As fluid moves through the capillary, the hydrostatic pressure decreases from the arteriole end to the venule end (fluid exits the capillary along the gradient). The osmotic pressure in the interstitium is relatively constant, and will be stronger than capillary hydrostatic pressure near the venule end. As a result, an increase in the interstitial osmotic pressure would cause less fluid to enter the interstitium, because there is less area in the bed where the capillary hydrostatic pressure is greater than the interstitial osmotic pressure.

The capillary is the site of fluid exchange with the body's tissues. This fluid transfer is moderated by two factors: hydrostatic pressure and osmotic pressure. Hydrostatic pressure is the "pushing" force on water due to the presence of more fluid in one region than another. In general, larger fluid volumes generate higher hydrostatic pressure. Osmotic pressure is the "pulling" force on water due to the presence of solutes in solution. Albumin proteins are the main source of osmotic pressure in capillaries, pulling water into the blood.

At the arteriole end of the capillary, the hydrostatic pressure is stronger than the interstitial osmotic pressure and fluid is forced into the interstitium. Osmotic pressure remains relatively constant over the length of the capillary, but hydrostatic pressure drops sharply as it nears the venule end due to the initial loss of fluid volume. At that point, the interstitial osmotic pressure becomes stronger than the capillary's hydrostatic pressure. This forces fluid back into the capillary.

Hydrostatic pressure is the force of the fluid volume against a membrane, while osmotic pressure is related to the protein concentration on either side of a membrane pulling water toward the region of greater concentration.

When fluid enters the capillaries, it is initially pushed out because the hydrostatic pressure pressing outward is greater than the osmotic pressure pushing inward. Although osmotic pressure stays constant throughout the capillary length, hydrostatic pressure decreases towards the venule end of the capillary. This makes the osmotic pressure larger than the hydrostatic pressure, and pushes the fluid back into the capillary. 

Ocean water has a higher osmolarity (more units of solute per unit of solvent) than human blood. When James drank the ocean water, it was absorbed into his circulatory system and it pulled water from the cells. Water flows from an area of low osmolarity to an area of high osmolarity. When water was pulled from the cells, the fluid volume in James’ blood increased. As blood reached James’ kidneys, the extra fluid from the tissues was filtered into the urine and caused him to urinate more frequently.