Can you explain the process of ultrafiltration and selective readsorption?

Ultrafiltration and selective reabsorption are processes which occurs in the kidney to filter the blood. In ultrafiltration, the blood is under a high pressure in the glomerulus which causes ions and small molecules such as water and glucose to be filtered out across a partially-permeable membrane into the Bowman's Capsule. If the body needs the ions or molecules, they are then reabsorbed selectively (as in only some of them are reabsorbed) in the convoluted tubules. If water is required, it is reabsorbed in the collecting ducts.