Three more rules...

Instead of 'rules,' let's call them equations.  But before we go on, let's (one more time) list those priorities (in order) of the CV System:

• Maintain Arterial Blood Pressure
• Maintain Cardiac Output
• Maintain Venous Pressure

And... a brief summary of why they are important?

• To ensure delivery of blood to the Heart and Brain
• To deliver oxygen/nutrients (via blood) and remove wastes
• To prevent fluid from being forced into tissues (particularly, lung tissue)

Excellent!  Now, we proceed to our 3 Important Equations of the CV System:

Our first equation is this:

Stoke Volume (SV) ≈ Preload, Contractility, and Afterload

What does all of this mean?  To start, Stroke Volume (SV) refers to how much blood is ejected with each heartbeat.  Contrasting to cardiac output (CO), CO not only describes how much but also how fast that blood is being ejected by the heart (we'll return to this).  Preload and Afterload both refer to a stretching force or "stress" in the heart chambers.  Well, what does that mean?  Thinking of it like a balloon (which you can thank my teacher for this example).  To fill the balloon with air (say, when you're blowing up balloons for a birthday party), you have to overcome the 'resistance' of the latex to force your breath inside of the balloon.  Think of this as preload.  During preload, the 'resistance' of the wall of the heart is overcome to allow the heart to fill with blood.  This corresponds to diastole (the top number in a blood pressure reading).  Now, how about afterload?  Well, back to our balloon:  if you were to pop a balloon, and you were to slow it down to ultra slow speed, you would see that the balloon shrinks back from the pinhole.  That would also be a 'force,' the force that retracts the latex back to a smaller, less stressed size.  Obviously, no one 'pops' the heart with a pin (or so we hope), but the thought is similar.  Afterload is the force that the heart must overcome to eject blood out of the heart chamber.  Does that make sense?  If the heart was a rubber duck that you had filled with water, the afterload would be how hard you would have to squeeze the duck to squirt all of the water out.  That's afterload.  Afterload corresponds to contraction in the heart (or the pressure of systole).  Systole is the bottom number of your blood pressure reading.  Hopefully, you may be able to understand why an increase in either number is bad news.  When your diastolic number is too high, that means that the heart has to work harder to fill the heart with blood.  In contrast, when your systolic number has increased, that means that the heart is working harder to eject blood out of your heart!

Here's an Ultra-Slow Video of a Balloon being Popped (note that the latex is 'shrinking' to normal size):

Now, that just leaves contractility.  Contractility just describes the strength or vigor with which the heart is contracting.  This number is independent of both preload and afterload.  Contractility is, more or less, going to be controlled by the nervous system.  Your contractility increases when you are frightened, for example, but decreases when you're relaxing.  I do want to make a note:  Do not mistake contractility for speed.  What I mean is, contractility is referring to how hard the heart is beating.  Imagine seeing a monster while chewing gum.  Yes, your heart rate is going to race, holy cow there's a monster!  But, it's also going to pump 'harder,' and in our case may be best exampled by 'chomping' on that gum while you're scared.  Similarly, if you were to be chewing gum while studying, you are just 'chewing' on the gum, not chomping.  Yeah?  

So, to summarize:  Stroke Volume (or the amount of blood ejected by the heart) is related to the preload (or the force that is overcome to allow filling of the heart), afterload (or the force that is used to empty the heart) and contractility (or the vigor with which the heart contracts, as controlled by the nervous system). 

Our second equation is: 

Cardiac Output = Heart Rate * Stoke Volume

Well, look here!  There is stroke volume again.  The good news is you now know a lot about that!  Stroke Volume is influenced by Preload, Afterload, and Contractility!  Heart Rate, I am sure many of you are familiar with.  This describes how fast the heart beats.  Many things change how quick the heart beats: Monsters, Excitement, Sleep, Spas, etc.  Heart rate most assuredly changes via stimulation of the nervous system by your environment (both externally and internally). 

So, if you take Stroke Volume (or the amount of blood being ejected by the heart) and multiply it by Heart Rate (or at what speed the heart is beating), you have Cardiac Output.

Big Picture Moment:  Maintenance of Cardiac Output is Priority number TWO of the CV System!

Our final equation is: 

Arterial Blood Pressure ≈ Total Peripheral Resistance * Cardiac Output

Alright!!  Here we are again with a term you know all about, Cardiac Output (or the amount of blood ejected [Influenced by Preload, Afterload, and Contractility] by the speed at which the heart is beating [influence by the nervous system]).  Excellent!  Now, let's talk about Total Peripheral Resistance (TPR).  TPR is going to be something that is going to be a little difficult to explain.  Bascially, TPR refers to how contracted or dilated the vessels (arteries and veins) in your body are.  Let's divert to an example:  In the cold weather, your body will be vasoconstricted.  What I mean by this is: the vessels in your body are going to contract to pull body away from your periphery (say your hands, feet, arms, legs) to concentrate as much blood as possible to the core of your body.  Why?  Well, what is located in the core of your body?  Well, all of your super important organs!  This, oftentimes, is why frost bite occurs.  When the weather is cold enough to cause vasoconstriction, there is not a lot of blood being delivered to the hands and feet due to vasconstriction of vessels.  This means that tissue of the hands and feet are not receiving oxygen, nutrients, or having waste removed.  This can kill those tissues!  In contrast, when the weather is too hot the body vasodilates.  This is just the opposite of vasoconstriction.  Vasodilation means that the vessels dilate (or expand) to allow as much blood as possible to the periphery.  This accounts for flushed faces and sweating in the summer.  By allowing a ton of blood (body temperature blood) to as many surfaces as available, the body hopes to lose much of the heat by evaporation.  Something that is also true is that the body does not want to cook all of those super important organs.  Total Peripheral Resistance, then, refers to how vasoconstricted or vasodilated the vessels in the body are.  

So, if we take Cardiac Output by the TPR (or the degree to which blood vessels are vasoconstricted of vasodilated), we have determined the Arterial Blood Pressure.  

And, NOW, for the even bigger picture, where else have we seen Arterial Blood Pressure before?  That's right!  This was Priority Number ONE of the Cardiovascular System.  As you can see, there is a TON of stuff that goes into making sure the Priority Number one is met, explaining much of the complexity of the CV System.  

Cheers!