quantny of Charge fowing in an unri time. i.e current, E = the difference of the potental. i .e. the voltage and R = the resistance agains the curre

equation, R. [This is analogous to the Ohm's law in electncity 1= E/R, where, 1= quantny of Charge fowing in an unri time. i.e current, E = the difference of the potental. i .e. the voltage and R = the resistance agains the current flow Indeed flow öf electnc currant is often visualized as flow of water in a tube] In our body, as stated above, the Q can be visualized as the cardiac output, n . the viscosity cofficient öf blood. l- the total length from the begiinning öf aortato the terminal part of vene cave (in case ofsystemic circulation) and r the mean diameter of all the vessels Normally, the 1 does not change in a grown up man, A also does not change frequently. Thcrefore, it is the r, the radius. which changes readily and frequently (due to such causes like syrmpathehic stimulation leading to vasoconstnction and so on) Further. the resistance R changes as (the 4th power of the r. the radius 'Seit' of the perpheral resistanc. Every segment of the vascular tree (e.g Windkessel vessels. arterioles, capillaries, capacitance vessels) offers some resistance However. the maximum resistance is encountered at the level of the arterioies. Therefore. as explained earlier, the totat pressure, P. falls maximally at the level öf the arterioles (figure 5.7. 4) Further. because of the fact that the arteriolar walls contain heavy investments of smooth muscies and are Süsceptible to. (i) sympathetic stimulation äs well äs to the actions of rmany (ll) vasoactive agents. the arteriolar diameter alhert very frequently (according to the needs of the body) producing alterations of resistance Therafore, to repeat (i) pressure drop -occurs maximally at the level of artenoles, and (H) arteriolar resistance varies frequently and very effectively For all these reasons. artenoles have captured the atttention ofthe physiologis. Fig 5. 7 4 Blood pressure values in the differnt segments of the vascular tree. Note. the sharpest fall of BP occurs at the level of arterioles, indicating the arterioles offer maximum resistance. The situation changes äs ine blood erters the zone of ca-pillsries As Ihe number of capilaries is very great, ther total rasislance offered agaimst tha flow falls