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Questions and Answers
What is the most abundant molecule in the human body?
Water
Which ions are predominantly found in the extracellular and intracellular fluids?
The Na+/K+ ATPase pump moves two Na+ ions into the cell for each cycle.
False
What does osmolality measure?
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What is the normal range of ECF osmolality?
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Match the mechanisms regulating hydration status with their sources:
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ADH is released in response to increased renal perfusion.
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What condition can severe water depletion cause in the brain?
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What occurs when there is an increase in plasma osmolality due to sodium?
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The equation for calculated osmolality is ____.
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Study Notes
Water and Electrolyte Basics
- Water is the most abundant molecule in the human body
- Electrolyte composition differs in ECF and ICF
- Extracellular fluid contains predominantly sodium
- Intracellular fluid contains predominantly potassium
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Na+/K+ ATPase pump
- Most important active transporter in cells
- Moves 3 sodium ions out of the cell, and 2 potassium ions into the cell
- Responsible for generating Na+ and K+ gradients across the cell membrane
- Na+ gradient powers coupled transport of glucose and other substances
- Consumes about 1/3 of all ATP produced in a resting body
- If the pump stops functioning, Na+ and K+ gradients will be disrupted, which can interrupt cell signals
Water Distribution
- Water moves from intravascular space based on pressure differences
- Hydrostatic pressure pushes fluid from vessels into interstitial space
- Oncotic pressure, driven by albumin, holds water in the intravascular compartment
- Osmotic pressure pulls water from low solute to high solute compartment
Osmolality
- Osmolality is a measurement of the concentration of osmotically active solutes in a solution
- Normal ECF osmolality is 275-295 mmol/kg water
- Water loss from ECF increases osmolality, causing water to move from ICF to ECF
- Osmolality can be measured directly or calculated using the formula: 2 x [Na+] + [urea] + [glucose]
Regulation of Hydration Status
- Mechanism: Kidney, Adrenal Glands, Hypothalamus, Cardiac Atria
- Stimuli: Renal perfusion, ECF tonicity, blood volume, and blood pressure
- Effects: Na+ and water excretion/retention, thirst, blood volume regulation
Osmolality vs Tonicity
- Hypothalamic osmostat regulates ADH release and thirst
- Osmostat is sensitive to small changes in plasma osmolality
- The cell membrane is selectively permeable to solutes
- Urea and alcohol freely permeable
- Increase in plasma osmolality due to sodium increases osmotic pressure across the cell membrane, leading to water leaving the cell
- Increase in plasma osmolality due to urea does not have this effect due to free permeability of urea
- Under physiological conditions, tonicity is mainly dependent on plasma sodium concentration
- Changes in cell volume are important in the brain
- Hypertonicity causes cerebral shrinkage and rupture of vessels
- Hypotonicity leads to brain swelling and compression
- The brain can adapt by altering its content of "osmolytes"
Water Depletion
- Severe water depletion causes cerebral dehydration and can lead to bleeding due to damaged blood vessels
- Cerebral shrinkage is limited in the short term by movement of ECF ions into cerebral cells
- If dehydration persists, brain cells produce osmotically active organic compounds called 'osmolytes' to adapt
- Excessive fluid replacement can cause cerebral edema due to rapid movement of water into the ICF
Osmolal Gap
- Measured osmolality and calculated osmolality are usually similar
- Normal gap is usually less than 10 mmol/kg water
- A larger gap suggests presence of unmeasured osmotically active substances, which may indicate a medical condition.
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Description
This quiz explores the fundamental concepts of water distribution and the roles of electrolytes in human physiology. You will learn about the differences between intracellular and extracellular fluids, the Na+/K+ ATPase pump, and the pressures affecting fluid movement within the body. Test your knowledge on these essential topics.
