Effects of vessel size, cell sedimentation and haematocrit on the adhesion of leukocytes and platelets from flowing blood

BACKGROUND: Leukocytes and platelets typically fulfil their functions through adhesion to the walls of vessels with different size, haematocrit and shear rate.

OBJECTIVE: We aimed to investigate differential effects of these variables on leukocyte and platelet adhesion.

METHODS: Blood with varying haematocrit was perfused at a range of wall shear rates through capillaries of depth 100 or 300 µm coated with P-selectin or collagen.

RESULTS: Adhesion of leukocytes was much more efficient in the smaller capillaries, but was equal on the upper and lower surfaces and showed nearly identical shear rate dependence for either size of vessel. Platelets also adhered more efficiently in the smaller vessels (although the effect of size was not so great), and equally on upper and lower surfaces, but their adhesion was much less sensitive to increasing shear rate. In previous studies using vertically-orientated capillaries, adhesion increased with increasing haematocrit (Am. J. Physiol.285 (2003), H229-H240). Here, in horizontal 100 µm capillaries, adhesion was highly efficient at haematocrit of 10% but restricted to the lower surface. Adhesion decreased initially as haematocrit was increased to 30% and then increased slightly again at 40% haematocrit. Increasing haematocrit supported a monotonic increase in platelet adhesion in the horizontal capillaries.

CONCLUSIONS: Platelets adhere efficiently over a wider range of sizes and shear rates, and at high haematocrit. Leukocytes adhere better in smaller vessels and at low haematocrit in horizontal vessels. The different behaviours may represent 'rheological adaptation' to functions in inflammation vs. haemostasis.