Body Fluids and Fluid Balance Regulation

Body Fluids and Fluid Balance Regulation

• Body fluids comprise approximately 60% (50-70%) of total body weight (TBW).
• Body fluids are water plus dissolved solutes.
• Solutes include electrolytes (e.g., Na+, K+, Cl-, HCO3-, Mg2+, Ca2+) and non-electrolytes (e.g., glucose, urea, creatinine).
• Total body water is approximately 60% of TBW.

Objectives

• Identify the body's main fluid compartments.
• Explain methods for measuring body fluid compartments.
• Describe functions of water.
• Describe the regulation of fluid and electrolyte balance.
• Describe types of intravenous fluid therapy.

Body Fluid Content

• Body fluids are divided into intracellular fluid (ICF) and extracellular fluid (ECF).

Percentage of H2O in Tissues

• Newborn infants have 80% water content.
• Adult males have 60% water content.
• Adult females have 50% water content.
• Elderly population has 45% water content.

Body Fluid Compartments

• A 70kg person has 42 litres of total water.
• Intracellular fluid (ICF) comprises approximately 28 litres.
• Extracellular fluid (ECF) comprises approximately 14 litres.
• ECF is further divided into interstitial fluid (ISF) and plasma.
• Interstitial fluid (ISF) is approximately 10.5 litres (15%).
• Plasma water is approximately 3.5 litres (5%).
• Fluid compartments include intracellular, extracellular, plasma, and interstitial compartments, as well as transcellular fluid.

Measurement of Body Fluid Volumes

• A known amount of dye or indicator is injected into the body.
• The dye concentration in the compartment is measured.
• The compartment volume is calculated using the formula: Volume of fluid = Amount of dye injected / Concentration of dye
• Indicators must mix evenly in the fluid compartment.
• Indicators must be non-toxic and not alter the body's physiological activity.
• Indicators must not be metabolized or excreted.
• Indicators must be relatively easy to measure.

Measurement of Total Body Water

• Total body water can be measured using deuterium oxide (D₂O), tritium oxide, and aminopyrine.
• To measure Total body water inject deuterium oxide, 4 mg.
• When Total Body Water is estimated, the serum/plasma concentration is 0.01 mg/dL.

Measurement of ECF and ICF Volumes

• ECF volume can be measured using inulin, mannitol, or sucrose.
• ICF volume is calculated by subtracting the ECF volume from the total body water.

Measurement of Plasma Volume

• Plasma volume can be measured using Evans blue dye or radioactive iodine bound to plasma proteins.

Measurement of ISF Volume

• ISF volume is calculated by subtracting plasma volume from the ECF volume.

Functions of Body Water

• Regulates body temperature.
• Lubricates joints.
• Prevents constipation.
• Dissolves minerals and other nutrients.
• Carries nutrients and oxygen to cells.
• Moistens tissues.
• Protects organs and tissues.
• Lessens the burden on kidneys and liver by flushing out waste products.

Water Balance

• Water intake is 2400ml/day.
• Exogenous water intake is 2200 ml/day.
• Endogenous water intake amounts to 200ml/day.
• Water output is approximately 2400 ml/day.
• This includes insensible water loss (approximately 700 ml), sweat (approximately 100 ml) and feces (approximately 100 ml), and urine (approximately 1500 ml).

Control of Water Input

• Reduced blood volume (hypovolemia) stimulates the thirst center.
• Increased plasma osmolarity (hypertonicity) stimulates the thirst center.
• Increased water intake leads to reduced plasma osmolarity and returning to normal blood volume.

Control of Water Output

• Reduced blood volume (hypovolemia) and increased plasma osmolarity (hypertonicity) stimulate the posterior pituitary to release ADH.
• ADH increases water reabsorption by the kidneys, resulting in reduced urine volume.
• Reduced plasma osmolarity and returning to normal blood volume.

Regulation of Fluid Balance

• Water deficit (low water intake) stimulates the hypothalamic thirst center and increases ADH to stimulate water reabsorption by kidneys.
• Water excess (high water intake) inhibits ADH secretion and increases urine output.

Edema

• Edema is the presence of abnormally large amounts of fluid in the extracellular tissue spaces.
• Causes of intracellular edema include hyponatremia, depressed metabolism, lack of nutrients to cells and inflammation.
• Causes of extracellular edema include heart failure and kidney disease.

Electrolytes

• Electrolytes are electrically charged ions in body fluids.
• Cations are positively charged ions (e.g. Na+, K+, Ca2+).
• Anions are negatively charged ions (e.g. Cl-, HCO3-, PO43-).
• Chief extracellular cation is Na⁺; and intracellular is K⁺; principal extracellular anion is Cl⁻; principal intracellular anion is PO4³⁻.

This quiz focuses on understanding body fluids, their composition, and the regulation of fluid balance in the human body. You'll learn about fluid compartments, measurement methods, and the role of water and electrolytes. It is essential for mastering the principles of human physiology related to hydration and therapies.