Fluid Balance and Dehydration Quiz

Body Water Content

• Total body water varies based on age, body mass, and body fat
• Infants have ~73% or more body water (low body fat).
• Body water content declines to ~45% in old age.
• Adult males generally have ~60% body water (higher muscle mass).
• Adult females generally have ~50% body water (higher fat content, less skeletal muscle mass).
• Skeletal muscle tissue is ~75% water.
• Adipose tissue is less than 20% water (lowest of all tissues).

Fluid Compartments

• Total body fluid in adults averages ~40 liters.
• Two major fluid compartments:
• Intracellular fluid (ICF): fluid inside cells, ~25 liters (~2/3 of total body fluid).
• Extracellular fluid (ECF): fluid outside cells, ~15 liters (~1/3 of total body fluid).
• Two major ECF compartments:
• Plasma: fluid part of blood (3 liters).
• Interstitial fluid (IF): fills spaces between cells (12 liters).

Composition of Body Fluids

• Water is the universal solvent for electrolytes and nonelectrolytes.
• Water moves from areas of lower osmolality to higher osmolality.
• Electrolytes are molecules that dissociate into ions in water (conduct an electrical current).
• Examples include inorganic salts, acids, bases, and some proteins.
• Electrolytes have greater osmotic power than nonelectrolytes.
• Nonelectrolytes are molecules that do not dissociate into ions (e.g., glucose, lipids, creatinine, urea).
• Solute concentration is expressed in milliequivalents per liter (mEq/L), a measure of electrical charges.
• 1 mEq of single-charged ions (like Na+) equals 1 mOsm.
• 1 mEq of bivalent ions (like Ca²+) equals 0.5 mOsm.

Composition of Extracellular and Intracellular Fluids

• Extracellular fluid (ECF) contains higher Na⁺ concentration than intracellular fluid (ICF).
• ECF major anion: Cl⁻.
• ICF (low in Na⁺ and Cl⁻): contains 3x more soluble proteins than plasma.
• ICF major cation: K⁺ .
• ICF major anion: HPO₄²⁻
• Proteins, phospholipids, cholesterol, and triglycerides make up a large portion of the mass of dissolved solutes.
• 90% of mass in plasma, 60% in interstitial fluid (IF), and 97% in intracellular fluid (ICF).

Fluid Movement Among Compartments

• Fluids continually exchange and mix between compartments via osmotic and hydrostatic pressures.
• Water moves freely between compartments along osmotic gradients.
• Many solutes cannot move freely due to size or charge.
• Any difference in solute concentration leads to net water movement.
• Exchanges occur between plasma and IF across capillary walls.
• Fluid leaks from the arteriolar end of capillaries, most is reabsorbed at the venule end.
• Exchanges between IF and ICF occur across plasma membranes.
• Large intake of salt leads to water shifting out of cells and into the ECF.

Exchange of Gases, Nutrients, Water and Wastes

• A continuous exchange of gases, nutrient, water, and wastes occurs between the three fluid compartments.

Regulation of Water Intake and Output

• Water intake equals water output (~2500 ml/day).
• Water gains primarily come from ingested foods and beverages.
• A small amount of water is produced via cellular metabolism.
• Insensible water loss occurs across skin and airways.
• Sensible water loss occurs via urine (~60%), sweat, and feces
• Osmolality should remain around 280-300 mOsm.
• Increased osmolality stimulates thirst and ADH release.
• Decreased osmolality inhibits thirst and ADH release.

Regulation of Water Intake

• Hypothalamic thirst centers control thirst, stimulated by:
• Osmoreceptors (activated by elevated plasma osmolality).
• Dry mouth.
• Decreased blood volume/pressure.

The Thirst Mechanism

• The thirst center (in the hypothalamus) is triggered by ECF osmolality, resulting in increased water intake.

Regulation of Water Output

• Obligatory water losses:
• Insensible water loss across skin and airways.
• Sensible water loss via urine (~500 ml/day), sweat, and feces.
• Volume and solute concentration of urine depend on intake, diet, and variable water loss via sweat (substantial in heat) and feces (substantial with diarrhea).
• Kidneys begin to eliminate excess water within 30 minutes of ingestion (peaks at ~1 hour).
• ADH release takes time to be inhibited.

Influence of Antidiuretic Hormone (ADH)

• Water reabsorption in collecting ducts is proportional to ADH release.
• Fall in ADH decreases body fluid volume.
• Rise in ADH concentrates urine.
• ADH release is stimulated by:
• Rise in ECF osmolality.
• Drop in blood volume/pressure.

Disorders of Water Balance

• Dehydration: ECF fluid loss due to hemorrhage, severe burns, prolonged vomiting, diarrhea, or excessive sweating.
• Symptoms include thirst, dry skin, reduced urine output (oliguria).
• Hypotonic hydration: excess water ingestion or renal insufficiency leads to a decrease in ECF osmolality.
• Symptoms include nausea, vomiting, muscular cramping, and cerebral edema.
• Edema: accumulation of interstitial fluid; causes tissue swelling.
• Causes include conditions that increase fluid outflow of blood or reduce fluid return to blood.

Regulation of Sodium Balance

• Na⁺ is important in maintaining water balance and blood pressure.
• Aldosterone plays a major role in regulating Na⁺ content.
• High levels of aldosterone lead to active reabsorption of Na⁺ in the distal convoluted tubule (DCT) and collecting duct (CD).
• Water follows Na⁺, leading to increased ECF volume
• Low levels of aldosterone lead to excretion of Na⁺ in urine.
• Water follows Na⁺, leading to decreased ECF volume
• Various factors influence aldosterone release, including renin-angiotensin-aldosterone system (RAAS), changes in blood pressure, and K⁺ concentration.
• ANP, secreted by atria of heart, inhibits aldosterone, renin, and ADH; this reduces Na⁺ retention and increases excretion.
• Other hormones like estrogens (increase Na⁺ retention), progesterone (mild diuresis), and glucocorticoids (increase Na⁺ retention) also influence Na⁺ balance.

Regulation of Potassium Balance

• K⁺ is the major intracellular cation regulating cellular activity and resting membrane potential (RMP).
• High ECF K⁺ causes depolarization which lowers excitability.
• Hypokalemia (low ECF K⁺) cause hyperpolarization, reducing excitability.
• Most important factor affecting K⁺ secretion is its ECF concentration.
• High K⁺ intake increases K⁺ secretion and vice versa.
• Aldosterone directly stimulates K⁺ secretion by principal cells; so, rising K⁺ concentrations trigger aldosterone release, increasing K⁺ secretion and promoting Na⁺ reabsorption.

Regulation of Calcium and Phosphate Balance

• 99% of calcium is in bones as calcium phosphate salts.
• ECF concentration of calcium is primarily regulated by parathyroid hormone (PTH).
• PTH increases calcium levels by targeting bone (increasing resorption), kidneys (increasing calcium reabsorption, and decreasing phosphate reabsorption), and small intestine (increasing calcium absorption).

Clinical-Homeostatic Imbalances

• Severe electrolyte deficiencies can lead to cravings for salty foods (Addison's disease)
• Pica: abnormal cravings for non-nutritive substances (caused by mineral deficiencies).
• Sodium imbalances impact thirst and ECF volume.
• Potassium and calcium imbalances alter neuromuscular excitability.

Regulation of Anions

• Chloride (Cl⁻) is the most abundant ECF anion; it helps maintain normal osmotic pressure of blood.
• When acidosis occurs, less Cl⁻ is reabsorbed in lieu of bicarbonate.
• Most other anions have transport maximums and are excreted in urine when excess occurs.

Chemical Buffer Systems

• Chemical buffers act as the first line of defense against pH changes.
• The three major buffer systems include bicarbonate, phosphate, and proteins buffers.
• Bicarbonate buffer system is the most important ECF buffer and the only one in blood.
• Strong acid is added → HCO3⁻ ties up H⁺, forming weak acid H2CO3
• Weak acid added → weakly reducing the pH decreases (or alkaline reserve) is used up
• Strong base is added → H2CO3 dissociates and donates H⁺.
• Phosphate buffer system is most significant intracellular buffer; important in urine
• Strong acid added → HPO₄²⁻ ties up H⁺, forming dihydrogen phosphate.
• Strong base added → dihydrogen phosphate forms monohydrogen phosphate.
• Proteins act as both weak acids and weak bases; they tie up H⁺ and help buffer both sides of the pH range.

Respiratory Regulation of H⁺ concentration

• Respiratory system eliminates CO2, lowering the concentration of carbonic acid (a weak acid) and H⁺.
• A rise in arterial pCO₂ stimulates more ventilation eliminating more CO₂ lowering H⁺.
• A fall in arterial pCO₂ inhibits ventilation, accumulating CO₂ which increases H⁺ production.

Renal Regulation of Acid-Base Balance

• Kidneys regulate the acid-base balance by adjusting plasma bicarbonate (HCO3⁻) levels.
• They do so by adjusting the excretion of acids and incorporating HCO3⁻ into the blood supply, thereby replenishing the alkaline reserve.

Developmental Aspects of Fluid, Electrolyte, and Acid-Base Balance

• Infants have higher ECF volume and metabolic rates relative to adults, making them more vulnerable to fluid, electrolyte, and acid-base imbalance.
• Inefficient kidneys make newborns at risk for dehydration.