BIOL 1104 Lecture Notes - Angiotensin-Converting Enzyme, Zona Glomerulosa, Macula Densa

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11 Feb 2013
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Control of ECF & Composition III
Sodium Balance
I. Regulation of ECF volume & composition
a. Glomerular-tubular balance—small changes in GFR won’t have large effects
on Na excretion b/c renal tubules compensate for changes in GFR by proportional
changes in Na reabsorption
b. Physical Factors—changes in concentration of solutes (osmolarity) or volume,
may be monitored by sensors
i. Hydrostatic pressure differences—↑volume => ↑GFR (more “push”)
ii. Oncotic pressure differences—as [H2O] ↓, [proteins] ↑ => ↑reabsorption
c. Hormonal Factors—released in response to GFR change or diff. rates of
substance reabsorption
Segment Hormone Effect
Proximal Tubule Angiotensin ↑ NaCl reabsorption
(vasoconstriction)
TAL/ DCT / CCT Aldosterone ↑ NaCl reabsorption
Distal/Collecting
Duct
Atrial Natriuretic
Factor (ANF)
↓ NaCl reabsorption (↑GFR =>
↑excretion)
i. Renin-Angiotensin System
1. BP => macula densa cells signal J-G cells release renin
converts angiotensinogen to angiotensin I at lungs, angiotensin
converting enzyme (ACE) converts angiotensin I to angiotensin II
BP in several ways: 1) stimulates ADH release, 2) stimulates
aldosterone release, 3) promotes vasoconstriction
2. Aldosterone
a. Synthesized/released in zona glomerulosa cells of adrenal
cortex
b. Fxn: ↑NaCl reabsorption in DCT/ Collecting ducts by ↑
transcription of proteins that ↑ Na channel activity
c. Regulation: ↑[Na]plasma => ↓ aldosterone secretion
i. ↑[K] & ↑[ACTH] => ↑ aldosterone production
d. long acting hormone
ii. Atrial Natriuritic Peptide/Factor (ANP/ANF)
1. Purpose—increases natriuresis (excretion of Na by kidneys)
2. Function
a. ANF antagonizes action of Angiotensin II—causes renal
vasodialation, ↑ GFR, ↑ blood flow to medulla
b. ANF reduces aldosterone production—directly inhibits
production & indirectly via inhibiting angiotensin II action
c. ANF reduces Na reabsorption—↑cGMP (a negative
modulator of Na channels) => ↓Na reabsorption
d. Neural Factors
i. Sympathetic NS control stimulation causes:
1. control smooth muscle tone of afferent arterioles
1
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