The fluid within the body can be seen as contained in distinct compartments. Some fluid resides in our cells in an intracellular compartment, some resides around the cells – this is interstitial fluid - and some resides within our blood vessels. There is a continuous exchange between these compartments which provides nourishment to our cells and removes waste products.
Respiration involves the exchange of oxygen for carbon dioxide to and from the body's tissues. Since 02 and CO2 are carried by the blood, they diffuse in and out of the interstitial fluid through the pores in the capillary wall. They do this through the movement of blood through the capillaries.
To see how this happens requires an understanding of the forces that affect movement across the capillary wall. Two forces are involved that work against each other to produce this movement. These are hydrostatic pressure, and oncotic pressure.