renal (urinary) system chapter 17 function maintains homeostasis in ecf removing metabolic waste...
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Renal (Urinary) System
Chapter 17
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Function
Maintains homeostasis in ECF • Removing metabolic waste (except CO2)
– e.g. ammonia, urea, uric acid
• Removing foreign compounds – e.g. drugs, food additives, pesticides
• Regulating salt concentrations, fluid volume, and pH
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Anatomy• Kidneys (2)
– process plasma into urine
• Ureters (2)– tubes that carry urine to
bladder
• Urinary bladder– storage of urine
• Urethra– carries urine to exterior
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Anatomy
• Cortex – outer granular region
• Medulla – inner striated region
– renal pyramids
• Renal Pelvis– central collecting cavity
• Renal artery and vein
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Anatomy
• Nephron– 1 million per kidney
– functional unit of the kidney
– smallest unit capable of forming urine
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Nephron
Vascular Component– conducts blood
• Renal Artery • Afferent Arteriole
• Glomerulus • Efferent Arteriole• Peritubular Capillaries • Venules • Renal Vein
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Nephron
Tubular Component– forms urine
• Bowman’s capsule• Proximal Convoluted
Tubule• Loop of Henle• Distal Convoluted
Tubule
• Collecting duct
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Urine Formation• Urine - water and waste solutes• Nephrons conduct three processes to convert
blood plasma into urine1. filtration
• filter blood plasma to retain cells/proteins
2. reabsorption• remove valuable materials from filtrate
3. secretion• transfer additional wastes to filtrate
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Filtration
• Occurs in the glomerulus– Fenestrated capillaries
• 3 layers of podocytes form capillary walls
• Small pores (fenestrae)
– filters plasma • proteins + cells stay in blood
• forms ultrafiltrate
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Filtration
• Filtration driven by blood pressure
• Glomerular filtration is non-selective – Small particles pass (glucose,
Na+, urea, H2O)
– Large ones do not
• 20% of plasma enters tubule– plasma filtered 65x/day
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Reabsorption
• Occurs in remainder of nephron tubule
• Selective movement of substances from tubule into plasma– Return of valuable substances
to peritubular caps
• Active or passive– Passive (no energy)
– Active transport (requires energy)
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Secretion
• Also occurs in tubules
• Additional materials transported from plasma in peritubular capillaries into tubule– excess K+, Ca2+ and H+,
uric acid
– foreign compounds
• By passive diffusion or active carrier transport
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Proximal Tubule
Proximal Tubule Reabsorbs: • 2/3 of plasma Na+
• 2/3 of plasma Cl-
• 2/3 of plasma H2O
• 100 % of plasma glucose
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Active Transport in theProximal Tubule
• Na+ actively transported from cell to blood
• Creates Na+ gradient favoring Na+ flow from lumen
• Na+ gradient used to transport glucose against concentration gradient (cotransport)
• Glucose diffuses into blood passively
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Passive Reabsorption in theProximal Tubule
• Cl- to be reabsorbed passively along electrical gradient
• Water reabsorbed along osmotic gradient
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Acid Base Balance
• Proximal tubule also secretes H+ and absorbs HCO3
-
– used to regulate pH– with pH, H+
secretion and HCO3-
reabsorption
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Loop of Henle
• Kidneys produce a hyperosmotic urine– less H2O than blood
plasma
– concentrating mechanism occurs in the Loop of Henle
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Loop of Henle
• Countercurrent Multiplication – generates osmotic
gradient that draws H2O out of the tubules to be reabsorbed
– due to active reabsorption of Na+ & Cl-
Exchange = 30
Exchange = 50
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Loop of Henle
• descending limb– permeable to water
– no active transport
• ascending limb– impermeable to water
– lined w/ Na+-K+ pumps
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Loop of Henle
• Pumping of ions out of ascending limb creates osmotic gradient
• Water flows out of descending limb
• Absorbed by peritubular capillaries
• Fluid becomes more concentrated as it passes down descending limb
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Loop of Henle
• Removal of ions without water causes fluid to become less concentrated in the ascending limb
• Less concentrated than blood in distal convoluted tubule
• 25% of initial Na+ and water reabsorbed by loop of Henle
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Distal Convoluted Tubule and Collecting Duct
• Secretion of K+ and H+
• Reabsorption of Na+
and water• Generation of
hyperosmotic urine– final ~8% of water
and Na+ reabsorbed
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Hormonal Regulation of Reabsorption
• Aldosterone– increases Na+ reabsorption and K+ secretion by distal &
collecting tubules salt retention and BP (H2O retention)
• ADH– induces implantation of aquaporins (water channels)
into tubule cell membranes permeability of Distal and Collecting tubules to water H2O reabsorption = urine volume
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Triggering of Aldosterone Release
• release induced by juxtaglomerular apparatus– region of afferent arteriole that comes
into contact w/ascending limb of Loop of Henle
• releases renin (enzyme) into blood in response to BP
• renin converts angiotensinogen angiotensin I
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Triggering of Aldosterone Release
• angiotensin I converted to angiotensin II (fully activated) by angiotensin converting enzyme in lungs
• angiotensin II stimulates aldosterone release
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Urination• Ureters
– transfer urine to pelvic region
• Urinary Bladder– Stores urine – smooth muscle, stretchable walls
• Two sphincters– internal urethral sphincter
(involuntary)– external urethral sphincter
(voluntary)
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Urination
Urination Reflex:• Stretch receptors in bladder
wall spinal cord – efferents to smooth muscle
contraction
• Internal sphincter relaxes• External sphincter relaxes