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The renal circulation is the flow of blood to and through the kidneys to allow them to filter it before returning it to the rest of the cardiovascular system. Approximately 20% of the heart's output of blood reaches the kidneys, far more than these organs need to sustain themselves. The extra blood travels through the filtration systems inside these organs to remove waste products and adjust the balance of the blood chemistry. Problems with renal circulation can lead to health conditions like high blood pressure, hormone imbalances, and edema, a swelling caused by buildups of fluid.
Blood reaches the kidneys through the abdominal aorta, which branches off into the renal arteries to deliver blood to the organs. Most of the blood travels directly into the cortex, which contains a complex network of structures that process the blood. Various compounds pass freely across a permeable membrane, allowing the kidneys to extract waste products and balance the composition of the blood. If salts in the blood are too high, the kidneys can filter them out, and if they are too low, they can remove excess liquid to restore the balance.
Unwanted blood products are eliminated in the form of urine, which travels to the bladder for excretion. The urine can become more concentrated when people are dehydrated and the kidneys must conserve water. People with adequate or excessive hydration tend to have more dilute urine. A doctor can learn about kidney health by examining a urine sample to determine how well the organs are functioning.
In addition to cleaning the blood and balancing the blood chemistry, the renal circulation can produce hormones and trigger hormone production elsewhere in the body. The freshly cleaned blood travels out and up the ascending vena cava for circulation. Renal circulation relies on the ability to process high volumes of blood in short periods of time to keep the blood chemistry within a range of tolerable values. Kidney damage can slow this process or overload the kidneys when it comes to processing compounds like medication in the blood, which can in turn adversely affect cardiovascular health.
Some problems with renal circulation can be temporary. Patients may be able to recover on their own, without medical intervention, by staying hydrated. In other cases, the kidneys need assistance. Renal dialysis can temporarily take over for injured or failing organs to clean the blood and restore it to circulation while the kidneys rest. A transplant to replace an entirely failed kidney is another option for patients who do not respond to treatment and are unlikely to recover, even with dialysis and medication.