Integration and coordination of the complex osmoregulatory processes described in Chapter 10 appear to be largely under endocrine control. As we saw earlier, management of body fluids is based on two principal mechanisms: (1) alteration of membrane permeability to water and particular ions; and (2) active ion pumps transporting ions against their concentration gradient. Adequate functioning of osmoregulatory tissues that incorporate these properties is ultimately dependent on blood supply. Hormones may thus influence the activity of osmoregulatory organs in several ways: 1. By altering the active transport of ions (Na+, K+, H+, Cl−, and HCO3−) through direct actions on the cells involved. Adrenal steroids, posterior pituitary peptides, and catecholamines influence active sodium and potassium transport in renal and extrarenal tissues in most vertebrates. 2. By changing osmotic movement of water and ion diffusion across epithelia by adjusting membrane permeability. Pituitary prolactin alters sodium permeability of the teleost gill, limiting diffusional loss in freshwater. Posterior pituitary hormones may similarly alter the water permeability of a number of tissues. 3. By altering blood flow to and within osmoregulatory organs by adjusting vascular smooth muscle tone and thus blood vessel diameter. In this way catecholamines, angiotensins, and neurohypophyseal peptides may modify the rate of glomerular filtration and, accordingly, urine flow in the non-mammalian kidney. Variation in regional blood flow distribution can also influence the secretory activity of tissues like the teleost gill.