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What is the formula for calculating osmolality?
The hypothalamic osmostat is not sensitive to changes in plasma osmolality.
False
What is the primary function of the Na+/K+ ATPase pump?
Osmolality refers to the pressure exerted by the albumin in the blood.
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What is the primary function of aldosterone?
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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?
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The predominant electrolyte in the extracellular space is ______.
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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?
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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:
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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.
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What may excessive fluid replacement lead to?
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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.
