Mechanisms of Blood Pressure

If someone were to ask you, “what is blood pressure?”, how would you reply? You might have no real idea, or you might describe it as the pressure on your arteries, or you might describe it as systole and diastole, but none of those answers truly offer an answer to the question. In this article, we will go into detail about what exactly makes up blood pressure; we will not discuss the details of hypertension, simply the mechanisms that make up blood pressure.
 

What is Blood Pressure?
The term “blood pressure” is likely known to you, and it may also be known that blood pressure is, indeed, the force of blood being pushed against the arteries by the heart [1]. Blood pressure is further broken down between systole and diastole as two different times of force measurement.

Systole
Systole is what people read as the top number on a blood pressure monitor and it represents the contraction phase of the left ventricle of the heart [2]. It is the height of force on the arteries.

Diastole
Meanwhile, diastole is the bottom number on a blood pressure reading and represents the relaxation of the left ventricle of the heart as it prepares for its next contraction (systole)[2]. It is the lowest point of force on the arteries.



 

So, we understand how to read blood pressure and we understand what that reading means, but we still do not understand what influences change in blood pressure. Why is it that one day these numbers are, for example, 116/66, and months later they are 130/70? In simple terms, what mechanisms influence how hard the heart must force blood throughout the body? Let us find out.

Mechanisms that Impact Blood Pressure
There are four mechanisms that make up blood pressure.

Peripheral Resistance
Peripheral resistance is the amount of “catch” that occurs across the vascular system walls. Think of it as if you were to move your hand across a clean counter top – there is little resistance. Now, if you were to move your hand across a counter top that had dried pancake syrup on it, your hand would “catch” and stick; this is similar to how good or bad one’s peripheral resistance. The more resistance, the more force the heart must use to push nutrients, blood cells, and the like, throughout the body [3]. The less resistance, the less force is necessary to fulfill demands of the body.

What can decrease or increase peripheral resistance?
Vasodilation (widening, opening) and vasoconstriction (closing) of the vessels in the body will decrease and increase peripheral resistance, respectively [4]. This can be due to a multitude of factors including, but not limited to pharmacological reasons, exercise, etc  [4]. There are other, more direct, impact factors on peripheral resistance, as well, but they are beyond the scope of this article. 

Vessel Elasticity
As blood is ejected from the heart into the arteries and out to the body, there is a stretch effect that the arteries are able to use to mitigate the force being placed upon them [3]. So, as the left ventricle contracts (systole), the blood shoots out of the heart through the arteries and as this is the height of force being placed on the arteries, the arteries can stretch a bit to reduce the pressure placed during any given high force moment (usually systole) [3].

What influences vessel elasticity?
Anything that leads to the hardening of the arteries, like arteriosclerosis, because as the arteries harden and calcify, they offer less of that stretch effect and are less forgiving of the force being exerted upon them and as such an increase in blood pressure is witnessed [3].

Blood Volume
Bloodvolume is exactly what it implies by name – the amount of blood. It is possible to manipulate blood volume, but the most impactful way is with water. If blood volume is too high, there is more overall pressure against the vasculature [3]. This is one of the most important mechanisms in the body and has the most influence on general blood pressure, but can also have a profound effect in chronic blood pressure measures. If the body has a reason to add water to the circulatory system, blood volume increases.

What influences blood volume?

As mentioned, water has the most profound impact on blood volume, but blood volume does not spike up and down needlessly in a closed system (meaning, if you haven’t been shot or stabbed, for example – ha!). That said, the higher serum sodium levels, the more water will follow sodium, and the more water, the more blood volume [3]. Or, temporary decreases can also come from circumstances like sweating in which the body loses water, draws some from the blood, reducing volume, therefor reducing blood pressure (ever heard of a person passing out while in an intense sport? Possibly due to lack of blood volume).

Cardiac Output
Cardiac output is typically characterized by the amount of blood pushed out of the heart per minute; it is also characterized as heart rate and stroke volume (HR: number of beats of heart and SV: amount of blood released per beat)[5]. However, in this context, we are most interested in stroke volume. We are interested in stroke volume, because stroke volume is characterized by systole [3]. The more blood is pumped out of the heart (increase in stroke volume), the harder the left ventricle of the heart must contract (systole), leading to an increase in blood pressure.

What influences cardiac output?
Any situation in which the body requires more blood increases cardiac output; be that exercise, acute stress, or any other circumstance.
 

SUMMARY

Now, when someone asks you what blood pressure is, you will be able to reply with an educated, researched answer. Blood pressure originates in the heart, is measured in the vasculature, and is made up of both. Blood pressure is made with a combination of peripheral resistance, vessel elasticity, blood volume, and cardiac output (specifically, stroke volume).

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[1] Description of High Blood Pressure - NHLBI, NIH. (n.d.). Retrieved from http://www.nhlbi.nih.gov/health/health-topics/topics/hbp

[2] Understanding Blood Pressure Readings. (n.d.). Retrieved from http://www.heart.org/HEARTORG/Conditions/HighBloodPressure/AboutHighBloodPressure/Understanding-Blood-Pressure-Readings_UCM_301764_Article.jsp#.Vq0UvbIrKUk

[3] Factors that Affect Blood Pressure. (n.d.). Retrieved from http://bk.psu.edu/clt/bisc4/ipweb/misc/assignmentfiles/cardiovascular/Fact_Aff_Blood_Pressure.pdf

[4] Cardiac Anesthesiology: Peripheral Resistance, Control of Blood Pressure, Cardiac Physiology, Starling’s Law. (n.d.). Retrieved from http://pie.med.utoronto.ca/CA/CA_content/CA_cardiacPhys_peripheralResistance.html

[5] Physiology & Psychology: Performance Benchmarks -- Cardiac Output. (n.d.). Retrieved from http://btc.montana.edu/olympics/physiology/pb01.html

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