In the proximal tubule chloride reabsorption occurs in two parts. Tubular secretion occurs simultaneously during reabsorption of filtrate. These actions increase sodium reabsorption and potassium secretion. Ciclosporin causes hypertension by inducing vasoconstriction in the kidneys and increasing sodium reabsorption. The nephron uses four mechanisms to convert blood into urine&colon; filtration, reabsorption, secretion, and excretion. Angiotensin II has a direct effect on the proximal tubules to increase Na reabsorption. The reabsorption of bone by osteoclasts also plays a role in calcium homeostasis. The presence of cystine in urine is often indicative of amino acid reabsorption defects. The system produces urine by a process of filtration, reabsorption, and tubular secretion. Aldosterone is responsible for the reabsorption of about 2% of filtered glomerular filtrate. This will promote sodium reabsorption and fluid retention, causing diuretic resistance. In total this segment accounts for approximately 25-30% of total Na reabsorption along the nephron. Little or no tubular reabsorption of creatinine occurs. It works by decreasing the reabsorption of sodium by the kidneys. This leads to increased enterohepatic circulation, resulting in increased reabsorption of bilirubin from the intestines.