How about the top three equations when discussing the CV System?
Do you remember the equation that allows you to assess wall stress?
How about hypertrophy? What can you tell me about that?
Finally, can you give me a brief summary in regards to the PV Loop of the LV?
To allow you a moment to marinate on these questions, here is a random screencap of one of my favorite movies:
That's right, Toothless! And, yes, I'm too old for this cartoon. But, I just cannot help but feel a little love for this dragon! Such a nice story for children. But, I digress....
The Top Three Priorities of the CV System:
- Maintain Arterial Blood Pressure
- Maintain Cardiac Output
- Maintain Venous Pressure
Great! The Top Three Equations in the CV System:
- ArtBP ≈ TPR * CO
- CO = HR * SV
- SV ≈ Preload, Afterload, and Contractility
Excellent, again! The equation for wall stress:
Wall Stress = (Intracavitary Pressure * Radius)
Wall Thickness
Outstanding! How about Hypertrophy?
Hypertrophy describes an increase in cardiac mass. Hypertrophy can occur in one of two ways, either Concentric Hypertrophy or Eccentric Hypertrophy. Eccentric Hypertrophy describes an increase in cardiac mass that has an increase in volume but does not have an increase in wall thickness. Concentric Hypertrophy describes an increase in cardiac mass that has an increase in wall thickness, and either has no change or very little change in cardiac volume. Hypertrophy, in particular Concentric Hypertrophy, is the easiest way by which the heart can return wall stress to a normal value once increased.
Fantastic job!! Finally, how about a quick summary of the PV Loop of the LV?
The PV loop is the depiction of a single heart beat in the body. Along the x-axis, volume is depicted. Along the y-axis, pressure is depicted. The line that originates from the (0,0) point on the graph and connects to the Upper Left corner of the PV Loop is a depiction of contractility. Beginning with the Upper Left corner, this is the point at which the Aortic Valve closes. The Bottom Left corner is the point at which the mitral valve opens. The vertical line that connects the Upper Left and Bottom Left corner depict the period of isovolumetric relaxtion in the heart (a point of relaxation that has no change in volume). The Bottom Right corner is the point at which the mitral valve closes. The Upper Right corner is the point at which the aortic valve opens. The vertical line that connects the Bottom Right corner to the Upper Right corner depicts the period of isovolumetric contraction in the heart (a point of contraction that has no change in volume). Starting that the Upper Left corner and ending at the Bottom Right corner, diastole has occurred. Starting at the Bottom Right corner and ending at the Upper Left corner, systole has occurred. The width of the PV Loop corresponds to stroke volume and is easily influenced by change in preload, afterload and contractility. Something to remember is that the PV Loop is not a "snapshot" but rather a line that is traveled to depict a single heartbeat. The pressure is constantly changing along the PV Loop, while the volume has two points (isovolumetric relaxation and isovolumetric contraction) at which the volume is unchanging. Also, the PV Loop is incapable of accounting for wall thickness changes, as it only depicts one heartbeat and changes in wall thickness occur over a long period of time.
And there you have it! Great job answering those review questions! They will be important to keep in mind as we proceed through our studies of the CV System!
So, where are we today? Well, using all of the knowledge from the review questions, what two issues to you feel dominate the CV System/ You've got it, blood pressure and blood flow! So, how do you suppose the body keeps such tight control over either of these two issues? Well, as you're about to see, there are many intricate ways in which the body (read: brain) ensures that changes in blood pressure or blood flow are addressed as quickly and diligently as possible. You see, it must ensure that Priority #1 is maintained, otherwise it will be incapable of controlling any changes that when ensure the body stays alive.
In regards to ArtBP, a series of receptors (known as baroreceptors) exist in the body. Well, what are baroreceptors? Baroreceptors can be best thought of a cells that detect stretch in the arteries. How stretched (or distended - dictated by how full of blood the vessel is) the artery is results in a nervous signal being sent to the brain to communicate that stretch. Let me see if I can give you an example... Pretend it's Thanksgiving. What do most people do at Thanksgiving? They eat, and they eat a lot! So, as your stomach fills with food, it becomes distended. Now, what happens when you overeat? You feel terrible! With good cause, too. If you we unaware that your stomach was painfully full (or distended) with food, your stomach might burst (and, for the record, it really would take a whole lot of food to cause this to happen). Well, there are stretch receptors on your stomach that tell the brain that the stomach, "Hey! The stomach is too full, stop eating." What about the opposite? What happens when there is no stretch in the stomach? That's right! Those stretch receptors say, "Hey Brain! There is no stretch in the stomach, you need to find food!"
Now, the stomach receptors aren't baroreceptors (the baroreceptors exclusively measure the stretch in blood vessels), but the idea is the same. When the arteries are overstretched because they are too full of blood, a message is sent to the brain that says, "Hey! Brain, better make some adjustments in the heart (via CO, SV, Preload, Afterload, Contractility) to decrease the stretch in the baroreceptors. Just the opposite is true too. When there is not enough stretch in the baroreceptors, the baroreceptors send a message to the brain that says, "Hey! The arteries are not full enough! Better make some changes!"
Now, baroreceptors are not in all arteries. Actually, the baroreceptors are located in specific arteries. There are the aortic baroreceptors, in the aorta, and the carotid baroreceptors in the carotid artery.
For simplicity, I have edited this photo to highlight only the location of the aorta and carotid arteries. See how close they are to the heart? This is much of the reason that these arteries were chosen to possess the baroreceptors. Remember, Priority #1 is designed to ensure that ArtBP gets blood to the brain and heart (very important organs).
So, how can the brain respond, then? We've already mentioned causing a change in CO, SV, Preload, Afterload, and Contractility. But, how? How does the brain cause those to change? Well, basically, there are three types of changes that can occur:
- Autonomic nerve changes
- Endocrine (or hormonal) changes
- Structural changes
Starting with the autonomic nerve changes: It's important to establish that there are two different types of autonomic nerves in the body. There are the sympathetic nerves, while the others are the parasympathetic nerves. What's the difference? Well, the sympathetic nerves control your "Fight or Flight" response. Well, what does that mean?! If you encounter a monster, what would you hope to have happen? Well, you can either fight the monster or you can run fast (of flee) from the monster. In order for you fight or flight to be successful, you will need a series of things to change in your body to help that out. Can you think of some things that would help you fight or fly better? Well, you probably need to breathe faster (to get more oxygen), you will need you heart to pump faster, so that the blood is getting oxygen and nutrients, to the tissues quicker. You might need your pupils to dilate (or expand) so that you can see more. And, you definitely don't want to have to use the restroom. So, you'd like for your intestinal motility to decrease and for you bladder to stay relaxed (rather than contracted, which is what occurs when you urinate). All of those things, those are the things the the sympathetic nervous system (SNS) causes to happen (or, those are the things the SNS controls).
The other system is the parasympathetic nervous system (PNS). The PNS does just the opposite of everything I have listed above. So, the PNS would reverse those changes once you are done fighting or flying from the monster. Does that make sense?
Back to the heart, as you can imagine, when the baroreceptors sense that there is too much stretch in the arteries, the PNS is activated to cause a decrease in CO, SV, Preload, Afterload, or Contractility. But, wait! I thought Priority #2 was to maintain Cardiac Output, not decrease it!!! Well, it is. You see, if the baroreceptors are too stretched this means that there has already been an unwanted increase in CO or ArtBP. Yeah?
Now, autonomic nerve changes are by far the quickest response to a change in baroreceptor input. Followed by those changes, you have endocrine (or homonal) changes. These, certainly, are slower than any nerve change. But, they aren't too bad. The hormone epinephrine is one that will increase heart rate. The hormone renin is going to affect how the kidneys function. Again, the kidneys?? What do they have to do with anything? Well, if the kidneys are triggered by the hormone renin, there are going to excrete (or get rid of) less salt. For now, just know that water follows salt. So, if salt is retained by the body, so is water. This water can then be used to increase blood volume. There we are, back to the heart! So, for example, if the baroreceptors sense the the arteries are too empty, the brain may cause a release of renin, which will cause the kidneys to conserve salt (and therefore conserve water), which will end in the increase of blood volume (or, pretty much, result in there being more blood in the arteries).
Finally, we have structural changes. Well, we've already talked about this! You know that hypertrophy causes an increase in cardiac mass. You know that it can happen in two different way, but you also know that it takes a long time. Therefore, structural changes are the slowest changes that will occur in response to a change in baroreceptor stretch (and, while I'm here, structural changes in the heart are going to be initiated by the brain through both nervous change and endocrine change).
So, why three different responses to a change in the baroreceptor, isn't that inefficient? Well, actually, it is very important for the body to have this three-tier system in response to baroreceptor changes. You see, each time you sit, stand, exercise, get stressed, there is a change in all of the factors of the CV System (meaning CO, SV, Preload, Afterload, Contractility). These changes are normally controlled by the nervous change. In more chronic situations, the endocrine changes happen initially, followed by structural changes. If it weren't for this three-tier response to change, each time you sat or stood, a structural change could be initiated and cause hypertrophy! Well, that wouldn't be good at all!
As you can see, though complex, the CV System is intricate for a very important reason: Not only does it keep you alive, but it also prevents unwanted or unneeded changes from occurring)!
Cheers! - JD
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