Overview proximal tubules relative concentrations updated Introduction Most small substances are freely filtered by the glomerulus meaning login to view 2 more bullets Substances are subsequently secreted or reabsorbed by the proximal tubule so that login to view 2 more bullets The balance between secretion and reabsorption of each substance controls login to view 2 more bullets 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 login to view 2 more bullets 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 login to view 2 more bullets Na+ reabsorption is the main driver of H2O reabsorption so login to view 1 more bullet Cl- reabsorption has two phases including a login to view 2 more bullets Cl- TF/P therefore ↑ before it plateaus