maximum velocrty and it may be visualized that these molecules are dragging' the olher laminae This dragging force.that is,the force which causes

means the drop of total pressure ofthe blood dueto its passage through the capiHaries ia not great (fig 5. 7 4). In the venous system pressure is very low (fig 5.7 .4) and hence veins belong to the Iow pressure sysiem' In this connection. it should be remembered that our body is able to constrinct. setectively. the artenioles supplying a particular organ. Thus. arteroles supplying the skin undergo selectiver vasospasm (for example. during early phase of muscular exercise this causes ischemia ofthe skin As arterioles supplying the other organs (e .g skeleta muscles) remain unaffected or are even dilated, the blood is diverted to these argans Thus. in this way, the body achieves 'redustributions'of the blood flow Viscosity At the outset, some comrnonly used terms will be evplained Let mn is a tube (fig 5. 7 513) through which water is flowing in such cases, Sir isaac Newton visualized nearly three ceturies ago. that the flowing water may be considered to be consisting öf large number of concentric taminae (Iike the concentrinc laminae of an onion äs in A of fig 5.75 Each lamna or 'shell' is in intimate comaci with its adjacent laminae Wher such a liquid (water in this example) moves the Velocrty profile' of the laminae takes a parabolaid shape (B in fig 5_7 5) The lamina adjacent to the wall of the tuba, does not move at all, where as the velocity Increases more and more äs the lamina in question is more and more near the central longtudinal axis That is, the moiecules in the central longrtudinal axis has the maximum velocrty and it may be visualized that these molecules are dragging' the olher laminae This dragging force.that is,the force which causes the velocrly gradient in the laminae. is called the shearno forte Fig 575 Across sectiona! view of a laminar flow B Longitudinal view Note, tha velocity is
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