Urine concentration is the collection of waste materials in the urine for excretion by the body. The ability to concentrate soluble waste is important to overall health, as it allows the body to eliminate it without losing vital fluids. Very young organisms tend to have trouble concentrating their urine and need more water to sustain themselves. Similarly, the disease can interfere with this process and can lead to the production of very dilute urine.
This process takes place in the kidneys. Blood flows to the kidneys and through a network of structures known as nephrons that allow fluid and salts to flow through a semi-permeable membrane. When the body has excess salts and other materials that it needs to get rid of, they pass through easily, leaving behind fluid. If there is too much fluid, the nephrons can excrete this through the membrane.
Waste materials are prepared for disposal, while materials still used by the body can be recycled and sent into the bloodstream through the reabsorption process. The amount of urine concentration that occurs depends on hydration levels in the body and the production of a compound known as antidiuretic hormone, or vasopressin. This hormone causes the kidneys to retain water, leading to more concentrated urine.
The body regulates the concentration of urine to maintain a stable balance of fluids and salts in the blood chemistry. People who are dehydrated tend to produce extremely concentrated urine that can be dark, with a strong odor. Dehydrated bodies need water, and can afford to excrete only a small amount to remove waste materials. After loading water, such as drinking a lot of water during sports to stay hydrated, the body can excrete relatively dilute urine.
Health conditions can negatively affect urine concentration. Kidney damage can make it harder to filter your blood, leading to excessive thirst and very dilute urine. Patients who cannot concentrate their urine need large amounts of water to remove unnecessary salts from the bloodstream. Eventually, they can become so impaired that they require external hemodialysis to filter their blood. Such patients may need a kidney transplant.
In a urinalysis test, the laboratory usually issues a report on the concentration of urine. Patients may also be asked to perform a urine concentration test after doing something like abstaining from drinking, carrying water, or taking antidiuretic hormone. This test shows how well the kidneys are working under stress, which can provide important information about a patient's health.