The filterability of leukocytes in undiluted blood

A filtrometer is described for measuring the flow of fluids through microfilters. The flow of Newtonian fluids through the filters can be predicted from the diameter, length and number of pores. There are no physical artefacts such as turbulent flow or a significant lag period before steady‐state flow is achieved. The instrument has been used as a viscometer and has been used to record and analyse the flow of undiluted blood through 5 $\mu$ m polycarbonate filters. The calculated viscosity of Newtonian fluids agrees well with those measured by a more conventional viscometer (Ostwald). Flow profiles of blood have been analysed to give both the numbers and the flow properties of a small population of slow leukocytes which equate numerically with the monocytes. They are subdivided into three distinct sub‐populations, according to their rheological properties, and these are termed $\mathit{SL}_{1}$ , $\mathit{SL}_{2}$ and $\mathit{PB}$ . The concentration of these cells, in blood, are $0.12\pm0.02 \times 10^{6}$ ml $^{-1}$ , $0.11\pm0.02 \times 10^{6}$ ml $^{-1}$ , $0.09\pm0.02 \times 10^{6}$ ml $^{-1}$ in young females aged about 25 years. The transit time of these cells, through 5 $\mu$ m pores, is $34.8\pm1.4$ s, $147.5\pm2.5$ s and $> 300$ s, respectively. Analysis of blood from older men (53–79 years) gives essentially the same results although the concentration of $SL_{1}$ is slightly higher at $0.19\pm0.09 \times 10^{6}$ ml $^{-1}$ .