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.
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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 fold whereas halving of a segments radius will increase its resistance by 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. Thus, additional segments added to the series simply serve to increase the total resistance.
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 Overview Several special values of resistance are frequently used in clinical settings and refer to the total resistances of entire segments of the 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. Rociletinib in EGFR-mutated non-small-cell lung cancer. Rosell, R. Erlotinib versus standard chemotherapy as first-line treatment for European patients with advanced EGFR mutation-positive non-small-cell lung cancer EURTAC : a multicentre, open-label, randomised phase 3 trial.
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