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Renal Functions
- Filter waste from blood
- Maintain blood volume
- Maintain blood osmolarity
- Uses: Filtration, Reabsorption, Secretion
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Nephron
- Functional Unit of the kidney
- Has vascular system and tubular system
- All but cells and proteins in blood can be filtered
- Most reabsorbed, remainder= urine
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Vascular System
- 2 capillary systems: 1 for filtering and 1 for reabsorption
- Afferent arteriole-> glomerus (filtration) -> efferent arteriole->peritubular capillaries (reabsorption) -> venules
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Tubular System
- S- shaped
- Bowman's Capsule (receives filtrate) -> proximal tubule -> Loop of Henle -> distal tubule -> collecting duct -> ureter
- Variable reabsorption
- Hormonal control of volume
- (80% of plasma that enters the glomerus is NOT filtered and leaves through efferent arteriole; 20% DOES)
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Glomerular Filtration
- From glomerulus into Bowman's capsule
- Glomerular capillaries have very wide pores
- Only cells and proteins are not filtered
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Inulin
- Fructose polymer
- Filtered, not reabsorbed or secreted
- Used to measure glomerular filtration rate (GFR)
- Inject in blood- measure in urine- proportional to amount of filtration
- GFR ~125ml/min, 1/40 of total blood volume
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Hydrostatic/ Osmotic Pressures
- Hydrostatic: BP force filtrate into Bowman's Capsule
- Osmotic Pressures: so much fluid is filtered, remaining proteins have higher than normal osmotic pressure
- Net: lots of filtration
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Control of GFR
- Afferent arteriole radius controls entry to glomerulus
- Afferent Arteriole dilation increases GFR
- Afferent arteriole constriction decreases GFR
- Sympathetic nerve constrict afferent arteriole, decreases GFR
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Tubular Reabsorption
- Must recover most filtrate
- 125 filtered ml/min reabsorbed -> 1ml/min urine, 1.44 L/day
- 125 filtered, 123 ml/min reabsorbed -> 2ml/min urine, 2.88 L/day
- Excess urine loss in diabetes -> decrease BP -> shock -> death
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Transport Maximum
- Different carriers for different molecules
- TM is limit of transport due to limited number of carriers
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Glucosuria
- 3 times more carrier capacity than normal filtered load
- If G in urine, blood must have at least 3 times more G than normal
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Sodium Reabsorption
- Controls reabsorption of many other molecules
- Na+ pump only on basolateral side of tubular cells
- Pump Na+ out, create gradient for Na+ entry into cells
- ATP needed for energy
- Tight junctions prevent flow in spaces between cells
- Caffeine decreases Na+ reabsorption
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Cotransport
- Carries for Na+ and cotransported molecule
- Glucose, amino acids, bicarbonate, Cl- are cotransported with Na+ during reabsorption
- Energy use is Na+ movement down gradient into cells
- H2O follows osmotically at proximal tube
- Variable H2O reabsorption at distal tube an collecting duct
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Glucose Reabsorption
- Binds to carrier with Na+ on luminal side to enter tubular cell
- Separate, non-Na+ glucose carrier moves G into interstitial space
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Proximal Tubule Water Reabsorption
- 60-70% water reabsorbed in proximal tubule
- 180L/day filtered ~1L of urine, variable (distal tubule and collecting duct)
- 500mL of urine minimum per day to remove toxins
- Osmotic reabsorption of water follows solutes, especially Na+
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