Water and Electrolyte Balance Quiz

Water, Electrolyte, & Acid-Base Balance

• Body Water Content:
  • Infants have 73% or more water (low body fat, low bone mass)
  • Adult males have ~60% water
  • Adult females have ~50% water (higher fat content, less skeletal muscle mass)
  • Adipose tissue is the least hydrated tissue
  • Total body water in adults averages ~40 liters
  • Water content declines to ~45% in old age

Body Fluid Compartments

• Total body water: 40 liters, 60% of body weight
• Intracellular fluid (ICF): 25 liters, 40% of body weight
• Interstitial fluid (IF): 12 liters, 80% of extracellular fluid (ECF)
• Plasma: 3 liters, 20% of ECF
• Extracellular fluid (ECF): 15 liters, 20% of body weight

Composition of Body Fluids

• Electrolytes:
  • Dissociate into ions in water
  • Examples: inorganic salts, all acids and bases, some proteins
  • Conduct electrical current
  • Greater osmotic power than nonelectrolytes
  • Greater ability to cause fluid shifts
  • NaCl → Na+ + Cl− (electrolyte; 2 particles)
  • MgCl2 → Mg2+ + 2Cl− (electrolyte; 3 particles)
  • Glucose → glucose (nonelectrolyte; 1 particle)

Electrolyte Composition of Body Fluids

• Sodium (Na+) is super abundant in ECF
• Potassium (K+) abundance is super in ICF
• Other electrolytes (calcium, magnesium, chloride, bicarbonate, phosphate, sulfate) present in both ECF and ICF, but with different concentrations.

Exchange of Gases, Nutrients, Water, and Wastes

• Exchange occurs between lungs, gastrointestinal tract, and kidneys, as well as blood plasma, interstitial fluid, and intracellular fluid.

Water Balance and ECF Osmolality

• Water intake must equal water output (~2500 ml/day)
• Water intake comes from ingested foods and beverages, and a small amount from metabolism (metabolic water)
• Water output includes urine (60%), insensible water loss (skin and lungs), perspiration, and feces

Water Balance and ECF Osmolality Regulation

• Osmolality is maintained around 280-300 mOsm
• Rise in osmolality:
  • Stimulates thirst
  • Causes ADH release
• Decrease in osmolality:
  • Causes thirst inhibition
  • Causes ADH inhibition

Major Sources of Water Intake and Output

• Average daily intake: 2500 ml
• Average daily output: 2500 ml
• Intake sources: beverages (60%), foods (30%), metabolism (10%)
• Output sources: urine (60%), insensible loss (skin, lungs) (28%), sweat (8%), feces (4%)

Regulation of Water Intake

• Thirst is the driving force for water intake
• Governed by the hypothalamic thirst center
• Hypothalamic osmoreceptors detect ECF osmolality and are activated by:
  • Increased plasma osmolality of 1-2%
  • Dry mouth
  • Decreased blood volume or pressure
  • Angiotensin II or baroreceptor input

Regulation of Water Intake: Mechanisms and Consequences of ADH Release

• Osmoreceptors detect changes in ECF osmolality and trigger ADH release.
• Increased ADH increases water reabsorption in the kidneys, resulting in concentrated urine and increased blood volume.
• Decreased blood ADH release results in dilute urine, decreased blood volume

Water Balance Disorders

• Dehydration: ECF water loss due to hemorrhage, burns, prolonged vomiting/diarrhea, sweating, water deprivation, or endocrine disturbances; symptoms include “cottony” oral mucosa, thirst, dry/flushed skin, and oliguria; can lead to noticeable weight loss, fever, mental confusion, hypovolemic shock, and loss of electrolytes
• Hypotonic Hydration: Cellular overhydration (water intoxication); occurs with renal insufficiency or rapid water ingestion; symptoms include severe metabolic disturbances, nausea, vomiting, muscular cramping, cerebral edema, and possible death; treated with hypertonic saline
• Edema: Atypical accumulation of interstitial fluid, not cell swelling; impairs tissue function; caused by increased fluid flow out of blood or decreased return of fluid to blood

Central Role of Sodium in Fluid and Electrolyte Balance

• Sodium (Na+) is the most abundant cation in extracellular fluid (ECF)
• Controls extracellular fluid (ECF) volume and water distribution due to water following salt
• Changes in Na+ levels affect plasma volume, blood pressure, and intracellular and extracellular fluid volumes
• There are no known receptors monitoring Na+ levels directly in body fluids, so Na+ and water balance is linked to pressure and volume control mechanisms.
• Changes in blood pressure or volume trigger neural and hormonal controls to regulate Na+ content.

Regulation of Sodium Balance

• Influence of aldosterone and angiotensin II:
  • Renin (from kidneys) catalyzes angiotensin II production.
  • Angiotensin II prompts aldosterone release from adrenal cortex.
  • Increased aldosterone results in increased Na+ reabsorption and thus water retention by the kidneys.
  • Aldosterone release is also triggered by elevated potassium (K+) levels.
• Influence of atrial natriuretic peptide (ANP):
  • Released by atrial cells in response to stretch caused by increased blood pressure.
  • Decreases blood pressure and blood volume.
  • Inhibits ADH, renin, and aldosterone production.
  • Increases excretion of Na+ and water.
• Influence of other hormones, such as female sex hormones (estrogen and progesterone) and glucocorticoids. Estrogen—increases Na+ reabsorption, leads to water retention; progesterone—decreases Na+ reabsorption, blocks aldosterone; glucocorticoids increase Na+ reabsorption.

Potassium Balance Regulation

• Potassium (K+) in ECF affects resting membrane potential (RMP).
  • Elevated ECF K+: decreased RMP, and depolarization—reduced excitability
  • Decreased ECF K+: hyperpolarization and nonresponsiveness
• Disruption of K+ (hyperkalemia or hypokalemia) in the heart can interfere with electrical conduction.
• K+ balance is controlled by regulating the amount secreted into the filtrate in cortical collecting ducts.

Regulation of Anions

• Chloride (Cl−) is the major anion accompanying Na+ in ECF.
• Helps maintain osmotic pressure and blood pH.
• 99% of Cl− is reabsorbed passively in the PCT.
• Follows Na+ transport; related to active transport of Na+.
• When acidosis occurs, fewer chloride ions are reabsorbed.

Acid-Base Balance

• pH affects all functional proteins & biochemical reactions.
• Normal pH of body fluids:
  • Arterial blood: 7.4
  • Venous blood/interstitial fluid: 7.35
  • Intracellular fluid: 7.0
• Alkalosis: arterial pH >7.45
• Acidosis: arterial pH <7.35

Acid-Base Balance: Regulation

• Regulation involves three mechanisms:
  • Chemical buffer systems (fast, first line)
    • Bicarbonate buffer system: HCl + NaHCO3 → H2CO3 + NaCl
    • Phosphate buffer system: HCl + Na2HPO4 → NaH2PO4 + NaCl
    • Protein buffer system: Intracellular proteins are major buffers; plasma proteins also important.
  • Brain-stem respiratory centers (acts within 1-3 minutes)
  • Renal mechanisms (hours to days to effect change)

Respiratory Regulation of H+

• Alkalosis depresses the respiratory center, causing a decreased respiratory rate and depth, thus increasing H+ concentration.
• Respiratory impairments cause acid-base imbalances (hypoventilation causes respiratory acidosis, and hyperventilation causes respiratory alkalosis).

Conserving Filtered Bicarbonate Ions: Bicarbonate Reabsorption

• Kidneys replenish (reabsorb) bicarbonate to maintain the alkaline reserve.
• Tubule cells are impermeable to bicarbonate, but permeable to CO2.
• Bicarbonate gains entry in a roundabout way, converting to CO2 and back into bicarbonate.
• The mechanism is coupled to H+ secretion.

Generating New Bicarbonate Ions

• Metabolism generates H+, which is balanced by new bicarbonate.
• Secreted H+ reclaims filtered HCO3-.
• Body has the same amount of HCO3− and H+

This quiz focuses on the water content in the human body, including variations across different age groups and genders. It also covers fluid compartments and the composition of body fluids, particularly the role of electrolytes. Test your understanding of these essential concepts in physiology!