Function of the kidneys (1) The kidneys are on either side of the spine under the rib cage. The average adult male kidney is 10 cm long, 5 cm wide and 4 cm thick, and slightly smaller in women. (2) The most important function of the kidney is to produce urine. The renal artery enters the kidney and branches repeatedly, eventually coiling to form a vascular mass called the glomerulus. There are about 1.3 million glomeruli in each kidney. These glomeruli have many holes in them, like a “sieve”. Red blood cells, white blood cells and proteins in the blood vessels cannot pass through these sieves, while small molecules such as water molecules, sodium ions, glucose and human metabolic waste can pass through the sieves. The fluid filtered out by the glomerulus at this time is called proto-urine, which contains many nutrients. When the primary urine flows through the renal tubules, these nutrients are reabsorbed and 99% of the water is also reabsorbed, finally forming urine for discharge. A person filters out about 180 liters of raw urine per day, forming about 1.8 liters of urine. By producing urine, the kidneys regulate water, ion and acid-base balance to maintain the normal internal environment of the body. (3) The kidneys also secrete hormones, such as erythropoietin, which stimulates bone marrow hematopoiesis, and active vitamin D3, which promotes intestinal absorption of calcium; the kidneys also secrete hormones that regulate blood pressure, causing it to rise. (4) When abnormalities occur in the kidneys, they cause red blood cells and protein to leak out, and hematuria and proteinuria will be present in laboratory tests. In severe cases, water and metabolites cannot be excreted adequately, which can lead to uremia, and anemia, bone abnormalities, and hypertension can also occur.