Water diffuses from the lumen into the interstitial space during the reabsorption of water in the proximal convoluted tubule due to an increase in the interstitium's osmolarity.
Reabsorption is the process by which water and solutes from the PCT are injected into the blood. From the proximal convoluted tubule, the solutes and water go to the interstitium before entering the peritubular capillaries. The majority of the solutes and 99 percent of the water filtered by the nephron must be reabsorbed; all of these chemicals were "absorbed" in the digestive tract. The peritubular and vasa recta capillaries return reabsorbed fluids and chemicals to the circulation.
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One easy way to measure the "size" of a ball is how far it is from side to side. Your tennis ball example is a good one: a typical tennis ball is about 2.5 inches form side to side.
