A few short microvilli are present , but they may be difficult to distinguish in the light microscope . The epithelium can not be penetrated by water. Consequently, the transport of ions over the epithelium sets up a gradient in osmotic pressure, which serves as driving force in the further concentration of the urine. The straight portion of the distal tubule contacts the glomerulus forming the macula densa . Cells in the distal tubulus are sensitive to the hormone aldosterone, which is produced in the zona glomerulosa of the adrenal glands.
Aldosterone stimulates the active resorption of sodium ions and the excretion of potassium ions.
The convoluted distal tubule merges with the collecting ducts of the kidney. Osmotic forces move water out of the lumen of the tubules as they pass through the medulla of the kidney, where cells of the ascending thin and straight distal tubules of the loop of Henle have established high concentrations of sodium in the extracellular space. Both transversely or longitudinally cut tubules are suitable. Note that it will be almost impossible to identify ALL tubules that are visible. In most of our sections only little medulla is present - try to scan along the margins of the tissue and see if you can find some medulla.