 Capillary Dynamics In this learning object, we will discuss the movement of fluid within the vascular system. Capillaries are part of the vascular system and connect arterioles with venules. Remember, within the vascular system, only capillaries have walls thin enough to let solutes pass through. This movement of fluid and solutes plays a critical role in maintaining fluid balance within the body. The primary critical function of the capillaries is to deliver nutrients and remove wastes between the blood and the tissue cells of the body. This movement of nutrients and wastes is not simply dependent on the porous capillary. Hydrostatic and osmotic pressures inside the capillary and outside the capillary in the tissues work together to maintain fluid balance. Let's take a look. When the hydrostatic pressure builds inside a capillary, it causes filtration, forcing fluids and solutes out through the capillary walls into the tissues, thus delivering nutrients. When the pressure inside the capillary is less than the pressure outside it, fluids and solutes move back into the capillary. Don't forget that osmotic pressures are at work within the capillary also. Albumin, a plasma colloid, constitutes about 7% of the blood plasma. It is a large protein molecule that can't pass through the walls of the capillary. Think of albumin as a water magnet, attracting water and holding it inside the capillary vessel. Albumin helps to maintain the capillary blood pressure by preventing too much fluid from leaving. However, as long as the hydrostatic pressure is slightly greater than the albumin colloid and the osmotic pressure at the arterial end, water and nutrients will leave the capillaries and move to the tissues. Capillary hydrostatic pressure starts to fall below the albumin colloidal pressure at the venial end of the capillary, resulting in waste fluids moving or being absorbed into the capillary. These waste fluids ultimately drain into the venous system. In summary, capillary filtration occurs along the first half of the vessel and reabsorption along the second half of the vessel. This delicate dance can be altered by any number of factors that influence fluid or solute balance such as renal and heart function. Let's take a closer look. A indicates the amount of plasma hydrostatic pressure. This is an outward force. Fluids and nutrients are being pushed from the vessel to the tissues. This is happening at the arterial end. B indicates the amount of plasma osmotic pressure. This is an inward force. Abumin with the capillary is pulling water from the tissues. This is happening throughout the capillary but exerts greater pressure at the venial end. C indicates the amount of tissue hydrostatic pressure. This is an inward force. Waste fluids are being pushed by the tissues into the capillary. This is happening at the venial end. D indicates the amount of tissue osmotic pressure. This is an outward force. Fluids and nutrients are being pulled by the tissues from the capillary. This is happening at the arterial end. Let's illustrate the factors we discussed by calculating some pressure gradients. In doing so, you can determine which direction fluids will be moving within the capillary bed. Remember, you always total the outward forces and subtract the total inward forces. After we subtract the total inward forces from the total outward forces, we can determine the total forces moving the liquid. If the total forces moving the liquid is a positive number, fluid moves out of the vessel. When the total forces moving the liquid is a negative number, the net movement of fluid will be into the vessel. This concludes Capillary Dynamics.