Your kidneys are comparable to a set of advanced multifunctional machines. They regulate your blood pressure, maintain your body's water volume, release essential hormones and, most importantly, filter about 200 qts. of blood every day and remove 2 qts. of waste products and excessive water from your body. The kidneys usually filter important nutrients such as protein, glucose and certain minerals, then reabsorb them back into the bloodstream. While glucose is an important nutrient for the body, excessive levels in the blood can cause problems for your kidneys.
Normal Glucose Metabolism
In a healthy person, glucose derived from food is first transported in the blood to the cells for energy. At normal blood glucose levels, usually between 70 to 130 mg per dl, the kidneys usually filter glucose the cells do not use. The proximal convoluted tubule, a loop-like structure of the kidney, then reabsorbs about 98 percent back into the blood. In some situations, people with renal glucosuria have kidneys that excrete glucose in the urine despite normal or low levels of glucose in the blood. This is often due to a defect in some cells of the kidneys that decrease the reabsorption of glucose. Mostly a benign condition, renal glucosuria may be hereditary or a sign of diabetes.
Handling of High Blood Glucose
Your kidneys have a rate-limited filtration and reabsorption capacity known as the glucose Tm system. This system depends on the filtration rate of the kidneys, called glomerula filtration rate, GFR, typically at 1.25 dL per minute. At this rate, the maximum glucose load your kidney can handle is 375 mg per minute, which is derived from multiplying 1.25 dL per minute by 300 mg glucose per minute. At blood glucose levels beyond 300 mg per dl, the proximal convoluted tubule fails to reabsorb the filtered glucose and glucose will start spilling into the urine.
Consequence of Chronic High Blood Glucose
Consistent high blood glucose levels in the blood initiate a process called diuresis, where your kidneys reabsorb water to increase urine production to maintain the right concentration of fluid in your body. This pushes your kidneys to work very hard to continuously filter large volume of fluids. Eventually, your kidneys may become exhausted and start to lose their function. Excessive glucose in the blood also creates chemical reactions and damages the structures of the kidneys. People with uncontrolled diabetes often develop kidney disease where the damaged kidneys start to leak protein into the urine.
Keeping your Kidneys Healthy
To keep your kidneys healthy, eat right and control your blood pressure and blood glucose levels, if you have diabetes. People of certain ethnic groups are at higher risk for developing kidney disease; thus, discuss with your doctor whether you belong to those groups and how to reduce your risk.
- National Kidney & Urologic Diseases Information Clearinghouse (NKUDIC): The Kidneys and How They Work
- "Vander's Renal Physiology, 7th Edition"; Douglas Eaton, et al.; March 2009