Essentials of Anatomy and Physiology

creatinine, and ammonia, which must be removed
from the blood before they accumulate to toxic levels.
As the kidneys form urine to excrete these waste prod-
ucts, they also accomplish several other important
functions:

1. Regulation of the volume of blood by excretion or
   conservation of water


2. Regulation of the electrolyte content of the blood
   by the excretion or conservation of minerals


3. Regulation of the acid–base balance of the blood by
   excretion or conservation of ions such as Hions
   or HCO 3 ions


4. Regulation of all of the above in tissue fluid

The process of urine formation, therefore, helps
maintain the normal composition, volume, and pH of
both blood and tissue fluid by removing those sub-
stances that would upset the normal constancy and
balance of these extracellular fluids.

KIDNEYS

The two kidneysare located in the upper abdominal
cavity on either side of the vertebral column, behind
the peritoneum (retroperitoneal). The upper por-
tions of the kidneys rest on the lower surface of the
diaphragm and are enclosed and protected by the
lower rib cage (see Fig. 18–1). The kidneys are
embedded in adipose tissue that acts as a cushion and

is in turn covered by a fibrous connective tissue mem-

brane called the renal fascia, which helps hold the

kidneys in place (see Box 18–1: Floating Kidney).

Each kidney has an indentation called the hilus

on its medial side. At the hilus, the renal artery enters

the kidney, and the renal vein and ureter emerge. The

renal artery is a branch of the abdominal aorta, and the

renal vein returns blood to the inferior vena cava (see

Fig. 18–1). The ureter carries urine from the kidney to

the urinary bladder.

INTERNAL STRUCTURE

OF THE KIDNEY

In a coronal or frontal section of the kidney, three

areas can be distinguished (Fig. 18–2). The lateral and

middle areas are tissue layers, and the medial area at

the hilus is a cavity. The outer tissue layer is called the

renal cortex; it is made of renal corpuscles and convo-

luted tubules. These are parts of the nephron and are

described in the next section. The inner tissue layer is

the renal medulla, which is made of loops of Henle

and collecting tubules (also parts of the nephron). The

renal medulla consists of wedge-shaped pieces called

renal pyramids. The tip of each pyramid is its apex or

papilla.

The third area is the renal pelvis; this is not a layer

of tissues, but rather a cavity formed by the expansion

of the ureter within the kidney at the hilus. Funnel-

shaped extensions of the renal pelvis, called calyces

(singular: calyx), enclose the papillae of the renal pyr-

amids. Urine flows from the renal pyramids into the

calyces, then to the renal pelvis and out into the ureter.

THE NEPHRON

The nephronis the structural and functional unit of

the kidney. Each kidney contains approximately 1 mil-

lion nephrons. It is in the nephrons, with their associ-

ated blood vessels, that urine is formed. Each nephron

has two major portions: a renal corpuscle and a renal

tubule. Each of these major parts has further subdivi-

sions, which are shown with their blood vessels in Fig.

18–3.

Renal Corpuscle

A renal corpuscleconsists of a glomerulus surroun-

ded by a Bowman’s capsule. The glomerulusis a cap-

illary network that arises from an afferent arteriole

and empties into an efferent arteriole. The diameter

The Urinary System 421

BOX18–1 FLOATING KIDNEY

A floating kidney is one that has moved out of its

normal position. This may happen in very thin

people whose renal cushion of adipose tissue is

thin, or it may be the result of a sharp blow to

the back that dislodges a kidney.

A kidney can function in any position; the

problem with a floating kidney is that the ureter

may become twisted or kinked. If urine cannot

flow through the ureter, the urine backs up and

collects in the renal pelvis. Incoming urine

from the renal tubules then backs up as well. If

the renal filtrate cannot flow out of Bowman’s

capsules, the pressure within Bowman’s capsules

increases, opposing the blood pressure in the

glomeruli. Glomerular filtration then cannot take

place efficiently. If uncorrected, this may lead to

permanent kidney damage.