An interesting thing about the removal of water from the blood by the kidneys is the relation it shows to the activity of the sweat glands. The total amount of water gotten rid of in any day is the amount taken in in drinking during the same time. The amount we drink is controlled chiefly by the sense of thirst, assuming that we are in a place where water can be had. The amount of water that passes out in the form of sweat is not under our control; in a later chapter we shall see how the formation of sweat is regulated. There is also a constant and fairly steady loss of water from the lungs, since every breath that we exhale carries with it all the water vapor which it can hold. What the kidneys have to do is to get rid of the water that is not passed out either from the sweat glands or the lungs. The amount of urine that is formed in a day is, therefore, extremely variable; on days that are warm and when a great deal of exercise is taken there will be so very much sweat formed that in spite of copious drinking comparatively little water will be discharged from the kidneys. On the other hand on cold days in which not much exercise is taken there will be a large amount of urine formed. It is interesting to note that some substances like asparagus, and to a less extent coffee, cause a large increase in the amount of urine. We do not know certainly whether these act by increasing filtration from the tips of the kidney tubes or by diminishing the reabsorption in the part farther along, but so far as our information goes either or both may occur.