Physiology of blood vessels

This chapter covers two major fields of the blood circulation: ‘distribution’ and ‘exchange’. After a short survey of the types of vessels, which form the circulation system together with the heart, the chapter describes how hydrostatic pressure derived from the heartbeat and vascular resistance determine the volume of blood that is locally delivered per time unit. The vascular resistance depends on the length of the vessel, blood viscosity, and, in particular, on the diameter of the vessel, as formulated in the Poiseuille-Hagen equation. Blood flow can be determined in vivo by different imaging modalities. A summary is provided of how smooth muscle cell contraction is regulated at the cellular level, and how neuronal, humoral, and paracrine factors affect smooth muscle contraction and thereby blood pressure and blood volume distribution among tissues. Subsequently the exchange of solutes and macromolecules over the capillary endothelium and the contribution of its surface layer, the glycocalyx, are discussed. After a description of the Starling equation for capillary exchange, new insights are summarized(in the so-called glycocalyx cleft model) that led to a new view on exchange along the capillary and on the contribution of oncotic pressure. Finally mechanisms are indicated in brief that play a role in keeping the blood volume constant, as a constant volume is a prerequisite for adequate functioning of the circulatory system.