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Questions and Answers
What is the formula for calculating osmolality?
What is the formula for calculating osmolality?
The hypothalamic osmostat is not sensitive to changes in plasma osmolality.
The hypothalamic osmostat is not sensitive to changes in plasma osmolality.
False
What is the primary function of the Na+/K+ ATPase pump?
What is the primary function of the Na+/K+ ATPase pump?
Osmolality refers to the pressure exerted by the albumin in the blood.
Osmolality refers to the pressure exerted by the albumin in the blood.
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What is the primary function of aldosterone?
What is the primary function of aldosterone?
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Cerebral dehydration due to ________ causes osmolar imbalance leading to movement of water.
Cerebral dehydration due to ________ causes osmolar imbalance leading to movement of water.
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What happens to water movement when ECF osmolality increases?
What happens to water movement when ECF osmolality increases?
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The predominant electrolyte in the extracellular space is ______.
The predominant electrolyte in the extracellular space is ______.
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Match the following regulators of hydration status with their sources:
Match the following regulators of hydration status with their sources:
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Which of the following pressures drives fluid from vessels into the interstitial space?
Which of the following pressures drives fluid from vessels into the interstitial space?
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What effect does an increase in plasma osmolality due to sodium have on cells?
What effect does an increase in plasma osmolality due to sodium have on cells?
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Match the following concepts with their definitions:
Match the following concepts with their definitions:
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In cases of severe water depletion, brain cells can adapt by synthesizing organic compounds called osmolytes.
In cases of severe water depletion, brain cells can adapt by synthesizing organic compounds called osmolytes.
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The Na+/K+ ATPase pump consumes about one-third of all ATP in a resting body.
The Na+/K+ ATPase pump consumes about one-third of all ATP in a resting body.
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What may excessive fluid replacement lead to?
What may excessive fluid replacement lead to?
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What role does RAAS play in water and electrolyte balance?
What role does RAAS play in water and electrolyte balance?
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Study Notes
Water and Electrolytes Balance
- Water is the most abundant molecule in the human body.
- The electrolyte compositions of the extracellular fluid (ECF) and intracellular fluid (ICF) are different.
- The extracellular space is predominantly made up of sodium.
- The intracellular space is primarily composed of potassium.
Na/K ATPase pump
- Most widespread and physiologically important active transporter in cells.
- Moves three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell with each ATP hydrolysis cycle.
- Responsible for generating the typical Na+ and K+ gradients found across the cell membrane.
- The Na+ gradient is used to power coupled transport of glucose and many other substances.
- In a body at rest, the activity of the Na+/K+-ATPase consumes about a third of all ATP.
- If the Na/K-ATPase stops working, the concentration gradients of Na+ and K+ on the inside and outside of the cell may be affected.
- This can interrupt cell signals.
Water Distribution
- Water moves from the intravascular space based on pressure differences.
- Hydrostatic pressure drives fluid from vessels into the interstitial space.
- Oncotic pressure, driven by albumin, holds water in the intravascular compartment.
- Osmotic pressure pulls water from a low solute to a high solute compartment.
Osmolality
- Osmolality is a physical property of a solution based on the concentration of osmotically active solutes.
- Normal ECF osmolality is in the range of 275–295 mmol/kg water.
- Water loss from the ECF increases osmolality, resulting in the movement of water from the ICF to the ECF.
- Osmolality can be directly measured or calculated using the formula: 2x [Na+] + [urea] + [glucose].
Regulation of Hydration Status
- Glomerular Filtration Rate (GFR): Regulated by the kidney. Permits Na+ and water excretion.
- Aldosterone: Secreted by the adrenal glands. Increases renal Na+ and water retention in response to decreased renal perfusion.
- Antidiuretic Hormone (ADH): Secreted by the hypothalamus. Increases pure water retention in response to increased ECF tonicity and decreased blood volume.
- Atrial Natriuretic Factor (ANF): Released by cardiac atria. Increases renal Na+ and water excretion in response to increased blood volume.
Osmolality vs. Tonicity
- The hypothalamic osmostat controls both ADH release and the sensation of thirst.
- The hypothalamic osmostat is acutely sensitive to small changes in plasma osmolality.
- The cell membrane is selectively permeable to various solutes.
- Urea and alcohol are freely permeable.
- An increase in plasma osmolality due to sodium implies an increase in osmotic pressure across the cell membrane and withdraws water from the cell to equalize osmolalities.
- An increase in plasma osmolality due to urea does not have this effect because of the free permeability of urea between the ICF and ECF.
- Effective osmolality or tonicity under physiological conditions is primarily dependent on plasma sodium concentration.
- Changes in cell volume are particularly important in the case of the brain.
- Cerebral dehydration due to hypertonicity causes osmolar imbalance, leading to extracellular movement of water and cerebral shrinkage, which can rupture vessels.
- Hypertonicity also leads to intracellular movement of water and cerebral swelling (oedema), which causes compression.
- The brain can adapt by altering the content of “osmolytes”.
Blood Volume
- Severe water depletion causes cerebral dehydration, leading to cerebral bleeding through damage to blood vessels.
- In the short term: cerebral shrinkage is limited by the movement of extracellular ions into cerebral cells, resulting in an osmotic shift of water.
- If dehydration persists, brain cells adapt by synthesizing osmotically active organic compounds (“osmolytes”).
- Excessive fluid replacement may cause cerebral oedema because of rapid intracellular movement of water.
Osmolal Gap
- Measured osmolality (in mmol/kg of water) and calculated osmolarity (in mmol/L of solution) are normally very similar.
- The normal gap is <10 mmol/kg.
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Description
Test your knowledge on water and electrolyte balance in the human body. This quiz covers key concepts like the differences in electrolyte compositions and the role of the Na/K ATPase pump. Assess your understanding of how these factors influence cell function and signaling.
