Chapter 15

The Urinary System

Functions of the Urinary System

• Kidneys dispose of waste products in urine
• Nitrogenous wastes
• Toxins
• Drugs
• Excess ions
• Kidneys’ regulatory functions include:
• Production of renin to maintain blood pressure
• Production of erythropoietin to stimulate red blood cell production
• Conversion of vitamin D to its active form

Organs of the Urinary System

• Kidneys
• Ureters
• Urinary bladder
• Urethra

Kidneys
• Location and structure
• The kidneys are situated against the dorsal body wall in a retroperitoneal
position (behind the parietal peritoneum)
• The kidneys are situated at the level of the T 12 to L3 vertebrae
• The right kidney is slightly lower than the left (because of position of the liver)

Kidneys
• Kidney structure
• An adult kidney is about 12 cm (5 in) long and 6 cm (2.5 in) wide
• Renal hilum
• A medial indentation where several structures enter or exit the kidney
(ureters, renal blood vessels, and nerves)
• An adrenal gland sits atop each kidney

Kidneys
• Kidney structure (continued)
• Three protective layers enclose the kidney
• Fibrous capsule encloses each kidney
• Perirenal fat capsule surrounds the kidney and cushions against blows
• Renal fascia is the most superficial layer that anchors the kidney and
adrenal gland to surrounding structures

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Kidneys
• Kidney structure (continued)
• Three regions revealed in a longitudinal section
1. Renal cortex—outer region
2. Renal medulla—deeper region
• Renal (medullary) pyramids—triangular regions of tissue in the
medulla
• Renal columns—extensions of cortexlike material that separate
the pyramids

Kidneys
• Kidney structure (continued)
• Three regions (continued)
3. Renal pelvis—medial region that is a flat, funnel-shaped tube
• Calyces form cup-shaped “drains” that enclose the renal
pyramids
• Calyces collect urine and send it to the renal pelvis, on to the
ureter, and to the urinary bladder for storage

Kidneys
• Blood supply
• One-quarter of the total blood supply of the body passes through the kidneys
each minute
• Renal artery provides each kidney with arterial blood supply
• Renal artery divides into segmental arteries → interlobar arteries → arcuate
arteries → cortical radiate arteries

Kidneys
• Venous blood flow
• Cortical radiate veins → arcuate veins → interlobar veins → renal vein
• There are no segmental veins
• Renal vein returns blood to the inferior vena cava

Nephrons
• Structural and functional units of the kidneys
• Each kidney contains over a million nephrons
• Each nephron consists of two main structures
1. Renal corpuscle
2. Renal tubule

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Nephrons
• Renal corpuscle consists of:
1. Glomerulus, a knot of capillaries made of podocytes
• Podocytes make up the inner (visceral) layer of the glomerular capsule
• Foot processes cling to the glomerulus
• Filtration slits create a porous membrane—ideal for filtration
2. Glomerular (Bowman’s) capsule is a cup-shaped structure that surrounds the
glomerulus
• First part of the renal tubule

Nephrons
• Renal tubule
• Extends from glomerular capsule and ends when it empties into the collecting
duct
• From the glomerular (Bowman’s) capsule, the subdivisions of the renal tubule
are:
1. Proximal convoluted tubule (PCT)
2. Nephron loop (loop of Henle)
3. Distal convoluted tubule (DCT)

Nephrons
• Cortical nephrons
• Located entirely in the cortex
• Include most nephrons
• Juxtamedullary nephrons
• Found at the cortex-medulla junction
• Nephron loop dips deep into the medulla
• Collecting ducts collect urine from both types of nephrons, through the renal
pyramids, to the calyces, and then to the renal pelvis

Nephrons
• Two capillary beds associated with each nephron
1. Glomerulus
2. Peritubular capillary bed

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Nephrons
• Glomerulus
• Fed and drained by arterioles
• Afferent arteriole—arises from a cortical radiate artery and feeds the
glomerulus
• Efferent arteriole—receives blood that has passed through the
glomerulus
• Specialized for filtration
• High pressure forces fluid and solutes out of blood and into the glomerular
capsule

Nephrons
• Peritubular capillary beds
• Arise from the efferent arteriole of the glomerulus
• Low-pressure, porous capillaries
• Adapted for absorption instead of filtration
• Cling close to the renal tubule to receive solutes and water from tubule cells
• Drain into the interlobar veins

Urine Formation and Characteristics

• Urine formation is the result of three processes
1. Glomerular filtration
2. Tubular reabsorption
3. Tubular secretion

Urine Formation and Characteristics

• Glomerular filtration
• The glomerulus is a filter
• Filtration is a nonselective passive process
• Water and solutes smaller than proteins are forced through glomerular
capillary walls
• Proteins and blood cells are normally too large to pass through the
filtration membrane
• Once in the capsule, fluid is called filtrate
• Filtrate leaves via the renal tubule

Urine Formation and Characteristics

• Glomerular filtration (continued)
• Filtrate will be formed as long as systemic blood pressure is normal
• If arterial blood pressure is too low, filtrate formation stops because
glomerular pressure will be too low to form filtrate

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Urine Formation and Characteristics
• Tubular reabsorption
• The peritubular capillaries reabsorb useful substances from the renal tubule
cells, such as:
• Water
• Glucose
• Amino acids
• Ions
• Some reabsorption is passive; most is active (ATP)
• Most reabsorption occurs in the proximal convoluted tubule

Urine Formation and Characteristics

• Tubular secretion
• Reabsorption in reverse
• Some materials move from the blood of the peritubular capillaries into the
renal tubules to be eliminated in filtrate
• Hydrogen and potassium ions
• Creatinine

Urine Formation and Characteristics

• Tubular secretion (continued)
• Secretion is important for:
• Getting rid of substances not already in the filtrate
• Removing drugs and excess ions
• Maintaining acid-base balance of blood
• Materials left in the renal tubule move toward the ureter

Urine Formation and Characteristics

• Nitrogenous wastes
• Nitrogenous waste products are poorly reabsorbed, if at all
• Tend to remain in the filtrate and are excreted from the body in the urine
• Urea—end product of protein breakdown
• Uric acid—results from nucleic acid metabolism
• Creatinine—associated with creatine metabolism in muscles

Urine Formation and Characteristics

• In 24 hours, about 1.0 to 1.8 liters of urine are produced
• Urine and filtrate are different
• Filtrate contains everything that blood plasma does (except proteins)
• Urine is what remains after the filtrate has lost most of its water, nutrients, and
necessary ions through reabsorption
• Urine contains nitrogenous wastes and substances that are not needed

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Urine Formation and Characteristics
• Urine characteristics
• Clear and pale to deep yellow in color
• Yellow color is normal and due to the pigment urochrome (from the
destruction of hemoglobin) and solutes
• Dilute urine is a pale, straw color
• Sterile at the time of formation
• Slightly aromatic, but smells like ammonia with time
• Slightly acidic (pH of 6)
• Specific gravity of 1.001 to 1.035

Urine Formation and Characteristics

• Solutes normally found in urine
• Sodium and potassium ions
• Urea, uric acid, creatinine
• Ammonia
• Bicarbonate ions

Urine Formation and Characteristics

• Solutes NOT normally found in urine
• Glucose
• Blood proteins
• Red blood cells
• Hemoglobin
• WBCs (pus)
• Bile

Ureters
• Slender tubes 25–30 cm (10–12 inches) attaching the kidney to the urinary bladder
• Continuous with the renal pelvis
• Enter the posterior aspect of the urinary bladder
• Run behind the peritoneum
• Peristalsis aids gravity in urine transport

Urinary Bladder
• Smooth, collapsible, muscular sac situated posterior to the pubic symphysis
• Stores urine temporarily
• Trigone—triangular region of the urinary bladder base based on three openings
• Two openings from the ureters (ureteral orifices)
• One opening to the urethra (internal urethral orifice)
• In males, the prostate surrounds the neck of the urinary bladder

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Urinary Bladder
• Wall of the urinary bladder
• Three layers of smooth muscle collectively called the detrusor muscle
• Mucosa made of transitional epithelium
• Walls are thick and folded in an empty urinary bladder
• Urinary bladder can expand significantly without increasing internal pressure

Urinary Bladder
• Capacity of the urinary bladder
• A moderately full bladder is about 5 inches long and holds about 500 ml of
urine
• Capable of holding twice that amount of urine

Urethra
• Thin-walled tube that carries urine from the urinary bladder to the outside of the body
by peristalsis
• Function
• Females—carries only urine
• Males—carries urine and sperm

Urethra
• Release of urine is controlled by two sphincters
1. Internal urethral sphincter
• Involuntary and made of smooth muscle
2. External urethral sphincter
• Voluntary and made of skeletal muscle

Urethra
• Length
• In females: 3 to 4 cm (1.5 inches long)
• In males: 20 cm (8 inches long)
• Location
• Females—anterior to the vaginal opening
• Males—travels through the prostate and penis
• Prostatic urethra
• Membranous urethra
• Spongy urethra

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Micturition
• Micturition
• Voiding, or emptying of the urinary bladder
• Two sphincters control the release of urine, the internal urethral sphincter and
external urethral sphincter
• Bladder collects urine to 200 ml
• Stretch receptors transmit impulses to the sacral region of the spinal cord
• Impulses travel back to the bladder via the pelvic splanchnic nerves to cause
bladder contractions

Micturition
• When contractions become stronger, urine is forced past the involuntary internal
sphincter into the upper urethra
• Urge to void is felt
• The external sphincter is voluntarily controlled, so micturition can usually be delayed

Fluid, Electrolyte, and Acid-Base Balance

• Blood composition depends on three factors
1. Diet
2. Cellular metabolism
3. Urine output

Fluid, Electrolyte, and Acid-Base Balance

• Kidneys have four roles in maintaining blood composition
1. Excreting nitrogen-containing wastes (previously discussed)
2. Maintaining water balance of the blood
3. Maintaining electrolyte balance of the blood
4. Ensuring proper blood pH

Maintaining Water Balance of the Blood

• Normal amount of water in the human body
• Young adult females = 50%
• Young adult males = 60%
• Babies = 75%
• The elderly = 45%
• Water is necessary for many body functions, and levels must be maintained

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Maintaining Water Balance of the Blood
• Water occupies three main fluid compartments
1. Intracellular fluid (ICF)
• Fluid inside cells
• Accounts for two-thirds of body fluid
2. Extracellular fluid (ECF)
• Fluids outside cells; includes blood plasma, interstitial fluid (IF), lymph,
and transcellular fluid
3. Plasma (blood) is ECF, but accounts for 3L of total body water.
• Links external and internal environments (Figure 15.9)

Maintaining Water Balance of the Blood

• The link between water and electrolytes
• Electrolytes are charged particles (ions) that conduct electrical current in an
aqueous solution
• Sodium, potassium, and calcium ions are electrolytes

Maintaining Water Balance of the Blood

• Regulation of water intake and output
• Water intake must equal water output if the body is to remain properly
hydrated
• Sources for water intake
• Ingested foods and fluids
• Water produced from metabolic processes (10%)
• Thirst mechanism is the driving force for water intake

Maintaining Water Balance of the Blood

• Thirst mechanism
• Osmoreceptors are sensitive cells in the hypothalamus that become more
active in reaction to small changes in plasma solute concentration
• When activated, the thirst center in the hypothalamus is notified
• A dry mouth due to decreased saliva also promotes the thirst mechanism
• Both reinforce the drive to drink

Maintaining Water Balance of the Blood

• Sources of water output
• Lungs (insensible since we cannot sense the water leaving)
• Perspiration
• Feces
• Urine

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Maintaining Water Balance of the Blood
• Hormones are primarily responsible for reabsorption of water and electrolytes by the
kidneys
• Antidiuretic hormone (ADH) prevents excessive water loss in the urine and
increases water reabsorption
• ADH targets the kidney’s collecting ducts

Maintaining Electrolyte Balance

• Small changes in electrolyte concentrations cause water to move from one fluid
compartment to another
• A second hormone, aldosterone, helps regulate blood composition and blood volume
by acting on the kidney
• For each sodium ion reabsorbed, a chloride ion follows, and a potassium ion
is secreted into the filtrate
• Water follows salt: when sodium is reabsorbed, water follows it passively back
into the blood

Electrolyte Balance
• Renin-angiotensin mechanism
• Most important trigger for aldosterone release
• Mediated by the juxtaglomerular (JG) apparatus of the renal tubules
• When cells of the JG apparatus are stimulated by low blood pressure, the
enzyme renin is released into blood

Electrolyte Balance
• Renin-angiotensin mechanism (continued)
• Renin catalyzes reactions that produce angiotensin II
• Angiotensin II causes vasoconstriction and aldosterone release
• Result is increase in blood volume and blood pressure

Maintaining Acid-Base Balance of Blood

• Blood pH must remain between 7.35 and 7.45 to maintain homeostasis
• Alkalosis—pH above 7.45
• Acidosis—pH below 7.35
• Physiological acidosis—pH between 7.0 and 7.35

Maintaining Acid-Base Balance of Blood

• Kidneys play greatest role in maintaining acid-base balance
• Other acid-base controlling systems
• Blood buffers

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 • Respiration

Maintaining Acid-Base Balance of Blood

• Blood buffers
• Acids are proton (H+) donors
• Strong acids dissociate completely and liberate all of their H + in water
• Weak acids, such as carbonic acid, dissociate only partially
• Bases are proton (H+) acceptors
• Strong bases dissociate easily in water and tie up H +
• Weak bases, such as bicarbonate ion and ammonia, are slower to
accept H+

Maintaining Acid-Base Balance of Blood

• Molecules react to prevent dramatic changes in hydrogen ion (H +) concentrations
• Bind to H+ when pH drops
• Release H+ when pH rises
• Three major chemical buffer systems
1. Bicarbonate buffer system
2. Phosphate buffer system
3. Protein buffer system

Maintaining Acid-Base Balance of Blood

• The bicarbonate buffer system
• Mixture of carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3)
• Carbonic acid is a weak acid that does not dissociate much in neutral
or acid solutions
• Bicarbonate ions (HCO3−) react with strong acids to change them to
weak acids
HCl + NaHCO3 → H2CO3 + NaCl
strong acid weak base weak acid salt

Maintaining Acid-Base Balance of Blood

• The bicarbonate buffer system (continued)
• Carbonic acid dissociates in the presence of a strong base to form a weak
base and water
NaOH + H2CO3 → NaHCO3 + H2O
strong base weak acid weak base water

Maintaining Acid-Base Balance of Blood

• Respiratory mechanisms
• Respiratory rate can rise and fall depending on changing blood pH to retain

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 CO2 (decreasing the blood pH) or remove CO2 (increasing the blood pH)

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Maintaining Acid-Base Balance of Blood
• Renal mechanisms
• When blood pH rises:
• Bicarbonate ions are excreted
• Hydrogen ions are retained by kidney tubules
• When blood pH falls:
• Bicarbonate ions are reabsorbed
• Hydrogen ions are secreted
• Urine pH varies from 4.5 to 8.0

Developmental Aspects of the Urinary System

• The kidneys begin to develop in the first few weeks of embryonic life and are
excreting urine by the third month of fetal life
• Common congenital abnormalities include polycystic kidney and hypospadias
• Common urinary system problems in children and young to middle-aged adults
include infections caused by fecal microorganisms, microorganisms causing sexually
transmitted infections, and Streptococcus

Developmental Aspects of the Urinary System

• Control of the voluntary urethral sphincter does not start until age 18 months
• Complete nighttime control may not occur until the child is 4 years old
• Urinary tract infections (UTIs) are the only common problems before old age
• Escherichia coli (E. coli), a bacterium, accounts for 80 percent of UTIs

Developmental Aspects of the Urinary System

• Renal failure is an uncommon but serious problem in which the kidneys are unable
to concentrate urine, and dialysis must be done to maintain chemical homeostasis of
blood
• With age, filtration rate decreases and tubule cells become less efficient at
concentrating urine, leading to urgency, frequency, and incontinence
• In men, urinary retention is another common problem

Developmental Aspects of the Urinary System

• Problems associated with aging
• Urgency—feeling that it is necessary to void
• Frequency—frequent voiding of small amounts of urine
• Nocturia—need to get up during the night to urinate
• Incontinence—loss of control
• Urinary retention—common in males, often the result of hypertrophy of the
prostate gland

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