Monday, February 25, 2013

Blood viscosity

Blood viscosity is a concept that is important in understanding blood flow.  It is, after all, one of the factors that affects peripheral resistance to blood flow.

One major factor influencing blood viscosity is hematocrit.  You may be interested in using the analogy of ketchup outlined a few months ago in my article for students Blood viscosity and peripheral resistance at

Recently, researchers also looked at the viscosity of the blood plasma alone (without the formed elements).  They found that blood plasma has unique characteristics of flow found only in non-Newtonian fluids, becoming less viscous with increasing pressure.  Again, just like ketchup. Plasma, unlike plain water, exhibits both viscous and elastic behaviors.

Researchers found in recent experiments that this characteristic of plasma may promote swirling where blood vessels diameters change—both at the beginning and end of a narrowed segment.  Thus, this could have an effect on formation of clots at stenoses or where a stent has been placed.

So, as you may have suspected all along, blood is not only thicker than water—it's weirder than water.

Want to know more?

  • Blood viscosity and peripheral resistance
    • Kevin Patton
    • The A&P Student 12 September 2012
    • [Analogy of ketchup flow for students.  Includes video.]
  • Blood Is Thicker Than Water – And Blood Plasma Is, Too
    • Science Daily Feb. 18, 2013
    • [Brief, plain-language article outlining the recent research.]
  • Rheology of human blood plasma: Viscoelastic versus Newtonian behavior.
    • M. Brust, et al.
    • Phys. Rev. Lett, 110, 078305 (2013) DOI: 10.1103/PhysRevLett.110.078305
    • [Original journal article.  See photos from the experiments below.]

Recording from one of the "drop-experiments": If blood plasma is placed between two plates and then they pulled apart, high-speed cameras show in conjunction with high-resolution microscope objectives that strands and droplets form. This demonstrates that plasma is elastic and viscous and does not behave like water.
Photo: Christof Schaefer, Phys. Rev. Lett. 110, 2013, 078305th Copyright (2013) by the American Physical Society

Plasma turbulence affects the blood. In one experiment, the researchers had plasma flow through a microfluidic constriction as in vasoconstriction. They showed turbulence at the end of the contraction, but also - as seen here in the pictures - sticking to its beginning. This turbulence is caused by the viscoelastic properties of blood plasma.
Photo: Mathias chest, Phys. Rev. Lett. 110, 2013, 078305th Copyright (2013) by the American Physical Society

Monday, February 11, 2013

Finger wrinkles

You know that dramatically wrinkling that occurs when your fingers and and toes get wet?

The classic explanation has been that such wrinkling is caused by osmosis.  But if you think about it, why does it occur only on the palmar and plantar skin surfaces?  My face doesn’t wrinkle when it’s wet.  Well, OK, my face is always a bit wrinkled—wet or not.

Some scientists are now thinking that this is not an osmotic effect but, instead, a nervous response to wetness.  According to this latest theory, such a response helps us avoid slipping and injuring ourselves in wet conditions.  It would also improve our ability to make and use tools under a variety of conditions.

Want to know more?

  • Pruney digits help people get a grip: Wrinkling may have evolved as an adaptation to wet conditions
    • Tanya Lewis 
    • Science News Web edition: January 9, 2013 Print edition: February 9, 2013; Vol.183 #3 (p. 11) 
    • [Brief article explaining the recent development in plain English.]

  • Water-induced finger wrinkles improve handling of wet objects.
    • K. Kareklas et al.
    • Biology Letters. Published online January 8, 2012.
    • [Journal article outlined proposed theory.]

  • For ancient hominids, thumbs up on precision grip.
    • B. Bower  
    • Science News, Vol. 177, May 8, p. 15. 
    • [Brief related article on evolution of human grip.]

  • Fingerprints filter the vibrations fingers feel
    • L. Sanders.
    • Science News, Vol. 175, February 28, p. 10
    • [Brief related article on sensory function of human epidermal friction ridges.]