Final answer:
Na+, urea, phosphate, and Cl− have clearances higher than zero due to their partial filtration and secretion by the kidneys. These substances are managed through glomerular filtration and hormonal regulation to maintain homeostasis.
Step-by-step explanation:
Na+, urea, phosphate, and Cl− have clearances that are higher than zero because they are either filtered out of the blood by the kidneys and not fully reabsorbed or they are substances that the body needs to get rid of.
Glomerular filtration is a process by which the kidneys filter out wastes, excess substances, and other chemicals from the blood. These include Na+ (Sodium), urea, phosphate, and Cl− (Chloride). Some of these are reabsorbed back into the bloodstream to maintain homeostatic plasma concentrations, whereas others, like urea and some drugs, are secreted as waste. This complexity helps the body balance electrolyte concentrations and pH, and remove toxins.
Moreover, substances like urea are partially reabsorbed, but not completely, hence why there is always some level of clearance from the body. Hormones like ADH can adjust the amount of reabsorption as needed by altering the number of transport proteins and channels in the kidney's collecting ducts.