The kidneys are organs specialized to filter the blood. As such, they make an important contribution to the removal of metabolic waste products as well as to maintenance of fluid and electrolyte balance. Specific functions of the kidneys include:

• Regulation of extracellular fluid volume

• Regulation of inorganic electrolyte concentration in extracellular fluid

• Regulation of the osmolarity of extracellular fluid

• Removal of metabolic waste products

• Excretion of foreign compounds

• Maintenance of acid-base balance

• Hormone and enzyme production

The regulation of extracellular fluid volume, in particular, plasma volume, is important in the long-term regulation of blood pressure. An increase in plasma volume leads to an increase in blood pressure and a decrease in plasma volume leads to a decrease in blood pressure. Plasma volume is regulated primarily by altering the excretion of sodium in the urine. Other inorganic electrolytes regulated by the kidneys include chloride, potassium, calcium, magnesium, sulfate, and phosphate.

The kidneys also regulate the osmolarity of extracellular fluid, in particular plasma osmolarity. The maintenance of plasma osmolarity close to 290 mOsm prevents any unwanted movement of fluid into or out of the body's cells. An increase in plasma osmolarity causes water to leave the cells, leading to cellular dehydration; a decrease in plasma osmolarity causes water to enter the cells, leading to cellular swelling and possibly lysis. Plasma osmolarity is regulated primarily by altering the excretion of water in the urine.

As the major excretory organs in the body, the kidneys are responsible for the removal of many metabolic waste products. These include urea and uric acid, which are nitrogenous waste products of amino acid and nucleic acid metabolism, respectively; creatinine, a breakdown product of muscle metabolism; and urobilinogen, a metabolite of hemoglobin that gives urine its yellow color. Foreign compounds excreted by the kidneys include drugs (e.g., penicillin, nonsteroidal anti-inflammatory drugs); food additives (e.g., saccharin, benzoate); pesticides; and other exogenous nonnutritive materials that have entered the body. If allowed to accumulate, these substances become quite toxic.

Along with the respiratory system, the renal system maintains acid-base balance by altering the excretion of hydrogen and bicarbonate ions in the urine. When the extracellular fluid becomes acidic and pH decreases, the kidneys excrete H+ ions and conserve HCO3- ions. Conversely, when the extracellular fluid becomes alkaline and pH increases, the kidneys conserve H+ ions and excrete HCO3- ions. Normally, the pH of arterial blood is 7.4.

Although the kidneys are not considered endocrine glands per se, they are involved in hormone production. Erythropoietin is a peptide hormone that stimulates red blood cell production in bone marrow. Its primary source is the kidneys. Erythropoietin is secreted in response to renal hypoxia. Chronic renal disease may impair the secretion of erythropoietin, leading to development of anemia. The kidneys also produce enzymes. The enzyme renin is part of the renin-angiotensin-aldosterone system. As will be discussed, these substances play an important role in the regulation of plasma volume and therefore blood pressure. Other renal enzymes are needed for the conversion of vitamin D into its active form, 1,25-dihydroxyvitamin D3, which is involved with calcium balance.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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