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2e Collecting Tubules and Collecting Ducts: How Tubular Fluid Becomes Urine

2e Collecting Tubules and Collecting Ducts: How Tubular Fluid Becomes Urine

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Chapter Twenty-Seven  Urinary System












convoluted tubule



Renal corpuscle




convoluted tubule

LM 100x

(b) Histology of renal cortex

Nephron loop

Tall microvilli

Short, sparse microvilli



Proximal convoluted tubule

(a) Nephron components



Distal convoluted tubule

(c) Convoluted tubule epithelia

Thick limbs of

nephron loops

Collecting ducts

Thin limbs of

nephron loops

LM 100x

Vasa recta

(d) Histology of renal medulla

Figure 27.8

The Convoluted Tubules and Nephron Loop. The convoluted tubules and nephron loop carry tubular fluid that is being modified to form urine. (a) In this drawing,

each of the components of the nephron is distinguished by a different color. (b) A photomicrograph of a section through the cortex compares transverse sections of the

proximal and distal convoluted tubules. (c) Comparisons of the simple cuboidal epithelium lining the proximal and distal convoluted tubules show differences in the

sizes and numbers of microvilli. (d) A photomicrograph of a transverse section through the medulla compares nephron loops and collecting ducts.

(b) © Keith Wheeler/Science Photo Library/Corbis; (d) © McGraw-Hill Education/Al Telser, photographer

through the renal medulla toward the renal papilla (not shown) (see

figures 27.5 and 27.8d). Both collecting tubules and collecting ducts

are lined by a simple epithelium. The epithelial cells are cuboidal

in the tubules, but very tall columnar cells in the ducts near the

renal papilla.

The collecting ducts are the last structures that have the capacity to modify the tubular fluid further, and can do so under the influence of ADH and aldosterone. Secretion of these hormones results

in increased water and sodium absorption from the tubular fluid in

the collecting ducts, thereby reducing water and sodium loss from


Chapter Twenty-Seven  Urinary System

Table 27.2

Parts of a Nephron and Their Functions




Renal corpuscle

Capillary ball or tuft covered by podocytes and surrounded by

an epithelial capsule; capsular space is between the parietal and

visceral layers of the capsule

Produces a filtrate of blood that must be modified as it passes

through the convoluted tubules and nephron loop

Proximal convoluted tubule

Tubule lined with simple cuboidal epithelium; has a prominent

brush border (microvilli); cytoplasm tends to stain more brightly

than DCT cells

Reabsorbs ions (especially Na+), nutrients (glucose and

amino acids), plasma proteins, vitamins, and water; secretes

some H+ ions

Nephron loop

Tubule that forms a hairpin/U-shaped loop; has thick and thin

ascending and descending portions; the most distal part of the

loop often extends into the medulla

Thick limbs are lined with simple cuboidal epithelium

Thin limbs are lined with simple squamous epithelium

Reabsorbs water in tubular fluid; also reabsorbs Na+ and Cl−;

secretes some H+ ions

Distal convoluted tubule

Tubule lined with simple cuboidal epithelium with only a sparse

brush border; cytoplasm of cells tends to be paler than that of

PCT cells

Secretes H+ and K+ into tubular fluid; reabsorbs Na+ and

water from tubular fluid (in presence of aldosterone and


the kidneys. ADH is secreted in response either to a rise in the concentration of ions in the blood or a fall in blood volume, as when the

body is dehydrated and needs to conserve water. If an individual is

well hydrated, the collecting ducts merely transport the tubular fluid

and do not modify it. However, if an individual is dehydrated, water

conservation must occur, and more-concentrated urine is produced.

ADH acts on the collecting duct epithelium, making it more able to

absorb water from the tubular fluid.

Once the tubular fluid leaves the collecting duct, the tubular

fluid now may be called urine. The remaining structures in the kidney simply transport the urine and cannot modify this fluid further.

Several collecting ducts merge to empty into a papillary duct that

opens at the edge of the renal papilla into minor calyx. Urine leaves

the renal papilla and enters the minor calyces, which then transport

the urine to major calyces, which in turn transport the urine to the

renal pelvis. Because the calyces and the renal pelvis are in contact

with urine, they are lined with transitional epithelium. The renal

pelvis conducts urine into the ureter.



Is ADH secreted when the body is dehydrated or well


27.2f  Juxtaglomerular Apparatus

Associated with the nephron are some structures collectively

referred to as the juxtaglomerular (jŭks′tă-glō-mer′yū-lăr;

juxta = near) apparatus (see figure 27.7a, b). Components of the

juxtaglomerular apparatus are juxtaglomerular cells, macula densa,

and extraglomerular mesangial cells. Juxtaglomerular cells are

modified smooth muscle cells of the afferent arteriole located near

the entrance to the renal corpuscle. The macula (mak′yū-lă; spot)

densa (den′să; dense) is a group of modified epithelial cells in a

distal convoluted tubule that touch the juxtaglomerular cells. These

cells, which are located only on the tubule side next to the afferent arteriole, are narrower and taller than other distal convoluted

tubule epithelial cells. Extraglomerular mesangial cells, located

between the juxtaglomerular cells and the arterioles, can contract

and phagocytize filtered particles.

The structures of the juxtaglomerular apparatus work ­together

to regulate blood pressure. The macula densa cells continuously

monitor ion concentration in tubular fluid. Thus, if either blood

volume or solute concentration is reduced, tubular fluid reflects

this reduction, and the macula densa cells detect this change and

stimulate the juxtaglomerular cells to release renin. Recall from

section 20.9a that renin activates the renin-angiotensin pathway,

resulting in aldosterone production, which causes increases in blood

ion concentrations and blood volume. This arrangement is important in  maintaining blood volume and blood pressure homeostasis

in the body.




1 0

What are the components of the nephron, and how do they

modify the filtrate?

Where is ADH produced, and how specifically does it affect

water concentration in the urine?

What is the structure and function of the juxtaglomerular


27.3  Urinary Tract

✓✓Learning Objectives

6. Explain the anatomy and location of the ureters, urinary

bladder, and the male and female urethras.

7. Outline the blood vessels and nerves that supply the organs of

the urinary tract.

The urinary tract consists of the ureters, urinary bladder, and urethra.

27.3a  Ureters

The ureters (yū-rē′tĕr, yū′rē-ter) are long, fibromuscular tubes that

conduct urine from the kidneys to the urinary bladder. Each tube

averages 25 centimeters in length and is retroperitoneal. The ureters

originate at the renal pelvis as it exits the hilum of the kidney, and

then extend inferiorly to enter the posterolateral wall of the base

of the urinary bladder. The wall of the ureter is composed of three

concentric tunics. From innermost to outermost, these tunics are the

mucosa, muscularis, and adventitia (figure 27.9). (The ureter does

not have a submucosa.)

The mucosa is formed from transitional epithelium, which

is both distensible (stretchy) and impermeable to the passage of

urine. External to the transitional epithelium of the mucosa is the

Chapter Twenty-Seven  Urinary System


Clinical View 27.2

Renal Failure, Dialysis,

and Kidney Transplants

Renal failure refers to greatly diminished or absent renal function

caused by the destruction of about 90% of the tissue in the kidney.

Renal failure often results from a chronic disease that affects the

glomerulus or the small blood vessels of the kidney, as a result of

autoimmune conditions, high blood pressure, or diabetes. Once

the kidneys have been destroyed, there is no chance they will

regenerate or begin functioning again. Thus, the two main treatments are dialysis or a kidney transplant.

The term dialysis (dī-al′i-sis; dialyo = to separate) comes

from a Greek word meaning “to separate agents or particles

on the basis of their size.” Two forms of dialysis are commonly

used today: peritoneal dialysis and hemodialysis. In peritoneal

dialysis, a catheter is permanently placed in the peritoneal cavity, to which a bag of dialysis fluid can be attached externally.

As this fluid enters the peritoneal cavity, harmful waste products

are transferred, or dialyzed, from the blood into the fluid. After

several hours, the fluid is drained from the peritoneal cavity and

replaced with fresh fluid.

In hemodialysis, the patient’s blood is cycled through

a machine that filters the waste products across a specially

designed membrane. The patient must remain stationary for the

time it takes to cycle the blood through the dialysis unit while the

metabolic waste products are removed. Hemodialysis must be

performed three to four times a week, and each treatment takes

about 4 hours.

A kidney transplant from a genetically similar person may

successfully restore renal function. The kidney is generally

removed from the donor by a laparoscopic procedure that

removes the kidney through the umbilicus (navel, or belly button) after making a single small incision. This decreases the

donor’s recovery time from about 3 months to just under a

month. The replacement kidney is attached to an artery and

lamina propria, composed of a fairly thick layer of dense irregular

connective tissue. The continuous production of urine ensures that

the ureters are rarely completely empty, but as peristaltic waves

propel urine through the ureter, the ureter may be temporarily

empty at specific places along its length. At these locations, the

mucosa folds to fill the lumen. Thereafter, when the ureter is distended, the mucosa can be stretched; this folding of the mucosa

allows for considerable increase in the luminal diameter when


The middle muscularis consists of two smooth muscle layers: an inner longitudinal layer and an outer circular layer. (The

arrangement of muscle layers is opposite that in the GI tract,

where the circular layer is the inner layer.) The presence of urine

within the renal pelvis causes these muscle layers to produce

peristaltic waves that propel the urine through the ureters into the

urinary bladder.

vein in the inferior abdominopelvic region, where it is relatively

easy to establish a vascular connection. The new kidney rests

either on the superior surface of, or immediately lateral to, the

urinary bladder. Because a pelvic artery and vein connect the

donor kidney to the patient’s blood supply, having the kidney

near the bladder means only a short segment of a ureter is

needed for connection to the bladder. The diseased kidney is

not removed. Because the transplanted kidney is a foreign tissue, immunosuppressant drugs are administered to suppress

the immune system’s activity.

Donor kidneys may come through an organ procurement

program or from a living person. Because most people have two

kidneys, and normal renal function actually requires only one,

some people have donated a kidney (while still alive) to a relative

or even to a total stranger.

Diseased kidneys


abdominal aorta

Inferior vena cava

Transplanted kidney

Transplanted ureter

Urinary bladder

Location of a transplanted kidney in the abdominopelvic cavity.



Why do the ureters use peristalsis to actively pump urine to

the urinary bladder? Why don’t the ureters rely on gravity to

move the urine to the inferiorly located bladder?

The external layer of the ureter wall is the adventitia, which

is formed from areolar connective tissue. Some extensions of this

areolar connective tissue layer also anchor the ureter to the posterior

abdominal wall.

The ureters project through the posteroinferior bladder wall

obliquely, and some smooth muscle fibers of the inner longitudinal layer of the muscularis insert into the lamina propria of the

bladder. Because of the oblique course the ureters take through

the bladder wall, the ureteral walls are compressed as the bladder

distends, decreasing the likelihood of urine refluxing into the


­ureters from the bladder.


Chapter Twenty-Seven  Urinary System










circular longitudinal epithelium







LM 18x

(a) Ureter cross section

(b) Histology of ureter

Figure 27.9

Ureters. The ureters conduct urine from the kidneys to the urinary bladder for storage prior to voiding from the body. (a) Features of a ureter in cross-sectional

view. (b) A photomicrograph of a ureter in cross section shows its mucosal folds and thick muscularis.

(b) © McGraw-Hill Education/Al Telser, photographer

Multiple blood vessels supply blood to the ureters. In general,

a segment of ureter receives its blood supply from a branch of the

nearest artery. Thus, the superior aspects of the ureter are supplied by the renal arteries, whereas the more inferior aspects may

receive their arterial supply from the aorta, common iliac, and/or

internal iliac arteries. Venous drainage is through the companion


The ureters are innervated by the autonomic nervous system.

Parasympathetic axons (see section 18.3) come from CN X (which

supplies the superior region of the ureter) and from the pelvic

splanchnic nerves (which supply the inferior region of the ureter).

There are no known effects of this innervation. Sympathetic axons

(see section 18.4) come from the T11–L2 segments of the spinal cord.

Pain from the ureter (e.g., due to a kidney stone lodged in the ureter)

is referred to the T11–L2 dermatomes. These dermatomes are along a

“loin-to-groin” region, so “loin-to-groin” pain typically means ureter

and/or kidney discomfort.


1 1

1 2

Describe the structure and function of the middle tunic of

the ureter.

What nerves innervate the ureters?

27.3b  Urinary Bladder

The urinary bladder is an expandable, muscular container that

serves as a reservoir for urine (figure 27.10). The bladder is positioned immediately posterior to the pubic symphysis. In females,

the urinary bladder is anteroinferior to the uterus and directly

anterior to the vagina; in males, the bladder is anterior to the rectum and superior to the prostate gland. The urinary bladder is a

retroperitoneal organ, because only its superior surface is covered

with peritoneum.

When empty, the urinary bladder exhibits an upside-down

pyramidal shape. Filling with urine distends it superiorly until it

assumes an oval shape (figure 27.10c). A fibrous, cordlike median

umbilical ligament extends toward the umbilicus from its origin on

the anterosuperior border of the urinary bladder. It is a remnant of

the embryologic structure called the urachus. Ureters enter the posterolateral wall of the urinary bladder through the oblique ureteral

openings. The constricted neck of the bladder is located inferiorly

and connected to the urethra.

A posteroinferior triangular area of the urinary bladder wall,

called the trigone (trī′gōn; trigonum = triangle), is formed by imaginary lines connecting the two ureteral openings and the urethral

opening. The trigone does not move and remains in place as the urinary bladder fills and evacuates. It functions as a funnel to direct the

stored urine into the urethra as the bladder wall contracts. The trigone

is embryologically different from the rest of the urinary bladder.

While the urinary bladder forms from a structure called the cloaca,

the trigone forms from the distal parts of the ureters, which become

incorporated into the posterior wall of the urinary bladder.

The four tunics that form the wall of the bladder are the mucosa, submucosa, muscularis, and adventitia. The mucosa lines the

bladder lumen; it is formed by a transitional epithelium that accommodates the shape changes occurring with distension, and by a highly

vascularized lamina propria that supports the mucosa. Additionally,

mucosal folds, called rugae, allow for even greater distension. Within

the trigone region, the mucosa is smooth, thick, and lacking rugae.

The submucosa lies immediately external to the mucosa and is

formed by dense irregular connective tissue that supports the urinary

bladder wall.

The muscularis consists of three layers of smooth muscle,

collectively called the detrusor (dē-trū′sŏr, -sōr; detrudo = to drive

away) muscle. These smooth muscle bundles exhibit such complex

orientations that it is difficult to delineate individual layers in random

Chapter Twenty-Seven  Urinary System

Median umbilical





Detrusor muscle

of muscularis

Ureteral openings


Neck of urinary bladder




Lamina propria


Internal urethral


Detrusor muscle

of muscularis


External urethral sphincter

(in urogenital diaphragm)


(a) Urinary bladder, anterior view

Transitional epithelium

Lamina propria

LM 100x



Superior wall of

distended full bladder

Detrusor muscle

of muscularis

Superior wall of

empty bladder


LM 18x

(b) Histology of urinary bladder


Figure 27.10

Urinary Bladder. (a) The urinary bladder is an expandable, muscular sac. This view depicts the female bladder. (b) Photomicrographs of the urinary bladder

wall show its tunics. (c) Sagittal views show that the urinary bladder expands superiorly and becomes more oval in shape as it fills with urine.

(b, top) © Victor P. Eroschenko; (b, bottom) © McGraw-Hill Education/Al Telser, photographer



Chapter Twenty-Seven  Urinary System

Clinical View 27.3

Renal Calculi

A renal calculus (kal′kyū-lŭs, pl., calculi; pebbles), or kidney stone,

is formed from crystalline minerals that build up in the kidney. Over

75% of calculi contain calcium, in combination with either oxalate

or phosphate.

Causes and risk factors for kidney stone formation include

inadequate fluid intake and dehydration, frequent urinary tract

infections, reduced urinary flow and volume, and certain abnormal chemical or mineral levels in the urine. Males tend to develop

stones more often than females.

If the renal calculi are very small, they are asymptomatic,

and the person excretes them without ever realizing it. However,

a larger stone can become obstructed in the kidney, renal

pelvis, or ureter. The term urolithiasis (yūr′ō-li-thī′ă-sis; lithos =

stone) refers to the presence of renal calculi anywhere along

the urinary tract. Symptoms include severe, cramping pain

along the “loin-to-groin” region and possibly nausea and

vomiting. The epithelium of the ureter becomes inflamed as it

tries to push the stone along its path, resulting in blood in the

urine, called hematuria (hē′mă-tyū′rē-a).

Most stones smaller than about 4 millimeters in diameter

eventually pass through the urinary tract on their own once

the patient drinks plenty of water (2–3 quarts per day) to assist

movement. But if the stone is too large (more than 8 millimeters

in diameter) and doesn’t pass on its own, medical intervention is required. The most common treatment is ­lithotripsy

histologic sections. At the neck of the urinary bladder, an involuntary

internal urethral sphincter is formed by the smooth muscle that

encircles the urethral opening.

The adventitia is the outer layer of areolar connective tissue of

the urinary bladder. A peritoneal membrane covers only the superior

surface of the urinary bladder, and in this superior region, the peritoneum plus the connective tissue forms a serosa.

Arterial blood vessels extend to the urinary bladder and penetrate its wall from branches of the internal iliac artery. Venous blood

drains into the internal iliac veins.


The expulsion of urine from the bladder is called micturition

(mik′chū-rish′ŭn; micturio = to desire to make water) or urination

(yūr′i-nā′shŭn). Micturition is initiated by a complex sequence of

events called the micturition reflex. The bladder is supplied by both

parasympathetic and sympathetic axons of the autonomic nervous

system. The parasympathetic axons (pelvic splanchnic nerves) come

from the micturition reflex center located in spinal cord segments

S2–S4. The pelvic splanchnic nerves relax the internal urethral

sphincter so that urine can pass through and stimulate contraction

of the detrusor muscle. Thus, the parasympathetic axons stimulate

micturition. The sympathetic axons are from the T11–L2 segments

of the spinal cord. These axons cause contraction of the internal urethral sphincter and inhibit contraction of the detrusor muscle. Thus,

sympathetic axons inhibit micturition.

(lith′ō-trip′sē; tresis = boring), whereby ultrasound or shock

waves are directed toward the stones to pulverize them into

smaller particles that can be expelled in the urine. Alternatively,

using ureteroscopy, a scope is inserted from the urethra into the

urinary bladder and ureter to break up and remove the stone.

If these treatments aren’t viable, traditional surgery may be


Renal calculi








Renal calculi may become lodged at

various sites along the urinary tract.

The micturition reflex occurs in a series of steps:

1. When the bladder fills with urine and becomes distended, stretch

receptors in the bladder wall are activated, and they signal the

micturition reflex center.

2. Impulses within the parasympathetic division of the autonomic

nervous system travel to both the internal urethral sphincter and

the detrusor muscle.

3. The smooth muscle in the internal urethral sphincter relaxes,

and the smooth muscle in the detrusor muscle contracts.

4. The person’s conscious decision to urinate causes relaxation of

the external urethral sphincter.

5. In addition to the squeezing action of the detrusor muscle on

the volume of the urinary bladder, the expulsion of urine is

facilitated by contraction of the abdominal wall muscles.

6. Upon emptying of the urinary bladder, the detrusor muscle

relaxes, and the neurons of the micturition reflex center are


27.3c  Urethra

The urethra (yū-rē′thră) is a fibromuscular tube that originates at the

neck of the urinary bladder and conducts urine to the exterior of the

body (figure 27.11). The luminal lining of the urethra is a protective mucous membrane that houses clusters of mucin-producing cells

called urethral glands. Bundles of smooth muscle fibers surround the

mucosa and help propel urine to the outside of the body.


Chapter Twenty-Seven  Urinary System

Ureteral openings

Ureteral openings


Urinary bladder

Urinary bladder

Internal urethral sphincter

External urethral sphincter

Internal urethral


Prostate gland

External urethral


Bulbourethral gland






Bulb of the vestibule











urethral orifice





(a) Female urethra


Figure 27.11

Urethra. The urethra conducts urine from the urinary bladder to the

external urethral orifice. These coronal views compare (a) a female urethra

and (b) a male urethra.


urethral orifice

(b) Male urethra

Two urethral sphincters restrict the release of urine until the

pressure within the urinary bladder is high enough and voluntary

activities needed to release the urine are activated. The internal

urethral sphincter is the involuntary, superior sphincter surrounding the neck of the bladder, where the urethra originates.

This sphincter is a circular thickening of the detrusor muscle

and is controlled by the autonomic nervous system. The external

urethral sphincter is inferior to the internal urethral sphincter and

is formed by skeletal muscle fibers of the urogenital diaphragm.

This sphincter is a voluntary sphincter controlled by the somatic

nervous system. This is the muscle children learn to control when

they become “toilet-trained.”

The male and female urethras differ in length and morphology.

Female Urethra

The female urethra has a single function: to transport urine to the exterior of the body. The lumen of the female urethra is primarily lined

with a stratified squamous epithelium. The urethra is 3 to 5 centimeters long, and opens to the outside of the body at the external urethral

orifice located in the female perineum.

Male Urethra

The male urethra has two functions—urinary and reproductive—

because it serves as a passageway for both urine and semen, but not at

the same time. It is about 18 to 20 centimeters long and is partitioned

into three segments: the prostatic urethra, the membranous urethra,

and the spongy urethra.


Chapter Twenty-Seven  Urinary System

The prostatic (pros-tat′ik) urethra (figure 27.11b) is about

3 to 4 centimeters long and is the most dilatable portion of the

urethra. It extends through the prostate gland, immediately inferior

to the male bladder, where multiple small prostatic ducts enter it.

The urethra in this region is lined by a transitional epithelium, with

many blood vessels in the underlying dense irregular connective tissue. Two smooth muscle bundles surround the mucosa: an internal

longitudinal bundle and an external circular bundle. The external

circular muscular bundles are a continuation of the thickened circular region of smooth muscle forming the internal urethral sphincter

at the bladder outlet.

The membranous (mem′bră-nŭs) urethra is the shortest and

least dilatable portion of the male urethra. It extends from the inferior surface of the prostate gland through the urogenital diaphragm.

As a result, it is surrounded by striated muscle fibers that form the

external urethral sphincter of the urinary bladder. The epithelium

in this region is often either stratified columnar or pseudostratified


The spongy (spŭn′jē) urethra is the longest part (15 centimeters) of the male urethra. It is encased within a cylinder of

erectile tissue in the penis called the corpus spongiosum, and

extends to the external urethral orifice. The proximal part of

Clinical View 27.4

Urinary Tract Infections

A urinary tract infection (UTI) occurs when bacteria (most

commonly E. coli) or fungi enter and multiply within the urinary

tract. Women are more prone to UTIs because they have a

short urethra that is close to the anus, allowing bacteria from

the GI tract to more readily enter the female urethra. Sexual

intercourse or medical use of a urinary catheter also increases

the risk of UTIs in both sexes.

A UTI often develops first in the urethra, an inflammation called urethritis (yū′rĕ-thrī′tis). If the infection spreads

to the urinary bladder, cystitis (sis-tī′tis) results. Occasionally,

bacteria from an untreated UTI can spread up the ureters to the kidneys, a condition termed pyelonephritis


Symptoms of a UTI include difficult and painful urination, called dysuria (dis-yū′rē-ă; dys = bad, difficult); frequent

urination; and a feeling of uncomfortable pressure in the

pubic region. If the infection spreads to the kidneys, then

sharp back and flank pain, fever, and occasionally nausea

and vomiting occur. A UTI can be diagnosed through urinalysis (yūr′i-nal′i-sis), a test of the urine that can reveal the

presence of inflammatory cells, blood, and bacteria or fungi.

Appropriate antibiotic therapy cures most UTIs caused by


the spongy urethra is lined by a pseudostratified columnar epithelium, whereas the distal part has a lining of stratified squamous



1 3

1 4

1 5

Where is the trigone located? How does it assist urinary

bladder function?

What is the initiating stimulus that activates the micturition


Which portion of the male urethra is the shortest and least


27.4  Aging and the Urinary System

✓✓Learning Objective

8. Describe the changes in the urinary system resulting from

increasing age.

Changes in the size and functioning of the kidneys normally begin

in our third decade of life and continue thereafter. With age comes

a reduced blood flow to the kidneys and a decrease in the number

of functional nephrons. The reduction in blood flow contributes

to a decreased glomerular filtration rate; consequently, both reabsorption and secretion are reduced. The loss of nephrons results

in a diminished ability to filter and cleanse the blood as well as a

decrease in the number of target cells within the kidneys that are

capable of responding to stimulation by aldosterone or antidiuretic

hormone. Thus, the ability to control blood volume and blood pressure is reduced.

Structural changes in the urinary bladder also affect the

storage and voiding of urine. The bladder decreases in size, and

smooth muscle tone in the bladder wall gradually diminishes,

which may lead to more frequent urination. Sometimes, there is a

delay in n­ oticing the urge to urinate. Additionally, control of the

urethral sphincters—and eventually even control of micturition—

may be lost.

The inability to control the expulsion of urine is called

­incontinence. It most commonly occurs among the elderly, and

most frequently in women. The extent of the incontinence ranges

from occasional urine leakage to complete inability to retain urine.

There are two categories of incontinence: (1) stress incontinence

often occurs during vigorous exercise or strenuous coughing, and

(2) urge incontinence results from immediate bladder ­contraction

after feeling a strong need to urinate. Causes of incontinence include severe weight gain, complications following pelvic surgery,

coincidence with uncontrolled diabetes, severe constipation, pelvic

prolapse (in females) as a result of pregnancy, and enlarged prostate

(in males).


1 6

What are the possible consequences of aging on urine


Chapter Twenty-Seven  Urinary System

27.5  Development of the Urinary System

✓✓Learning Objective

9. Explain the embryonic and fetal development of the urinary


Most of the urinary system and the reproductive system (discussed in

section 28.5) form from the intermediate mesoderm of the embryo.

Intermediate mesoderm is located between the somites (paraxial

mesoderm) and the lateral plate mesoderm. Thus, the urinary system

and the reproductive system are linked in both their development and

their postnatal functions.

27.5a  Kidney and Ureter Development

Some of the intermediate mesoderm forms bilateral longitudinal

ridges, each called a urogenital ridge. The urogenital ridges

ultimately will form parts of the urinary system and internal

reproductive organs. The urinary parts of the urogenital ridges

give rise to three sets of excretory organs, which become increasingly more advanced. The first excretory organs, each called a

pronephros (prō-nef′ros; before kidney; pl., pronephroi), appear

in the cervical region of the embryo and seem to be vestigial—

that is, they have no known function in humans. They appear in

the early fourth week and quickly degenerate by the end of that

same week.

The second set of organs, each called a mesonephros (pl.,

mesonephroi), appears just before the pronephroi degenerate. These

structures are formed from tissue comprising the urogenital ridge

in the thoracic and lumbar regions. Each mesonephros is composed

of multiple saclike segments. A mesonephric (mez′ō-nef′rik) duct

drains urine from each mesonephros to the developing urinary bladder. The mesonephros (or “intermediate” kidney) persists until about

week 10 of development.

Early in the fifth week, the final set of excretory organs, each

called a metanephros (or permanent kidney), forms (figure 27.12).

Each meta­nephros (pl., metanephroi) forms a functional adult kidney

and takes over urine production by week 10. The components of the

metanephros form from two specific structures: the ureteric bud and

the metanephric mesoderm (table 27.3). The ureteric (yū′rē-ter′ik)

bud forms as a separate outgrowth from the caudal end of the mesonephric duct and gives rise to the ureter, renal pelvis, calyces,

and collecting ducts of the kidney. The metanephric (met′ă-nef′rik)

­mesoderm develops from intermediate mesoderm in the sacral region of the embryo, and forms the nephron components of the kidney:

the glomerular capsule, the proximal convoluted tubule, the nephron

loop, and the distal convoluted tubule. The metanephric mesoderm

will not grow and develop unless the ureteric bud grows into and

merges with it.

As the ureteric bud grows and merges with the meta­

nephric meso­derm, the ureteric bud undergoes a branching pattern

­(figure 27.12b). The first set of branches forms the major calyces in

week  6, and the second set of branches forms the minor calyces

in week 7. This branching pattern continues until the tiny collecting ducts are formed in the kidney, so that by week 32, well over

1 million collecting tubules have formed. As the calyces and collecting ducts form, they signal the metanephric mesoderm to grow


and develop into the nephron components. By week 10 of development, the metanephros is able to both produce and expel urine. Fetal

urine supplements amniotic fluid production.

During weeks 6–9, the developing kidneys migrate from

their position in the pelvic cavity to a more superior position in

the lumbar portion of the abdominal cavity (figure 27.12c). The

mechanisms for this ascent are not known, but may be related to

differential growth of the embryo. As the kidneys ascend, they obtain temporary blood vessels from the nearby vasculature. When the

kidneys migrate further, the older temporary blood vessels regress

and degenerate, and new blood vessels for the kidney form. Eventually, by week 9, the kidneys complete migration to the lumbar

region and acquire their permanent renal arteries branching from

the descending abdominal aorta.

27.5b  Urinary Bladder and Urethra Development

The urinary bladder and urethra develop from the distal part of

the hindgut called the cloaca (klō-ā′kă; sewer). The cloaca forms

not only the epithelium of the urinary bladder and urethra, but

also the rectum and anal canal. The cloaca is separated from

the outside of the body by a thin cloacal membrane. Extending s­ uperiorly from the cloaca to the umbilicus is a thin, tubular

hindgut extension called the allantois (ă-lan′tō-is). (By week 6,

the allantois becomes a fibrous cord called the urachus, and in

adults the remnant of this cord is called the median umbilical


Between weeks 4 and 7 of development, a mass of mesodermal

tissue called the urorectal (yur′ō-rek′tăl) septum grows through the

cloaca and toward the cloacal membrane (figure 27.13). This septum

subdivides the cloaca into an anterior urogenital (yūr′ō-jen′i-tăl)

sinus and a posterior anorectal (ā′nō-rek′tăl) canal. The urogenital

sinus (formed from endoderm) and some surrounding mesoderm

develop into the urinary bladder and urethra. The urorectal septum

grows toward and attaches to the cloacal membrane, subdividing it into an anterior urogenital membrane and a posterior anal

membrane. Both of these membranes rupture by week 8, allowing

the urethra and the anal canal to each communicate with the outside

of the body.


1 7

What structures are formed by the cloaca?

Table 27.3

Embryonic Derivations of Ureter

and Kidney Structures

Ureteric Bud Origin

Metanephric Mesoderm Origin


Glomerular capsules

Renal pelvis

Proximal convoluted tubules

Major calyces

Nephron loops

Minor calyces

Distal convoluted tubules

Collecting tubules and ducts


Chapter Twenty-Seven  Urinary System





Ureteric bud

Week 5



Renal pelvis

Major calyx


Week 6


Renal pelvis


Major calyces





Minor calyces




Ureteric bud

Week 7

(a) Week 5

(b) Metanephric kidney formation

Figure 27.12


Urinary bladder


(c) Weeks 6–9: Kidney migrates from pelvis to lumbar region


Kidney migration


Kidney Development. The kidneys form from the

urogenital ridges of the embryo. (a) By week 5, the

pronephroi have regressed, and the mesonephroi begin

producing urine. The metanephric kidneys, which

eventually form the permanent kidneys, also begin to

form. (b) The development of the metanephric kidney is

shown between weeks 5 and 7. (c) Between weeks 6 and

9, the kidney migrates superiorly from its initial location

in the pelvic cavity to the lumbar region on the posterior

abdominal wall.

Chapter Twenty-Seven  Urinary System

Clinical View 27.5

Kidney Variations and Anomalies

Renal agenesis is the failure of a kidney to develop. Kidney

development ceases if the ureteric bud and the metanephric

mesoderm do not grow toward each other and meet. Failure of

one kidney to develop, called unilateral renal agenesis, occurs

in about 1 per 1000 births, whereas bilateral renal agenesis

occurs in about 1 per 3000 births. Unilateral renal agenesis is

often asymptomatic, whereas bilateral renal agenesis is invariably fatal.

A pelvic kidney can be present if the developing kidney fails

to migrate from the pelvic cavity to the abdominal cavity. The pelvic

kidney receives its blood supply from branches of the common

iliac artery, as opposed to a renal artery that branches from the

aorta. A pelvic kidney usually has normal function and causes no

problems for the individual.

A horseshoe kidney develops when the inferior parts of the

left and right kidneys fuse as they try to ascend from the pelvic

­cavity into the abdominal cavity. The single, large kidney looks like

a horseshoe. The kidney’s superior migration is usually halted as

it gets stuck around the origin of the inferior mesenteric artery.

Horseshoe kidneys are fairly common, occurring in about 1 per

600 births. Like a pelvic kidney, a horseshoe kidney typically is

asymptomatic and functions normally.

Supernumerary (sū′pĕr-nū′mĕr-ār′ē) kidneys are extra

kidneys that develop. They appear to be caused by ureteric bud

duplication. However, supernumerary kidneys are very rare. It is

more common to see a duplicated or bifid ureter, from a duplicated

ureteric bud, traveling to a single kidney. Typically, these have no

clinical significance.

Bifid ureter (arrows).

© McGraw-Hill Education/Christine Eckel

Horseshoe kidney is a fairly common variation.

© MedImage/Science Source

Finally, because the kidneys must migrate from the pelvic

cavity into the abdominal cavity, it is fairly common for multiple

renal vessels to supply the kidney. Normally, as the kidney

migrates, it acquires temporary vessels from the nearby blood

supply, and its older, inferior vessels regress and degenerate. If

these temporary vessels fail to degenerate, the kidney is left with

multiple vessels.


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