The force applied by fluids against a surface, known as hydrostatic pressure, initiates the transfer of fluid among different compartments. Within our blood vessels, the blood's hydrostatic pressure is a result of the heart's pumping action. At the arteriolar end of capillaries, hydrostatic pressure (capillary blood pressure) exceeds the opposing colloid osmotic pressure created primarily by plasma proteins like albumin. This discrepancy in pressure propels plasma and nutrients from the capillaries into the surrounding tissues. The fluid and cellular wastes from the tissues reenter the capillaries at the venule end, where hydrostatic pressure is lesser compared to the osmotic pressure inside the vessel. Filtration pressure pushes fluid from the intravascular compartment to the interstitial fluid (IF) around the tissue cells. The excessive fluid in the interstitial space not directly returned to the capillaries gets drained by the lymphatic system and re-joins the vascular system at the subclavian veins. Hydrostatic pressure plays a pivotal role in managing water movement in the kidney's nephrons to ensure the proper filtration of blood to produce urine. As hydrostatic pressure in the kidneys rises, more plasma exits the capillaries, resulting in increased urine filtrate. Conversely, if the kidneys' hydrostatic pressure drops excessively, as can occur in dehydration, kidney functions may be compromised, leading to reduced removal of nitrogenous wastes from the bloodstream. Severe dehydration can potentially lead to acute kidney injury. Movement of fluid between compartments also occurs along an osmotic gradient. This gradient is formed by the difference in solute concentration on either side of a semi-permeable membrane. The strength of the osmotic gradient corresponds to the difference in solute concentration across the membrane. Water, following the principles of osmosis, moves from an area of low solute concentration to an area of high solute concentration across a semipermeable membrane. In humans, water constantly shifts between the intravascular space and IF, and IF and intracellular fluid (ICF) based on varying conditions across different body parts. For instance, during perspiration, water is lost through the skin. If excess water is lost through this process, it results in dehydration. However, when a person rehydrates by drinking water, it is redistributed by the same gradient, replenishing all body tissues with water. The cellular movement of certain solutes is an active process that requires energy, while the movement of other solutes is passive and energy-independent. Active transport allows cells to move substances against their gradient through a membrane protein, utilizing ATP. The sodium-potassium pump is an example of active transport, expelling sodium from cells and importing potassium against their respective gradients. Conversely, passive transport relies on a molecule or ion's ability to pass through the membrane and the presence of a concentration gradient that allows the molecules to move from higher to lower concentration areas. Certain molecules, like dissolved gases, lipids, and water, can easily permeate the cell membrane, while others, like glucose, amino acids, and ions, cannot and need facilitated transport to move down a concentration gradient through specific protein channels in the membrane. This process does not require energy. Glucose, for instance, is moved into cells by glucose transporters through facilitated diffusion.