Fluids

• Body Fluids


  Catch-all term for all H2O based liquid in body
• Body water


  • Amount varies with Age
  • Sex
  • % body fat
  • More fat means less water
• 2 Main Fluid Compartments

  • Intracellular compartment
  • Extracellular compartment
• Intracellular compartment


  Cytosol holds ICF (intracellular fluid)
• Extracellular compartment


  • Holds ECF (extracellular fluid)
  • Plasma
  • Interstitial fluid
• Water


  • Universal solvent, solutes include Electrolytes
  • Non-electrolytes
• Non-electrolytes


  Covalent bonds
• Electrolytes


  • Ionic compounds - mostly salts
  • Ionic bonds dissociate in water
  • Greater osmotic pull than non-electrolytes
  • Conduct electric current in solution
• H2O moves from low molality to high molality


  Concentration expressed in mEq/L
• mEq


  • For single charged ions (Na+), 1 mEq = 1 mOsm
  • For bivalent ions (Ca2+), 1 mEq = 1/2 mOsm
  • 1 mEq of either provides same amount of charge
• Ion composition of the compartments

  • Main ECF cation: Na+
  • ECF anions: Cl- & protein anions in Plasma, Cl- in Interstitial fluid
  • Main ICF cation: K+
  • ICF anion: HPO42-
• Osmotic Movement of Water between Compartments

  1. Solutes move between compartments only under certain conditions, but H2O moves freely

  2. Any factor that changes solute concentration of a compartment results in movement of water

  3. H2O direction determined by Hydrostatic pressure - the 'push' of water

  4. Osmotic pressure - the 'pull' of solutes

  5. Osmolality of all body fluids is equal/equalizes quickly
• Osmotic Movement of Water between Compartments

  • French fries w/extra NaCl - Na+ concentration in ECF rises, water moves out of cells to equalize
  • Gallon of water, super-fast - Na+ concentration of blood drops, water moves into cells to equalize
• H2O Loss

  • Majority via urine/kidneys
  • Sensible water loss - ~1800 ml/day
  • Insensible water loss - ~700 ml/day
• H2O Gain

  • Metabolic water - ~250 ml/day
  • Food intake - ~750 ml/day
  • Liquid intake - ~1500 ml/day
• Water/Electrolyte balance

  • A healthy person maintains normal osmolality of body fluids: 280-300 mOsm
  • If too high - Thirst mechanism, we produce ADH
  • If too low - Thirst inhibited, ADH inhibited
  • H2O follows Na+
• Regulation of Water Intake

  1. Thirst mechanism - driving force

  2. Triggered by High plasma osmolality, Hypothalamic osmoreceptors, Decreased blood volume (pressure), Arterial baroreceptors

  3. Inhibited when Receptors in pharynx detect liquid intake, cease thirst

  4. Preemptive - Allows water time to work into system, Avoids overhydration
• Regulation of Water output

  1. Obligatory water loss

  2. Insensible - Lungs, insensible perspiration

  3. Sensible - 500ml/day of urine to flush out metabolic wastes

  4. If too much H2O intake - 1/2 hour to inhibit ADH, then get rid of excess by urination
• ADH

  • Biggest influence on water conservation
  • High ADH - Aquaporins (AQP) inserted in collecting ducts, Causes reabsorption
  • Low ADH - Very few AQPs in collecting ducts, Very little reabsorption of water, Dilute urine
  • Release is triggered by Osmoreceptors in hypothalamus, Atrial baroreceptors sense very low blood pressure
• Water Imbalances

  • Dehydration - Causes: Hemorrhage, severe burns, vomiting, profuse sweating, water deprivation
  • Dehydration - Effects: Confusion, hypovolemic shock
  • Overhydration (hypotonic hydration - hyponatremia) - Causes: Renal insufficiency, extreme intake (water intoxication)
  • Overhydration - Effects: Nausea, vomiting, cramps, cerebral edema, seizures, death
• Electrolyte Homeostasis


  • Usually talking about salts
  • Intake thru diet
  • Output thru Perspiration, Urination, Defecation, Vomiting
• Na+


  • Primary ion creating ECF osmotic pressure
  • Concentration affects cellular function
  • Accounts for 280 mOsm of ECF's normal 300 mOsm
  • Amount determines ECF volume, therefore blood pressure
  • Water follows Na+
• Na+ Regulation

  1. Aldosterone - primary

  2. Triggered by Low blood pressure triggers renin - RAAS

  3. Angiotensin II directly stimulates release

  4. Elevated K+ in ECF

  5. Renal reabsorption of Na+, excretion of K+

  6. Atrial Natriuretic Peptide (ANP) decreases blood pressure

  7. Estrogens mimic aldosterone

  8. Monitored indirectly by baroreceptors (BP)
• Na+ imbalances

  • Hypernatremia - Causes: dehydration, excess IV saline
  • Hypernatremia - Effects: confusion, lethargy
  • Hyponatremia - Causes: insufficient aldosterone(Addison's disease), overhydration, vomiting & diarrhea, kidney disease, SIADH
  • Hyponatremia - Effects: cerebral edema; confusion, giddiness, coma, twitching, hypovolemia
• K+


  • Necessary for neuromuscular function (RMP)
  • Heart is very sensitive to K+ levels
  • Part of body's buffer system
  • K+ levels rise with acidosis, drop with alkalosis
• K+ Regulation


  1. Mainly kidneys (DCT & collecting ducts)

  2. Aldosterone - Negative feedback mechanism

  3. Increases tubular secretion
• K+ Imbalances


  • Hyperkalemia - Causes: renal failure, insufficient aldosterone
  • Hyperkalemia - Effects: nausea, diarrhea, vomiting, bradycardia, arrythmia, cardiac arrest, sk. muscle weakness, paralysis
  • Hypokalemia - Causes: GI issues, starvation, excess aldosterone, diuretics
  • Hypokalemia - Effects: arrhythmia, weakness, acidosis, confusion
• Ca++


  • Stored in bones
  • Necessary for Clotting, Neuromuscular excitability, Membrane permeability, PO43-
• Ca++ Regulation


  Parathyroid hormone & PTH effects - Bones, Kidneys, Small intestine
• Ca++ imbalance


  • Hypercalcemia - Causes: excess PTH, renal disease, prolonged immobilization
  • Hypercalcemia - Effects: decreased neuromuscular excitability, cardiac arrythmias and arrest, confusion, kidney stones, coma
  • Hypocalcemia - Causes: insufficient PTH or vitamin D, renal failure, alkalosis
  • Hypocalcemia - Effects: neuromuscular hyperexcitability, tetany, weak heartbeat
• Cl-


  • Main anion of ECF
  • Regulation - Reabsorbed by kidneys - follows Na+ during reabsorption
  • During acidosis, HCO3- replaces Cl-
• Acid-Base Homeostasis


  • Physiological pH - Arterial blood between 7.35-7.45
  • Below 7.35 = acidosis, Above 7.45 = alkalosis
  • Affects all proteins, H bonds maintain shape
• Sources of acids/bases


  • Cell metabolism - Lactic acid, phosphoric acid, ketone bodies
  • Glucose metabolism/CO2 transport - CO2 + H2O carbonic acid bicarbonate + H+
  • Diet - Amino acids, fatty acids, citric acid, ascorbic acid, Bicarbonate ions (alkaline)
• Buffers


  • Maintain physiological pH
  • Chemical buffers - Quick, act first
  • Physiological buffers - Lungs, Kidneys - Powerful, but slow
• pH maintenance - ECF

  1. If a strong acid, like HCl, is added - Bicarbonate picks up H+, results in more carbonic acid

  2. If a strong base, like NaOH, is added - Carbonic acid releases H+, results in more sodium bicarbonate

  3. Weak base replaces strong base - very little pH change
• pH maintenance - ICF, urine

  1. Protein buffer system - Some amino acids have carboxylic acid groups that can release H+ when pH increases, some have groups that can act as bases and pick up H+ when pH decreases

  2. Most proteins are amphoteric - have both
• pH maintenance - Physiological buffers

  1. In lungs, CO2 unloaded - Equation shifts left, H+ gets reincorporated into H2O

  2. Medullary chemoreceptors sense increased PCO2 – cerebral acidosis, Respiratory rate & depth increases, Ventilation increases – more CO2 is cleared, PCO2 decreases, Reaction shifts left, reduces H+ concentration, pH is restored

  3. Medullary chemoreceptors are depressed when H+ concentration is low, Respiratory rate & depth slows, becomes shallower, Ventilation decreases – allows CO2 concentration to increase, Reaction shifts right, increases H+ concentration, pH is restored
• Physiological Buffers - Kidneys

  1. Renal regulation of blood pH - 2 methods

  2. Kidneys excrete fixed acids - Phosphoric, uric, lactic acids & ketone bodies

  3. Kidneys regulate pH by adjusting bicarbonate levels in the blood, by Excreting bicarbonate - Losing a bicarbonate frees up an H+, shifts right

  4. Reabsorbing or making a new bicarbonate - Gaining a bicarbonate ties up an H+, shifts left
• Physiological Buffers


  1. Secretion of H+

  2. Bicarbonate reabsorption

  3. Generating new bicarbonate - By excretion of buffered H+

  4. Generating new bicarbonate - By NH4+ secretion
• pH imbalances


  • Acidosis or alkalosis is either respiratory or metabolic, depending on cause
  • Respiratory acidosis - PCO2 not being cleared, too high
  • Respiratory alkalosis - PCO2 too low
  • Metabolic acidosis - pH below 7.35, bicarb low
  • Metabolic alkalosis - pH above 7.45, bicarb high