Which of the choices below reflects the balance (or imbalance) between the direction and amount of fluid that flows across the capillary walls?

A. plasma and formed element concentration
B. hydrostatic and osmotic pressure
C. hydrostatic pressure only
D. blood volume and viscosity

B. hydrostatic and osmotic pressure

Explanation:

The mass movement of fluids into and out of capillary beds requires a transport mechanism far more efficient than mere diffusion. This movement often referred to as bulk flow, involves two pressure-driven mechanisms: Volumes of fluid move from an area of higher pressure in a capillary bed to an area of lower pressure in the tissues via filtration. In contrast, the movement of fluid from an area of higher pressure in the tissues into an area of lower pressure in the capillaries is reabsorption. Two types of pressure interact to drive each of these movements: HYDROSTATIC PRESSURE AND OSMOTIC PRESSURE.

The primary force driving fluid transport between the capillaries and tissues is HYDROSTATIC PRESSURE, which can be defined as the pressure of any fluid enclosed in a space. Blood hydrostatic pressure is the force exerted by the blood confined within blood vessels or heart chambers. Even more specifically, the pressure exerted by blood against the wall of a capillary is called capillary hydrostatic pressure (CHP) and is the same as capillary blood pressure. CHP is the force that drives fluid out of capillaries and into the tissues.

The net pressure that drives reabsorption—the movement of fluid from the interstitial fluid back into the capillaries—is called OSMOTIC PRESSURE (sometimes referred to as oncotic pressure). Whereas hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws fluid back in. Osmotic pressure is determined by osmotic concentration gradients, that is, the difference in the solute-to-water concentrations in the blood and tissue fluid. A region higher in solute concentration (and lower in water concentration) draws water across a semipermeable membrane from a region higher in water concentration (and lower in solute concentration).