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What is Normal Kidney Function?

Why is normal kidney function pivotal to health and wellbeing?

In order to understand the course of chronic kidney failure it is necessary to know the reasons that normal kidney function is pivotal to health and wellbeing.  The following lists the most important functions of the kidneys and what can be done when they start to fail.

Excretion of waste products and fluid and electrolyte balance:

Kidneys play a critical role not only in excreting waste products, but also in maintaining electrolyte (sodium, potassium, bicarbonate) and water balance.  As kidney function declines, nitrogenous waste products—that is, blood urea nitrogen (BUN), and creatinine—accumulate.

Reducing the intake of protein, which is the source of these waste products, is helpful. The typical American diet contains much more protein then needed, but care must be taken to be sure that intake is sufficient to meet the recommended daily allowance.  This is especially important in children and teens.

Normal kidney function allows wide latitude in the amount of salt, potassium and fluid ingested because the kidneys make the proper adjustments, that is, they retain what the body needs and excrete what it does not need.  As kidney failure progresses and kidneys lose their ability to maintain homeostasis (balance), salt, potassium and fluid intake has to be decreased accordingly.  Working with a Renal Dietitian is essential.

Maintain acid-base balance: 

Normal metabolism produces large quantities of acid, mainly sulfuric acid, resulting from protein metabolism.  Healthy kidneys excrete this acid. Once renal excretion of acid is insufficient to offset its production, reducing its source by restricting dietary protein is helpful, but alone is insufficient to maintain acid-base balance. Thus, patients also need to take base (alkali) such as sodium bicarbonate to neutralize remaining body (not stomach) acid.  Failure to do so results in poor appetite, poor growth and weakened bones.

Production of red blood cells:

Bone marrow is dependent on a hormone, called erythropoietin that is produced by kidneys, and that directs the bone marrow to make red blood cells (RBC). Prior to the availability of human recombinant erythropoietin, blood transfusions were the only choice.  Today, the injection of erythropoietin makes that unnecessary.

Maintain bone health:

Healthy bones require modification of vitamin D before it is biologically active and able to control the intestinal absorption of calcium, the renal excretion of phosphorous, and bone deposition of calcium and phosphorous.  This chemical modification of vitamin D occurs in the kidneys. The parathyroid gland is next to the thyroid gland and produces a hormone called parathyroid hormone (PTH), which in turn is under control of the blood calcium level.  In health, PTH keeps blood calcium and phosphorous levels normal and bones healthy.  Low levels of blood calcium, low biologically active vitamin D, and high levels of phosphorous activate it.  But in kidney failure the hormone’s attempt to maintain a normal blood calcium level occurs at the expense of the bones.  That is, it draws calcium out of the bones.

Failure to adequately control hyperparathyroidism results in osteopenia, that is, reduced bone density and growth. This can be catastrophic for children and teens.  Treatment consists of the oral administration of vitamin D that has been modified in the laboratory to its biologically active form (1-25-vitamin D).  In addition, calcium supplementation, and phosphorous restriction (combined with binders that reduce absorption of phosphorous from the intestine), is necessary. The goal is to keep the PTH level no more than twice the upper limits of normal.  Achieving and maintaining this delicate balance requires close medical supervision and the help of a Renal Dietician.

Control blood pressure:

Blood pressure is, to a large extent, controlled by the kidneys, via the production of the proper balance of vasoconstrictive and vasodilatory chemicals and by maintaining normal salt and water balance. High blood pressure afflicts the majority of renal failure patients, but can often be controlled by maintaining normal sodium (salt) balance via dietary salt restriction and/ or the use of diuretics (water pills).  However, in most cases, this is insufficient, and the administration of antihypertensive drugs is also necessary.  Failure to maintain normal blood pressure increases the risk of heart failure, heart attack and stroke.

Maintain normal growth: 

Healthy kidneys are required for normal linear growth.  Children with even moderate degrees of chronic renal failure often fall behind in their growth.  As long as the bone growth centers have not yet closed, linear growth can be improved. First, it is necessary to assure adequate caloric and protein intake, to correct acidosis, and to control hyperparathyroidism. If this regimen does not adequately stimulate growth, human recombinant growth hormone (HGH) can be started.  For maximum growth, a daily injection of growth hormone is necessary. In almost all cases linear growth improves, and many experience enough catch-up growth to achieve normal adult height.