Transcapillary refill in hemorrhage and shock

Moderate or major hemorrhage leads to diminished cardiac output and to increased total peripheral resistance. These cannot be corrected fully until blood volume is restored, which, in the absence of therapy, requires the movement of fluid and of protein from the interstitium into the capillaries. This movement appears mediated entirely by changes in the Starling forces, dominated in the first phase by a fall in capillary hydrostatic pressure, which promotes a rapid shift of protein-free fluid from the interstitium into the capillaries. The second phase, temporally overlapping the initial phase, involves the return of protein to support plasma oncotic pressure. Interstitial albumin probably constitutes the source of the immediate restoration of plasma protein. The driving force for return of the protein depends on interstitial volume and pressure, which in turn appear to depend on movement of fluid from cells to the interstitium, mediated by an increase in solute bathing the cells, especially glucose. It is suggested that this solute is delivered to peripheral cells from the splanchnic bed, where it appears to be formed under the action of hormones whose secretion is stimulated by hemorrhage and injury.