UNIT 9

The Urinary System

Learning Outcomes

At the end of this lesson, you will be able to:

 describe the parts of the urinary system and give the functions of each;
 name the processes involved in urine formation; and
 explain the significance of maintaining fluid and electrolyte balances.

Pretest

The kidney is referred to as an excretory organ because it excretes_______________wastes. It is

also a major homeostatic organ because it maintains the electrolyte,_______________,
and_________________ balance of the blood. Urine is continuously formed by
the__________________and is routed down the___________________by the mechanism
of_________________to a storage organ called the____________________. Eventually the urine
is conducted to the body exterior by the___________________. In males, this tubelike structure is
about__________________inches long; in females, it is approximately_______inches long.

Content

The principal function of the urinary system is to maintain the volume and composition of body fluids
within normal limits. One aspect of this function is to rid the body of waste products that accumulate
as a result of cellular metabolism, and, because of this, it is sometimes referred to as the excretory
system.

Although the urinary system has a major role in excretion, other organs contribute to the excretory
function. The lungs in the respiratory system excrete some waste products, such as carbon dioxide
and water. The skin is another excretory organ that rids the body of wastes through the sweat
glands. The liver and intestines excrete bile pigments that result from the destruction of hemoglobin.
The major task of excretion still belongs to the urinary system. If it fails the other organs cannot take
over and compensate adequately.

The urinary system maintains an appropriate fluid volume by regulating the amount of water that is
excreted in the urine. Other aspects of its function include regulating the concentrations of various
electrolytes in the body fluids and maintaining normal pH of the blood.

In addition to maintaining fluid homeostasis in the body, the urinary system controls red blood cell
production by secreting the hormone erythropoietin. The urinary system also plays a role in
maintaining normal blood pressure by secreting the enzyme renin.

Components of the Urinary System

The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys form
the urine and account for the other functions attributed to the urinary system. The ureters carry the
urine away from kidneys to the urinary bladder, which is a temporary reservoir for the urine. The
urethra is a tubular structure that carries the urine from the urinary bladder to the outside.

Kidneys
Ureters
Urinary Bladder
Urethra

Kidneys
The kidneys are the primary organs of the urinary system. The kidneys are the organs that filter the
blood, remove the wastes, and excrete the wastes in the urine. They are the organs that perform
the functions of the urinary system. The other components are accessory structures to eliminate the
urine from the body.

The paired kidneys are located between the twelfth thoracic and third lumbar vertebrae, one on
each side of the vertebral column. The right kidney usually is slightly lower than the left because the
liver displaces it downward. The kidneys, protected by the lower ribs, lie in shallow depressions
against the posterior abdominal wall and behind the parietal peritoneum. This means they are
retroperitoneal. Each kidney is held in place by connective tissue, called renal fascia, and is
surrounded by a thick layer of adipose tissue, called perirenal fat, which helps to protect it. A tough,
fibrous, connective tissue renal capsule closely envelopes each kidney and provides support for the
soft tissue that is inside.

In the adult, each kidney is approximately 3 cm thick, 6 cm wide, and 12 cm long. It is roughly bean-
shaped with an indentation, called the hilum, on the medial side. The hilum leads to a large cavity,
called the renal sinus, within the kidney. The ureter and renal vein leave the kidney, and the renal
artery enters the kidney at the hilum.

The outer, reddish region, next to the capsule, is the renal cortex. This surrounds a darker reddish-
brown region called the renal medulla. The renal medulla consists of a series of renal pyramids,
which appear striated because they contain straight tubular structures and blood vessels. The wide
bases of the pyramids are adjacent to the cortex and the pointed ends, called renal papillae, are
directed toward the center of the kidney. Portions of the renal cortex extend into the spaces
between adjacent pyramids to form renal columns. The cortex and medulla make up the
parenchyma, or functional tissue, of the kidney.

The central region of the kidney contains the renal pelvis, which is located in the renal sinus, and is
continuous with the ureter. The renal pelvis is a large cavity that collects the urine as it is produced.
The periphery of the renal pelvis is interrupted by cuplike projections called calyces. A minor calyx
surrounds the renal papillae of each pyramid and collects urine from that pyramid. Several minor
calyces converge to form a major calyx. From the major calyces, the urine flows into the renal
pelvis; and from there, it flows into the ureter.

Each kidney contains over a million functional units, called nephrons, in the parenchyma (cortex
and medulla). A nephron has two parts: a renal corpuscle and a renal tubule.The renal corpuscle
consists of a cluster of capillaries, called the glomerulus, surrounded by a double-layered epithelial
cup, called the glomerular capsule. An afferent arteriole leads into the renal corpuscle and an
efferent arteriole leaves the renal corpuscle. Urine passes from the nephrons into collecting ducts
then into the minor calyces.

The juxtaglomerular apparatus, which monitors blood pressure and secretes renin, is formed from
modified cells in the afferent arteriole and the ascending limb of the nephron loop.

Ureters
Each ureter is a small tube, about 25 cm long, that carries urine from the renal pelvis to the urinary
bladder. It descends from the renal pelvis, along the posterior abdominal wall, which is behind the
parietal peritoneum, and enters the urinary bladder on the posterior inferior surface.

The wall of the ureter consists of three layers. The outer layer, the fibrous coat, is a supporting layer
of fibrous connective tissue. The middle layer, the muscular coat, consists of the inner circular and
outer longitudinal smooth muscle. The main function of this layer is peristalsis: to propel the urine.
The inner layer, the mucosa, is transitional epithelium that is continuous with the lining of the renal
pelvis and the urinary bladder. This layer secretes mucus, which coats and protects the surface of
the cells.

Urinary Bladder
The urinary bladder is a temporary storage reservoir for urine. It is located in the pelvic cavity,
posterior to the symphysis pubis, and below the parietal peritoneum. The size and shape of the
urinary bladder varies with the amount of urine it contains and with the pressure it receives from
surrounding organs.

The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is
continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds
called rugae. The rugae and transitional epithelium allow the bladder to expand as it fills.

The second layer in the walls is the submucosa, which supports the mucous membrane. It is
composed of connective tissue with elastic fibers.

The next layer is the muscularis, which is composed of smooth muscle. The smooth muscle fibers
are interwoven in all directions and, collectively, these are called the detrusor muscle. Contraction
of this muscle expels urine from the bladder. On the superior surface, the outer layer of the bladder
wall is parietal peritoneum. In all other regions, the outer layer is fibrous connective tissue.
There is a triangular area, called the trigone, formed by three openings in the floor of the urinary
bladder. Two of the openings are from the ureters and form the base of the trigone. Small flaps of
mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it
from backing up from the bladder into the ureters. The third opening, at the apex of the trigone, is
the opening into the urethra. A band of the detrusor muscle encircles this opening to form the
internal urethral sphincter.

Urethra
The final passageway for the flow of urine is the urethra, a thin-walled tube that conveys urine from
the floor of the urinary bladder to the outside. The opening to the outside is the external urethral
orifice. The mucosal lining of the urethra is transitional epithelium. The wall also contains smooth
muscle fibers and is supported by connective tissue.

The internal urethral sphincter surrounds the beginning of the urethra, where it leaves the urinary
bladder. This sphincter is smooth (involuntary) muscle. Another sphincter, the external urethral
sphincter, is skeletal (voluntary) muscle and encircles the urethra where it goes through the pelvic
floor. These two sphincters control the flow of urine through the urethra.

In females, the urethra is short, only 3 to 4 cm (about 1.5 inches) long. The external urethral orifice
opens to the outside just anterior to the opening for the vagina.

In males, the urethra is much longer, about 20 cm (7 to 8 inches) in length, and transports both
urine and semen. The first part, next to the urinary bladder, passes through the prostate gland and
is called the prostatic urethra. The second part, a short region that penetrates the pelvic floor and
enters the penis, is called the membranous urethra. The third part, the spongy urethra, is the
longest region. This portion of the urethra extends the entire length of the penis, and the external
urethral orifice opens to the outside at the tip of the penis.
 Learning Activities

I.
Circle the term that does not belong in each of the following groupings.

1. Intraperitoneal Kidney Retroperitoneal Superior lumbar region

2. Drains kidney Ureter Urethra Renal pelvis

3. Peritubular capillaries Reabsorption Glomerulus Low-pressure vessels

4. Juxtaglomerular apparatus Distal tubule Glomerulus Afferent arteriole

5. Glomerulus Peritubular capillaries Blood vessels Collecting duct

6. Cortical nephrons Juxtamedullary nephrons Cortex/medulla junction

Long nephron loops

7. Nephron Proximal convoluted tubule Distal convoluted tubule

Collecting duct

8. Medullary pyramids Glomeruli Renal pyramids Collecting ducts

9. Glomerular capsule Podocytes Nephron loop Glomerulus

II.
Circle the term that does not belong in each of the following groupings.

1. Bladder Kidney Transitional epithelium Detrusor muscle

2. Trigone Ureter openings Urethral opening Bladder Forms urine

3. Surrounded by prostate gland Contains internal and external sphincters

Continuous with renal pelvis Urethra

4. Prostatic Male Female Membranous Spongy

III.
Using the key choices, identify the structures that best fit the following descriptions. Insert the
correct term(s) or corresponding letter(s) in the answer blanks.

Key Choices:

A. Bladder B. Urethra C. Ureter

_________________________ 1. Drains the bladder

_________________________ 2. Storage area for urine
_________________________ 3. Contains the trigone
_________________________ 4. In males has prostatic, membranous, and spongy parts
_________________________ 5. Conducts urine by peristalsis
_________________________ 6. Substantially longer in males than in females
_________________________ 7. A common site of “trapped” renal calculi
_________________________ 8. Contains transitional epithelium
_________________________ 9. Also transports sperm in males

Mastery Test

___1. A radiologist is examining an X-ray of the lumbar region of a patient. Which of the
following is (are) indicative of normal posi- tioning of the right kidney?
A. Slightly lower than the left kidney
B. More medial than the left kidney
C. Closer to the inferior vena cava than the left kidney
D. Anterior to the 12th rib
___2. Which of the following encloses both kidney and adrenal gland?
A. Renal fascia
B. Perirenal fat capsule
C. Fibrous capsule
D. Visceral peritoneum
___3. Microscopic examination of a section of the kidney shows a thick-walled vessel with
renal corpuscles scattered in the tissue on one side of the vessel but not on the other
side. What vessel is this?
A. Interlobar artery
B. Cortical radiate artery
C. Cortical radiate vein
D. Arcuate artery
___4. Structures that are at least partly composed of simple squamous epithelium include:
A. collecting ducts
B. glomerulus
C. glomerular capsule
D. nephron loop
___5. Where are glucose and amino acids reab- sorbed?
A. Descending limb of nephron loop
B. Glomerulus
C. Proximal convoluted tubule
D. Distal convoluted tubule
___6. Which process is used to excrete sodium ions?
A. Osmosis
B. Facilitated diffusion
C. Active transport
D. Passive diffusion
___7. What is the function of the juxtaglomerular apparatus?
A. Detects fall in blood pressure
B. Releases angiotensin
C. Releases renin
D. Releases aldosterone
___8. Urine passes through the ureters by which mechanism?
A. Ciliary action
B. Peristalsis
C. Gravity alone
D. Suction
___9. Sodium deficiency hampers reabsorption of:
A. glucose
B. albumin
C. creatinine
D. water
___10. The main function of transitional epithelium in the ureter is:
A. protection against kidney stones
B. secretion of mucus
C. reabsorption
D. stretching
___11. Jim was standing at a urinal in a crowded public restroom and a long line was
forming behind him. He became anxious (sym- pathetic response) and found he
could not micturate no matter how hard he tried. Use logic to deduce Jim’s problem.
A. His internal urethral sphincter was con- stricted and would not relax.
B. His external urethral sphincter was con- stricted and would not relax.
C. His detrusor muscle was contracting too hard.
D. He almost certainly had a burst bladder.
___12. What is the typical urine output per day?
A. 150 mL
B. 500 mL
C. 1500 mL
D. 3000 mL
___13. What is the normal range for urine specific gravity?
A. 500–1000 mL
B. pH 6.0–7.2
C. 60–80
D. 1.001–1.035

___14. The urinary bladder:

A. is lined with transitional epithelium
B. has a thick, muscular wall
 C. receives the ureteral orifices at its superior aspect
D. is innervated by the renal plexus
___15. Which of the following are controlled voluntarily?
A. Detrusor muscle
B. Internal urethral sphincter
C. External urethral sphincter
D. Levator ani muscle
___16. In movement between IF and ICF:
A. water flow is bidirectional
B. nutrient flow is unidirectional
C. ion flow is selectively permitted
D. ion fluxes are not permitted
___17. Loss of water from the body via the lungs is termed as:
A. respiratory water loss
B. insensible water loss
C. micturition
D. respiratory diuresis
___18. The smallest fluid compartment is the:
A. ICF C. plasma
B. ECF D. IF
___19. Which of the following are electrolytes?
A. Glucose
B. Lactic acid
C. Urea
D. Bicarbonate
___20. Chloride ion reabsorption:
A. exactly parallels sodium ion reabsorption
B. fluctuates according to blood pH
C. increases during acidosis
D. is controlled directly by aldosterone
___21. Respiratory acidosis occurs in:
A. asthma
B. emphysema
C. barbiturate overdose
D. cystic fibrosis

___22. Hyperkalemia:
A. triggers secretion of aldosterone
B. may result from severe alcoholism
C. disturbs acid-base balance
 D. results from widespread tissue injury
___23. Renal tubular secretion of potassium is:
A. obligatory
B. increased by aldosterone
C. balanced by tubular reabsorption
D. increased in alkalosis
___24. Which buffer system(s) is (are) not important urine buffers?
A. Phosphate C. Protein
B. Ammonium D. Bicarbonate