vasodilate in response (known as metaboreflexes).
The next was in an indirect measurement. If metaboreflexes have occured (or, if tissues have vasodilated or vasoconstricted due to changes in oxygen, nutrient or waste levels), the subsequent change in blood pressure is going to be detected by baroreceptors, which communicate to the brain to cause nervous, endocrine or structural changes in response.
So, what's the big picture? In regards to the detection of changes in blood flow, the system is not as advanced as the system of baroreceptors that detect changes in arterial blood pressure. However, though weaker, the system that detects changes in blood flow can get big effects once metaboreflexes result in changes of blood pressure that stimulate the baroreceptors, recruiting the more powerful system for assistance.
Cheers! - JD
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