BIOL 1104 Lecture Notes - Plasma Osmolality, Circumventricular Organs, Osmoreceptor

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11 Feb 2013
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Control of ECF Volume I—Guggino
I. Conserving and balancing body H2O
a. Humans can excrete large or small quantities of water independently of solutes
b. Kidneys regulate water excretion over large range!
c. Sensing changes in Osmolality
i. Osmoreceptors—located in circumventricular organs in brain; contain non-selective
Ca channels
1. regulate thirst & secretion of ADH
2. only respond to changes in effective osmolality (ie if a freely permeable solute
is responsible for the change in osmolality, osmoreceptors won’t notice)
ii. Thirst
1. also very sensitive to changes in plasma osmolality, but can also be stimulated
by changes in bl. volume/pressure
2. Factors controlling thirst: osmolality, blood volume, blood pressure,
angiotensin II, gastric distention, dryness of mouth
II. ADH (Antidiuretic Hormone)
a. Synthesized in paraventricular & supraoptic nuclei of hypothalamus
b. Stored in and released from the posterior pituitary
c. ADH secretion is very sensitive to small changes (↑) in osmolality but
d. Mechanism of ADH action
i. Triggers cascade that => aquaporins insertion in to apical cell membranes of distal
tubules & collecting ducts
ii. Increases water permeability of distal tubules & collecting ducts => ↑ reabsorption
e. KEY for concentrating urine (absent/decreased fxn in diabetes insipidus)
(Raymond has removed images summarizing the ADH and thirst response to an increase or decrease in ECF
osmolality.)
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