Fig. 7. Diagram of a possible mechanism for vasomotor control in the pigeon skin vasculature. We suggest that activation of ß-adrenergic receptors (ß-AR) constricts arterial vessels (A), while activation of ₂-ARs constricts vessels somewhere along the venous side (B). These effects can be direct or indirect. Thus propranolol (ß-AR) and clonidine (₂-AR) act jointly to raise capillary hydrostatic pressure by decreasing arterial resistance and inducing greater resistance at the venous side. The rise in pressure may induce water outflow from the capillary, thereby increasing cutaneous water evaporation.