There is a ball of capillaries that filters the plasma. There are a series of tubules that receive and modify the filtrate before excretion. There are peritubular capillaries that serving the tubules and help to establish their function. Barrier between intracellular fluid and extracellular fluid is cell membrane. Both active and passive transport processes are involved.
Secretion means moving from the blood into the tubule lumen.
Remember it is a mineralocorticoid whose purpose is to save NaCl in the plasma. Sometimes they produce a large volume of dilute urine for losing water. Sometimes they produce a small volume of concentrated urine for water conservation. These systems allow the surrounding tissue to become hyperosmotic so that water can be reabsorbed by osmosis. Vasopressin produced in hypothalamus, stored in posterior pituitary, secreted into blood by water deficit or inhibit secretion by water excess.