les and ions which are reabsorbed in the kidney are reabsorbed by active transport mechanisms (Applegate, 2000, 382). Active transport uses carrier molecules, and the rate of reabsorption is limited by the activity of the carrier molecules. Some negative ions passively follow actively transported positive ions to maintain electrical neutrality.
Glucose is reabsorbed by active transport mechanisms so that no glucose is present in the urine under normal conditions. In diabetes, glucose concentrations in the blood and filtrate may exceed the capacity of the active transport mechanisms to absorb it, and glucose may be present in the urine. Active transport mechanisms require the input of energy in the form of ATP to function. Passive transport is by simple diffusion down a concentration gradient, and requires no energy input. It occurs because the concentration of the solute is higher in the filtrate than in blood.
9.4 The peritubular capillaries form a network around the tubular portion of the nephron, and are lined by fenestrated epithe
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