the interval in a homeostatic process between a change of the trait controlled and the appropriate response, due to afferent, efferent, and central components. The lag may be a pure random variable, e.g., the waiting time of an exponential process or the sum of several such processes taking any value greater than zero but with a mean considerably greater than zero; sometimes it may be deterministic or almost so and with a minimum sharply defined and greater than zero for anatomic reasons. For instance, the partial pressures of oxygen and carbon dioxide are controlled in the lungs but based on afferent information obtained from the carotid body that is already dated because of the circulation time of 10 seconds or so between the two sites.