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collateral channels) to given body region–Common at joints, in abdominal organs, brain, and heart; none in retina, kidneys, spleen•Vascular shunts of capillaries are examples of arteriovenous anastomoses•Venous anastomoses are commonPhysiology of Circulation: Definition of Terms•Blood flow–Volume of blood flowing through vessel, organ, or entire circulation in given period•Measured as ml/min•Equivalent to cardiac output (CO) for entire vascular system•Relatively constant when at rest•Varies widely through individual organs, based on needs•Blood pressure (BP)–Force per unit area exerted on wall of blood vessel by blood •Expressed in mm Hg•Measured as systemic arterial BP in large arteries near heart–Pressure gradient provides driving force that keeps blood moving from higher to lower pressure areas•Resistance (peripheral resistance)–Opposition to flow
–Measure of amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation•Three important sources of resistance–Blood viscosity–Total blood vessel length–Blood vessel diameter•Factors that remain relatively constant:–Blood viscosity•The "stickiness" of blood due to formed elements and plasma proteins•Increased viscosity = increased resistance –Blood vessel length•Longer vessel = greater resistance encountered•Blood vessel diameter–Greatest influence on resistance•Frequent changes alter peripheral resistance•Varies inversely with fourth power of vessel radius–E.g., if radius is doubled, the resistance is 1/16 as much–E.g., Vasoconstriction increased resistance•Small-diameter arterioles major determinants of peripheral resistance•Abrupt changes in diameter or fatty plaques from atherosclerosis dramatically increase resistance –Disrupt laminar flow and cause turbulent flow•Irregular fluid motion increased resistanceRelationship Between Blood Flow, Blood Pressure, and Resistance•Blood flow (F) directly proportional to blood pressure gradient (P) –If P increases, blood flow speeds up•Blood flow inversely proportional to peripheral resistance (R)–If R increases, blood flow decreases:F = P/R•R more important in influencing local blood flow because easily changed by altering blood vessel diameterSystemic Blood Pressure•Pumping action of heart generates blood flow
•Pressure results when flow is opposed by resistance•Systemic pressure–Highest in aorta–Declines throughout pathway–0 mm Hg in right atrium•Steepest drop occurs in arteriolesArterial Blood Pressure•Reflects two factors of arteries close to heart–Elasticity (compliance or distensibility)–Volume of blood forced into them at any time•Blood pressure near heart is pulsatile •Systolic pressure: pressure exerted in aorta during ventricular contraction–Averages 120 mm Hg in normal adult •Diastolic pressure