Physiology Lecture 8: Water and Electrolytes

Lecture 8: Water, Electrolytes, and Body Fluid

• The lecture covers water, electrolytes, and body fluid, a crucial aspect of human physiology.
• The lecturer is Dr. Isabel Hwang, a Senior Lecturer and Year 1 Coordinator in Medicine at the Faculty of Medicine of CUHK.
• The lecture outlines include defining total body water, distribution in fluid compartments, osmosis and osmotic pressure, definitions and applications of osmolarity/osmolality, examples of effective and ineffective osmoles in plasma, homeostatic regulation of water balance, thirst reflex mechanism, ADH mechanism, and fluid movement across capillary walls.
• Formation of edema and its common causes are also discussed.
• Pre-class assignment on Blackboard is included. Students will answer "What is the role of electrolytes in our bodily fluids?" and provide examples of major electrolytes in bodily fluids.
• Regulation of body fluid and composition is fundamental to physiology.
• Cells require defined composition for normal function.
• Intracellular fluid (ICF) composition remains relatively constant.
• External environment can affect inner environment.
• Blood volume impacts venous return and mean arterial blood pressure.
• Fluid disturbances happen in disease states.
• A fundamental understanding of body fluids is helpful for patient management and drug dosage.
• Water balance is vital; cells, especially brain cells, are sensitive to altered water status.
• Water balance disturbance occurs in two forms: dehydration and overhydration.
• Dehydration is due to insufficient intake or excessive fluid loss: affects elderly and all ages.
• Overhydration is linked to excessive water intake. Athletes might drink too much to prevent dehydration.
• Dehydration symptoms: dry mouth, thirst, headache, fatigue, dizziness, and confusion.
• Overhydration symptoms: nausea, vomiting, headache, muscle weakness, confusion, disorientation, seizures, and coma.

Water is kept in different body fluid compartments.

• Body fluid is divided into compartments: extracellular and intracellular.
• Compartments are separated by boundaries like blood vessel walls and cell membranes.
• Cell membranes are semi-permeable (selectively permeable).
• Capillary walls are more diffusible.
• Transport occurs passively and actively between compartments.

Total body water (TBW)

• TBW is the total water volume in a person's body, expressed as a percentage of total body weight.
• In adults, TBW accounts for approximately 60% of total body weight.
• Water content in muscle cells, bone cells, and fat cells differ.
• Water content lowers to ~45% in old age and dehydration.
• Body water content is a significant determinant for drug dosage. Women and older people have less body water, leading to higher drug concentration in their plasma.

Osmolarity

• Osmotic pressure reflects total solute concentration, rather than the substances' identities.
• It represents the ability of solutes to "pull" water towards areas with higher solute concentration.
• Higher solute concentration leads to higher osmotic pressure.
• Total body water (TBW) is influenced by both intracellular and extracellular compartments. For a 70kg healthy adult, TBW is 42L.

Plasma Volume vs. Blood Volume

• Hematocrit (Hct) measures the volume of red blood cells relative to total blood volume. Men tend to have higher percentages.
• Plasma volume is not the same as blood volume.

Definition of Electrolytes

• All body fluids have no net charge. Cations and anions balance each other.
• Solutes are divided into electrolytes and non-electrolytes.
• Electrolyte solutions dissociate (break into smaller particles) into ions (e.g., Na⁺, K⁺, Cl⁻, and others). The solute exerts a stronger osmotic effect.
• Non-electrolyte solutes are non-dissociating (do not break into ions in solution) such as glucose, and urea.

Distribution of fluid in ECF and ICF

• Fluid and electrolyte movement between compartments is determined by various driving forces.
• Movement across cell membranes is driven by osmotic pressure.
• Movement across capillary walls is driven by both hydrostatic and colloid osmotic pressures (oncotic pressure).

Osmosis

• Osmosis is the passive diffusion of water molecules down a concentration gradient across a semi-permeable membrane. Water moves from an area of high water concentration to one of low concentration.
• Osmolarity reflects total solute concentration.
• Semi-permeable membrane distinguishes between solutes which can cross the membrane and those which cannot.
• Osmotic pressure is a driving force that pushes water to enter the compartment with higher solute concentration.
• Hydrostatic pressure is the pressure a liquid exerts that balances the osmotic pressure.

Effective and Ineffective Osmoles

• Non-penetrating solutes (effective osmoles) cannot readily cross the plasma membrane. Examples include sodium chloride (NaCl), and others.
• Penetrating solutes (ineffective osmoles) can cross the plasma membrane. Examples of ineffective osmoles include urea and glucose.

Osmolarity

• Osmolarity reflects the total solute concentration (number of particles) per liter of solution.
• Physiological osmolarity is usually expressed as mOsm/L.
• Osmolality represents total solute concentration (number of particles) per kilogram of solvent, and it is temperature independent.

Osmoreceptors, Thirst Reflex, and ADH

• Osmoreceptors in the hypothalamus detect changes in ECF osmolarity.
• An increase in plasma osmolarity (1-2%) and/or decreased blood volume or MAP trigger the thirst reflex, leading to increased fluid intake.
• The hypothalamus activates ADH secretion by the posterior pituitary gland, promoting water reabsorption in the kidney.

Movement of Fluid Across Compartments

• Fluid movement between plasma and interstitial fluid depends on the balance between hydrostatic and oncotic pressures (oncotic pressure is also called plasma colloid osmotic pressure).
• At the arterial end of a capillary, hydrostatic pressure dominates, causing fluid filtration (movement out of capillaries).
• At the venous end, oncotic pressure dominates, causing absorption (fluid movement into capillaries).
• The lymphatic system returns excess filtered fluid to the circulatory system.

Edema

• Edema is the accumulation of excess fluid in the interstitial space.
• Edema occurs when the balance between hydrostatic and oncotic pressures is disrupted.
• A decrease in oncotic pressure (e.g., malnutrition, loss of plasma proteins) or an increase in hydrostatic pressure (e.g., hypertension) can lead to edema formation.

Application of concepts and learning outcomes.

• Students are expected to apply the acquired knowledge to scenarios related to electrolyte and solute/osmolarity balance. These concepts are fundamental learning outcomes.

Required Reading

• A biomedical science textbook, Chapter 4, covers water, electrolytes and body fluids. Specific page numbers (55-98) are noted.