Lecture13 OutlineWater & Electrolyte Balance

Water volume regulation involves

  • Kidneys
  • Integumentary System
  • Lungs
  • Gastrointestinal System

Coordinated by Nervous System & Endocrine System

Distribution of Fluids

  1. Intracellular Fluids (ICF)
  2. Enclosed by cell membrane
  3. 40% of total body weight
  4. Extracellular Fluid (ECF)
  5. Outside cell membrane
  6. 20% of total body weight
  7. Plasma = within blood vessels
  8. Interstitial fluid = surrounds blood vessels
  9. Transmembrane fluids: Cerebral spinal fluid,, synovial fluid, vitreous & aqueous humor of eye, ect.
  • Each compartment differs slightly, but roughly equal osmotic pressure in all

Distribution of electrolytes

  1. Intracellular Fluid
  2. K+, Mg2+, PO4-2, Protein
  1. Extracellular Fluid
  2. Na+, Cl-, Ca2+, HCO3-, some proteins in plasma

Movement of fluids

  1. Extracellular Fluids
  2. Hydrostatic pressure (example: blood pressure)
  3. Colloid osmotic pressure (unequal protein distribution creates osmotic gradient)
  1. Intracellular Fluids
  2. Osmotic pressure (unequal solute gradient across cell membrane creates osmotic pressure)
  3. Hypotonic ECF – water enters cell, cell swells
  4. Hypertonic ECF – water leaves cell, cell shrinks

Water Balance

Water intake (2500mL) = Water output (2500mL)

  1. Water IntakeII. Water Output
  2. Drinking, 60%1. Urine, 60%
  3. Foods, 30%2. Evaporation, 34%
  4. Cell metabolism, 10% - skin & lungs

(e.g. dehydration synthesis) - sweat

3. Feces, 6%

Regulation of Water Intake

  1. Habits & Social Settings influence consumption
  2. Regulatory Mechanisms influence consumption
  3. Thirst = primary regulation of water intake
  4. Thirst Center in Hypothalamus
  5. Osmoreceptors detect changes in osmolarity & respond
  1. Thirst Mechanism
  2. Body looses 1% H2O, and osmotic pressure increases
  3. Osmoreceptors detect increased osmotic pressure
  4. Hypothalamus stimulates thirst sensation
  5. Drinking water causes stomach to distend
  6. Stomach distension inhibits thirst center
  7. Water is absorbed from stomach & small intestines

Regulation of Water Output

  1. Antidiuretic Hormone (ADH) –
  2. Stimulated by insufficient water in extracellar fluid
  3. Water loss is detected by osmoreceptors in hypothalamus
  4. Hypothalamus stimulates ADH secretion from posterior pituitary gland
  5. ADH circulates in blood
  6. ADH promotes water reabsorption in Distal convoluted tubules & collecting ducts
  7. Decreased urine output
  8. Excessive water in ECF decreases ADH secretion, increases urine output
  9. Alcohol = diuretic; inhibits ADH secretion

Homeostasis of fluid

Thirst center & ADH work together

  1. As blood osmolarity increases
  2. thirst increases – increased water intake
  3. ADH secretion increases – decreased urine output
  1. As blood osmolarity decreases
  2. Thirst decreases – lowers water intake
  3. ADH secretion decreases – increases urine output

Disorders of water balance

  1. Dehydration
  2. Water is lost from ECF
  3. Water from intracellular fluid leaves cells, & cells shrink
  4. Symptoms:
  5. Dry skin & mouth
  6. Disorientation, odd behavior, kidney failure, heart arrhythmia
  7. Water Intoxication
  8. Hypotonic ECF
  9. Water from ECF enters cells & cells swell
  10. Common in marathon runners with excessive water intake
  11. Edema
  12. Accumulation of fluid in interstitial spaces
  13. Ascites
  14. Low plasma protein prevents colloid osmotic movement of plasma back into blood
  15. Fluid builds up in interstitial space
  16. Distended belly
  17. Commonly seen in cirrhosis & malnourished children (lack of protein in diet)

Electrolyte Balance

  1. Major electrolytes: Na+, K+, Ca2+, Cl-, Mg2+, H+, PO4-2, HCO3-
  2. Electrolyte intake = Electrolyte output
  1. Regulation of Sodium (Na+)Sodium = Natrium (Na)
  2. Na+ is dominant extracellular cation
  3. 90% of osmotic pressure in extracellular fluid is due to sodium and anions associated with it (Cl-)
  1. Na+ loss
  2. Excreted by kidneys
  3. Aldosterone = increases Na+ reabsorption in DCT & CD
  4. Alsosterone is secreted when [Na+] is low
  5. Decreased [Na+] in ECF stimulates renin-angiotensin-aldosterone system
  6. Sweat
  7. Small amounts of Na+ are lost by sweat
  8. Hyponatremia = low sodium
  9. Caused by prolonged diarrhea, vomiting, sweating
  10. Leads to water intoxication
  11. Hypernatremia = excessive sodium
  12. Thirst, dry mouth
  1. Regulation of Potassium (K+) Potassium = Kalium (K)
  2. Aldosterone
  3. Increases K+ secretion in distal convoluted tubules & collecting ducts
  4. Secreted with excessive K+
  5. K+ imbalance affects membrane potential
  6. Hyperkalemia = excessive K+
  7. Intestinal cramping, neuromuscular irritability, muscle weakness
  8. Hypokalemia = insufficicent K+
  9. Skeletal muscle weakness, bradycardia
  10. Regulation of Calcium (Ca2+)
  11. 99% of Calcium is stored in bones
  12. Parathyroid Hormone – increases blood Calcium
  13. Stimulates osteoclast activity
  14. Promotes Ca2+ reabsorption in kidneys
  15. Increases Active vitamin D synthesis
  16. Active Vitamin D promotes Calcium absorption in small intestine
  17. Calcitonin – decreases blood Calcium
  18. Stimulates osteoblast activity (and inhibits osteoclasts)

Acid & Base Balance

  1. Overview
  2. Acids release H+ in solution: HCl → H+ + Cl-
  3. Bases release OH- in solution:NaOH → Na+ + OH-
  4. Acid + Base yields Salt + Water: HCl + NaOH → NaCl + H2O
  1. pH measurements
  2. pH measures [H+] (as pH decreases, [H+] increases)
  3. Acids: pH < 7.0 [H+] > [OH-]
  4. Bases: pH > 7.0[H+] < [OH-]
  5. Neutral: pH = 7.0[H+] = [OH-]
  6. Blood pH = 7.35 – 7.45, slightly basic
  1. Strong/ Weak Acids & Bases
  2. Strong Acids dissociate completely & release H+: HCl → H+ + Cl-
  3. Strong Bases dissociate completely & release OH- : NaOH → Na+ + OH-
  1. Weak Acids & Bases dissociate reversibly, example: CO2 in plasma

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

H2CO3 (Carbonic Acid) = weak acid

HCO3- (Bicarbonate) = weak base

  1. Weak acids release, or donate H+: H2CO3 → H+ + HCO3-
  2. Weak bases accept H+: HCO3- + H+ → H2CO3

Acid/Base Buffer System

  • Buffers prevent changes in pH
  • Buffers absorb H+ and OH- released from strong acids & strong bases
  • Weak acids & weak bases are formed as products; minimized pH changes

1. Bicarbonate Buffer System

Weak Acid = H2CO3 (Carbonic Acid)Weak Base = HCO3- (Bicarbonate)

  • Addition of a strong acid: H+ + HCO3- → H2CO3 (proton is absorbed)
  • Addition of a strong base: OH- + H2CO3→ HCO3- + H2O(hydroxide is absorbed)

2. Phosphate Buffer System

Weak Acid = H2PO4- (dihydrogen phosphate)Weak Base = HPO4-2 (monohydrogen phosphate)

  • Addition of strong acid: H+ + HPO4-2 → H2PO4-
  • Addition of strong base: OH- + H2PO4- → HPO42- + H2O

3. Protein Buffer System

  • Carboxyl terminal (-COOH)
  • Addition of strong acid: H+ + COO- → -COOH
  • Addition of strong base: OH- + -COOH→ -COO- + H2O
  • Amino terminal (-NH2)
  • Addition of strong acid: H+ + NH2 → -NH3+
  • Addition of strong base: OH- + NH3+ → -NH2 + H2O
  • Hemoglobin
  • Oxygen transport protein, also important buffer against H+ ions from CO2 buildup
  • CO2 from metabolism diffuses through plasma into red blood cells
  • Reaction 1: CO2 + H2O → H2CO3

catalyzed by carbonic anhydrase from RBCs

  • Reaction 2: H2CO3→ H+ + HCO3-

Overall reaction: CO2+ H2O ↔ H2CO3 ↔ H+ + HCO3-

  • * As CO2 builds up, [H+] increases
  • Deoxyhemoglobin binds excess [H+]

Respiratory Excretion of CO2

  1. Respiratory Centers in medulla oblongata
  2. Regulates rate & depth of breathing
  3. As CO2 accumulates, H+ levels rise: CO2+ H2O ↔ H2CO3 ↔ H+ + HCO3-
  4. H+ stimulates medulla, which increase rate of breathing
  5. Increased CO2 = increased rate & depth of breathing, raises pH

Renal Excretion of H+

  1. As pH decreases, excessive H+ can be secreted in renal tubules (PCT & DCT)
  2. Tubule cells also produce ammonia (NH3)
  3. NH3 combines with H+ to form ammonium (NH4) in filtrate
  4. H+ + NH3→ NH4
  5. Ammonium is impermeable to cell membranes, so NH4 leaves with urine (prevents reabsorption of H+)

Acid/Base Imbalances

Acidosis: pH < 7.35 Alkalosis: pH > 7.45

  1. Respiratory Acidosis
  2. Accumulation of CO2
  3. Often caused by hindered ventilation
  4. Obstruction of airway
  5. Pneumonia, emphysema
  6. Injury to respiratory centers in brainstem
  1. Metabolic acidosis
  2. Accumulation of other acids (H2CO3), or loss of base
  3. Kidney disease
  4. Prolonged vomiting or diarrhea
  5. Diabetes mellitus
  1. Respiratory Alkalosis
  2. Hyperventilation = too great loss of CO2
  3. Lightheadedness, dizziness, agitation
  1. Metabolic Alkalosis
  2. Great loss of H+, or great gain in a base.
  3. Accompanied by raise in blood pH
  4. Prolonged vomiting, following gastric lavage (pumping stomach), excessive consumption of antacids.
  5. Decreased breathing rate & depth