Final answer:
None of the solutes were present in the filtrate due to the process of reabsorption in the nephron, whereby essential substances are taken back into the bloodstream and waste products are excreted.
Step-by-step explanation:
The reason none of the solutes were present in the filtrate is primarily due to the process of reabsorption that takes place in the kidneys, particularly within the nephrons. The glomerulus filters a variety of substances including water and small solutes from the blood. However, as the filtrate passes through various parts of the nephron, such as the proximal tubule, loop of Henle, distal convoluted tubule (DCT), and the collecting duct, a significant portion of water and solutes are reabsorbed back into the bloodstream. This reabsorption is highly selective and is based on the body's needs, which are regulated by hormones.
When the filtrate reaches the DCT, most of the reabsorption has already taken place; typically, up to 82 percent of the water, along with the solutes, has been reabsorbed. The composition of the final urine is adjusted further in the ascending and descending loops of Henle and the collecting duct, where water and solute concentration are carefully regulated through both reabsorption and tubular secretion. These processes ensure that crucial substances are retained and metabolic wastes such as urea, uric acid, and certain drugs are excreted.