Haematocrit distribution in rabbit tenuissimus muscle

Low values of mean capillary haematocrit have been reported in many tissues including skeletal muscle. The present study was undertaken to analyse haematocrit distribution in the transverse and terminal arterioles, capillaries and venules of the rabbit tenuissimus muscle preparation. Tube haematocrit, i.e. the volume fraction of red cells, in muscle capillaries (n = 85) was found to be 39% of systematic haematocrit Hsys. In part, this haematocrit reduction is due to the Fahraeus effect. Corresponding capillary discharge haematocrit HD was 56% of Hsys. Tenuissimus muscle capillaries are fed by terminal arterioles originating from transverse arterioles. The latter extend into and supply adjacent connective tissue septa in addition to the muscle tissue proper. In transverse arterioles leaving the muscle to enter the connective tissue, HD was found to be 127% of Hsys (n = 18), and in collecting venules at the muscle edge HD was 129% of Hsys (n = 18). These findings indicate that the connective tissue microcirculation represents a functional red-cell shunt in resting tenuissimus muscle. Since only about 20% of the inflow to the preparation passes through the connective tissue, this shunting is not sufficient to satisfy conservation of red-cell mass. In addition, it is likely that the observed low capillary haematocrit is in part due to a positive correlation between blood-flow velocity and HD in capillaries originating from individual terminal arterioles. This phenomenon is called the network Fahraeus effect.