Chapter 36: Transport of Urea, Glucose, Phosphate, Calcium, Magnesium, and Organic Solutes

The kidney filters these substances from plasma at the glomerulus and then selectively reabsorbs or secretes them according to physiological requirements, maintaining homeostatic balance throughout the body. The chapter explains how different nephron segments employ distinct transport mechanisms adapted to their specific functions. Glucose, normally filtered but essential for cellular metabolism, undergoes complete reabsorption in the proximal tubule through active transport mechanisms, preventing urinary loss. Phosphate reabsorption occurs primarily in the proximal tubule via sodium-dependent cotransport and is hormonally regulated by parathyroid hormone, which decreases phosphate reabsorption to maintain appropriate serum levels. Calcium transport is tightly controlled through multiple nephron sites, with hormonally regulated reabsorption mechanisms responding to parathyroid hormone and calcitriol, while magnesium follows a similar pattern of filtered load adjustment. Organic solutes including amino acids and other metabolic products are reabsorbed through specific carrier-mediated mechanisms in the proximal tubule, preventing their loss in urine. Urea handling differs substantially from other solutes, as some urea is reabsorbed to maintain medullary osmolarity while excess urea is excreted based on protein intake and metabolic state. The chapter integrates how filtration, reabsorption, and secretion work coordinately to balance solute excretion with physiological needs, examining the role of electrochemical gradients, specific transporter proteins, and hormonal regulation in controlling final urine composition. Understanding these transport mechanisms is fundamental to comprehending how the kidneys maintain mineral balance, prevent pathological electrolyte disturbances, and adapt renal function to changing dietary and metabolic demands.