The kidney is mainly composed of renal units, each with about 1 million nephrons. The renal units include the renal tubules and their corresponding tubules, which are located in the renal cortex and the tubules in the renal medulla. The renal tubules are composed of the glomerulus and the renal capsule, which is a mass of capillary network consisting of five to eight branches that branch off from the small arteries of the human glomerulus. Between these capillaries, there is a small amount of thylakoid tissue, which consists of thylakoid matrix and a small number of thylakoid cells. The glomerular capillary wall is divided into 3 layers: the inner layer is the endothelium, the middle layer is the basement membrane, and the outer layer is the epithelium, and these 3 layers together are called the “glomerular filtration membrane”, which completes the important physiological functions of the kidney. There is an important special structure between the small human glomerular artery and the small outgoing glomerular artery, which is collectively called the “glomerular paracellular apparatus”, which is mainly composed of paracellular cells and dense spots. The cytoplasm of paraglomerular cells contains secretory granules, and immunofluorescence has shown that these granules contain renin, which is secreted by paraglomerular cells in more than 90% of the body. When renal ischemia and the internal pressure of small renal arteries decreases it can stimulate renin secretion, so the paracellular cells are also called “pressure sensing cells”. The dense spot is evolved from the distal tubular epithelial cells, and the cytoplasm contains small particles, which can sense the change of fluid volume and sodium ion concentration in the distal tubule, so it is also called “sodium-sensitive cells”, which can regulate the secretion of renin by the paracellular cells. The renal tubules can be divided into proximal tubules (including proximal tubules and medullary collaterals descending thick segment), medullary collaterals thin segment, and distal tubules (including medullary collaterals ascending thick segment and distal collaterals). The proximal tubule can reabsorb almost all glucose, amino acids and proteins, as well as 65% of Na+, 85% of water and 50% of urea in the glomerular filtrate (primary urine), and it can also secrete H’, ammonia, creatinine, and muriatic acid into the tubular lumen, which is an important part for reabsorbing a large amount of useful substances in the primary urine and secreting certain wastes. The thick segment of the ascending branch of the medullary collaterals is the key part of urinary dilution. The epithelial cells in this segment actively reabsorb NaCl without the reabsorption of water, thus making the tubular fluid in this segment hypotonic and making urinary dilution possible. The function of the distal tubule is to continue to reabsorb water and Na and to secrete K+, H+, and ammonia into the lumen, which plays an important role in maintaining the acid-base balance of the blood. Aldosterone promotes the reabsorption of Na’ and excretion of K+ from the epithelium of this segment; antidiuretic hormone increases its reabsorption of water, resulting in concentrated urine and reduced urine volume. The main physiological function of the kidney is to produce urine and excrete the end products of human metabolism and certain wastes and poisons, while retaining useful substances by reabsorption and regulating water, electrolytes and acid-base balance. In addition, the kidneys secrete some important endocrine hormones (such as renin, erythropoietin, prostaglandins, etc.) and are also the inactivation site for hormones such as insulin, gastrin and parathyroid hormone.