Urine concentration is the collection of waste materials in urine for excretion by the body. The ability to concentrate soluble waste is important to one's 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 support themselves. Likewise, illness can interfere with this process and may result in the production of very dilute urine.
This process takes place in the kidneys. Blood flows into the kidneys and through a network of structures known as nephrons that allow fluid and salts to flow across a semi-permeable membrane. When the body has excess salts and other materials it needs to eliminate, these pass readily through, leaving fluid behind. If there is too much fluid, the nephrons can excrete this across the membrane.
Waste materials are prepared for elimination, while materials the body still uses can be recycled and sent into the bloodstream through the process of reabsorption. The amount of urine concentration that occurs depends on the hydration levels in the body and the production of a compound known as antidiuretic hormone, or vasopressin. This hormone triggers the kidneys to retain water, leading to more concentrated urine.
The body regulates urine concentration to maintain a stable balance of fluids and salts in the blood chemistry. People who are dehydrated tend to produce extremely concentrated urine which may be dark, with a strong odor. Dehydrated bodies need water, and can afford to excrete only a small amount to get rid of waste materials. After water loading, like drinking lots of water during sports to stay hydrated, the body may excrete relatively dilute urine.
Health conditions can adversely affect urine concentration. Kidney damage may make it harder to filter the blood, which creates excessive thirst and very dilute urine. Patients who cannot concentrate their urine need ample supplies of water to flush unnecessary salts out of their bloodstream. Eventually, they may become so impaired that they require external hemodialysis to filter their blood. Such patients may ultimately need a kidney transplant.
In a urinalysis test, the lab usually issues a report on the urine's concentration. Patients may also be asked to take a urine concentration test after doing something like refraining from drinking, water loading, or taking antidiuretic hormone. This test shows how the kidneys perform under strain, which can provide important insight into a patient's health.