Usually blood pressure is determined for the big arteries relatively close to the aorta since that is the pressure that determines blood flow through the systemic circulation. The pulse represents the pressure wave transmitted through the fluid-filled arteries as the ventricle pushes blood into the aorta with each heartbeat. The capillaries offer a huge surface area for exchange of nutrients, water, oxygen, carbon dioxide, waste materials, etc. While small molecules like those in the previous sentence can easily move across capillary walls, usually plasma proteins and blood cells are trapped inside the capillaries. The movement of water and other small molecules between the inside of the capillary and the interstitial fluid surrounding the capillary is due to a balance of forces across the capillary wall .
Bulk flow is a passive transport process similar to simple diffusion.
Movement of water and solutes out of the capillary is called filtration , while movement of water and solutes into the capillary is called absorption . The direction for the flux of water and solutes across the capillary wall is determined by comparing the hydrostatic pressure difference across the capillary wall and the osmotic pressure difference across the capillary wall. The osmotic pressure difference is the difference between the osmotic pressure of the liquid inside the capillary and the osmotic pressure of the liquid outside the capillary. Normally due to the trapped plasma proteins inside the capillary, the osmotic pressure due to large protein molecules there is considerably higher than in the interstitium surrounding the capillary. Excess water and solutes in the interstitium due to filtration out of the capillaries are removed by the lymphatic system.