Notes
Vascular Resistance
Sections
Vascular resistance = The impediment to blood flow.
Total peripheral resistance (aka, systemic vascular resistance) = Describes the resistance to blood flow throughout the entire systemic vasculature (throughout the entire body).
Resistance within an organ for example, resistance within in the kidney.
Determinants
Three key determinants of resistance:
- Blood viscosity
- Vessel length
- Vessel radius
Blood viscosity
Directly proportional to vascular resistance.
Hematocrit (the volume of red blood cells in the blood) is the primary determinant of blood viscosity.
Clinical correlation: patients with abnormally elevated levels of blood products often manifest strokes from blood clots as a part of a broader hyperviscosity syndrome.
Vessel length
Directly proportional to resistance.
Blood flow passing through a longer vessel will encounter greater friction, and, therefore, more resistance.
Vessel radius
Indirectly proportional to resistance. Recall that the radius is the length of a line from the center of a circle to its perimeter; it is half the length of the diameter, which extends from one side to the other.
The inverse relationship between vessel radius and resistance is NOT linear:
When the radius decreases, resistance increases exponentially by the fourth power.
Poiseuille equation: Describes how the determinants of blood resistance interact.
Resistance = 8 * blood viscosity * vessel length / Pi * radius to the 4th power.
Series vs Parallel Resistance
The arrangement of vessels affects resistance.
Series resistance: Illustrated by the blood vessels of a single organ.
Equal to the sum of the individual resistances that blood encounters as it flows through vasculature.
Pressure decreases as blood moves through the series of vessels because of increasing resistance; it decreases most significantly in the arterioles.
Parallel resistance: Illustrated by the branching of the systemic circulation.
Each parallel artery receives a portion of the total blood flow. Addition of parallel vessels decreases the total resistance. If resistance within any one of the individual vessels increases, so will total vascular resistance.
Memory aid:
If this is confusing, think of blowing through multiple straws: the more straws you add, the less resistance there is; but, if one of those straws becomes blocked, overall resistance increases.
Full-Length Text
Here we will learn the determinants of vascular resistance to blood flow.
To begin, start a table, and denote that vascular resistance is the impediment to blood flow.
Physiologists and clinicians are interested in:
Total peripheral resistance (aka, systemic vascular resistance), which describes the resistance to blood flow throughout the entire systemic vasculature (throughout the entire body), and,
Vascular resistance within a single organ; for example, resistance within in the kidney.
To begin, let's illustrate the three key determinants of resistance:
- Blood viscosity
- Vessel length
- Vessel radius
We'll focus on blood viscosity, first.
Write that it is indicated by the symbol eta, which resembles an "n," and is directly proportional to vascular resistance.
Write that hematocrit (the volume of red blood cells in the blood) is the primary determinant of blood viscosity.
To illustrate this, draw two tubes.
Fill one with blood that has a low hematocrit level.
Fill the other with blood that has a high hematocrit.
Which will have greater resistance?
The one with a higher hematocrit – thus, high hematocrit produces high blood viscosity, which is a key determinant of resistance.
As a clinical correlation, consider that patients with abnormally elevated levels of blood products often manifest strokes from blood clots as a part of a broader hyperviscosity syndrome.
Next, write that vessel length, indicated by the letter "l," is also directly proportional to resistance.
Blood flow passing through a longer vessel will encounter greater friction, and, therefore, more resistance.
To show this, draw a short blood vessel and a longer blood vessel.
Indicate the direction of blood flow, and write that the shorter vessel creates less resistance than does the longer vessel.
Finally, write that vessel radius, "r," is indirectly proportional to resistance. Recall that the radius is the length of a line from the center of a circle to its perimeter; it is half the length of the diameter, which extends from one side to the other.
The inverse relationship between vessel radius and resistance is NOT linear:
When the radius decreases, resistance increases exponentially by the fourth power.
To show this, draw one large and one small vessel in cross-section.
Indicate that the large one has a longer radius, and, therefore, less resistance to vascular blood flow.
The smaller vessel has a shorter radius and creates greater resistance to blood flow.
Elsewhere, we'll learn how the vessel radius is altered to redirect blood flow.
Next, let's write out the Poiseuille equation, which describes how the determinants of blood resistance interact:
Write that resistance = (8 x blood viscosity x by vessel length)/(Pi x radius to the 4th power)
Notice that this equation reflects what we illustrated above:
Resistance is directly proportional to blood viscosity and vessel length, and indirectly proportional to vessel radius.
So far, we have addressed key determinants of resistance within a vessel, now let's take a step back and look at the vasculature, as a whole, and see how the arrangement of vessels affects resistance.
We describe this arrangement as series vs. parallel resistances.
First, write that series resistance is illustrated by the blood vessels of a single organ.
To show this, draw an artery that branches to form arterioles, which deliver oxygenated blood to capillary beds;
Venules carry deoxygenated away from the capillary beds and drain into veins.
Indicate that series resistance is the sum of the individual resistances that blood encounters as it flows through these vessels.
To show this mathematically, write that:
Total resistance is equal to the sum of the resistances in the arteries, arterioles, capillaries, venules, and veins.
Pressure decreases as blood moves through the series of vessels because of increasing resistance; it decreases most significantly in the arterioles.
Next, write that parallel resistance is illustrated by the branching of the systemic circulation.
To show this, draw a large artery that branches into smaller arteries, then into arterioles.
Indicate that each parallel artery receives a portion of the total blood flow.
To illustrate this mathematically:
Label the first set of parallel arterioles in our diagram R1-R4.
Write the equation for parallel resistance, as follows:
1/ total resistance = 1/the resistance of each arteriole; in our example, R1-R4.
Write that addition of parallel vessels decreases the total resistance, but, Iif resistance within any one of the individual vessels increases, so will total vascular resistance.
If this is confusing, think of blowing through multiple straws: the more straws you add, the less resistance there is; but, if one of those straws becomes blocked, overall resistance increases.