Porth's Essentials of Pathophysiology, 4e

613

Structure and Function of the Kidney

C h a p t e r 2 4

to nonionized drug depends on the pH of the urine. Aspirin, for example, is highly ionized in alkaline urine and in this form is rapidly excreted in the urine, and it is largely nonionized in acid urine and, as such, reab- sorbed rather than excreted. Measures that alkalinize or acidify the urine may be used to increase elimination of drugs, particularly in situations of toxic overdose. Endocrine Functions of the Kidney In addition to their role in regulating fluid and electro- lytes, the kidneys function as an endocrine organ in that they produce chemical mediators that travel through the blood to distant sites where they exert their actions. The kidneys participate in control of blood pressure through the renin-angiotensin-aldosterone mechanism, in cal- cium metabolism by activating vitamin D, and in regu- lating red blood cell production through the synthesis of erythropoietin. The renin-angiotensin-aldosterone mechanism is impor- tant in short- and long-term regulation of blood pressure (see Chapter 18). Renin is an enzyme that is synthesized and stored in the juxtaglomerular cells of the kidney. This enzyme is thought to be released in response to a decrease in renal blood flow or a change in the com- position of the distal tubular fluid, or as the result of sympathetic nervous system stimulation. Renin itself has no direct effect on blood pressure. Rather, it acts enzy- matically to convert a circulating plasma protein called angiotensinogen to angiotensin I. Angiotensin I, which has few vasoconstrictor prop- erties, leaves the kidneys and enters the circulation; as it is circulated through the lungs, the angiotensin-con- verting enzyme catalyzes its conversion to angiotensin II. Angiotensin II is a potent vasoconstrictor, and it acts directly on the kidneys to decrease salt and water excre- tion. Both mechanisms have relatively short periods of action. Angiotensin II also stimulates the secretion of aldosterone by the adrenal gland, thereby exerting a more long-term effect on maintenance of blood pressure by increasing the reabsorption of sodium in the distal tubule. Renin also functions via angiotensin II to produce constriction of the efferent arteriole as a means of preventing a serious decrease in glomerular filtration pressure. Erythropoietin Erythropoietin is a polypeptide hormone that regulates the differentiation of red blood cells in the bone marrow (see Chapter 13). Between 89% and 95% of erythropoi- etin is formed in the kidneys. The synthesis of erythro- poietin is stimulated by tissue hypoxia, which may be brought about by anemia, residence at high altitudes, or impaired oxygenation of tissues due to cardiac or pulmonary disease. Persons with chronic kidney disease often are anemic because of an inability of the kidneys to The Renin-Angiotensin-Aldosterone Mechanism

produce erythropoietin. This anemia usually is managed by the administration of a recombinant erythropoietin (epoetin alfa) produced through DNA technology to stimulate erythropoiesis. Vitamin D Activation of vitamin D occurs in the kidneys. Vitamin D increases calcium absorption from the gastrointestinal tract and helps to regulate calcium deposition in bone. It also has a weak stimulatory effect on renal calcium absorption. Although vitamin D is not synthesized and released from an endocrine gland, it is often considered as a hormone because of its pathway of molecular acti- vation and mechanism of action. Vitamin D exists in two forms: natural vitamin D (cho- lecalciferol), produced in the skin from ultraviolet irra- diation, and synthetic vitamin D (ergocalciferol), derived from irradiation of ergosterol. The active form of vita- min D is 1,25-dihydroxycholecalciferol. Cholecalciferol and ergocalciferol must undergo chemical transforma- tion to become active: first to 25-hydroxycholecalciferol in the liver and then to 1,25-dihydroxycholecalciferol in the kidneys. Persons with end-stage renal disease are unable to transform vitamin D to its active form and may require pharmacologic preparations of the active vitamin (calcitriol) for maintaining mineralization of their bones. ■■ The kidneys have multiple functions, including maintaining the volume and composition of body fluids through the regulation of electrolyte levels and excretion of various by-products of metabolism. ■■ Sodium and potassium levels are regulated by the GFR and by humoral agents such as aldosterone, which controls the final steps in regulating their absorption or elimination. ■■ The kidneys regulate the pH of body fluids by eliminating H + and conserving or generating bicarbonate ions. ■■ Various products of metabolism and exogenous organic anions, such as drugs, are bound to plasma proteins and are therefore unavailable for filtration in the glomerulus.Thus, secretion of substances into tubular fluid provides the route for elimination in the urine. ■■ The kidneys also function as endocrine organs. They participate in control of blood pressure by way of the renin-angiotensin-aldosterone mechanism, help regulate red blood cell production through the synthesis of erythropoietin, SUMMARY CONCEPTS

and aid in calcium metabolism through the conversion of vitamin D to its active form.