Tissue fluid

Tissue fluid is the fluid that surrounds cells in tissues. It fills the spaces between body cells. For cells, tissue fluid is their immediate environment. In mammals, this fluid plays a crucial role as part of the specialized mass transport system that carries raw materials to body cells and removes metabolic wastes.

Formation of Tissue Fluid

Tissue fluid is made from small molecules that leave the blood plasma. This process occurs in capillary beds by pressure filtration. At the arteriole end of a capillary bed, the hydrostatic (liquid) pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid. This difference in pressure forces fluid out of the capillaries and into the spaces around the cells.

Substances like oxygen, water, and nutrients (such as glucose and amino acids) are pushed out of the capillaries. Capillary walls are remarkably thin, only one cell thick (squamous epithelium), which shortens the diffusion pathway for efficient exchange. The narrow lumen of capillaries also slows down blood flow, increasing time for exchange. Small gaps exist between the endothelial cells of capillary walls, allowing plasma to leak out.

Composition of Tissue Fluid

Tissue fluid is very similar in composition to blood plasma. However, it differs from blood in that it does not contain red blood cells or big proteins, because these components are too large to be pushed out through the capillary walls. While it contains fewer protein molecules than blood plasma, it still contains some, just not large ones. Some white blood cells, however, can squeeze through capillary walls and move freely in tissue fluid. The components of blood not found in tissue fluid are mainly blood proteins (like albumin), red blood cells, and platelets.

Tissue fluid is mainly water, which serves as a solvent for polar substances like glucose, amino acids, salts, and water-soluble vitamins, making it ideal for transport. Water's high specific heat capacity in tissue fluid also helps the body maintain a relatively constant temperature.

Function of Tissue Fluid

Tissue fluid forms the immediate environment of body cells. It is the primary site for efficient exchange of substances between the blood and the surrounding tissue cells. Cells take in oxygen and nutrients from the tissue fluid, and release metabolic waste (like carbon dioxide) into it. Maintaining constant conditions in tissue fluid (such as pH, temperature, water potential, and glucose concentration) is vital for efficient cell function and enzyme activity.

Return of Tissue Fluid and Lymphatic System

As fluid leaves the capillaries, the hydrostatic pressure within them reduces. Due to this fluid loss and the increasing concentration of plasma proteins (which remain in the capillaries), the water potential at the venule end of the capillary bed is lower than the water potential in the tissue fluid. This difference means that some water re-enters the capillaries from the tissue fluid by osmosis. Approximately 90% of the tissue fluid returns to the blood capillaries by this route.

Any excess tissue fluid that does not return to the capillaries is drained into the lymphatic system. Once it enters the lymphatic vessels, it is called lymph.

The lymphatic system is a network of blind-ending vessels. Like veins, lymphatic vessels contain valves at intervals to prevent the backflow of fluid. The flow of lymph is aided by the contraction of surrounding body muscles. Lymphatics eventually return the lymph back to the circulatory system, typically draining into the subclavian veins near the heart.

Lymph is virtually identical to tissue fluid but may contain more large proteins (those too big for blood capillaries) and white blood cells (lymphocytes and macrophages). Lymph nodes are present along the lymphatic vessels, where phagocytic macrophages engulf bacteria and cell detritus, and immune system cells (lymphocytes and plasma cells) are found. After digestion, lipids are also transported from the intestines to the bloodstream via the lymph system.

Consequences of Imbalance (Oedema)

If the volume of fluid that filters out of capillaries exceeds what is reabsorbed, it leads to a greatly increased volume of tissue fluid, causing swelling of the tissues, known as oedema. This can be caused by increased blood pressure (high hydrostatic pressure in capillaries) or increased permeability of capillary walls. A diet chronically deficient in protein (e.g., in conditions like kwashiorkor) can also cause oedema, as low plasma protein concentrations lead to less water being drawn back into capillaries by osmosis.