chapter 26: the urinary system
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Chapter 26: The Urinary System. An Introduction to the Urinary System. Figure 26–1. 3 Functions of the Urinary System. Excretion : removal of organic wastes from body fluids Elimination : discharge of waste products Homeostatic regulation : - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 26: The Urinary System
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An Introduction to the Urinary System
Figure 26–1
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3 Functions of the Urinary System
1. Excretion: – removal of organic wastes from
body fluids
2. Elimination:– discharge of waste products
3. Homeostatic regulation:– of blood plasma volume and solute
concentration
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Kidneys• Organs that excrete urine
Urinary Tract• Organs that eliminate urine:
– ureters (paired tubes)– urinary bladder (muscular sac)– urethra (exit tube)
Urination or Micturition• Process of eliminating urine • Contraction of muscular urinary
bladder forces urine through urethra, and out of body
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5 Homeostatic Functions of Urinary System
1. Regulate blood volume and blood pressure:
– by adjusting volume of water lost in urine– releasing erythropoietin (increase RBC
production) and renin (regulates blood pressure)
2. Regulate plasma ion concentrations:– sodium, potassium, and chloride ions (by
controlling quantities lost in urine)– calcium ion levels (through synthesis of
calcitriol)3. Help stabilize blood pH:– by controlling loss of hydrogen ions and
bicarbonate ions in urine
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4. Conserve valuable nutrients:– by preventing excretion while excreting
organic waste products
5. Assist liver to detoxify poisons
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The Position of the Kidneys
Figure 26–2
• Are located either side of vertebral column:– left kidney lies superior to
right kidney– superior surface capped by
adrenal gland• Position is maintained by:
– overlying peritoneum– contact with adjacent
visceral organs– supporting connective
tissues
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Typical Adult Kidney
• Is about 10 cm long, 5.5 cm wide, and 3 cm thick
• Weighs about 150 g
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Hilum• Point of entry
for renal artery and renal nerves
• Point of exit for renal vein and ureter
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Renal Sinus• Internal cavity within kidney• Lined by fibrous renal capsule
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Renal Capsule• Bound to outer surfaces of structures in
renal sinus• Stabilizes positions of ureter, renal blood
vessels, and nerves
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Renal Cortex • Superficial portion of kidney in contact
with renal capsule• Reddish brown and granular
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Renal Lobe • Consists of:
– renal pyramid– overlying area of renal cortex– adjacent tissues of renal columns
• Produces urine
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Renal Papilla• Ducts discharge urine into minor
calyx: – cup-shaped drain
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Major Calyx• Formed by 4 or 5 minor calyces
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Renal Pelvis• Large, funnel-shaped chamber• Consists of 2 or 3 major calyces • Fills most of renal sinus• Connected to ureter, which drains kidney
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Functional Anatomy of Nephron & Collecting System
Figure 26–6
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Nephron• Consists of
renal tubule and renal corpuscle
• Microscopic, tubular structures in cortex of each renal lobe
• Where urine production begins
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Renal Tubule • Long tubular
passageway• Begins at renal
corpuscle
Renal Corpuscle• Spherical
structure consisting of:– Bowman’s capsule– cup-shaped
chamber– capillary network
(glomerulus)
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Filtration• Occurs in renal
corpuscle• Blood pressure:
– forces water and dissolved solutes out of glomerular capillaries into capsular space
– produces protein-free solution (filtrate) similar to blood plasma
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3 Functions of Renal Tubule1. Reabsorb useful organic nutrients that
enter filtrate2. Reabsorb more than 90% of water in
filtrate3. Secrete waste products that failed to
enter renal corpuscle through filtration at glomerulus
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Cortical and Juxtamedullary Nephrons
Figure 26–7
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Cortical Nephrons (1 of 2 types)• 85% of all
nephrons• Located mostly
within superficial cortex of kidney• Loop of Henle is relatively short
• Efferent arteriole delivers blood to a network of peritubular capillaries:– which surround
entire renal tubule
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The Renal Corpuscle• Each renal corpuscle:
– is 150–250 µm in diameter– includes Bowman’s capsule and glomerulus
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Filtration• Blood pressure:
– forces water and small solutes across membrane into capsular space
• Larger solutes, such as plasma proteins, are excluded
Filtration at Renal Corpuscle • Is passive• Solutes enter capsular space:
– metabolic wastes and excess ions– glucose, free fatty acids, amino acids,
and vitamins
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Reabsorption
• Useful materials are recaptured before filtrate leaves kidneys
• Reabsorption occurs in proximal convoluted tubule
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•The Thick Descending Limb – Has functions similar to PCT:
• pumps sodium & chloride ions out of tubular fluid
•Ascending Limbs – Of juxtamedullary nephrons in medulla:
• create high solute conc. in peritubular fluid
•The Thin Segments – Are freely permeable to water, not to solutes– Water movement helps conc. tubular fluid
•The Thick Ascending Limb – Ends at a sharp angle near the renal corpuscle
- where DCT begins
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3 Processes of the DCT
1. Active secretion of ions, acids, drugs, and toxins
2. Selective reabsorption of sodium and calcium ions from tubular fluid
3. Selective reabsorption of water:– concentrates tubular fluid
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• Transports tubular fluid from nephron to renal pelvis
• Adjusts fluid composition
• Determines final osmotic concentration and volume of urine
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Renal Physiology
• The goal of urine production:– is to maintain homeostasis– by regulating volume and composition of
blood– including excretion of metabolic waste
products• Urea
– Due to breakdown of aa
• Creatinine– Due to breakdown of creatinine kinase (important in
muscle contraction)
• Uric acid– Formed due to recycling of ATGCU
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Organic Waste Products
• Are dissolved in bloodstream• Are eliminated only while dissolved
in urine• Removal is accompanied by water
loss• Concentrated urine:
– 1200–1400 milliosmols/L (4 times plasma concentration)
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Differences between Solute Concentrations in Urine and
Plasma
Table 26–2
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3 Basic Processes of Urine Formation 1. FILTRATION
• Hydrostatic pressure forces water through membrane pores:
– small solute molecules pass through pores– larger solutes & suspended materials are
retained• Occurs across capillary walls:– as water and dissolved materials are pushed
into interstitial fluids• In some sites (ie –liver), pores are large:
– plasma proteins can enter interstitial fluids
• At the renal corpuscle:– specialized mem. restricts all circulating
proteins
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2 & 3 Reabsorption and Secretion
• At the kidneys involve:– Diffusion– passive molecular movement from
an area of high conc to area of low conc– Osmosis-movement of water across semi-
perm mem from area of low conc of solute to higher con of solute
– channel-mediated diffusion- specific channel used, no energy, conc. dependent
– carrier-mediated transport – next slide
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The Transport Maximum (Tm)
• Concentration higher than transport maximum:– exceeds reabsorptive abilities of nephron– some material will remain in the tubular fluid
and appear in the urine• Determines the renal threshold
– the plasma concentration at which:– a specific compound or ion begins to appear in
urine
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Renal Threshold for Glucose • Is approximately 180 mg/dl• If plasma glucose is greater than 180
mg/dl:– Tm of tubular cells is exceeded
– glucose appears in urineGlycosuria
• Is the appearance of glucose in urine
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Renal Threshold for Amino Acids • Is lower for glucose (65 mg/dl)
• Amino acids commonly appear in urine:– after a protein-rich meal
Aminoaciduria
• Is the appearance of amino acids in urine
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Aldosterone• Is a hormone produced by adrenal cortex• Reduces Na+ lost in urine
Hypokalemia• Produced by prolonged aldosterone
stimulation• Dangerously reduces plasma
concentrationNatriuretic Peptides • Oppose secretion of aldosterone
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Parathyroid Hormone and Calcitriol• Circulating levels regulate reabsorption at
the DCT
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Acidosis• Lactic acidosis:
– develops after exhaustive muscle activity• (bulging muscles can cut off blood supply)
– due to anaerobic respiration
• Ketoacidosis:– Lower blood pH, higher acid, due to presence of
ketones– develops in starvation or diabetes– Body does not have suff. glucose/glycogen to sustain
met activity• Muscle loss can occur - dieting
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Control of Blood pH• By H+ removal and bicarbonate
production at kidneys • Is important to homeostasis
Alkalosis• Abnormally high blood pH• Can be caused by prolonged aldosterone
stimulation:– which stimulates secretion
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Response to Acidosis• PCT and DCT deaminate amino acids:
– ties up H+
– yields ammonium ions (NH4+) and HCO3
—
(carbonic acid)
• Ammonium ions are pumped into tubular fluid
• Bicarbonate ions enter bloodstream
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ADH – antidiuretic hormone• Hormone causes special water channels
to appear• Increases rate of osmotic water
movement• Higher levels of ADH increases:– number of water channels– water permeability of DCT and collecting
system• No ADH, water is not reabsorbed – All fluid reaching DCT is lost in urine
producing large amounts of dilute urine
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The Hypothalamus • Continuously secretes low levels of ADH:• At normal ADH levels:
– collecting system reabsorbs 16,800 ml fluid/ day (9.3% of filtrate)
• A healthy adult produces:– 1200 ml urine per day (0.6% of filtrate)
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Diuretics
• Are drugs that promote water loss in urine (diuresis)
• Diuretic therapy reduces:– blood volume– blood pressure– extracellular fluid volume
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The Composition of Urine • Results from filtration, absorption, and
secretion activities of nephrons• Some compounds (such as urea) are
neither actively excreted nor reabsorbed along nephrons
• Organic nutrients are completely reabsorbed:– other compounds missed by filtration process
(e.g., creatine)
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• The Concentration of components – in a urine sample depends on osmotic
movement of water
•Normal Urine•Is a clear, sterile solution•Yellow color (pigment urobilin) generated in kidneys from urobilinogens
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A Summary of Renal Function
Figure 26–16a
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Step 1: Glomerulus • Filtrate produced
at renal corpuscle has the same composition as blood plasma:– without plasma
proteins
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Step 2: Proximal Convoluted Tubule (PCT)• Active removal of
ions and organic substrates:– produces osmotic
water flow out of tubular fluid
– reduces volume of filtrate
– keeps solutions inside and outside tubule isotonic
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Step 3: PCT and Descending Limb• Water moves
into peritubular fluids, leaving highly concentrated tubular fluid
• Reduction in volume occurs by obligatory water reabsorption
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Step 4: Thick Ascending Limb• Tubular cells
actively transport Na+ and Cl— out of tubule
• Urea becomes higher proportion of total osmotic concentration
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Step 5: DCT and Collecting Ducts• Final
adjustments in composition of tubular fluid
• Osmotic concentration is adjusted through active transport (reabsorption or secretion)
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Step 6: DCT and Collecting
Ducts• Final
adjustments in volume and osmotic concentration of tubular fluid
• Exposure to ADH determines final urine concentration
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Step 7: Vasa Recta• Absorbs solutes
and water reabsorbed by loop of Henle and the ducts
• Maintains concentration gradient of medulla
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Urine Transport, Storage, and Elimination
• Takes place in the urinary tract:– ureters– urinary bladder– urethra
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Organs for the Conduction and Storage of Urine
Figure 26–18a
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Organs for the Conduction and Storage of Urine
Figure 26–18b
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Wall of the Urinary Bladder • Contains mucosa, submucosa, and
muscularis layers:– form powerful detrusor muscle of urinary
bladder – contraction compresses urinary bladder and
expels urine
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The Urethra• Extends from neck of urinary
bladder• To the exterior of the body
The Male Urethra • Extends from neck of urinary bladder• To tip of penis (18–20 cm)
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The Female Urethra
• Is very short (3–5 cm)• Extends from bladder to vestibule• External urethral orifice is near
anterior wall of vagina
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The External Urethral Sphincter• In both sexes:
– is a circular band of skeletal muscle– where urethra passes through urogenital diaphragm
• Acts as a valve • Is under voluntary control:
– via perineal branch of pudendal nerve
• Has resting muscle tone• Voluntarily relaxation permits micturition
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How is urination regulated voluntarily and involuntarily and what is the micturition reflex?
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The Micturition Reflex• Coordinates the process of
urination• As the bladder fills with urine:– stretch receptors in urinary bladder (>500 ml):
• stimulate pelvic nerve
– stimulus travels from pelvic nerves:• stimulate ganglionic neurons in wall of bladder
– postganglionic neuron in intramural ganglion:• stimulates detruscor muscle contraction
– interneuron relays sensation to thalamus and deliver sensation to cerebral cortex
– voluntary relaxation of external thus internal urethral sphincter
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Infants • Lack voluntary control over urination• Corticospinal connections are not
established
Incontinence-Is the inability to control urination voluntarily
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Age-Related Changes in Urinary System• Decline in number of functional nephrons• Reduced sensitivity to ADH• Problems with micturition reflex
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3 Micturition Reflex Problems1. Sphincter muscles lose tone:
– leading to incontinence
2. Control of micturition can be lost due to:– a stroke– Alzheimer’s disease– CNS problems affecting cerebral cortex or
hypothalamus
3. In males, urinary retention may develop if enlarged prostate gland compresses the urethra and restricts urine flow