Intercellular cement and capillary permeability.

The passage of materials across a membrane consisting of cells, e.g., the cap& lary wall, requires a consideration of several structural tors. Environmental effects may be mediated through the exertion of may be purely nt of material passed through in a permeability characteristics of the and environmental f acchemical action on the structure of pressure on the membrane the membrane. or they may be mechanical through The effects of the mechanical factor a matter of degree, variations given time occurring without in the amou changing the membrane; or they may be qualitative, distending and thereby increasing the Even when considering the interrelations of purely porosity of the membrane. extrinsic factors such as hydrostatic and osmotic pressure, it is necessary to take into account both qualitative and quantitative effects on the fluid-exchange across the membrane. The action of specific chemical substances obviously involves qualitative changes. In this regard an understanding of the structural components of the membrane is of prime importance. Moreover, since we are dealing with a hemodynamic system, we must treat the permeability of the membrane in relation to the multiplicity of factors which control the peripheral distribution of the blood. On the basis of structural factors conditioning the passage of material across a cellular membrane we may distinguish three types (1). The permeability of one depends entirely on the cells constituting the membrane. This type is exemplified by secretory epithelium in which the constituent cells perform work in selectively absorbing material from the environment on the one side, passing it through the cytoplasm, and expelling it to the environment on the other side of the cell. An example of this kind is the wall of the renal proximal tubule in which a one-way permeability is so pronounced that even water is driven through The hydrostatic presthe tubule distends and in one direction against an osmotic pressure sure developed within such a tubule may be gradient (2). so high that the constituent cells become compressed from their normal cuboidal to a flat Moreover, this can occur without affecting the one-way, selecpavement shape. tive permeability of the membrane. Any cellular membrane which depends for its permeability on its constituent cells must be highly selective. The life of a cell is conditioned by its ability to maintain a differential between the environment of the cell and its interior. Concerning the penetration of substances into and their escape out of an individual cell, much investigation has been done and speculation a&an&i. But even the speculations have never gone so far as to assume that a cell can serve as