What makes the nephron produce the urine?

the nephron is made of 4 parts... the glomerulus, proximal convuluted tubules,the loop of henle, and the distal convuluted tubules.  the blood of the afferent arterioles enters the glomerulus, and the water and most of its contents are "squeezed" out of the capillary bed except the large particles... the filtrates (the squeezed out liquid) now enters the bowman's capsule and travels down the proximal convuluted tubules...where much of the sodium and water are reabsorbed, and it goes through the loop of henle, as it descends more water is reaborbed... and it ascends, and it reaches the distal converluted tubules... as it ascends waist products are excreeted into the filtrate... lastly it leaves the nephron into collecting duct aka renal pelvis... the end product here is what we called urine.

see the illustration for a better idea of urine's journey...
http://www.ivy-rose.co.uk/Topics/Urinary/Kidney_Nephron_cIvyRose.jpg