FUNCTIONS

A popular view considers the kidney to be an organ primarily responsible for the removal of metabolic waste from the body. Although this is certainly a major function of the kidneys, there are other functions that are arguably more important.
Function 1: Regulation of Water and Electrolyte Balance
The balance concept states that our bodies are in balance for any substance when the inputs and outputs of that substance are matched. Any difference between input and output leads to an increase or decrease in the amount of a substance within the body. Our input of water and electrolytes is enormously variable and is only sometimes driven in response to body needs. For example, we drink water when thirsty but we drink much more because it is a component of beverages that we consume for reasons other than hydration. We also consume food to provide energy, but food often contains large amounts of water. The kidneys respond by varying the output of water in the urine, thereby maintaining balance for water (ie, constant total body water content). Minerals
like sodium, potassium, magnesium, and so on are components of foods and generally present far in excess of body needs. As with water, the kidneys excrete minerals at a highly variable rate that, in the aggregate, matches input. One of the amazing feats of the kidneys is their ability to regulate each of these minerals independently (ie, we can be on a high-sodium, low-potassium diet or low-sodium, high-potassium diet, and the kidneys will adjust excretion of each of these substances appropriately).

Function 2: Excretion of Metabolic Waste
Our bodies continuously form end products of metabolic processes. In most cases, those end products serve no function and are harmful at high concentrations. Some of these waste products include urea (from protein), uric acid (from nucleic acids), creatinine (from muscle creatine), the end products of hemoglobin breakdown (which give urine much of its color), and the metabolites of various hormones, among many others.

Function 3: Excretion of Bioactive Substances (Hormones and Many Foreign Substances, Specifically Drugs) That Affect Body Function
Drugs and hormones in the blood are removed in many ways, mostly in the liver, but a number of them are removed in parallel by renal processes. Physicians have to be mindful of how fast the drugs are excreted in order to prescribe a dose that achieves the appropriate body levels.

Function 4: Regulation of Arterial Blood Pressure
Although many people appreciate at least vaguely that the kidneys excrete waste substances like urea (hence the name urine) and salts, few realize the kidneys’ crucial role in controlling blood pressure. Blood pressure ultimately depends on blood volume, and the kidneys’ maintenance of sodium and water balance achieves regulation of blood volume. Thus, through volume control, the kidneys participate in blood pressure control. They also participate in regulation of blood pressure via the generation of vasoactive substances that regulate smooth muscle in the peripheral vasculature.

Function 5: Regulation of Red Blood Cell Production
Erythropoietin is a peptide hormone that is involved in the control of erythrocyte (red blood cell) production by the bone marrow. Its major source is the kidneys, although the liver also secretes small amounts. The renal cells that secrete it are a particular group of cells in the interstitium. The stimulus for its secretion is a reduction in the partial pressure of oxygen in the kidneys, as occurs, eg, in anemia, arterial hypoxia, and inadequate renal blood flow. Erythropoietin stimulates the bone marrow to increase its production of erythrocytes. Renal disease may result in diminished erythropoietin secretion, and the ensuing decrease in bone marrow activity is one important causal factor of the anemia of chronic renal disease.

Function 6: Regulation of Vitamin D Production
When we think of vitamin D, we often think of sunlight or additives to milk. In vivo vitamin D synthesis involves a series of biochemical transformations, the last of which occurs in the kidneys. The active form of vitamin D (1,25-dihydroxyvitamin D3) is actually made in the kidneys, and its rate of synthesis is regulated by hormones that control calcium and phosphate balance.

Function 7: Gluconeogenesis
Our central nervous system is an obligate user of blood glucose regardless of whether we have just eaten sugary doughnuts or gone without food for a week. Whenever the intake of carbohydrate is stopped for much more than half a day, our body begins to synthesize new glucose (the process of gluconeogenesis) from noncarbohydrate sources (amino acids from protein and glycerol from triglycerides). Most gluconeogenesis occurs in the liver, but a substantial fraction occurs in the kidneys, particularly during a prolonged fast.

Most of what the kidneys actually do to perform the functions just mentioned involves transporting water and solutes between the blood flowing through the kidneys and the lumina of tubules (nephrons and collecting tubules that comprise the working mass of the kidneys). The lumen of a nephron is topologically outside the body, and any substance in the lumen that is not transported back into the blood is eventually excreted in the urine. As we explore renal function in more detail, we will constantly refer to tubular structure and the surrounding vasculature. Therefore, in the following section, we present the essential aspects of renal anatomy that are necessary to understand function.