Overview Introduction Most small substances are freely filtered by the glomerulus meaning filtration is in general a non-selective process a diverse set of substances enter the proximal tubule Substances are subsequently secreted or reabsorbed by the proximal tubule so that toxins can be cleared faster than filtration alone would allow useful substances can be retained by the body despite filtration The balance between secretion and reabsorption of each substance controls the concentration of that substance at the end of the proximal tubule the concentration that is presented to downstream portions of the nephron The proximal tubule also absorbs the majority of filtered water Relative Tubular Concentrations Substance concentrations in the proximal tubule can be defined relative to the plasma concentration of the same substance (tubular fluid/plasma = TF/P) so that the action of the proximal tubule on the substance can be easily determined substances can be analyzed relative to those with known behavior Interpretation of TF/P TF/P Meaning Causes Examples > 1 Solute is reabsorbed less quickly than water Active secretion No reabsorption PAH Inulin Urea = 1 Solute and water are reabsorbed at the same rate Solute reabsorption drives water reabsorption Sodium < 1 Solute is reabsorbed more quickly than water Active reabsorption Cotransport Glucose Amino acids Bicarb Key Proximal Tubule Changes Inulin is neither secreted nor reabsorbed in the proximal tubule so it ↑ in concentration along the proximal tubule its concentration can be viewed as a proxy for water reabsorption Na+ reabsorption is the main driver of H2O reabsorption so sodium TF/P remains close to 1 throughout the proximal tubule Cl- reabsorption has two phases including a slower rate than Na+ in the proximal 1/3 of the proximal tubule equivalent rate to Na+ in the rest of the proximal tubule Cl- TF/P therefore ↑ before it plateaus