Facts on File Encyclopedia of Health and Medicine

other substances across the membranous glomeru-
lar walls and into an encapsulated structure called
Bowman’s capsule. Specialized proteins called
transporters carry these substances across the cap-
illary membranes. The collected mixture, called
filtrate, funnels from the Bowman’s capsule into
the tubules. A separate capillary network, the per-
itubular capillaries, entwines the filtration tubules
to allow water and other substances to return to
the blood circulation (reabsorption). By the time a
heartbeat’s surge of blood completes its passage
through the nephrons, about 99 percent of the
water and electrolytes originally filtered from the
blood (sodium, potassium, magnesium, calcium,
and others) have returned to the circulation.
Kidney function remains at a fairly constant
level for much of life. By the 40s, the kidneys
begin to lose nephrons at a rate of about 10 per-
cent per decade. As a single kidney can meet the
body’s needs having only about one third of its
capacity, the kidneys are well structured to func-
tion the length of the lifespan. The primary threats
to kidney function are diseases that affect other
body systems or the body as a whole, such as DIA-
BETES, HYPERTENSION (high blood pressure), and
CARDIOVASCULAR DISEASE(CVD).
The kidneys also produce two vital hormones:
RENIN, which controls blood pressure, and EPO,
which regulates erythropoiesis (the synthesis of
new erythrocytes in the BONE MARROW). Health
conditions that damage the kidneys also affect
their ability to produce these hormones. Intersti-
tial cells (cells in the body of the kidney) produce
both hormones.
The pressure of blood flowing through the
glomeruli helps determine whether the kidney
releases renin, which is a process of perpetual bal-
ance. Renin sets in motion the cascade of events
through which the body produces angiotensin II, a
potent vasoconstrictor (chemical that narrows and
stiffens the blood vessels to raise blood pressure).
The kidneys also respond to the release of ANTIDI-
URETIC HORMONE (ADH) from the HYPOTHALAMUS,
another mechanism of blood pressure regulation
that influences the amount of water the kidneys
withhold in the blood or excrete in the urine. The
hypothalamus releases ADH to cause the kidneys
to withhold more water, which increases the
blood volume and thus the blood pressure.

The kidneys also detect the level of erythrocytes

that are in the blood as it passes through them, and

release erythropoietin when the erythrocyte level

drops. Erythropoietin stimulates the production of

reticulocytes in the bone marrow and their release

into the blood circulation, where they mature to

become oxygen-bearing erythrocytes.

HEALTH CONDITIONS THAT AFFECT THE KIDNEYS

ALPORT’S SYNDROME END-STAGE RENAL DISEASE(ESRD)

GLOMERULONEPHRITIS GLOMERULOSCLEROSIS

GOODPASTURE’S SYNDROME HEMOLYTIC UREMIC SYNDROME

HEPATORENAL FAILURE HORSESHOE KIDNEY

HYDRONEPHROSIS kidney cyst

MINIMAL CHANGE DISEASE NEPHRITIS

NEPHROLITHIASIS NEPHROPATHYof DIABETES

nephropathy of HYPERTENSION NEPHROTIC SYNDROME

POLYCYSTIC KIDNEY DISEASE pyelonephritis

RENAL CANCER RENAL FAILURE

renal osteodystrophy RENAL TUBULAR ACIDOSIS

rhabdomyolysis WILMS’S TUMOR

For further discussion of the kidneys within the

context of the urinary system’s structure and

function please see the overview section “The Uri-

nary System.”

See also BLADDER; ERYTHROCYTE; HEMATOPOIESIS;

HORSESHOE KIDNEY; RETICULOCYTE; URETHRA.

kidney stone See NEPHROLITHIASIS.

kidney transplantation A surgical OPERATION to

place a healthy, functioning kidney into a person

whose own kidneys have permanently failed. The

first successful kidney transplantation took place

between identical twin brothers in 1954. How-

ever, until the discovery of the immunosuppres-

sive DRUGcyclosporine in 1983, the risk of organ

rejection was very high, and kidney transplanta-

tion was a treatment of last resort. With current

IMMUNOSUPPRESSIVE THERAPYthe recipient of a trans-

planted kidney can expect to live 5 to 20 years or

longer with relatively normal kidney function.

Since the mid-1980s kidney transplantation has

become the standard of treatment for END-STAGE

RENAL DISEASE(ESRD), also called permanent kidney

failure (RENAL FAILURE). Transplant surgeons in the

United States perform about 15,000 kidney trans-

plant operations each year. However, a severe

202 The Urinary System