Viscosity

• Viscosity:

  • All fluids have a degree of viscosity
  • Essentially viscosity is an internal resistance to flow, produced as components of the fluid interact with each other causing friction
  • Blood is a reasonably viscous (thick) fluid and has a relatively high degree of internal resistance
  • It contains many cells, platelets and plasma proteins etc, suspended in water, all of which physically interact with each other and the endothelial wall as blood flows
• Viscosity:

  • In health, viscosity is generally constant, although it can change under certain circumstances for example; if you are severely dehydrated there will be proportionally more formed elements in your blood and less fluid
  • Your blood would be more viscous with greater internal resistance
• Viscosity:

  • The viscosity of blood is primarily determined by haematocrit - i.e. the percentage of blood volume composed of erythrocytes
  • An average haematocrit is ~ 40% (males = 42%, females = 38%)
  • circumstances in which haematocrit can increase or decrease, which will affect viscosity and therefore total peripheral resistance:
  • increase haematocrit
  • severe dehydration
  • polycythaemia - altitude response or COPD
  • decrease haematocrit
  • malnutrition (inadequate protein for red blood cell synthesis)
  • severe anaemia