Water and Electrolytes Overview

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Questions and Answers

What is the formula to calculate osmolality?

[Na+] + [urea] + [glucose]

2x [Na+] - [glucose] + [urea]

2x [Na+] + [urea] + [glucose] (correct)

2x [urea] + [Na+] + [glucose]

Which of the following hormones leads to renal sodium and water retention?

ADH

Aldosterone (correct)

ANF

GFR

What condition primarily leads to the release of ADH?

Increased blood volume

Decreased renal perfusion

Increased extracellular fluid tonicity (correct)

Decreased extracellular fluid tonicity

What happens during cerebral dehydration due to hypertonicity?

Cerebral shrinkage leading to bleeding Signup and view all the answers

How does the brain adapt to persistent dehydration?

By synthesizing osmotically active compounds Signup and view all the answers

What can result from excessive fluid replacement after dehydration?

Cerebral edema Signup and view all the answers

Which mechanism allows Na+ and water excretion in the kidneys?

GFR Signup and view all the answers

What role does effective osmolality play under physiological conditions?

It is primarily dependent on plasma sodium concentration Signup and view all the answers

What primarily drives water movement from the intravascular space to the interstitial space?

Hydrostatic pressure Signup and view all the answers

Which electrolyte is predominantly found in the intracellular fluid (ICF)?

Potassium Signup and view all the answers

What is the typical range of normal extracellular fluid (ECF) osmolality?

275–295 mmol/kg water Signup and view all the answers

How does the Na+/K+-ATPase pump function in terms of ion transport?

Pumps three Na+ ions out of the cell and two K+ ions in Signup and view all the answers

What is the main consequence if the Na+/K+-ATPase pump stops functioning?

Disruption of ion concentration gradients Signup and view all the answers

What occurs to water movement from the intracellular fluid (ICF) to the extracellular fluid (ECF) when ECF osmolality increases?

Water moves from the ICF to the ECF Signup and view all the answers

What role does albumin play in fluid movement across compartments?

It creates an osmotic gradient that holds water in the intravascular compartment Signup and view all the answers

What is the primary effect of water loss from ECF on osmolality?

Increases ECF osmolality Signup and view all the answers

Study Notes

Water and Electrolytes

Water is the most abundant molecule in the human body.
Electrolyte compositions within and outside of cells differ. Sodium is primarily found in the extracellular space, while potassium is found within cells.

Na/K ATPase Pump

The Na/K ATPase pump is crucial for cell function and the most widely distributed active transporter.
It moves 3 sodium ions out of the cell and 2 potassium ions into the cell simultaneously with each ATP hydrolysis cycle.
This generates the typical sodium and potassium gradient across cell membranes.
The Na/K ATPase pump uses the sodium gradient to power the coupled transport of glucose and other substances.
In a resting body, the Na/K ATPase pump uses approximately 1/3 of the body's ATP.
If the Na/K ATPase pump fails, the concentration gradients of sodium and potassium may be disrupted, interrupting cell signaling.

Water Distribution

Water moves based on pressure gradients between the intravascular and interstitial spaces.

Types of Pressure

Hydrostatic Pressure: Drives fluid out of vessels into the interstitial space.
Oncotic Pressure: Driven by albumin and holds water within the intravascular compartment.
Osmotic Pressure: Pulls water from a low solute to a high solute compartment.

Osmolality

Osmolality is a measure of the solute concentration in a solution.
Normal extracellular fluid osmolality is between 275-295 mmol/kg water.
Water loss from the extracellular fluid increases osmolality, resulting in movement of water from the intracellular fluid to the extracellular fluid (ICF to ECF) to equalize osmolality.
Osmolality can be measured directly or calculated using the formula:
- Osmolality = 2 x [Na+] + [urea] + [glucose]

Regulation of Hydration Status

Mechanism | Source | Stimulus | Effect
-------------------- | --------------- | ----------------------------- | -------------------------------------
GFR | Kidney | - | Permits Na & water excretion
Aldosterone | Adrenal | ↓ Renal perfusion | Renal Na & water retention
ADH | Hypothalamus | ↑ ECF tonicity, ↓↓↓ blood volume | Pure water retention
ANF | Cardiac atria | ↑ Blood volume | Renal Na & water excretion

Osmolality vs Tonicity

The hypothalamus controls both ADH release and thirst sensation.
The hypothalamus is acutely sensitive to small changes in plasma osmolality.
Cell membranes are selectively permeable to various solutes.
Urea and alcohol are freely permeable.
Increased plasma osmolality due to sodium increases osmotic pressure across the cell membrane, drawing water out of the cell to equalize osmolalities.
Increased plasma osmolality due to urea does not have this effect because urea freely passes between the ICF and ECF.
Under physiological conditions, effective osmolality or tonicity is primarily dependent on plasma sodium concentration.

Changes in Cell Volume

Cell volume changes are particularly important for brain function.
Cerebral dehydration due to hypertonicity leads to an osmotic imbalance, causing water movement outside of cells (EC) and cerebral shrinkage, which can damage blood vessels.
Hypertonicity also leads to water movement inside of cells (IC), causing cerebral swelling (edema) and potential compression.
The brain adapts by altering its “osmolyte” content.

Blood Volume

Water depletion severely dehydrates the brain, potentially causing cerebral bleeding from blood vessel damage.
In the short term, cerebral shrinkage is limited by the movement of extracellular ions (EC) into brain cells, creating an osmotic shift of water.
If dehydration persists, brain cells adapt by synthesizing osmotically active organic compounds ("osmolytes").
Excessive fluid replacement can cause cerebral edema due to rapid intracellular movement of water.

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Description

This quiz explores the critical roles of water and electrolytes in the human body, including the function of the Na/K ATPase pump. You'll learn about the transport mechanisms and the importance of sodium and potassium gradients for cellular processes. Test your understanding of how these elements contribute to overall health.