Final answer:
The raw material processed by the renal tubules that includes substances smaller than most proteins is glomerular filtrate. This filtrate is formed by glomerular filtration, omitting cells and proteins, and is then modified by tubular reabsorption and secretion to create urine.
Step-by-step explanation:
The plasma derived fluid that is the raw material processed by the renal tubules and includes substances smaller than most proteins is glomerular filtrate. This is a product of the first step in urine formation where the blood is filtered through the capillary network in the glomerulus. Unlike blood plasma, which contains cells and proteins, glomerular filtrate excludes these larger elements.
Substance such as water, inorganic ions, and small organic molecules like glucose and urea are passed through, while large proteins are retained in the circulatory system. The filtrate then travels through the nephron, where tubular reabsorption and tubular secretion further modify its composition to eventually form urine, which is distinct from lymph, interstitial fluid, and blood plasma.