Unit 2

• Body fluid compartments


  Intracellular fluid, Extracellular fluid, Plasma, Interstitial fluid
• Part of the fluid in the human body is water – (cytoplasm)
• Cell's operations rely on water


  Chemical reactions of life take place in aqueous solution: solution (fluid) and dissolved solutes
• Water


  • Distributes gases, nutrients, and waste products
• The body is mostly water
• Water content


  Varies in different organs and tissues
• Water content changes with

  • Age
  • Sex
  • Lean body mass
  • Hydration levels
• Infants: 75% or more (low body fat and bone mass)
• Adult males: 60% (more muscles)
• Adult females: 55% (higher fat, less skeletal muscle mass)
• Old age: declines to ~45%
• Affected by weather and exercise
• Body water content in percentage of a body weight is lowest in


  • Well build man
  • Fat woman/man
  • Well nourished child
• Total body water (TBW)

  = body weight x (0.6 male) ("0.5 female)"
• Intracellular


  TBW x 2/3
• Extracellular


  TBW x 1/3
• Interstitial


  TBW of ECF x 3/4
• Plasma


  TBW of ECF x 1/4
• For a 70kg man, % body water is 60
• % body water differs in individuals (age, gender, degree of obesity)
• Activity


  1. Weighs 100kg, assuming that 65% of the body mass is water and ICF is approximately 40 L

  2. Find the approximate ECF volume, plasma volume and IF volume
• Other water-based ECF


  • CSF
  • Lymph
  • Synovial fluid
  • Pleural fluid
  • Pericardial fluid
  • Peritoneal fluid
  • Aqueous humour
• Role of water in human physiology


  • Involved in biochemical reactions – reactant in metabolic pathways
• Fluid input and output (adults)

  • Fluid ingestion (2100ml/day)
  • Insensible (700ml/day)
  • Production (metabolism) (200ml/day)
  • Urine (1400ml/day)
  • Feaces (100ml/day)
  • Sweat (100ml/day)
• Electrolyte


  Substances when dissolved in solution dissociate into ions (able to carry electrical current)
• Electrolytes in plasma and IF are similar – continuous exchange of substances
• Higher conc. of proteins (in plasma) than in IF - due to highly permeable capillary membrane
• ICF and ECF are separated by cell membrane - highly permeable to water (not to most electrolytes)
• Major cations and anions of the intracellular and extracellular fluids

  • Na+
  • Cl-
  • K+
  • Ca2+
  • Mg2+
• Functions of body electrolytes

  • Produce an osmotic pressure
• To maintain electrical neutrality in each fluid compartment: Number cations = number of anions
• Characteristics of blood


  • pH: 7.35 - 7.45 (slightly alkaline)
• Components of whole blood

  • Blood volume
  • Hematocrit
  • Packed cell volume
• Blood plasma


  • > 90% is water
  • ~ 7% is proteins
  • Albumin, Globulins, Fibrinogen, Prothrombin
  • Dissolved solutes: electrolytes, nutrients, gases
• Fluid movement between compartments

  1. Blood passes through the blood capillary

  2. Pressure exerted by blood against the walls of the blood vessels by the pumping action of the heart

  3. Hydrostatic pressure

  4. Fluid and dissolved substances are forced through capillary pores into interstitial spaces

  5. Osmotic gradient (colloid osmotic pressure) - force generated by large plasma proteins in plasma

  6. Fluid moves by osmosis from interstitial spaces into blood

  7. Prevents significant loss of fluid from blood into interstitial spaces
• Fluid Filtration


  1. Hydrostatic pressure is higher at the arteriolar end than at the venules end

  2. Hydrostatic pressure is higher than colloid osmotic pressure at the arteriolar end
• Solute movement between compartments

  1. Sodium-Potassium pump

  2. Simple diffusion

  3. Facilitated diffusion - carrier proteins, i.e glucose, amino acids
• Movement between compartments


  Water moves through a semi-permeable membrane by osmosis in response to solute concentration gradient
• Concentration of solution


  Expressed in moles (Molar concentrations)
• Tonicity


  The ability of a solution (extracellular) to modify the volume of the cells by moving the water content