Lecture 5

• Body fluid


  About 60% of health adult body
• Body fluid


  • Compartmentalized
  • 2/3 intracellular ICF (inside cells)
  • 1/3 extracellular ECF (outside cells)
  • Intercellular or interstitial (between cells)
  • Intravascular (within the blood and lymphatic vessels)
• Factors that affect body fluid

  • Tissue type
  • Age
  • Sex
• Composition of body fluid

  • Mainly water
  • Solutes
  • Electrolytes such as sodium, potassium etc
  • Non-electrolytes such as glucose, urea, proteins etc
• Roles of water


  • Dissolves and transports nutrients (e.g., glucose) and waste products
  • Maintain structure of large molecules (e.g., glycogen)
  • Involved in metabolic reactions (hydrolysis and condensation)
  • Acts as lubricant (e.g., saliva) and cushion around joints, in the spinal cord and in the brain
  • Aids in thermoregulation (i.e., sweat)
  • Maintains blood volume
• Electrolytes


  • Carry electrical charges
  • Sodium and potassium are positively charged (cations)
  • Chloride and phosphate are negatively charged (anions)
  • Fluids have an overall neutral charge due to the balances between electrolytes
  • In intracellular fluid potassium and phosphate are the predominant electrolytes
  • In extracellular fluid, sodium and chloride are the predominant electrolytes
• Roles of electrolytes


  • Action potential (electrochemical gradient involving Na and K)
  • Nerve impulse transmission (action potential, Na and K)
  • Muscle contraction (action potential, movement of Ca)
  • Regulate fluid balance by maintaining the appropriate distribution of body fluids
  • Regulate the acidity of body fluids (acid-base balance)
• Water homeostasis


  Need to control total body water and water location (ratio)
• Sources of total body water

  • Metabolic reactions
  • Dietary
• Losses of total body water

  • 60% through excretion (urine and feces)
  • 30% through thermoregulation (sweat)
  • 10% through respiration (lungs)
• Water needs


  • Dependent on diet (water intake), activity (water loss), environment (temperature and humidity - water loss), body size (water loss)
  • 1.0-1.5 mL/kcal expended (adult)
  • About 2-4 L/day for adults
  • 1.5mL/kcal expended (infants)
• Water sources


  • Water and beverages
  • Foods (fruits and vegetables 95% water, meats and cheeses 50% water)
• Caffeinated beverages increase water loss since caffeine is a diuretic
• Body really good at water homeostasis
• Situations requiring attention for water homeostasis

  • Illness
  • Injury/trauma (blood loss)
  • Diuretics
  • Physical activity
  • Environment
  • Pregnancy
  • Lactation
• Body regulates water balance by excreting amount needed to maintain equilibrium
• Body must excrete at least 500 mL each day as urine
• Lungs, feces, and skin also carry away water
• Dehydration


  Caused by not drinking enough water
• Water intoxication


  Rare condition in which body water content is too high
• Thirst


  Driven by signals sensed by brain's hypothalamus such as concentration of solutes, electrolytes in the blood and blood volume
• Hormones regulating fluid maintenance

  • Antidiuretic hormone (ADH)
  • Renin
  • Angiotensin
  • Aldosterone
• Antidiuretic hormone (ADH)


  Secreted by pituitary gland in response to high electrolyte concentration in blood or low blood volume/pressure, triggers kidneys to reabsorb water
• Renin


  Enzyme secreted by kidney in response to low blood pressure, triggers activation of angiotensin l
• Angiotensin l/ll


  1. Angiotensin l: blood (plasma) protein activated by renin

  2. Angiotensin l converted to angiotensin ll

  3. Angiotensin ll triggers secretion of aldosterone and vasocontraction of blood vessels
• Aldosterone


  Hormone secreted by adrenal glands in response to angiotensin ll, triggers kidneys to reabsorb more sodium and chloride and retain water
• Maintains about 2/3 water inside cells (intracellular) and about 1/3 outside cells (extracellular)
• Electrolytes dissolved in water

  • Cations (positive, e.g., Na+)
  • Anions (negative, e.g., Cl-)
  • Move electrolytes and water will follow
• Protein pumps in cell membranes

  Move ions into or out of the cells, maintaining specific amounts of sodium outside and potassium inside
• Acid-base balance


  Ions help regular acidity (pH) of fluids by acting as buffers, kidneys play the primary role
• Sodium (Na+)


  • Principal electrolyte in extracellular fluid, helps maintain acid-base balance, essential to muscular contraction and nerve transmission
  • Excess sodium (salt) in diet can contribute to hypertension
  • Deficiencies unlikely
  • Easily digested and absorbed, easily obtained, 10% naturally occurring in foods, 15% added during cooking, 75% added by manufacturers
• Sodium content of foods


  • Processing of food changes the sodium content
  • Processed foods e.g., canned soup, are high in sodium
• Hyponatremia


  Abnormally low blood sodium level, result from prolonged vomiting, diarrhea, or sweating e.g., during exercise
• Hypernatremia


  Abnormally high blood sodium concentration, high blood pressure
• High sodium intakes correlate with hypertension, heart disease, and stroke
• DASH eating pattern aims to reduce sodium and increase potassium intakes since potassium lessens the effects of sodium
• Increasing physical activity reliably lowers blood pressure
• Chloride


  Primary anion (negative ion) regulator of extracellular volume, can move freely to the intracellular space, critical in maintaining fluid, electrolyte, acid-base balance, make up part of stomach acid (HCl)
• Potassium


  • Major positive ion inside the body cells, primary cation regulator of intracellular volume, major role in maintaining fluid and electrolyte balance, involved in maintaining a steady heartbeat, transmitting nerve impulses, and contracting muscle
  • Deficiency usually related to excessive losses rather than low intakes, toxicity can result from excessive intake of potassium salts or supplements
• Low potassium intake, especially when combined with high sodium intake, increases blood pressure and the risk of death from stroke