BIOL-244 Human Anatomy & Physiology: Chapter 26 Body Fluids & Electrolytes

Water Balance and Intake

• Water intake must equal water output, approximately 2500 ml/day
• Water intake comes from ingestion and metabolism (Krebs cycle and electron transport chain)
• Water output includes urine (60%), insensible water loss (through skin and lungs), perspiration (sensible), and feces

Disorders of Water Balance

• Dehydration: ECF water loss due to various causes (hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, diuretic abuse, endocrine disturbances)
• Signs and symptoms of dehydration: "cottony" oral mucosa, thirst, dry flushed skin, oliguria (reduced or low urine output)
• Oliguria may lead to weight loss, fever, mental confusion, hypovolemic shock, and loss of electrolytes

Edema

• Atypical accumulation of interstitial fluid, resulting in tissue swelling (not cell swelling)
• May impair tissue function by increasing distance for diffusion of oxygen and nutrients from blood into cells
• Can be caused by increased fluid flow out of blood (hydrostatic) or decreased return of fluid (osmotic) to blood

Electrolyte Balance

• Electrolyte balance usually refers to salt balance
• Salts control fluid movements, provide minerals for excitability, secretory activity, and other functions
• Salts enter the body through ingestion and metabolism
• Salts are lost via perspiration, feces, urine, and vomit

Regulation of Sodium Balance

• Hyponatremia: Decreased sodium decreases ECF osmolarity
• Causes of hyponatremia: Renal failure, Vomiting & Diarrhea, Diuretic use, Excessive fluids, Low Aldosterone
• Hyponatremia can lead to cellular edema (especially in the brain)

Body Water Content

• Infants: 73% or more water (low body fat, low bone mass)
• Adult males: ~60% water
• Adult females: ~50% water (higher fat content, less skeletal muscle mass)
• Adipose tissue is the least hydrated of all
• Water content declines to ~45% in old age

Body Fluid Compartments

• Two main fluid compartments: Intracellular fluid (ICF) and Extracellular fluid (ECF)
• ICF: fluid inside cells (2/3 of total body fluid)
• ECF: fluid outside cells (1/3 of total body fluid)
• Plasma (Intravascular), Interstitial fluid (IF), Lymph, CSF, Eye humor (fluids), synovial fluid, serous fluid, and gastrointestinal secretions

Composition of Body Fluids

• Water is the universal solvent
• Solutes are classified as non-electrolytes and electrolytes
• Non-electrolytes: most are organic molecules that do not dissociate in water (e.g., glucose, proteins, amino acids, lipids, creatinine, and urea)
• Electrolytes: dissociate into ions in water, have greater osmotic power than non-electrolytes (e.g., inorganic salts, all acids and bases, some proteins)

Electrolytes and Osmosis

• Electrolytes conduct electrical current
• Water moves to the area with greater solute (electrolyte) concentration
• ECF vs ICF: each fluid compartment has a distinctive pattern of electrolytes
• ECF: electrolyte contents mostly similar, major cation: sodium (Na+), major anion: chloride (Cl-)
• ICF: contains more soluble proteins than plasma, major cation: potassium (K+), major anion: Hydrogen Phosphate (HPO42-)

Regulation of Sodium Balance

• Na+-water balance is linked to blood pressure and blood volume control mechanisms
• Changes in blood pressure or volume trigger neural and hormonal controls to regulate Na+ content
• Aldosterone plays the biggest role in regulating Na+ by the kidneys, causing Na reabsorption

Hypokalemia and Hyperkalemia

• Hypokalemia: Low potassium levels, affects cardiovascular/neuromuscular resting membrane potentials
• Causes of hypokalemia: fluid loss due to vomiting, diarrhea, prolonged diuretic use (most common)
• Hyperkalemia: Excessively high potassium levels, renal failure, tissue destruction (burns, trauma, & flesh-eating bacteria)

Calcium and Phosphate Regulation

• Calcium balance is controlled by parathyroid hormone (PTH) and rarely deviates from normal limits
• PTH promotes an increase in blood calcium levels by targeting the small intestine, kidneys, and bones
• Calcium is important for blood clotting, cell membrane permeability, and neuromuscular excitability

Acid-Base Balance

• pH affects all functional proteins and biochemical reactions, so it is closely regulated by the body
• Normal pH of body fluids: 7.35-7.45
• Alkalosis: arterial pH >7.45, acidosis: arterial pH <7.35
• Respiratory acidosis: due to decreased ventilation or gas exchange, CO2 accumulates in blood, and blood pH drops
• Respiratory alkalosis: due to hyperventilation, often resulting from stress or pain, CO2 is eliminated faster than produced, and blood pH increases

Metabolic Acidosis and Alkalosis

• Most important indicator of metabolic function is HCO3-
• Normal HCO3-: 22-26 mmol/L
• HCO3- < 22 mmol/L: Metabolic acidosis, causes: 3-ketoacidosis from diabetes or renal failure, excessive alcohol consumption, lactic acid accumulation, diarrhea, and HCO3- loss
• HCO3- > 26 mmol/L: Metabolic alkalosis, less common, causes: excessive vomiting, excessive antacid use

Effects of Acidosis and Alkalosis

• Blood pH below 6.8 causes CNS depression, may lead to coma and death
• Blood pH above 7.8 causes overexcitation of the nervous system, leading to muscle tetany, extreme nervousness, convulsions, and death, often from respiratory arrest

pH Compensation Mechanisms

• If acid-base imbalance is due to malfunction of one physiological buffer system, the other system tries to compensate
• Respiratory system attempts to correct metabolic acid-base imbalances by increasing or decreasing respiration
• Kidneys attempt to correct respiratory acid-base imbalances by increasing or decreasing bicarbonate reabsorption/acid secretion