Blood Pressure Drop: Understanding Mechanisms

[Smachine]

Hello!

Ive been trying to understand mechanisms of blood pressure and so far the more I read the more confusing it gets. What I really don't understand is this - why does the pressure drop is largest in arterioles? The physiology book I use says that they constitute a high resistance and as the total blood flow is constant at all levels of the cardiovascular system the pressure is suppose to drop and this equation is given Q = ΔP/R, or ΔP = Q × R. I just don't get it, if the R is to increase then to sustain the same Q the pressure should actually increase from the equation. I always imagined that as larger blood vessels branch the total crossection area increases and so the total resistance decreases and that's the reason for pressure drop. I am really confused with this. :(

 

[sameeralord]

Hello welcome to the forum, you are in the same boat as me asking lost of blood pressure questions My knowledge is very poor as as well but after reading this is what I think. This can most surely be wrong.

Why do you say blood flow is constant in every region in cardiovascular system? Don't some regions get more blood. Also even if there is a pressure loss there would still be a flow, it is not a massive pressure drop right? Also isn't it like current in a series circuit with resistors. The current is same through all of them? Please excuse me if this is illogical. Thanks

 

[kathyt.25]

Are you using Principles of Human Physiology by Stanfield and Germann, by any chance?

First off, in cardiovascular physiology, when they refer to pressure "drop", this means pressure CHANGE. Therefore, arterioles have the greatest pressure change, relative to the other vessels.

Also, think of blood flow at two different levels:
(a) NET blood flow
(b) Blood flow through each individual circuit (systemic and pulmonary)

You are right in saying that as resistance (R) increases due to a decrease in radius (r) of the arterioles relative to the arteries, pressure (P) should increase. However, when you say that blood flow (Q) must remain constant throughout the system, it doesn't mean that flow through the circuit of vessels (ie. arteries > aterioles > capillaries > venoules > veins) is constant. Blood flow is far from constant as it travels through individual circuits.

For example, if you're exercising, the GI tract arterioles will constrict, therefore increasing resistance (R), and reducing flow (Q) to the GI tract capillary beds. However, the arterioles in the skeletal muscles will dilate, therefore decreasing resistance (R) and increasing flow (Q) to your skeletal muscles. This regulation/change in flow throughout the body makes intuitive sense, since you wouldn't want blood flowing to your intestines while there's a higher demand for blood in your skeletal muscles when you're exercising. This constant blood flow is referring to how blood through through the systemic circuit must be equal to blood flow in the pulmonary circuit.
Although Q increases and decreases throughout components of each circuit, the net blood flow from the pulmonary circuit will always equal the net blood flow from the systemic circuit. This also makes sense because oxygenated blood from lungs (pulmonary circuit) goes into the heart, to be pumped out to the rest of the body (systemic circuit). What goes in, must come out!