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Nephron Types:
- cortical: (short) make dilute urine (fish that secrete ammonia)
- juxtamedullary: (long) make concentrated urine
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Renal Processes
- 1- filtration
- 2- reabsorption (into blood)
- 3- secretion (into tubule)
- 4- excrition (end result of process)
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Filtration
- occurs in: glomerulus/ bowmans capsule (covers glomerulus)
- podocytes: contain gaps for movement of plasma content
- capsule damage allows RBCs, and proteins into urine (blood in urine=sign of capsule damage)
- 20% blood plasma filtered per round
- kidney recieves 25% of CO
- amount of fluid filtered = 180
- urinary output =1.5
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Glomerular Filtration Rate (GFR)
- = rate of plasma filtration
- changes in GFR often change urine output
- low GFR generally indicates renal failiure
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Filtration Forces
- PROMOTED by: blood pressure
- OPPOSED by:
- osmotic (oncotic) pressure - more proteins re absorbed in blood
- tubule pressure (fluid already in capsule)
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Regulation of GFR
- GFR autoregulation =maintenance of constant GFR over a wide range of BP
- ruquires altering vascular resistance
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INTRINSIC CONTROL
- MYOGENIC:
- sense by: high blood pressure opening stretch sensitive ion channels in afferent arterioles
- response: constriction thus lower blood flow
- TUBULOGLOMERULAR FEEDBACK:
- flow sensed by: macula densa
- response: paracrine hormones constrict afferent arteriole
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EXTRINSIC CONTROL:
- sympathetic innervation:
- reduces GFR
- occurs during hemorrage
- mechanism: afferent arteriole vaso constriction
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Controlling Permeability
- mesangial cells surround glomerular capillaries and contract to secrete inflammatory mediators to cause podocytes to swell thus blocking surface of capillaries and decreasing glomerular permeability
- contract when stimulated by - angiotensin 2
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MORE-----Measuring GFR
- measured by CLEARANCE of a filtered substance which is not reabsorbed, metabolized or secreted
- clearance: the volume of filtered fluid that had all solutes removeed from it
- characteristics of test substance for filtration
- insulin clearance used experimentally to find GFR
- creatinine more practical - a muscle metabolism byproduct, fairly constant plasma level
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MORE-----Calculating GFR
- filtration rate x Plasma conc = Urine conc x Urine Volume
- aka (mass filtered) = (mass excreted)
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MORE-----filtration
- filtered fluid volume returns to the plasma without the solute (the volume is cleared)
- V and GFR are RATES (usually measured over 24 hr)
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Measuring Renal Plasma Flow
- PAH is totally cleared so its used to measure RPF
- measured by agents that are both filtered and secretec but not absorbed
- total clearance: no agent remains in renal vein
- RPFxP(pah)=U(pah)xV/t
- or RPF=[U(pah)xV/t] P(pah)
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Renal Blood Flow
- calculation of renal blood flow depends on the hematocrit (% RBCs in blood):
- RBF =RPF/1-HTC (hematocrit)
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