Arterial End of Capillaries

Whether fluid leaves a capillary depends on the balance between two opposing pressures: hydrostatic pressure, which pushes fluid out, and oncotic pressure, which draws water back in.At the arterial end of a capillary, the blood has just come from an arteriole, so its hydrostatic pressure is high. Here the hydrostatic pressure is greater than the oncotic pressure. The difference between them is the net outward pressure, and it forces fluid out of the capillary through the gaps between the endothelial cells. This is called pressure filtration.net outward pressure = hydrostatic pressure − oncotic pressureWorked example. Hydrostatic pressure at the arterial end is 4.6 kPa and oncotic pressure is 3.3 kPa.net outward pressure = 4.6 − 3.3 = 1.3 kPaThe fluid forced out contains water and small dissolved substances such as glucose, amino acids, fatty acids, ions and oxygen. This fluid becomes tissue fluid, which delivers these substances to the cells. Red blood cells and plasma proteins stay in the capillary because they are too large to pass through the gaps.The larger the net outward pressure, the faster tissue fluid is formed. If blood pressure rises, hydrostatic pressure at the arterial end rises while oncotic pressure stays the same, so more fluid is forced out.