Chapter 26: The Urinary System
BIO 211 Lecture
Instructor: Dr. Gollwitzer
1
• Today in class we will discuss:
– The interrelationship between the CVS and urinary
system
– The major functions of the urinary system
• Excretion
• Elimination
• Homeostatic regulation
– The basic principles of urine formation
– Major functions of each portion of the nephron and
collecting system
– The 3 basic processes involved in urine formation
• Glomerular filtration
– Filtration pressures
• Tubular reabsorption
• Tubular secretion
2
CVS and Urinary System
• CVS delivers nutrients (from digestive tract)
and O2 (from lungs) to cells in peripheral
tissues
• CVS carries CO2 and waste products from
peripheral tissues to sites of excretion
– CO2 removed at lungs
– Most physiological waste products removed by
urinary system
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Major Functions of Urinary System
• Excretion
• Elimination
• Homeostatic regulation of:
– Blood plasma volume
– Solute concentration
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Major Functions of Urinary System
• Excretion
– Removal of organic wastes (e.g., urea, uric acid,
creatinine) from body fluids (= urine formation)
– Performed by kidneys which act as filtering units
• Elimination
– Discharge of waste products into environment
(urination)
– Occurs when urinary bladder contracts and forces
urine through urethra and out of body
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Major Functions of Urinary System:
Homeostatic Regulation
• Regulation of blood volume (water balance)
and BP
– Adjusts volume of water lost in urine
– Releases
• Renin
– Involved in production of angiotensin II that affects BP, thirst,
and other hormones (ADH, aldosterone) that affect water
retention by kidneys
• Erythropoietin
– Stimulates erythropoiesis in bone marrow, maintains RBC
volume
6
Major Functions of Urinary System:
Homeostatic Regulation
• Regulation of plasma ion concentrations
(electrolyte balance)
– Controls amounts lost in urine (e.g., Na+, K+, Cl-)
– Controls Ca2+ levels by synthesis of calcitriol
• Reabsorption (conservation) of valuable
nutrients
– Recycles valuable nutrients
• e.g., amino acids, glucose
– Prevents excretion in urine
7
Major Functions of Urinary System:
Homeostatic Regulation
• Stabilization of blood pH (acid-base balance)
– Controls loss of H+ and HCO3- in urine
• Detoxification
– Of poisons, e.g., drugs
• Deamination
– Removes NH2 (amino group) so amino acids can be
metabolized
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Basic Principles of Urine Formation
• Urine = fluid containing:
– Water
– Ions
– Soluble compounds
• Goal of urine production
– To maintain homeostasis
– By regulating volume and composition of blood
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Basic Principles of Urine Formation
• Involves excretion of solutes
(i.e., metabolic/organic waste products)
– Urea
• Most abundant
• Produced by breakdown of amino acids
– Creatinine
• Generated in skeletal muscle by breakdown of
creatine phosphate (CP, high energy compound that
plays a role as energy source in muscle contraction)
– Uric acid
• Formed by recycling nitrogenous bases from RNA
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Basic Principles of Urine Formation
• Waste products dissolved in bloodstream can
only be eliminated when dissolved in urine
– Thus removal accompanied by unavoidable
water loss
• To avoid dehydration, kidneys concentrate
filtrate (i.e., reabsorb water) produced by
glomerular filtration
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Functional Anatomy of
Nephron and Collecting System
Figure 26–6
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3 Processes Involved in Urine
Formation
• Glomerular filtration
– Forces water and solutes out of blood in
glomerulus into capsular space
–  filtrate
• Tubular reabsorption
– Recovers useful materials from filtrate
• Tubular secretion
– Ejects waste products, toxins, and other
undesirable solutes into tubules
13
Glomerular Filtration
• Occurs in renal corpuscle
• Hydrostatic pressure forces water and solutes:
– Out of blood in glomerulus
– Into capsular space  filtrate
• Occurs solely on basis of size
– Small solute molecules carried with filtrate
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Glomerular Filtration
• Involves passage across filtration membrane
which is composed of 3 cellular units
– Glomerular capillary endothelium
– Lamina densa
– Filtration slits
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Glomerular Filtration
• Glomerular capillary endothelium
– Filtered through pores in fenestrated capillaries
– Least selective filter
• Pores too small for RBCs to pass through
• Large enough for plasma proteins
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Renal Corpuscle
Figure 26–8
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Glomerular Filtration
• Lamina densa
– Basement membrane of glomerular capillaries
– More selective filter
• Blocks passage of large proteins
• Only small polypeptides, nutrients, and ions can cross
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Figure 26–10, 7th edition
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Glomerular Filtration
• Filtration slits
– Gaps between pedicels of podocytes (visceral
epithelium around glomerulus)
– Finest filter
• No polypeptides pass through
• Only nutrients, ions into capsular space
• Thus, glomerular filtrate:
– Does not contain plasma proteins or polypeptides
– Does contain small organic molecules (e.g.,
nutrients) and ions in same concentration as in
plasma
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Filtration Pressures
• Filtration pressure = balance between:
– Hydrostatic (fluid) pressures
• Glomerular hydrostatic pressure (GHP) in capillaries (50
mmg Hg)
• Capsular hydrostatic pressure (CHP) (15 mm Hg)
– Blood osmotic pressure (BOP) (25 mm Hg)
21
Filtration Pressures
• Hydrostatic (fluid) pressures
– Glomerular hydrostatic pressure (GHP) (50 mm Hg)
• = BP in glomerular capillaries
• Higher in glomerulus than in peripheral capillaries (35
mm Hg)
– Because efferent arteriole smaller in diameter than afferent
arteriole, need higher BP to force blood into it
• Promotes filtration – pushes water and solutes out of
plasma in capillaries into filtrate
• Opposed by…
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Filtration Pressures
• Hydrostatic (fluid) pressures
– Capsular hydrostatic pressure (CHP) (15 mm Hg)
• Opposes filtration – pushes water and solutes out of
filtrate into plasma in capillaries
• Results from resistance to flow along nephron and
conducting system that causes water to collect in
Bowman’s capsule
• More water in capsule  more pressure
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Filtration Pressures
• Blood osmotic pressure (BOP) (25 mm Hg)
– Results from presence of suspended proteins in
blood
– Promotes return of water into glomerulus
– Opposes filtration
– Tends to draw water out of filtrate and into plasma
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Figure 26–10, 7th edition
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Summary of Filtration Pressures
• Hydrostatic pressures
– GHP (pushing out of glomerulus) = 50 mm Hg
– CHP (pushing into glomerulus) = 15 mm Hg
– Net = 35 mm Hg (pushing out of glomerulus)
• Osmotic pressure
– BOP (draws into glomerulus) = 25 mm Hg
• Filtration pressure = 10 mm Hg
– Difference between net hydrostatic pressure and
blood osmotic pressure
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Summary of Filtration Pressures
• Problems that affect filtration pressure
– Can seriously disrupt kidney function
– Can cause a variety of clinical symptoms, e.g.,
• Drop in systolic pressure from 120 to < 110 mm Hg
would eliminate filtration pressure (10 mm Hg)
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• Today in class we will discuss:
– The 3 basic processes involved in urine formation
• Glomerular filtration
– Glomerular Filtration Rate
– Renal Failure
• Tubular reabsorption
– PCT, Loop of Henle & Countercurrent Exchange,DCT
– Collecting System
• Tubular secretion
– PCT, DCT and Collecting system
– Urine
• Compare/contrast to plasma
• General characteristics
• Hormone influence of volume and concentration
– Voluntary & involuntary regulation of urination and
the micturition reflex
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Glomerular Filtration Rate (GFR)
• Gomerular filtration
– Vital first step essential to all other kidney functions
– Must occur so:
• Waste products excreted
• pH controlled
• Blood volume maintained
• GFR = amount of filtrate kidneys produce per minute
• Avg GFR = 125 mL/min or 50 gal/day (out of 480 gallons
of blood flow/day)
– 10% of fluid delivered by renal arteries enters capsular spaces
– 99% of this reabsorbed so urinate only 0.5 gallons/day
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Glomerular Filtration Rate (GFR)
• Measured using creatinine clearance test (CCT)
– Breakdown of CP in muscle  creatinine
– Creatinine enters filtrate at glomerulus and is not
reabsorbed so is excreted in urine
– Can compare amount of creatinine in blood vs. in
urine during 24 hour and estimate GFR
– If glomerulus damaged, GFR will be altered (have
more or less creatinine in urine than normal)
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Glomerular Filtration Rate (GFR)
• GFR depends on:
– Adequate blood flow to glomerulus
– Maintenance of normal filtration pressures
• Affected by anything that reduces renal blood
flow or BP, e.g.,
– Hypotension, hemorrhage, shock, dehydration
• Decreased renal blood volume and/or BP 
decreased filtration pressure  decreased GFR
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Control of GFR
• GFR increased by:
– EPO (relatively minor)
– Renin-angiotensin system
– Natriuretic peptides (ANP and BNP)
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Control of GFR
• Decreased BP and/or blood volume 
– Decreased O2  JGA  EPO 
• Increased RBCs 
– Increased O2 delivery
– Increased blood volume  increased BP 
» Increased filtration pressure
» Increased GFR
– Decreased renal blood flow  JGA  reninangiotensin system 
• Increased blood volume  increased BP 
– Increased filtration pressure
– Increased GFR
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EPO and Renin
Figure 18–19b
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Renin-Angiotensin System
• Renin (enzyme)  (prohormone)
angiotensinogen  (hormone) angiotensin I (in
liver)
• Angiotensin I  angiotensin II (in lung
capillaries)
• Angiotensin II  increased blood volume and
BP  increased GFR
35
Primary Effects of Angiotensin II
• Stimulates constriction of efferent arterioles 
increased glomerular pressure
• Directly stimulates reabsorption of Na+ and H2O in DCT
 increased blood volume and BP
• Stimulates adrenal cortex  aldosterone 
reabsorption of Na+ (and H2O)  increased blood
volume and BP
• Stimulates posterior pituitary  ADH  reabsorption
of H2O  increased blood volume and BP
• Stimulates thirst  increased blood volume and BP
• Stimulates vasoconstriction of arterioles
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Renin-Angiotensin System: Response to Reduction in GFR
Figure 26–11-0
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Control of GFR
• Increased blood volume or BP  stretched
cardiac muscle cells  natriuretic peptides
– ANP = atrial NP
– BNP = brain NP (produced by ventricles)
• Natriuretic peptides
– Increase GFR
– Decrease blood volume and BP
– Via 2 mechanisms
38
Natriuretic Peptides Increase GFR
• Act opposite to angiotensin II
– Increase Na+ and H2O loss
• Inhibit renin release
• Inhibit secretion of aldosterone and ADH
– Suppress thirst
– Prevent increased BP by angiotensin II and NE
• Increase glomerular pressures
– Dilate afferent arterioles
– Constrict efferent arterioles
• Also increase tubular reabsorption of Na+
– Decreases blood volume and BP
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Renal Failure
• When filtration (GFR) slows, urine production
decreases
• Symptoms appear because water, ions, and metabolic
wastes retained rather than excreted
• Almost all systems affected: fluid balance, pH,
muscular contraction, neural function, digestive
function, metabolism
• Leads to:
– Hypertension (due to blood “backing up”)
– Anemia due to lack of erythropoietin production
– CNS problems (sleepiness, seizures, delirium, coma,
death)
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Renal Failure
• Acute renal failure
– From exposure to toxic drugs, renal ischemia,
urinary obstruction, trauma
– Develops quickly, but usually temporary
– With supportive treatment can survive
• Chronic renal failure
– Condition deteriorates gradually
– Cannot be reversed
– Dialysis or kidney transplant may prolong life
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Reabsorption and Secretion
• Occur in all segments of renal tubules
• Relative importance changes from segment to
segment
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Tubular Reabsorption
• Molecules move from filtrate  across tubular
epithelium into peritubular interstitial fluid and
blood
– Water, valuable solutes (e.g., nutrients, proteins,
amino acids, glucose)
• Occurs through diffusion, osmosis (H2O), active
transport by carrier proteins
• Occurs primarily along PCT (also along renal
tubule and collecting system)
43
Tubular Secretion
• Molecules move from peritubular fluid into
tubular fluid
• Lowers plasma concentration of undesirable
materials
• Necessary because filtration does not force all
solutes out of plasma
• Primary method of excretion for many drugs
• Occurs primarily at PCT and DCT
44
Reabsorption and Secretion: PCT
• Primarily reabsorption
– 60-70% of filtrate
– Includes:
• Organic nutrients (99-100%), e.g., glucose, amino acids,
proteins, lipids, vitamins
• Water (60-70%)
• Ions (60-70%), e.g., Na+, Cl-; also K+, Ca2+, HCO3-
– Reabsorbed materials enter peritubular fluid and
capillaries
• Secretion
– H+, NH4+, creatinine, drugs, toxins
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Reabsorption: Loop of Henle
• Reabsorption
– Na+, Cl– Water
• Accomplished by countercurrent exchange
– Refers to exchange by tubular fluids moving in
opposite directions
– Fluid in descending limb flows toward renal pelvis
– Fluid in ascending limb flows toward cortex
46
Countercurrent Exchange
• Occurs because of different permeabilities of
segments of LOH
• Descending limb (thin)
– Permeable to water
– Relatively impermeable to solutes
• Ascending limb (thick)
– Relatively impermeable to water and solutes
– Has active transport mechanisms
• Pump Na+ and Cl- from tubular fluid into peritubular fluid
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Countercurrent Exchange
• Na+ and Cl- pumped out of thick ascending limb into
peritubular fluid
• Increases osmotic concentration in peritubular fluid
around thin descending limb
• Results in osmotic flow of H2O out of thin descending
limb into peritubular fluid  increased solute
concentration in thin descending limb
• Arrival of concentrated solution in thick ascending
limb increases transport of Na+ and Cl- into
peritubular fluid
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Overview of Urine Formation
Figure 26–16
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Reabsorption and Secretion: DCT
• Reabsorption (by vasa recta)
– Na+ (under influence of aldosterone), Cl– Ca2+(under influence of PTH and calcitriol)
– H2O (under influence of ADH)
• Secretion
– K+ (in exchange for Na+), H+
– NH4+ (from deamination; produces lactic acid,
ketone bodies  acidosis)
– Creatinine, drugs, toxins
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Reabsorption and Secretion:
Collecting System
• Makes final adjustments to ion concentration
and urine volume
• Reabsorption
– Na+ (under influence of aldosterone)
– H2O (under influence of ADH)
– HCO3-
– Urea (distal portion)
• Secretion
– K+, H+
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Figure 26–15
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Summary: Urine Formation
• Involves all parts of nephron and collecting
system
• Processes occur primarily in certain areas
– Glomerular filtration at the renal corpuscle
– Nutrient reabsorption in the PCT
– Water and salt conservation in loop of Henle
– Tubular secretion in the DCT
• Regulation of final volume and solute
concentration occurs in loops of Henle and
collecting system
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Normal Kidney Function
• Continues as long as filtration,
reabsorption, and secretion function within
narrow limits
• Disruption of kidney function has
immediate effects on composition of
circulating blood
• If both kidneys affected, death occurs
within few days
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Normal Kidney Function
• Glomeruli produce approx 48 gallons (180 L)
of filtrate/day
– 70X plasma volume!
• Almost all fluid volume must be reabsorbed
to avoid fatal dehydration
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Urine
• Clear, sterile solution
• Yellow (“straw”) color due to pigment
(urobilin)
• Urinalysis = analysis of urine sample
• Results from filtration, absorption,
secretion activities of nephron
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Table 26–5
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Urine vs. Plasma
• Little to no metabolites and nutrients (glucose,
lipids, amino acids, proteins)
• Slightly increased Na+, greatly increased K+ and Cl-,
and greatly decreased HCO3• Very high levels of nitrogenous wastes (creatinine,
urea, ammonia, uric acid)
• Lower pH (6.0 vs. 7.4)
• Much greater water content (95% vs. 50%)
58
Table 26–2
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Diuresis
• Elimination of urine
• Usually used to indicate production large
volumes of urine
• Diuretics
– Drugs that promote water loss in urine
– Reduce
• Blood volume
• Blood pressure
• Extracellular fluid volume
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Micturition Reflex
• Coordinates the process of urination
• Begins when stretch receptors in bladder
stimulate parasympathetic neurons
– Results in contraction of detrusor muscle
contraction
• Voluntary relaxation of external urethral
sphincter causes relaxation of internal urethral
sphincter
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Micturition Reflex
Figure 26–20
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Voluntary Control
• Infants
– Lack voluntary control over urination
– Corticospinal connections are not
established
• Incontinence =
– Inability to voluntarily control urination
– May be caused by trauma to internal or
external urethral sphincter
63
Age-Related Changes
in Urinary System
• Decline in number of functional nephrons
• Reduction in GFR
• Reduced sensitivity to ADH
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Age-Related Changes
in Urinary System
• Problems with micturition reflex
– Sphincter muscles lose tone  incontinence
– Lose control due to:
• Stroke
• Alzheimer’s disease
• CNS problems
– In males, enlarged prostate compresses
urethra, restricts urine flow urinary retention
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The Excretory System
• Includes all systems with excretory functions
that affect body fluids composition
– Urinary system
– Integumentary system
– Respiratory system
– Digestive system
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