Fluid, Electrolyte, and Acid-Base Balance

Overview

Fluid, electrolyte, and acid-base balance are essential for maintaining homeostasis in the human body. These processes regulate the distribution and composition of body fluids, the concentration of ions, and the pH of body fluids, all of which are critical for normal cellular function.

Water Distribution in the Body

Percentage and Composition of Body Fluids

• Water makes up approximately 99% of the fluid volume outside cells.

• Extracellular Fluid (ECF): Includes the interstitial fluid of peripheral tissues and plasma of circulating blood.

• Minor ECF Components:

  • Lymph

  • Cerebrospinal fluid (CSF)

  • Synovial fluid

  • Serous fluids (pleural, pericardial, peritoneal)

  • Aqueous humor

  • Perilymph and endolymph

• Major Ions in ECF: Sodium (Na+), chloride (Cl-), and bicarbonate (HCO3-).

• Intracellular Fluid (ICF): The cytosol inside cells, containing potassium (K+), magnesium (Mg2+), phosphate ions (PO43-), and negatively charged proteins.

• All cellular operations rely on water as a diffusion medium for gases, nutrients, and wastes.

• The digestive system is the primary source of water gain, while the urinary system is the primary route of water loss.

Electrolyte Function and Balance

Definition and Importance of Electrolytes

• Electrolytes are ions released through the dissociation of inorganic compounds. They can conduct electrical current in solution.

• Electrolyte balance is achieved when the gains and losses of each electrolyte are equal.

• Balance is primarily maintained by regulating absorption in the digestive tract and loss via the kidneys.

Acid-Base Balance

Mechanisms and Importance

• Acid-base balance involves the precise regulation of hydrogen ion (H+) production and loss.

• The body generates acids during normal metabolism, which can reduce pH.

• Kidneys secrete H+ into urine and generate buffers that enter the bloodstream.

• Lungs affect pH by eliminating carbon dioxide (CO2).

Hormonal Regulation of Fluid and Electrolyte Balance

Key Hormones

• Antidiuretic Hormone (ADH):

  • Stimulates water conservation at the kidneys, reducing urinary water loss and concentrating urine.

  • Stimulates the hypothalamic thirst center, promoting fluid intake.

• Aldosterone:

  • Secreted by the adrenal cortex in response to rising K+ or falling Na+ levels in blood.

  • Activated by the renin-angiotensin-aldosterone system.

  • Determines the rate of Na+ reabsorption and K+ loss in kidneys.

  • High aldosterone levels cause kidneys to conserve Na+ and stimulate water retention ("water follows salt").

• Natriuretic Peptides (ANP and BNP):

  • Released by cardiac muscle cells in response to abnormal stretching of heart walls.

  • Reduce thirst, block release of ADH and aldosterone, cause diuresis (fluid loss by kidneys), and lower blood pressure and plasma volume.

Fluid Balance

Water Gains and Losses

• About 2500 mL of water per day is required to balance losses.

• Water Gains:

  • Drinking: 1200 mL

  • Eating: 1000 mL

  • Metabolic generation: 300 mL (from oxidative phosphorylation in mitochondria)

• Water Losses:

  • Urine

  • Feces

  • Insensible perspiration

  • Sensible perspiration (sweat), which can be significant during activity (up to 4 L/hr)

  • Fever can increase water losses

Disorders of Fluid Balance

Definitions and Mechanisms

• Edema: Swelling caused by movement of abnormal amounts of water from plasma into interstitial fluid.

• Lymphedema: Edema resulting from blockage of lymphatic drainage.

• Dehydration (Water Depletion):

  • Occurs when water losses exceed gains.

  • If water is lost but electrolytes are retained, ECF osmotic concentration rises, causing water to move from ICF to ECF.

  • Severe water loss may result from excessive perspiration, inadequate water intake, vomiting, or diarrhea.

  • Homeostatic responses include increased ADH and renin secretion and increased fluid intake.

• Hyperhydration (Water Excess):

  • Occurs when water is gained but electrolytes are not.

  • ECF volume increases and becomes hypotonic to ICF, causing fluid to shift into cells (ICF), which can distort cells and disrupt normal functions.

  • Signs include abnormally low Na+ concentrations (hyponatremia) and effects on CNS function (water intoxication).

Major Ions and Their Functions

Key Electrolytes

• Sodium (Na+):

  • Dominant cation in ECF; sodium salts provide 90% of ECF osmotic concentration.

  • Disorders:

    • Hyponatremia: Low Na+ (usually from hyperhydration).

    • Hypernatremia: High Na+ (usually from dehydration).

• Potassium (K+):

  • Dominant cation in ICF; 98% of body potassium is in ICF.

  • Cells expend energy to recover K+ lost to ECF.

  • Disorders:

    • Hypokalemia: Low K+ in blood.

    • Hyperkalemia: High K+ in blood.

• Calcium (Ca2+):

  • Most abundant mineral in the body; 99% is in the skeleton.

  • Functions in muscle and neural activity, blood clotting, as enzyme cofactors, and as second messengers.

  • Regulated by parathyroid hormone (PTH), calcitriol (increase Ca2+), and calcitonin (decreases Ca2+).

  • Disorders:

    • Hypercalcemia: High Ca2+ in ECF, usually from hyperparathyroidism (excess PTH).

    • Hypocalcemia: Low Ca2+ in ECF, usually from hypoparathyroidism, vitamin D deficiency, or chronic renal failure.

Summary Table: Major Ions and Disorders

Acid-Base Disorders and Regulation

Key Terms and Mechanisms

• Acidosis: Physiological state from abnormally low blood pH (acidemia: pH < 7.35).

• Alkalosis: Physiological state from abnormally high blood pH (alkalemia: pH > 7.45).

• CO2 and pH:

  • Most CO2 in solution converts to carbonic acid (H2CO3), which dissociates into H+ and HCO3-.

  • The partial pressure of CO2 (PCO2) is the most important factor affecting blood pH.

• Respiratory Acidosis:

  • Develops when the respiratory system cannot eliminate all CO2 generated by peripheral tissues.

  • Primary sign: Low blood pH due to hypercapnia (high blood PCO2).

  • Primary cause: Hypoventilation (abnormally low respiratory rate).

Key Equations

• CO2 conversion to carbonic acid:

Examples and Applications

• Example (Dehydration): A person with severe diarrhea loses more water than electrolytes, leading to increased ECF osmotic concentration and water movement from ICF to ECF.

• Example (Hyperhydration): Drinking excessive water without electrolyte intake can cause hyponatremia and water intoxication, affecting CNS function.

• Example (Respiratory Acidosis): Chronic obstructive pulmonary disease (COPD) can cause hypoventilation, leading to CO2 retention and decreased blood pH.

Additional info: The above notes expand on the provided outline with definitions, mechanisms, and examples to ensure a comprehensive understanding suitable for exam preparation in Anatomy & Physiology.