Abstract
The balance of force in a cylindrical tube (Oka-Azuma equation) indicates existence of a negative circumferential tension in the wall of blood vessels in vivo. However, we conventionally consider only a positive wall tension due to a positive transmural pressure in blood vessels. In this paper, the nature of such a negative circumferential tension in the vascular wall was reconsidered. The same type of negative tension or compressive force was shown to exist in any hollow and solid biological tissues placed in the atmospheric pressure. In living tissues, which have abundant water and are incompressible to external pressure, the compressive force by atmospheric pressure is simply the internal fluid pressure. The fluid pressure is a scalar and does not produce any effective net force vector. For this reason, there is no need to consider the compressive force component produced by the environmental pressure in the balance of force in living tissues.
