CH15 Fluid, Electrolyte, and Acid-Base Balance

Body Fluids

• A typical adult body contains approximately 40 L of body fluids, with 25 L (63%) located inside body cells (intracellular fluid, ICF) and 15 L (37%) located outside of body cells (extracellular fluid, ECF)
• ECF consists of 80% interstitial fluid and 20% blood plasma
• ICF is mostly water and rich in K+, Mg++, HPO42-, SO42-, and protein anions
• ECF contains more Na+, Cl-, HCO3-, and Ca++
• The cell membrane is selectively permeable, maintaining a relatively unchanged distribution of substances in different body fluids

Fluid Balance

• Fluid balance refers to the proper levels of water and electrolytes in the various body compartments according to their needs
• Osmotic pressure and hydrostatic pressure are the main forces behind molecular movement between body compartments

Water Balance

• Water is the most abundant constituent in the body, varying from 45% to 75% of body weight
• Water balance occurs when water intake equals water output
• A normal adult consumes about 2,500 ml of water daily, including 1,500 ml in beverages, 750 ml in food, and 250 ml from cellular respiration and anabolic metabolism

Electrolyte Balance

• Electrolytes are chemical substances that release cations and anions when dissolved in water
• Electrolytes serve four primary functions in the body:
  • As essential minerals (e.g., iodine, calcium)
  • Controlling osmosis between body compartments by establishing proper osmotic pressure (e.g., sodium, chloride)
  • Helping maintain acid-base balance (e.g., hydrogen ion, bicarbonate ion)
  • Carrying electrical current that allows the production of action potentials (e.g., sodium, potassium)
• The most important electrolytes include Na+, K+, Cl-, Ca++, and HPO42-

Regulation of Electrolytes

• Na+ is the most abundant extracellular cation, involved in nerve impulse transmission, muscle contraction, and creation of osmotic pressure
• Cl- is a major extracellular anion, involved in regulating osmotic pressure, forming HCI in the stomach, and involved in the "chloride shift" process in blood
• K+ is the most abundant cation in ICF, involved in maintaining fluid volume, nerve impulse transmission, muscle contraction, and regulating pH
• Ca++ is the most abundant ion in the body, located mainly in ECF, and is a major structural component of bones and teeth
• HPO42- is an important intracellular anion, another major structural component of bones and teeth, and is required for synthesis of nucleic acids and ATP, and for buffering reactions
• Level of electrolytes are mainly regulated by hormones:
  • Aldosterone (from adrenal cortex) causes an increase in sodium reabsorption and potassium secretion at the kidney tubules
  • Parathyroid hormone (PTH) from the parathyroid glands and Calcitonin (CT) from the thyroid gland regulate calcium balance

Acid-Base Balance

• Acids are electrolytes that release hydrogen ions (H+) when dissolved in water
• Bases are electrolytes that release hydroxide ions (OH-) when dissolved in water
• Acid-base balance is primarily regulated by the concentration of H+ (or the pH level) in body fluids, especially ECF
• Normal pH range of ECF is from 7.35 to 7.45
• Most H+ comes from metabolism, including glycolysis, oxidation of fatty acids and amino acids, and hydrolysis of proteins
• Homeostasis of pH in body fluids is regulated by acid-base buffer systems, respiratory centers in the brain stem, and by kidney tubule secretion of H+

Acid-Base Buffer Systems

• Chemical acid-base buffer systems, such as the bicarbonate buffer system, help regulate pH levels
• The bicarbonate buffer system is important because two of its components are rigorously controlled by the body: the lungs control CO2 and the kidney control HCO3-

Sources of H+

• Aerobic respiration of glucose produces CO2, which reacts with water to form carbonic acid
• Anaerobic respiration of glucose produces lactic acid
• Incomplete oxidation of fatty acids releases acidic ketone bodies
• Oxidation of sulfur-containing amino acids produces sulfuric acid
• Hydrolysis of phosphoproteins and nucleic acids gives rise to phosphoric acid