Almost 2 pints of blood laden with waste products arrive at the kidneys via the renal arteries (one going to each kidney) every minute. This blood is carried through ever smaller renal arteries into the specialized filtering structures. The kidneys contain millions of nephrons, tiny filtering units that have two parts. The glomerulus filters waste and fluid from the blood to create urine, and the tubule concentrates and excretes the waste products into the urine while allowing water and essential salts, minerals, and nutrients to be reabsorbed back into the bloodstream. The glomerulus consists of a twisted mass of convoluted capillaries (the tiniest blood vessels) with sieve-like walls perforated by tiny filtration pores. Large particles, including blood cells and proteins, remain in the bloodstream while waste products, electrolytes, nutrients and some of the water cross into the tubule. When the kidneys become infected or inflamed, the glomeruli allow larger molecules to pass into the urine. That is why doctors test urine for protein, an indication that the kidneys are not functioning properly. Blood cells may also pass through if the holes are large enough. In the tubule, several sophisticated hormonal processes occur that determine which substances will be excreted and which will be reabsorbed into the bloodstream. In the tubules, the kidneys adjust the acidity of the blood as well as the water, salt, and mineral content. Once the blood has been filtered, it leaves the kidney through the renal vein. The collected waste fluid passes as urine through ever larger collecting tubes. The collecting tubes coalesce in the kidney’s core (the renal pelvis) and drain via the ureter to the bladder. The kidneys produce urine continually day and night, passing about 3 1/2 pints daily, though this can vary considerably depending upon how much you drink. Last Updated: 8/22/2002
The Johns Hopkins University 1996-2003. All rights reserved. This information is not intended to provide advice on personal medical matters, nor is it intended to be a substitute for consultation. | 
