Cardiac output and blood pressure

[neuroiscool42]

So we all know the equation BP= CO x Peripheral Resistance and know that if we increase cardiac output we increase BP. However, I was studying in TPR and they were talking about how if you increase BP you can decrease cardiac output due to increased afterload. Why is the relationship in the former directly proportional but the relationship in the latter inversely proportional?

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[sfgiants94]

I will give this a shot; afterload, from what I understand is "leftover" blood that remains in the heart after contraction. When resistance is greater more energy is needed to get all of the blood out of the heart in the same time span, because of that increased resistance some blood remains leftover in the heart. Essentially, the heart has a harder time forcing blood from it into circulation when systemic arteries and veins have increased blood pressure. This results in more blood that never leaves the heart during that contraction and thus a decreased C.O. and increased afterload.

 

[neuroiscool42]

I will give this a shot; afterload, from what I understand is "leftover" blood that remains in the heart after contraction. When resistance is greater more energy is needed to get all of the blood out of the heart in the same time span, because of that increased resistance some blood remains leftover in the heart. Essentially, the heart has a harder time forcing blood from it into circulation when systemic arteries and veins have increased blood pressure. This results in more blood that never leaves the heart during that contraction and thus a decreased C.O. and increased afterload.

But if you increase CO, you increase blood pressure. Wouldn't that increase in blood pressure lower cardiac output? I feel like I'm literally going in circles with this concept.

 

[sfgiants94]

But if you increase CO, you increase blood pressure. Wouldn't that increase in blood pressure lower cardiac output? I feel like I'm literally going in circles with this concept.

Increasing CO (more volume of blood into systemic circulation) would increase BP, think of filling a water balloon with water; more volume of water increases its pressure. If the balloon could never pop (hypothetically) eventually the pressure inside the balloon would be more than the water pressure of the hose filling it. On a smaller scale this is going on with your heart. Your heart is the hose filling your circulatory system (the balloon). When you have that increased BP your heart can't get as much blood into the circulatory system because it pushes back with greater force.

So, increased CO will increase BP because of the increased volume in the circulatory system (the balloon is now more full than it was prior to the contraction). When the BP is higher the heart can't put out as much blood, thus decreasing CO (the balloon is so full that it pushes against the hose that is filling it).