Before sweating (at baseline), we will make the assumption that the subject is in homeostasis with adequate strength of the plasma osmolarity-decreasing effect of water reabsorption (through the water channels) as well as normal plasma and urine osmolarities (isotonic) and urine volume.

Step 1: With sweating, water leaves the plasma and osmolarity increases (concentrated, hypertonic).

Step 2: This is in the opposite direction to the plasma osmolarity-decreasing effect of the water reabsorption. Thus, although the strength of the plasma osmolarity-decreasing effect of water reabsorption (through the water channels) was adequate before sweating (at baseline), this strength is now too weak (blue) to reverse the increased plasma osmolarity (concentrated, hypertonic) produced by sweating. Control by negative feedback is needed.

Step 4: With the increase in water reabsorption, diluted (hypotonic) solution enters the plasma. This will lead to a reverse in the increased plasma osmolarity (concentrated, hypertonic) produced by sweating, decreasing it towards normal (baseline, isotonic) osmolarity. Because diluted (hypotonic) solution leaves the tubule due to reabsorption, the fluid remaining in the tubule has increased osmolarity (concentrated, hypertonic). Also, with the increase in water reabsorption, there is less water remaining in the tubule. Overall, the urine becomes concentrated and decreases in volume.