Effect of osmolality on erythrocyte rheology and perfusion of an artificial microvascular network

Abstract

Plasma sodium concentration is normally held within a narrow range. It may however vary greatly under pathophysiological conditions. Changes in osmolality lead to either swelling or shrinkage of red blood cells (RBCs). Here we investigated the influence of suspension osmolality on biophysical properties of RBCs and their ability to perfuse an artificial microvascular network (AMVN). Blood was drawn from healthy volunteers. RBC deformability was measured by osmotic gradient ektacytometry over a continuous range of osmolalities. Packed RBCs were suspended in NaCl solutions (0.45, 0.6, 0.9, 1.2, and 1.5 g/dL), resulting in supernatant osmolalities of 179 ± 4, 213 ± 1, 283 ± 2, 354 ± 3, and 423 ± 5 mOsm/kg H2O. Mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC) were determined using centrifuged microhematocrit. RBC suspensions at constant cell numbers were used to measure viscosity at shear rates ranging from 0.11 to 69.5s(-1) and the perfusion rate of the AMVN. MCV was inversely and MCHC directly proportional to osmolality. RBC deformability was maximized at isosmotic conditions (290 mOsm/kg H2O) and markedly decreased by either hypo- or hyperosmolality. The optimum osmolality for RBC suspension viscosity was shifted toward hyperosmolality, while lower osmolalities increased suspension viscosity exponentially. However, the AMVN perfusion rate was maximized at 290 mOsm/kg H2O and changed by less than 10% over a wide range of osmolalities. These findings contribute to the basic understanding of blood flow in health and disease and may have significant implications for the management of osmotic homeostasis in clinical practice.

Keywords: Artificial microvascular network; Hyperosmolality; Hypo-osmolality; Microfluidics; Microvascular perfusion; Osmolality; Red blood cell deformability.

Figure 1

Scanning electron micrographs of RBCs at an original 2000x-magnification. (a) RBCs in hypotonic solution (189 mOsm/kg H₂O). (b) RBCs in hypertonic solution (431 mOsm/kg H₂O).

Figure 2

The centrifuged hematocrit (Hct_c, triangles) and the calculated RBC indices – MCV (circles) and MCHC (rectangles) – for constant RBC numbers at different osmolalities. Mean values ± SD, n = 6.

Figure 3

Viscosity of RBC suspensions at room temperature for a range of osmolalities measured at shear rates of 69.5, 11.02, 0.95 and 0.11 s⁻¹ with a couette viscometer. Mean values ± SD, n = 8.

Figure 4

Deformability index (DI) measured by osmotic gradient ektacytometry for RBCs across a range of osmolalities. Each line (osmoscan) corresponds to a separate sample, n = 6.

Figure 5

Bright-field micrographs of RBCs suspended in hypotonic (left), isotonic (middle) and hypertonic (right) solutions as they perfuse portions of the AMVN. Inlet arterioles with a diameter of 70 µm (upper row) and capillaries with diameters of 5 µm and 7 µm (lower row) are shown.

Figure 6

Normalized AMVN perfusion rates (relative to the AMVN perfusion rate under isosmotic conditions) for RBCs suspended in solutions with different osmolalities. Mean values ± SD, n = 5.