I. The urinary system and its functions The urinary system is composed of the left and right kidneys, the left and right ureters, the bladder and the urethra, and is the main excretory organ of the human body. Usually, the kidneys and ureters are called the upper urinary tract, and the bladder and urethra are the lower urinary tract. The kidney is the organ that produces urine, which is stored in the bladder via the ureter and, when stored in a certain amount, is excreted through the urethra by nerve reflex. The kidneys are about 10-12 cm long, 5-6 cm wide and 3-4 cm thick, one on each side of the spine in the posterior lumbar region. The kidney is shaped like a fava bean and has a deep depression on the inside called the hilum, through which the blood vessels, nerves and ureters of the kidney pass. If the kidney is cut open longitudinally, the outer part is the cortex and the inner part is called the medulla; then there is a cavity consisting of the renal calyces and the renal pelvis; the pelvis is connected to the ureter. The basic structure of the kidney is the renal unit. Each kidney has about 1 to 2 million nephron units, each of which is composed of renal tubules and renal vesicles. The renal tubules are mainly located in the renal cortex and are spherical structures consisting of glomeruli and glomerular capsules, which are responsible for the formation of primary urine and filtration. The renal tubules are connected to the glomerular capsule and consist of proximal tubules, medullary collaterals and distal tubules in order, which have reabsorption and excretion functions. The distal tubule converges into the collecting duct and then into the papillary duct, and urine is secreted from the papillae into the renal calyces, to the renal pelvis, and finally into the bladder via the ureter, and is eliminated from the body via the urethra. The main job of the kidneys is to continuously remove toxins and excess water from the body’s blood. The kidneys filter and cleanse about 200 liters of blood per day, which is 60 times more than the whole body plasma and equivalent to 10 buckets of drinking water. The kidneys also have important roles in regulating electrolytes, acid-base balance, controlling blood pressure, stimulating the production of red blood cells and maintaining healthy bones. If the kidney gradually loses its function due to various reasons, you have chronic kidney disease. In chronic kidney disease, the kidney function gradually deteriorates unknowingly and eventually becomes completely damaged, which is called “end-stage renal disease” (also known as uremia) and requires kidney replacement or long-term reliance on “artificial kidney” to survive. The K/DOQI “Clinical Practice Guidelines for Chronic Kidney Disease” developed by the American Kidney Foundation in 2002 has been generally accepted by the international nephrology community, which defines chronic kidney disease (CKD): kidney damage (abnormal blood or urine composition, or abnormal imaging, or abnormal pathology) for ≥ 3 months. or pathology) for ≥3 months, with or without a decrease in GFR.