Why aren’t glucose and protein normally found in urine?
This has to deal with how the nephron works.
Glucose is filtered through the glomerulus into the Bowman's capsule. However, in the proximal convoluted tubule (PCT), glucose molecules, almost 100% of them, are actively reabsorbed back into the blood.
Very high level of blood sugar level may not allow PCT to reabsorb 100% glucose.
Proteins, do not filter through the glomerulus under normal cicumstances. They are simply too large to do so.
If proteins are found in the urine, there is serious problem at the level of filters in the nephrons. Same goes for red blood cells; they are too large to pass through the glomerulus.