Lecture 8: Water and Electrolytes in Medicine

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Questions and Answers

What distinguishes electrolytes from non-electrolytes?

Electrolytes are primarily composed of organic compounds.

Non-electrolytes dissociate in water into ions.

Electrolytes dissociate into ions in water. (correct)

Electrolytes cannot dissolve in water.

How does osmotic pressure influence fluid movement across cell membranes?

It drives fluid movement according to hydrostatic pressure.

It is a driving force for diffusion of water molecules. (correct)

It causes the fluid to become positively charged.

It encourages water to move against concentration gradients.

What term describes the condition when the body has excess fluid?

Overhydration (correct)

Homeostasis

Dehydration

Electrolyte imbalance

Which factor primarily drives fluid movement across capillary walls?

Both hydrostatic pressure and colloid osmotic pressure Signup and view all the answers

Plasma volume differs from blood volume primarily because:

Blood volume comprises plasma and red blood cells. Signup and view all the answers

In terms of body fluids, what does the term 'electroneutrality' refer to?

The number of cations equals the number of anions. Signup and view all the answers

Which of the following solutes exerts a greater osmotic effect?

Sodium chloride (NaCl) Signup and view all the answers

Which compartment of body fluids typically has the highest water content?

Intracellular fluid (ICF) Signup and view all the answers

What is the primary mechanism through which the body regulates water balance?

Thirst reflex mechanism Signup and view all the answers

Dehydration is primarily caused by which of the following conditions?

Excess fluid loss Signup and view all the answers

What is the main difference between effective and ineffective osmoles in plasma?

Ineffective osmoles do not contribute to osmotic pressure. Signup and view all the answers

What is the primary cause of dehydration?

Loss of excessive water Signup and view all the answers

What condition related to water balance is more common?

Dehydration Signup and view all the answers

What is the primary factor that affects mean arterial blood pressure (MAP)?

Blood volume (BV) Signup and view all the answers

Which of the following symptoms is NOT associated with dehydration?

Nausea Signup and view all the answers

What is a common consequence of overhydration?

Seizures Signup and view all the answers

Which of the following best describes the composition of intracellular fluid (ICF)?

It remains relatively constant. Signup and view all the answers

What primarily causes formation of edema in the body?

Fluid movement across capillary walls Signup and view all the answers

How is total body water (TBW) expressed?

As a percentage of body mass Signup and view all the answers

Which statement about osmotic pressure is true?

It drives the movement of water across semi-permeable membranes. Signup and view all the answers

What percentage of total body weight does water account for in adults?

60% Signup and view all the answers

Which body fluid compartment has the highest water content?

Muscle cells Signup and view all the answers

What role do semi-permeable membranes play in fluid movement?

They selectively allow certain substances to pass. Signup and view all the answers

What is the impact of aging on body water content?

Decreases to approximately 45% Signup and view all the answers

Which of these factors affects drug dosage considerations?

Body water content Signup and view all the answers

What is NOT a symptom of overhydration?

Excessive thirst Signup and view all the answers

Study Notes

Lecture 8: Water, Electrolytes, and Body Fluid

The lecture is about water, electrolytes, and body fluid
Dr. Isabel Hwang is the lecturer for this topic
She is a Senior Lecturer and Year 1 Coordinator in Medicine
Her affiliation is the Division of Education, School of Biomedical Sciences, Faculty of Medicine at CUHK
Her email address is [email protected]
Her office number is 3943 6795
The slides contain copyrighted material, restricted to students of MEDF1011.

Lecture Outline

Definition of total body water
Distribution of body fluid in different fluid compartments
Relationship between osmosis and osmotic pressure
Definition and application of osmolarity or osmolality
Examples of effective and ineffective osmoles in plasma
Homeostatic regulation of water balance
Thirst reflex mechanism
ADH mechanism
Fluid movement across the capillary wall
Formation of edema and examples of its common causes

Pre-class Assignment

Micro Module Seven: What is the role of electrolytes in our bodily fluids?
The assignment asks to give examples of major electrolytes in bodily fluids.

Regulation of Body Fluid and its Composition

Regulation of composition and volume of body fluids is fundamental to physiology.
Body cells require a well-defined composition to function normally.
Composition of intracellular fluid (ICF) is relatively constant.
External environment affects the internal environment.
Blood volume affects venous return and thus mean arterial blood pressure (MAP).
Fluid disturbances are observed in disease states.
Understanding body fluids is helpful for patient management and drug dosing.

Why Water Balance is Important?

The amount of water in cells remains fairly constant but can change in extreme or pathological conditions.
Tissue cells, like brain cells, are sensitive to water status.
Water balance disturbances can occur as dehydration or overhydration.
Dehydration is common and due to inadequate fluid intake or excessive fluid loss (vomiting, diarrhea, sweating).
Overhydration is less common and due to excessive water intake.

Dehydration Symptoms

Dry mouth and tongue
Thirst
Headache
Fatigue
Dizziness
Confusion
Excessive water exit from body cells, including brain cells
Shrinkage of brain cells
Stretching of vascular connections to the skull
Intracranial bleeding

Overhydration (Water Intoxication) Symptoms

Nausea and vomiting
Headache
Muscle weakness
Confusion/Disorientation
Seizures
Coma
Excessive water entry into body cells, including brain cells
Increase in space occupied in the skull
Increase in intra-cranial pressure
Impaired blood flow to the brain

Water in Body Fluid Compartments

Body fluid is divided into different compartments (extracellular vs. intracellular).
Compartments are separated by boundaries like blood vessel walls and cell membranes.
Cell membranes are semi-permeable (selectively permeable).
Transport processes occur via passive and active processes between compartments.
Capillary walls are more diffusible and less selective.

Total Body Water (TBW)

TBW is the total volume of water in a person's body, expressed as a percentage of their total body weight.
TBW accounts for approximately 60% of total body weight in adults.
Water content differs in muscle, bone, and fat cells.
Water content declines to ~45% in older adults and in dehydration states.
Body water content influences drug dosage, especially for women and older individuals.

Osmolarity/Osmolality

Osmolarity: reflects the total solute concentration in a given volume of solution.
Osmolarity is useful for expressing the number of osmoles(or particles) per liter of solution.
Osmolarity is usually expressed in mOsm/L.
Osmolarity is temperature-dependent and is measured at normal body temperature.
Osmolality: reflects the total solute concentration in a given mass of solvent.
Osmolality is temperature-independent.
Osmolality is often preferred over osmolarity.
Effective osmoles can cross the membrane; ineffective osmoles cannot.

Electro-neutrality of Body Fluid

All body fluids have no net charge(electroneutral).
The positively charged ions (cations) are balanced by negatively charged ions(anions).
Example: Plasma and Urine are both electrically neutral.

Comparison of Solutes (Electrolytes and Non-electrolytes)

Non-electrolytes: Mostly organic substances, do not dissociate in water, do not create charged particles (e.g., glucose and urea).
Electrolytes: Mostly inorganic, dissociate in water into charged particles (ions), exert greater osmotic effect than non-electrolytes (e.g., NaCl, KCI).

Distribution of Fluid

Fluid and electrolyte move between compartments based on different driving forces.
Osmotic pressure drives movement across cell membranes.
Hydrostatic pressure and colloid osmotic pressure regulate movement across capillary walls.

Osmosis

Osmosis is the passive diffusion of water across a semipermeable membrane from high water concentration to low water concentration.
Osmosis is independent of membrane potentials.

Osmotic Pressure

Osmotic pressure is a driving force to enter compartment with more solutes.
At equilibrium, this pressure is balanced by hydrostatic pressure.
Osmotic pressure reflects total solute concentration rather than the identity of specific solutes.
Higher solute means higher osmotic pressure.

Effective Osmoles

Non-penetrating solutes cannot cross plasma membranes.
Examples include NaCl, KCl, and plasma proteins.
They generate the net driving force for water movement.

Ineffective Osmoles

Penetrating solutes can cross plasma membranes.
Examples include urea, glucose (in healthy people), and ethanol.
They do not create net driving force for water movement.

Molarity and Concentration

Molarity represents solute concentration in terms of moles of solute per liter of solution.
Molarity is often misleading because the number of particles in solution isn't always equal to the number of molecules.

Osmolarity Represents Total Solute Concentration

Osmolarity is defined as the number of particles (ions or intact molecules) per liter of solution.
Physiologically it is expressed as mOsm/L.

Osmolarity and Its Significance

Osmolarity is a measure of osmotic pressure. Increased concentration of solutes in a solution means higher osmolarity.
Normal extracellular fluid (ECF) osmolarity is 285-295 mOsm/L.

Homeostatic Control of Plasma Osmolarity

Both thirst and ADH secretion are stimulated by an increase in plasma Osmolarity (ECF).
Plasma osmolarity can increase due to dehydration.
Example of loss of more fluid than taken in – sweating, vomiting, diarrhoea, etc.

The Thirst Reflex

The thirst reflex is the primary defense against increases in plasma osmolarity.
It is integrated by the hypothalamus and activated by hypothalamic osmoreceptors.
Increased plasma osmolarity (1-2%) triggers thirst and reduced blood volume/MAP trigger thirst.
Relief of dry mouth and activation of stomach/intestinal stretch receptors help reduce thirst.

Antidiuretic Hormone (ADH)

ADH, also called vasopressin, is secreted by the posterior pituitary.
It is secreted in response to dehydration.
ADH causes increase in water reabsorption in the kidneys.
Increased ADH means increased water reabsorption and reduced urine output.

Movement of Fluid Across Compartments

Fluid movement between plasma and interstitial fluid is based on balance between hydrostatic and colloid osmotic pressure.
Hydrostatic pressure forces fluid out of capillaries (filtration)
Colloid osmotic pressure draws fluid back into capillaries (absorption).

Distribution of Extracellular Fluid

Plasma solutes are nearly equal to the concentrations in interstitial fluid except for proteins.
Albumin is a significant effective osmolar, drawing water into the plasma and generating colloid osmotic pressure.
Lymph system collects excess filtrate for return to blood circulation.

Edema Formation

Edema is caused by imbalances between hydrostatic & oncotic pressures.
Decrease in oncotic pressure (e.g. malnutrition, renal disease) or increase in hydrostatic pressure (e.g. hypertension).
Accumulation of excessive fluid in the interstitial space.

Applications of Concepts

Post-class assignment on Blackboard relates to cases of electrolyte and osmolarity balance.

Learning Outcomes

Defining total body water (TBW) for a person of 60kg
Calculating TBW, intracellular, extracellular, blood, and plasma volumes.
Understanding that osmotic pressure reflects total solute concentration and ability to cause fluid shifts.
Examples of effective and ineffective osmoles.
Formulas for ECF osmolarity.
Flow diagrams of thirst reflex and ADH mechanisms.
Defining edema and examples of its causes.
Description of how hydrostatic and osmotic pressure regulate fluid exchange in capillaries.

Required Readings

Chapter 4 Water, Electrolytes, and Body Fluids from Basic Concepts in Biomedical Sciences I and pages 55-98.

End of Lecture

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Description

This quiz covers key concepts from Lecture 8 on water, electrolytes, and body fluid in the context of MEDF1011. Explore the roles of osmosis, osmolarity, and homeostatic mechanisms in maintaining fluid balance. Essential for understanding human physiology in medical studies.