Ch 26, L1

Body Fluids and Compartments

• Body fluids consist of water and solutes located in three main compartments.
• Total body water (TBW) depends on age, body mass, sex, and the amount of adipose tissue.
• TBW decreases with age.
• As muscle mass decreases, so does water content.
• Adipose tissue is the least hydrated, containing less than 20% water.
• Skeletal muscle has more body water due to greater muscle mass.
• Males typically have 60% water content, while females have 50%.
• Infants have 73% or more water due to low body fat and bone mass.
• Older adults have approximately 45% TBW.
• Females have a higher percentage of body fat compared to males, which affects water content.

Intracellular and Extracellular Fluid

• Water occupies two main fluid compartments: the intracellular fluid (ICF) and the extracellular fluid (ECF).
• The intracellular fluid (ICF) compartment contains trillions of cells.
• The extracellular fluid (ECF) constitutes the body's "internal environment."
• ECF is regarded as the external environment of each cell.
• The ECF has two sub-compartments: plasma and interstitial fluid (IF).
• Plasma constitutes the fluid part of the blood.
• Interstitial fluid (IF) occupies the microscopic gaps between tissue cells.
• Lymph, cerebrospinal fluid, humors of the eye, synovial fluid, serous fluid, and gastrointestinal secretions are typically similar to IF.

Fluid Composition and Solutes

• Water serves as the universal solvent for various solutes within body fluids.
• Solutes can be classified as electrolytes or non-electrolytes.
• Electrolytes are chemical compounds that dissociate into ions when dissolved in water.
• Electrolytes include inorganic salts, inorganic and organic acids and bases, and certain proteins.
• Non-electrolytes have bonds (usually covalent) that prevent them from dissociating in solution.
• Non-electrolytes do not create electrically charged species when dissolved; organic molecules, such as glucose, lipids, creatinine, and urea, are examples.
• All dissolved solutes contribute to the osmotic activity of a fluid.
• Electrolytes have greater osmotic power compared to non-electrolytes because they dissociate into at least two ions per molecule.
• Water moves from areas of lower osmolality to areas of greater osmolality, this is known as osmosis.
• Electrolytes have the greatest ability to cause fluid shifts due to osmosis.

Electrolytes and Fluid Movement

• Electrolytes are the most abundant solutes in body fluids, determining their chemical and physical reactions.
• This includes, sodium, potassium, calcium, magnesium, bicarbonate, chloride, phosphate, and sulfate.
• Proteins, triglycerides, phospholipids, and cholesterol are also dissolved.
• Sodium is the major cation in extracellular fluid and chloride is the major anion.
• Potassium is the major cation in intracellular fluid with phosphate as the major anion.
• Osmotic and hydrostatic pressures regulate the exchange and mixing of body fluids.
• Water moves freely along osmotic gradients, but solutes distribute based on size, electrical charge, or transport proteins.
• Changes in the solute concentration in any compartment lead to net water flows.

Fluid Exchanges and Volume

• Exchanges between plasma and interstitial fluid (IF) occur across capillary walls.
• Hydrostatic pressure forces protein-free plasma out of the blood vessel into the interstitial space.
• Osmotic pressure of plasma proteins reabsorbs the filtered fluid back into the bloodstream.
• Lymphatic vessels collect any remaining leakage in the interstitial space and return it to the blood.
• Exchanges between IF and ICF occur across plasma membranes, based on permeability.
• Two-way osmotic flow of water is substantial.
• Ions move selectively through active transport or channels.
• Nutrients, respiratory gases, and wastes move unidirectionally.
• Glucose and oxygen move into cells, while metabolic wastes move out.
• The osmolalities of all body fluids are equal, except during the first few minutes after a change occurs in one of the fluids due to the fact water moves freely between compartments.
• Increasing ECF solute content (mainly sodium chloride) causes osmotic shifts in the ICF with water moving out of the cells.
• Decreasing ECF osmolality causes water to move into the cells.
• Extracellular fluid solute concentration determines intracellular fluid volume.