The first gap: intracellular fluid; Extracellular fluid existing between tissues and cells, excluding fluid in cavity tissues and organs;
The second space: a small part of extracellular fluid existing in various cavities in the body, including pleural fluid, pericardial fluid, peritoneal fluid, joint fluid, synovial fluid and anterior water.
The first gap has two meanings: broad sense and narrow sense. The first gap in a broad sense refers to the gap between cells of the same kind that the human body naturally clings to, and is generally filled with solid, liquid or gas substances. In a narrow sense, the first gap refers to the gap between tissues and cells, which contains slow-flowing interstitial fluid and is a liquid environment for tissues and cells to survive. The second gap is relative to the first gap. The first gap refers to interstitial fluid, and the second gap refers to plasma water moving rapidly in the blood circulation system. The increase or decrease of blood volume mainly refers to the increase or decrease of plasma water. The first gap and the second gap exchange components on the capillary wall, which are in a dynamic equilibrium state and belong to functional ECF. Surgical trauma and local inflammation can transfer ECF to the injured site or infected tissue, causing local edema; Or affected by disease, anesthesia and surgery, the visceral vascular bed is dilated and congested; Or body fluids are stagnant in the cavity (such as intestinal paralysis, intestinal obstruction, and a large amount of body fluids accumulate in the gastrointestinal tract). Although this part of fluid comes from ECF, it is no longer directly related to the first gap and the second gap in function, so it is called the third gap.