Essentials of Pathophysiology: Concepts of Altered Health States / Edition 4

Author:Carol M. Porth [Porth, Carol M.]
Language: eng
Format: epub
ISBN: 9780397549610
Barnesnoble:
Publisher: Lippincott Williams & Wilkins
Published: 1993-12-28T05:00:00+00:00

C H A P T E R 2 4

Structure and Function of the Kidney

6 1 3

to nonionized drug depends on the pH of the urine.

produce erythropoietin. This anemia usually is managed

Aspirin, for example, is highly ionized in alkaline urine

by the administration of a recombinant erythropoietin

and in this form is rapidly excreted in the urine, and it

(epoetin alfa) produced through DNA technology to

is largely nonionized in acid urine and, as such, reab-

stimulate erythropoiesis.

sorbed rather than excreted. M easures that alkalinize or

acidify the urine may be used to increase elimination of

Vitam in D

drugs, particularly in situations of toxic overdose.

Activation of vitamin D occurs in the kidneys. Vitamin

D increases calcium absorption from the gastrointestinal

En d o c r in e Fu n c t io n s o f t h e Kid n e y

tract and helps to regulate calcium deposition in bone.

It also has a weak stimulatory effect on renal calcium

In addition to their role in regulating uid and electro-

absorption. Although vitamin D is not synthesized and

lytes, the kidneys function as an endocrine organ in that

released from an endocrine gland, it is often considered

they produce chemical mediators that travel through the

as a hormone because of its pathway of molecular acti-

blood to distant sites where they exert their actions. The

vation and mechanism of action.

kidneys participate in control of blood pressure through

Vitamin D exists in two forms: natural vitamin D (cho-

the renin-angiotensin-aldosterone mechanism, in cal-

lecalciferol), produced in the skin from ultraviolet irra-

cium metabolism by activating vitamin D, and in regu-

diation, and synthetic vitamin D (ergocalciferol), derived

lating red blood cell production through the synthesis of

from irradiation of ergosterol. The active form of vita-

erythropoietin.

min D is 1,25-dihydroxycholecalciferol. Cholecalciferol

and ergocalciferol must undergo chemical transforma-

The Renin-Ang iotens in-Aldos te rone

tion to become active: rst to 25-hydroxycholecalciferol

Me chanis m

in the liver and then to 1,25-dihydroxycholecalciferol

in the kidneys. Persons with end-stage renal disease are

The renin-angiotensin-aldosterone mechanism is impor-

unable to transform vitamin D to its active form and may

tant in short- and long-term regulation of blood pressure

require pharmacologic preparations of the active vitamin

(see Chapter 18). Renin is an enzyme that is synthesized

(calcitriol) for maintaining mineralization of their bones.

and stored in the juxtaglomerular cells of the kidney.

This enzyme is thought to be released in response to a

decrease in renal blood ow or a change in the com-

S U M M A R Y C O N C E P T S

position of the distal tubular uid, or as the result of

sympathetic nervous system stimulation. Renin itself has

no direct effect on blood pressure. Rather, it acts enzy-

■ Th e kid n e ys h a ve m u ltip le fu n ctio n s , in clu d in g

matically to convert a circulating plasma protein called

m a in ta in in g th e vo lu m e a n d co m p o s itio n o f

angiotensinogen to angiotensin I.

bo d y u id s th ro ug h th e re g u la tio n o f e le ctro lyte

Angiotensin I, which has few

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