Final answer:
Plasma and interstitial space are balanced by several forces, including the selective permeability of capillary walls and the opposing forces within the thorax. Fluid movement between the capillaries and interstitial space is controlled by hydrostatic pressure and osmotic pressure.
Step-by-step explanation:
Plasma and interstitial space are balanced by several forces. The capillary walls, which separate plasma and interstitial fluid, are permeable to water, inorganic ions, and small organic molecules. However, they are not permeable to large organic molecules like proteins. This selective permeability helps maintain the balance between these two fluids.
The forces within the thorax help create the negative intrapleural pressure, which balances the pressure between the lungs and the thoracic wall. Elastic tissue pulls the lungs inward, while surface tension of alveolar fluid creates an inward pull. These forces are counteracted by opposing forces from the pleural fluid and the thoracic wall. The outward pull from the pleural cavity and the natural elasticity of the chest wall helps maintain the balance.
Fluid movement between the capillaries and interstitial space is controlled by hydrostatic pressure and osmotic pressure. Hydrostatic pressure in the capillaries pushes fluid out into the interstitial space, while osmotic pressure draws fluid back into the capillaries. These pressure differences help maintain the balance between plasma and interstitial fluid.