In a histological section of a kidney, the renal corpuscles can be easily recognized. In contrast, the various segments of the fine tubules of the nephrons are more difficult to identify. However, the cytological features of the epithelial cells lining these tubules, which help identify their segments, will be presented in tables accompanying Figure 13.13 and Figure 13.21. The components of the nephrons have been extensively investigated with the electron microscope and physiological methods. Thus, for example, electron microscopic studies have permitted a better understanding of the mode of formation of the glomerular filtrate. The histology textbooks that include electron-microscope images of the kidney, should thus be consulted to fully appreciate the structural and functional features of this organ. The kidney is not only involved in the formation of urine but also an endocrine organ which secretes into the blood a hormone (renin) that regulates blood pressure. It also secretes erythropoietin, a hormone which stimulates the formation of red blood cells in bone marrow.