Final answer:
The process by which fluid from the bloodstream enters the kidney tubules is known as filtration, the first step in the kidney's urine formation process that occurs in the glomerulus.D. filtration
Step-by-step explanation:
The process whereby fluid from the bloodstream enters the tubules of the kidneys is called filtration. This is the first step in urine formation, occurring in the glomerulus of the nephron, where almost all solutes, except for proteins, are filtered out of the blood and into the tubule.
As the filtrate moves along the nephron, it undergoes additional processes such as tubular reabsorption in the proximal convoluted tubule and loop of Henle where solutes and water are reabsorbed, and tubular secretion which primarily occurs in the distal convoluted tubule (DCT), adding unwanted substances to the filtrate. These combined actions lead to the formation of urine that is eventually excreted from the body.