Your blood transports nutrients and oxygen to the cells of the body and carries away waste material that the body does not need. These waste materials are then brought back to the kidney to be excreted.
Under normal resting conditions, about 25 percent, or onefourth, of your blood flows into the kidneys through the renal arteries. The kidneys filter the blood flow from your heart through a sophisticated system that removes unneeded waste products from the blood and returns clean blood to the body via the renal veins. In the process, it discards what is not needed as urine via the ureter. Then this process does not function as it should, it can make a person extremely unwell. However, when this process is working effectively, it is not something which the person is even aware is taking place.
The kidneys work by filtering the blood that comes into each kidney through a very efficient system of microscopic nephrons.
Each kidney has about one million nephrons, which are the workhorses of the kidney. The nephrons eventually join and lead into the collecting ducts that finally empty into the renal or kidney pelvis. By the time the fluid in the nephrons has passed through the collecting ducts to reach the kidney pelvis, it has become urine.
Urine is composed of water, salt, small amounts of acid, and a variety of waste products such as urea, oxalate, uric acid, potassium, magnesium, creatinine, and other unwanted things (e.g., lead).
Both the male and female urinary tracts consist of two kidneys, which drain through their respective ureters into a bladder that is a holding sack.
The difference between the male and female urinary tracts lies in the outflow from the bladder through the urethra. The female urethra is a short structure that drains easily to the outside world. The male urethra is much more complicated, as it must pass through the prostate gland and then through the penis before reaching the external orifice.
The composition and amount of urine change as the kidneys compensate for changes in what you eat and drink, as well as lifestyle changes and illness. Urine accumulates in the collecting system of the kidney, which includes the pelvis and calyces. The kidney pelvis has smooth muscle that periodically contracts and squeezes urine into the ureter. Additional muscular contractions of the ureter propel the urine into the bladder.
The bladder is where urine is stored until it is excreted. It is a temporary storage space that tells you when it is full. The detrussor muscle makes the bladder contract and expel urine.
Stones rarely become trapped in the female bladder because the outflow of the female bladder is quite simple. The outflow of the male bladder is more complicated because it involves the prostate gland and the penis. When there is an obstruction of urine, there is the potential for stones to become stuck in the male ureter and bladder.
Urine in the bladder should never go back “upstream,” which is called reflux. A triangular muscle structure called a trigone seals the ureters when you urinate and stops urine from reentering the kidneys. Some people, however, have a congenital defect in the trigone that allows reflux; this condition may predispose those people to kidney stones as well as kidney infections.
John S. Rodman, M.D., R. Ernest Sosa, M.D., and Cynthia Seidman, M.S., R.D., with Rory Jones
The Experts Tell You All You Need to Know about Prevention and Treatment
John S. Rodman, M.D., is an internationally recognized expert and researcher on kidney stone disease. He is associate clinical professor of medicine at the Weill Cornell University School of Medicine in New York City and a member of the attending staffs at New York Presbyterian Hospital and Lenox Hill Hospital. He has published extensively in the professional journals and medical textbooks and has lectured worldwide on kidney stone formation and treatment.
R. Ernest Sosa, M.D., is a leading urologist in New York City who specializes in surgical techniques for kidney stones. He is associate clinical professor of urology at the Weill Cornell University School of Medicine and attending urologist at New York Presbyterian Hospital. He served for 20 years as director of the Brady Stone Center at the Department of Urology at Weil Cornell – New York Presbyterian Hospital. He has written and lectured on and taught the diagnosis and treatment of stone disease internationally.
Cynthia Seidman, M.S., R.D., is director of dietary services at the Rockefeller University Hospital in New York City. She coordinates the design and development of all research diets at Rockefeller and has published in a number of professional journals. She is cochair of the Research Committee of the National GCRC Research Dietitians Association and is a member of the American Dietetic Association.
Rory Jones, an award-winning writer and television producer, has done extensive work on health and medical topics. She has developed educational programs as well as interactive multimedia for both adults and children.