Whimsy

Red blood cells (enlarged approximately 1500 times)

The term nun whistle is used to describe flow noises of the blood that can be heard when listening with a stethoscope and indicate changed flow conditions in anemia (too few [red] blood cells ).

The blood plasma - i.e. the liquid part of the blood without cells - has a constant viscosity that is lower than that of the blood (reduced viscosity). Only the proportion of cells in the blood give the blood its special viscosity (which increases with increasing flow). As the number of cells in the blood decreases, the viscosity decreases accordingly. There is turbulence in the blood stream that can be heard as a whirring noise.

Red blood cells ( erythrocytes ) in iron deficiency anemia

term

The origin of the term is often explained by the fact that the symptom was first or most often found in nuns. Because of their often meatless and otherwise low-iron diet, they were particularly prone to iron deficiency anemia . Rather but the whirring sound of the will gyroscope have been eponymous ( Latin nona , spinning ').

Physical background

According to the equation for determining the Reynolds number , the transition from a laminar flow to a turbulent one depends on the reciprocal of the viscosity. The Reynolds number results as follows:

${\displaystyle Re={\frac {\rho \cdot v\cdot L}{\eta }}={\frac {v\cdot L}{\nu }}}$with: . ${\displaystyle \eta =\nu \cdot \rho }$

The individual symbols stand for the following quantities :

• ${\displaystyle \rho }$( Rho ) - characteristic density of the application (kg m ⁻³ ).
• ${\displaystyle v}$- Amount of a velocity characteristic of the application (ms ⁻¹ ),
• ${\displaystyle L}$- characteristic length of the application (m),
• ${\displaystyle \eta }$( Eta ) - characteristic dynamic viscosity of the application (kg s ⁻¹ m ⁻¹ ),
• ${\displaystyle \nu }$( Ny ) - characteristic kinematic viscosity of the application (m ² s ⁻¹ ).

If a value of Re = 2,000 is exceeded, a laminar flow changes very quickly into a turbulent one. Since the viscosity is below the fraction line in this relationship , the number Re approaches this value with decreasing viscosity and exceeds it at a critical value for which is sometimes exceeded in anemias. ${\displaystyle \eta }$ ${\displaystyle \eta }$

Schmidt et al. write: "With increased flow velocities (e.g. with vascular stenoses) or with reduced blood viscosity (e.g. with severe anemia), turbulent flow occurs in arteries distant from the heart , which can lead to auscultable flow noises ." (emphasis added by the Authors)

Individual evidence

1. ↑ SWR2 knowledge broadcast manuscript from June 28, 2004 ( RTF ; 65 kB).
2. ↑ Robert F. Schmidt, Florian Lang, Gerhard Thews (all eds.): Physiologie des Menschen mit Pathophysiologie , Heidelberg (Springer) 2005, p. 607