30 Questions
What happens to the extracellular osmolarity when a hypertonic solution is added to the extracellular fluid?
It increases
What is the net effect of adding a hypertonic solution to the extracellular fluid?
Increase in extracellular volume and decrease in intracellular volume
What happens to the osmolarity of the extracellular fluid when a hypotonic solution is added?
It decreases
What is the primary measurement available to evaluate a patient's fluid status?
Plasma sodium concentration
What is the result of osmosis of water when a hypertonic solution is added to the extracellular fluid?
Water diffuses out of the cells
What happens to the intracellular volume when a hypotonic solution is added to the extracellular fluid?
It increases
What is the primary driving force for water intake in the body?
Thirst mechanism
Which of the following conditions can cause hyponatremia?
All of the above
What is the role of the OVLT and SFO in the neural pathways responsible for thirst?
They originate Ang II-mediated thirst signals
What is the response to drinking water that helps to regulate water intake?
Inhibition of the thirst center
What is the role of the MnPO in the neural pathways responsible for thirst?
It transmits thirst signals to the thalamus
Which of the following is a stimulus for the hypothalamic thirst center?
Substantial decrease in blood volume or pressure
Which of the following neurons in the SFO is activated by water drinking signals to suppress thirst?
CCK neurons
What is the main function of the OVLT in water intake control?
Sensing increases in [Na] and osmolality in body fluids
Which of the following is NOT a site where thirst-driving systems are present?
Spinal cord
What is the effect of systemic dehydration on the body?
Increase in [Na] and osmolality in body fluids
Which of the following neurons is NOT activated by water drinking signals to suppress thirst?
VP neurons in the PVN
What is the role of the Nax signal in the OVLT?
Controlling water intake behavior
Which of the following is involved in regulating water intake during pregnancy and/or other conditions?
Relaxin-3
What is the function of the medial prefrontal cortex in water intake control?
Decision-making of water ingestion
Which of the following hormones increases water intake through Ang II-dependent mechanisms?
Angiotensin II
What is the effect of a decrease in Na in body fluids on thirst?
Inhibition of thirst
Which of the following neurons is involved in the regulation of water intake through Ang II-dependent mechanisms?
SLC9A4-positive neurons
What is the role of the lateral parabrachial nucleus (LPBN) in water intake control?
Feedback regulation of thirst
What happens to brain cells when overhydration occurs quickly?
They become susceptible to confusion, seizures, and coma
What is the primary treatment for overhydration, regardless of its cause?
Restricting fluid intake to prevent further overhydration
What is the result of a primary loss of water from the extracellular fluid?
Hypernatremia, due to loss of water from the extracellular fluid
What is a common cause of hypernatremia?
Dehydration caused by water intake that is less than water loss
What is the result of excessive sodium chloride added to the extracellular fluid?
Hypernatremia, due to excess sodium in the extracellular fluid
What is a type of diabetes insipidus caused by certain types of renal diseases?
Nephrogenic diabetes insipidus
Study Notes
Osmosis and Fluid Balance
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When a hypertonic solution is added to the extracellular fluid, extracellular osmolarity increases, causing water to move out of cells into the extracellular compartment.
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This results in an increase in extracellular volume, a decrease in intracellular volume, and a rise in osmolarity in both compartments.
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When a hypotonic solution is added to the extracellular fluid, extracellular osmolarity decreases, and some of the extracellular water diffuses into the cells until the intracellular and extracellular compartments have the same osmolarity.
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Both intracellular and extracellular volumes are increased by the addition of hypotonic fluid, although the intracellular volume increases to a greater extent.
Clinical Abnormalities of Fluid Volume Regulation
- Hyponatremia can occur due to:
- Vomiting and diarrhea
- Overuse of diuretics
- Renal diseases
- Addison Disease
- Excessive secretion of antidiuretic hormone
- Hypernatremia can occur due to:
- Water loss or excess sodium
- Lack of antidiuretic hormone
- Dehydration caused by water intake that is less than water loss
- Excessive sodium chloride added to the extracellular fluid
Regulation of Water Intake
- Thirst mechanism is the driving force for water intake
- Thirst is stimulated by:
- Plasma osmolality of 2–3%
- Angiotensin II or baroreceptor input
- Dry mouth
- Substantial decrease in blood volume or pressure
- Drinking water creates inhibition of the thirst center through:
- Relief of dry mouth
- Activation of stomach and intestinal stretch receptors
- Thirst-driving systems are present in the forebrain circumventricular organs (CVOs)
Neural Pathways of Thirst
- Angiotensin II-mediated thirst signals originate from AT1a neurons in the SFO and OVLT
- Clock-driven thirst signals originate from VP neurons in the SCN
- These signals are integrated in the MnPO and transmitted to the thalamus
- Water drinking signals sensed by the oral cavity and/or gastrointestinal tract are relayed to the NTS to activate CCK neurons in the SFO, GLP1R neurons in the MnPO, and Oxtr and PDYN neurons in the PBN
Water Intake Control
- Activation mechanisms of thirst:
- Increase in [Na] and osmolality in body fluids
- Angiotensin II
- Dipsogenic hormones such as secretin and relaxin-3
- Inhibitory mechanisms of thirst:
- Na decrease in body fluids
- CCK-dependent modulation of water intake
- Feedback regulation through the lateral parabrachial nucleus (LPBN)
- Water-predicting cues
- Estrogen and atrial natriuretic peptide
Treatment of Overhydration
- Fluid intake usually must be restricted
- Brain cells are particularly susceptible to overhydration (as well as dehydration)
Understand the effects of hypertonic and hypotonic solutions on extracellular and intracellular fluid volumes and osmolarity.
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