BIOL 2420 Lecture Notes - Lecture 9: Cvcc, Adrenal Cortex, Osmotic Concentration
26 Apr 2018
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Unit 8 – Lecture 9
Dehydration Triggers Homeostatic Responses
- Table 20.1 – responses triggered by changes in volume, blood pressure, and osmolarity
- Homeostatic response to severe dehydration is an excellent example of how the body
works to maintain blood volume and cell volume in the face of decreased volume and
increased osmolarity
o Illustrates the role of neural and endocrine integrating centers
- Severe dehydration
o Adrenal cortex receives two opposing signals
1. Secrete aldosterone
2. Do not secrete aldosterone
o The body has multiple mechanisms for dealing with diminished blood volume, but
high ECF osmolarity causes cells to shrink and presents a more immediate threat
to well-being
o Decreased volume and increased osmolarity = adrenal cortex does not secrete
aldosterone
▪ If it was secreted, then aldosterone would cause Na+ reabsorption, which
could worsen the already-high osmolarity associated with dehydration
o Compensatory mechanisms are aimed at restoring normal blood pressure, ECF
volume, and osmolarity by
1. Conserving fluid to prevent additional loss
2. Triggering cardiovascular reflexes to increase blood pressure
3. Stimulating thirst so that normal fluid volume and osmolarity can be
restored
The figure on previous page
- The two yellow stimuli at the top of the map are caused by dehydration
o Decreased blood volume/pressure
o Increased osmolarity
- Decreased ECF volume causes decreased blood pressure
o Blood pressure acts both directly and as a stimulus for several reflex pathways
that are mediated through the carotid and aortic baroreceptors and the pressure-
sensitive granular cells
o Decreased volume is sensed by the atrial volume receptors
1. The carotid and aortic baroreceptors signal the cardiovascular control center
(CVCC) to raise blood pressure
▪ Sympathetic output from the CVCC increases while parasympathetic
output decreases
a. Heart rate goes up as control of the SA node shifts from predominantly
parasympathetic to sympathetic
b. The force of ventricular contraction also increases under sympathetic
stimulation
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Document Summary
Table 20. 1 responses triggered by changes in volume, blood pressure, and osmolarity. Homeostatic response to severe dehydration is an excellent example of how the body works to maintain blood volume and cell volume in the face of decreased volume and increased osmolarity. Illustrates the role of neural and endocrine integrating centers. The two yellow stimuli at the top of the map are caused by dehydration: decreased blood volume/pressure. Redundancy in the control pathways ensures that all four main compensatory mechanisms are activated: cardiovascular responses, ang ii, vasopressin, and thirst: cardiovascular responses, combine increased cardiac output and increased peripheral resistance to raise blood pressure. Increase in blood pressure does not necessarily mean that blood pressure returns to normal. Increases water permeability of the renal collecting ducts: allows water reabsorption to conserve fluid, without fluid replacement, vasopressin cannot bring volume and osmolarity back to normal, oral (or intravenous) intake of water.
