BLOOD VESSEL STRUCTURE AND FUNCTION

Diffusion, Convection, and the Circulation

Vertebrate cells depend on the molecular diffusion of nutrients for metabolism and function. Single-celled organisms such as bacteria or protists obtain nutrients directly from their respective environments. Here, distances between the medium and cell interiors are small and well within diffusion distance limitations (assuming sufficient local concentrations exist). In contrast, multicellular organisms have cells that are internal to the organism structure, making access to nutrients by simple diffusion from the external environment difficult or improbable. For example, the maximum distance oxygen (O2) can diffuse in mammalian tissues is 20 to 100 μm (due primarily to a relatively low solubility in aqueous environments) (1,2). This limitation is exacerbated for larger nutrient molecules with smaller diffusion coefficients.

Many multicellular organisms overcome this diffusion limitation through two general mechanisms. First, a transport medium is used to contain and even concentrate nutrients. Second, this medium is delivered to the internal cells through some convective means such as active “pumping” of the medium or current-driven permeation through extracellular spaces. In vertebrates, the transport medium is blood, which contains, in addition to a vast array of molecular and cellular components, hemoglobin-carrying red blood cells. Hemoglobin in the blood overcomes the relatively low solubility of O2 in aqueous environments and creates an “O2 reservoir” that optimizes O2 diffusion gradients from blood to tissue cells. In vertebrates, the cardiovascular system serves to move the blood throughout the body, bringing nutrients into close association with tissue cells, where diffusion can then be effective.

The cardiovascular system is a closed-loop distribution system comprising a series of interconnected conduits – blood vessels – that facilitate convective flow initiated by the heart.