BIOL 1104 Lecture Notes - Juxtaglomerular Apparatus, Baroreceptor, Hematocrit

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11 Feb 2013
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Control of ECF Volume II
I. Control of Blood Plasma Volume
a. Typical 70kg person has 5L blood (3L plasma, 2L hematocrit)
b. Glomerulotubular balance—a change in GFR (flow) => a proportional change in
Na reabsorption so that the fraction reabsorbed is constant
i. Blood volume is normally constant over a wide range
ii. ↑Na intake => ↑ ECF volume
iii. ↑Na intake => ↑ mean arterial blood pressure (but blood volume = const)
iv. [Na] is main determinant of ECF volume & [K] of ICF volume
c. Volume/Pressure Sensors
i. Extra-renal sensors
1. arterial baroreceptors
2. intrathoracic low pressure receptors
ii. Renal sensors
1. juxtaglomerular apparatus
2. baroreceptors/mechanoreceptors
3. renal tubules (sense altered hemodynamics)
d. ADH release
i. Very sensitive to small changes in osmolality
ii. Not sensitive to changes in blood volume until ~10% decrease, then has
steep response
II. “Rocking the boat”
a. addition of dry NaCl to ECF ↑ osmolarity => H2O shift to ECF (ICF
contraction)
b. Addition of H2O to ECF both ICF & ECF expand but with maintained ratio &
↓ osmolarity
c. Addition of Isotonic Saline to ECF only expands ECF
Principles of Na balance
-[Na] in ECF doesn’t correlate w/ ECF volume
-ECF volume determined by amount of Na & Cl in ECF
-Kidney controls excretion of Na to match change in ECF NOT [Na]
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