What Is Tissue Fluid?

Tissue fluid is a fluid that exists between cells, also known as interstitial fluid. Tissue fluid exists in a large amount in animals and plants. It is a liquid environment in which ordinary cells live directly, and can exchange materials with cells.

In animals, interstitial fluid is a portion of the fluid that penetrates into the interstitial space from the end of the capillaries of the arteries. After exchanging substances with the tissue cells, it returns to the blood or lymph through the capillaries or the capillary vessels.

Most of the interstitial fluid is in a gel state and cannot flow freely, so it will not flow to the low-hanging part of the body due to gravity. Inserting an injection needle into the interstitial space of the body cannot extract the interstitial fluid. However, the dispersed movement of water in the gel and dissolved in water and various solute molecules is not hindered by the gel, and it can still exchange materials with blood and intracellular fluid. Interstitial fluid is generated by filtering the capillary wall through the arterial end of the capillary. At the venous end of the capillary, most of the blood is absorbed back through the tube wall. ^[1]

Interstitial fluid includes matrix and macromolecule-free substances exuding from capillaries

Under normal circumstances, interstitial fluid is continuously produced from the arterial end of the capillaries. At the same time, part of the interstitial fluid returns to the capillaries through the venous end of the capillaries, and another part of the interstitial fluid returns to the blood circulation through the lymphatic vessels. Interstitial fluid is formed by filtering blood plasma through the walls of capillaries. The comparison of the two forces of filtration and reabsorption determines the direction of liquid movement. Specifically, the direction in which fluid moves inside and outside the capillary wall depends on four factors, namely capillary blood pressure, hydrostatic pressure of tissue fluid, plasma colloid osmotic pressure, and tissue fluid colloid osmotic pressure.

Among them, capillary blood pressure and interstitial fluid colloid osmotic pressure are the forces that promote the filtration of fluid from the inside of the capillary to the outside of the capillary, while plasma colloid osmotic pressure and hydrostatic fluid pressure are the forces that absorb liquid from the outside of the capillary into the blood vessel. The difference between the filtered force and the reabsorbed force is called the effective filtering pressure, which can be expressed by the following formula:

Effective filtration pressure = (capillary blood pressure ten tissue fluid colloid osmotic pressure) one (plasma colloid osmotic pressure tissue fluid hydrostatic pressure)

Human plasma colloid osmotic pressure is mainly formed by albumin in plasma, which is generally about 3.33kPa (25mmHg). Capillary blood pressure averages approximately 4.0 kPa (30 mmHg) at the arterial end, approximately 2.0 kPa (15 mm Hg) at the venous end, tissue fluid colloid osmotic pressure is approximately 2.0 kPa (15 mm Hg), and tissue hydrostatic pressure is approximately 1.33 kPa (10 mmHg). If these numbers are respectively substituted into the above formulas, the effective filtration pressure at the arterial end of the capillary is 1.33kPa (10mmHg), and the effective filtration pressure at the venous end is 1.07kPa (-8mmHg). Therefore, at the capillary arterial end, the liquid filters out the capillaries, and at the venous end, the liquid is reabsorbed. ^[2]

(1)

Comparison of similarities and differences between plasma , interstitial fluid and lymph

The same point: they are all liquid environments, which together form extracellular fluid, provide cells with a living environment, and have similar components.

difference:

1. The living cells are different. The tissue fluid is the environment in which the tissue cells live directly; plasma is the environment in which blood cells live; lymph is the environment in which lymphocytes and phagocytes live directly;

2. The three are in different environments: interstitial fluid exists in the interstitial space of tissue cells, also known as interstitial fluid; plasma exists in blood vessels; lymph exists in lymphatic vessels;

3. The protein content is different: there are more proteins in the plasma than in the lymph and interstitial fluid.

Contact: Plasma exudates from capillaries to become tissue fluid, tissue fluid enters capillary lymphatic vessels to become lymph fluid, and lymph fluid returns to the blood and becomes plasma.