Urinary System
Chapter 24 – Day 2
4/7/08
Review Nephron Structure
 Network with blood vessels
 Two types of nephrons
♦ Cortical Nephrons – loop of Henle does not extend below
cortex
♦ Juxtamedullary nephrons – loop of Henle extends into
medulla
♦ Vasa recta – feature of jextamedullary nephrons
♦ 85% are cortical nephrons
• Majority of process is done here
♦ 15% are juxtamedullary
• Important role in concentrating urine
• Conserving water in the body
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Renal corpuscle
 Renal arteries branch into afferent arterioles in the
cortex
 One afferent arteriole enters the renal corpuscle
 Wall of the renal corpuscle is the Bowman’s capsule
 Afferent arteriole branches into capillary network
♦ Glomerulus, inside the corpuscle within the bowman’s
capsule
 A second efferent arteriole connects to the
glomerulus – brings blood out of the renal corpuscle
♦ (Efferent arteriole is thinner than afferent)
 Efferent arteriole branches around nephron as
peritubular capillaries and vasa recta
 Eventually the efferent arteriole connects to venules
– carries blood to renal veins and out of the kidney
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Nephrons & blood supply
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Fig. 24.7
Kidneys – Blood supply
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Fig. 24.5
Glomerulus
 Surrounded by 2 layers
♦ Bowman’s capsule:
• Visceral epithelium
• Parietal epithelium
♦ Separated by capsular
space
 Blood in the glomerulus
is bounded by 2 walls
♦ Endothelial cells of the
blood vessels
♦ Visceral layer of the
bowman’s capsule
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Fig. 24.8
Glomerulus
 Endothelial cells of blood vessels
♦ Have large diameter pores:
• Fenestrations
• Basement membrane
 Visceral Layer of the Bowman’s capsule
♦ Special basement membrane = lamina densa
♦ Special cells = podocytes
• These have special feet like structures called pedicels
• Spaces between pedicels = filtration slit/slit pore
 Special structures of the endothelial cells and
visceral layer restrict the passage of particles
according to size (analogous to the filter paper)
 Capsular space = place where filtrate is released
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Glomerulus - barriers
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Fig. 24.8
Glomerulus blood boundaries
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Fig. 24.8c
Filtration in the Glomerulus
 Blood rushes into glomerulus via the afferent
arteriole
 Difference in diameter between afferent & efferent
arterioles results in High Blood Pressure inside the
Glomerulus
♦ = Effective Filtration Pressure (EFP)
 This pushes blood against the endothelial cells
♦ Filtration takes place
♦ Fenestrated capillaries – stops blood cells from passing
through, but all other contents go through
♦ Basement membrane – stops large proteins
♦ Pedicels/filtration slits – stops medium-sized proteins from
leaving blood
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Glomerular Filtration
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Fig. 24.10
Filtrate
 Remaining contents end up in the capsular space
and are called filtrate
 Contents of filtrate:
♦ Water
♦ Small proteins
♦ Glucose
♦ Amino acids
♦ Vitamins
♦ Citric acid
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♦ Ions: Na+, Cl-, K+, HCO3
♦ ammonia
♦ Urea
♦ Bilirubin
♦ Neurotransmitter
♦ Hormones
Filtrate - PCT
 Filtrate passes to proximal convoluted tubule (PCT)
♦ PCT walls = simple cuboidal epithelium
♦ Outside fluid = peritubular fluid
♦ Then peritubular capillaries
 PCT – reabsorption begins
♦ 99% of the reabsorption is of organic materials
• Glucose, amino acids, vitamins, citric acid
♦ Facilitated diffusion
♦ Transport of Ions = active transport & passive
♦ Water reabsorption = osmosis
 Enters peritubular fluid, then to capillaries
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Filtrate – Loop of Henle
 Tubular fluid with remaining contents flows into Loop
of Henle
♦ Descending & Ascending Limbs
 Juxtamedullary nephrons – in contact with the vasa
recta
 Reabsorption of water & solutes
♦ (specifically Na+ & Cl-)
 Descending & Ascending limbs have different
properties – very important in concentrating urine
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Loop of Henle
 Descending limb = thin
♦ Permeable to H2O
♦ Mostly impermeable to salts (Na+,Cl-)
 Ascending limb = thick
♦ Impermeable to H2O and solutes
♦ Prevents diffusion by concentration
gradient
♦ Allows selective permeability
♦ Only Na+& Cl- can leave via active
transport while wastes stay in
♦ Salts (Na+,Cl-) are pumped out by
active transport = reabsorption of Na+&
Cl- into blood
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Fig. 24.13
Loop of Henle
 As tubular fluid moves up, salts
enter peritubular fluid via active
transport into vasa recta
 Results in a high solute
concentration of peritubular fluid
 Know:
♦ Direction of H2O? Direction of salts?
♦ From which limb?
♦ What is the significance/importance?
♦ How does the countercurrent work?
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Fig. 24.13
Loop of Henle
 Countercurrent flow drives reabsorption
♦ Osmotic concentration is constantly maintained
♦ Allows reabsorption of H2O into blood b/c osmotic
concentration is high in the peritubular fluid (due to high salts)
♦ = contercurrent multiplication (exchange increases)
 Loss of H2O from descending limb concentrates
solutes in urine
 Tubular fluid containing mostly wastes & some ions
enters distal convoluted tubule (DCT)
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Filtrate – Distal Convoluted Tubule
 Further reabsorption of salts
+
(Na & Cl )
♦ Active and Passive
 Presence of aldosterone
alters rate of Na+ reabsorption
 Ca2+ may be reabsorbed
depending on calcitriol
 Secretion also occurs in DCT
 Wastes are not reabsorbed –
moves forward to collecting
tube
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Fig. 24.15
Filtrate – Distal Convoluted Tubule
 Further reabsorption of salts
+
(Na & Cl )
♦ Active and Passive
 Presence of aldosterone
alters rate of Na+ reabsorption
 Ca2+ may be reabsorbed
depending on calcitriol
 Secretion also occurs in DCT
 Wastes are not reabsorbed –
moves forward to collecting
tube
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Fig. 24.15
Nephron - Secretion
 Reabsorption & secretion take place in the DCT
 Secretion:
♦ Other wastes that did not enter the nephron during filtration
♦ Transported later by secretion
♦ Peritubular capillaries → nephron
♦ PCT & DCT are involved in secretion
♦ K+ and H+ ions – depends on blood levels/blood conditions
• Example: H+ ions increase from CO2 conversion, they are
secreted to maintain balance
• Diabetes: ketones are released = acidic, so more H+ is secreted
♦ NH4+ ions (comes from removal of NH3 group from amino
acids)
♦ Other secreted compounds = drugs, creatinine,
neurotransmitters
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Filtrate – Collecting Duct
 Urine moves into the collecting tubule (urine)
♦ Some reabsorption:
• Na+ ions
• HCO3 ions
• Some urea reabsorption (helps to maintain high salt
concentration in peritubular fluid)
 Concentration gradient is very important to keep the
flow of things moving properly
♦ Ions & H2O enter peritubular fluid, then go to vasa recta
• Keeps a constant concentration gradient of high salt levels in
the peritubular fluid
 H2O reabsorption in kidney = very important process
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♦ Helps to keep H2O balanced in the body
♦ Depends on solute concentration
Urine - hydration
 If the body is well hydrated H2O absorption occurs
only in the descending limb of Loop of Henle
 Solute concentration gradient in Loop of Henle &
Collecting Tubule
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Fig. 24.13
Urine - hydration
 Dilute urine = large volume because high amounts of
water are retained
 Only ions are reabsorbed heavily
 If the body is dehydrated – activates pituitary gland to
produce antidiuretic hormone (ADH)
♦
♦
♦
♦
♦
Body conserves H2O in the kidney & thirst response for more H2O intake
Special H2O channels open in collecting tubule
H2O is reabsorbed from collecting tubule
Driven by diffusion because salt concentration is already high outside
Yields concentrated urine – small volume (conc. urea)
 Diuretics like caffeine:
♦ Prevent H2O reabsorption in collecting tubule, force H2O out of cells
♦ Leads to dilute urine but reduces blood volume
♦ Are also used to alter blood pressure
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Fig. 24.6
Kidneys – Blood supply
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Fig. 24.7
Kidneys – Blood supply
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Fig. 24.8
Kidneys – Blood supply
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Fig. 24.5
Kidneys – Blood supply
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Fig. 24.5