how does sweat occur

Sweating is a mechanism by which the the body removes excess water and trace amounts of salts through the sweat pores present in the skin. The skin contains sweat glands which remove water and trace amounts of nitrogenous wastes  from the capillaries surrounding it. During summers or strenuous physical exercise the blood capillaries surrounding the sweat glands dilate (vasodilation) causing more sweat formation. When the sweat comes out of the sweat pore, it evaporates from the skin taking the latent heat required for the process from the skin. Thus, sweat formation results in cooling of the body. During winters, the blood capillaries narrows (vasoconstriction), resulting in less formation of sweat and thus conserving the body heat.