# Resistance

Overview |
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- Resistance to blood flow is a property dependent on the physical characteristics of a segment of vasculature and the blood flowing within it. Qualitatively, the resistance to blood flow is the "difficulty" of pushing blood through a section of blood vessel.

Measurement |
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- Because of the complex geometry of blood vessels and the heterogenous composition of blood, the resistance of a vascular segment cannot be mathematically predicted and instead must be empirically determined. To determine the resistance of a vascular segment, a blood pressure gradient must be established across the segment and the resultant volume of blood flow through the segment per unit time (Blood Flow Volume) must be measured. The resistance of the vascular segment is simply the ratio of the blood pressure gradient to the Blood Flow Volume
- Formally: Resistance = (Blood Pressure Gradient)/(Blood Flow Volume)

Conceptual Assistance |
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- As mentioned the hemodynamic concept of Resistance is useful for quantitating the "difficulty" of pushing blood through a segment of vasculature. By "difficulty" we mean that the greater the resistance of a vascular segment is, the larger blood pressure gradient is required to actuate the same amount of Blood Flow Volume through the segment. An important corollary to the concept of resistance is that once a pressure gradient is used to actuate blood flow through a resistant segment of vasculature, the pressure of the blood following exit from the segment is reduced by the value of that gradient. For example, if the blood pressure prior to a segment of vasculature is 100 mm Hg and a 20 mm Hg pressure gradient is required to push blood through the segment, then the pressure of the blood following the segment will now be 80 mm Hg. In a qualitative sense, then, resistant segments of vasculature "drop" the pressure of blood by offering resistance to the flow of blood through that segment

Influential Variables |
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**Overview**- The complex geometry of vasculature and the composition of blood means that precise mathematical predictions of a segment's resistance is impossible and that empirical means must be used. However, some rough relationships can be inferred from the field of Fluid Mechanics, which is an engineering discipline that studies the flow of fluid through physical spaces such as pipes. Importantly, for any individual vessel, the most important factor affecting its resistance is its luminal radius
**Radius**- The resistance of a segment of vasculature is inversely proportional to the fourth power of the segment's radius. Therefore, thin segments of vasculature offer tremendously more resistance than wide ones. Certainly, pumping 1 gallon of water per minute through a needle will require far more of a pressure gradient than achieving the same flow through a wide pipe. It should be pointed out that this fourth power relationship means that doubling the radius of a vascular segment will reduce its resistance by roughly 16-fold whereas halving of a segments radius will increase its resistance by 16-fold. The powerful effect of luminal radius on resistance is the principal mechanism used to dynamically regulate the resistance of particular vascular segments. Therefore, vasoconstriction is used to increase vascular resistance while vasodilation is used to decrease vascular resistance.
**Length**- The resistance of a segment of vasculature increases proportionally with the segment's length. Therefore, long segments of vasculature offer more resistance than short ones. Certainly, pumping 1 gallon of water per minute through a garden hose one mile long will require far more of a pressure gradient than achieving the same flow through a garden hose one foot long.

**Arrangement**- The arrangement of vascular segments exerts a powerful influence on the total resistance they offer. Vascular segments can be arranged in series, meaning that blood must flow through all the segments sequentially. Vascular segments can also be arranged in parallel, meaning that the segment branches and recombines such that blood can only flow through one branch and cannot flow through all of them. The total resistance offered by vascular segments arranged in series is simply the sum total of the resistance offered by each segment individually (R
_{total}= R_{1}+ R_{2}+ R_{3}...R_{n}). Thus, additional segments added to the series simply serve to increase the total resistance. - Alternatively, the total resistance offered by vascular segments arranged in parallel is given as follows: 1/R
_{total}= 1/R_{1}+ 1/R_{2}+ 1/R_{3}...1/R_{n}. Consequently, the total resistance offered by the total branching network of vasculature is always less than the resistance of any single branch. Conceptually, one can think of each branch simply adding to the total radius for blood flow if all the branches were fused into one large tube. Consequently, adding a branch will solely increase the total available radius and thus reduce the resistance. - Given the above discussion it is clear that the arrangement of vascular segments in a parallel arrangement serves to reduce the total resistance, whereas their arrangement in series serves to increase the total resistance.
**Blood Viscosity**- Resistance to blood flow is proportional to the viscosity of the blood flowing through the vasculature. Consequently, increases in blood viscosity serve to increase the resistance to blood flow through a given vascular segment. Certainly, pumping 1 gallon of viscous kethcup per minute through a garden hose will require far more of a pressure gradient than pumping the same amount of water through the garden hose.

Special Resistances |
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**Overview**- Several special values of resistance are frequently used in clinical settings and refer to the total resistances of entire segments of the circulation.
**Systemic Vascular Resistance (SVR)**- The SVR refers to the resistance offered by the entire systemic circulation. Recall that the blood pressure gradient across the entire systemic circulation is the difference between the systemic arterial pressure and the right atrial pressure. Furthermore, the blood flow volume through the entire systemic circulation is simply the cardiac output
- Therefore: SVR = (Systemic Arterial Pressure - Right Atrial Pressure)/Cardiac Output.
**Pulmonary Vascular Resistance (PVR)**- The PVR refers to the resistance offered by the entire pulmonary circulation. Recall that the Blood Pressure Gradient across the entire pulmonary circulation is the difference between the pulmonary arterial pressure and the left atrial pressure. Furthermore, the blood flow volume through the entire pulmonary circulation is simply the cardiac output
- Therefore: PVR = (Pulmonary Arterial Pressure - Left Atrial Pressure)/Cardiac Output