Interstitial fluid and the lymph

Definition The fluid found in the intercellular spaces composed of: water, amino acids, sugars, fatty acids, coenzymes, hormones, neurotransmitters, salts, and cellular products.

It bathes and surrounds the cells of the body Provides a means of delivering materials to the cells intercellular communication, and removal of metabolic waste

Plasma and interstitial fluid Plasma and interstitial fluid are very similar. Plasma, the major component in blood, communicates freely with interstitial fluid through pores and intercellular clefts in capillary endothelium

Capillaries are composed of a single layer of epithelium surrounded by a thin basement membrane. Most capillaries have pores (spaces) between the individual cells that make up the capillary wall. Plasma fluid and small nutrient molecules leave the capillary and enter the interstitial fluid through these pores, in a process called bulk flow. Bulk flow facilitates the efficient transfer of nutrient out of the blood and into the tissues. However, blood cells and plasma proteins, which are too large to fit through the pores, do not filter out of the capillaries by bulk flow.

Blood and interstitial fluid Red blood cells, platelets and plasma proteins cannot pass through the walls of the capillaries. The resulting mixture that does pass through is essentially blood plasma without the plasma proteins. Tissue fluid also contains some types of white blood cell, which help combat infection.

Together, blood plasma and interstitial fluid make up the extracellular fluid (ECF). Plasma constitutes 20%, while interstitial fluid constitutes 80% of the ECF. The distribution of extracellular fluid between these two compartments is determined by the balance between two opposing forces: hydrostatic pressure and osmotic pressure

The lymph

Definition Lymph is considered a part of the interstitial fluid. The lymphatic system returns protein and excess interstitial fluid to the circulation.

The beating of the heart generates hydrostatic pressure, which causes bulk flow of fluid from plasma to interstitial fluid through walls of the capillaries. The pressure in the system forces plasma to filter out into the interstitial compartment. The composition of the interstitial fluid and the plasma is essentially the same except that plasma also contains plasma proteins not found in the interstitial fluid. plasma has a higher solute concentration than does the interstitial fluid. Consequently, osmotic pressure causes interstitial fluid to be absorbed into the plasma compartment. In other words, the plasma proteins drive the reabsorption of water back into the capillaries via osmosis.

The magnitudes of filtration and absorption are not equal The magnitudes of filtration and absorption are not equal. The net filtration of fluid out of the capillaries into the interstitial compartment is greater than the net absorption of fluid back into the capillaries. The excess filtered fluid is returned to the blood stream via the lymphatic system. In addition to its roles in digestion and immunity, the lymphatic system functions to return filtered plasma back to the circulatory system. The smallest vessels of the lymphatic system are the lymphatic capillaries (shown in yellow). These porous, blind-ended ducts form a large network of vessels that infiltrate the capillary beds of most organs. Excess interstitial fluid enters the lymphatic capillaries to become lymph fluid

How does the lymphatic system works? The clear, watery blood plasma -- containing the oxygen, proteins, glucose and white blood cells -- "leaks" out through the capillary walls and flows around all the cells. The pores in the capillaries are too small to let red blood cells through, however All of the cells in your body are therefore bathed in lymph, and they receive their nutrients and oxygen from the lymph.

Somehow, all of this lymph has to end up somewhere, so it is recirculated. The lymph capillaries and vessels pick up the lymph fluid and start pumping it away from the cells. Lymph vessels do not have an active pump like the heart. Instead, lymph vessels have muscle motion that pumps the lymph. You have just as many lymph vessels and capillaries as you have blood vessels and capillaries!

Lymph in the lymph vessels eventually reaches a lymph node -- there are about 100 nodes scattered throughout the body. Lymph nodes filter the lymph and also contain large numbers of white blood cells (a big part of the immune system), which remove foreign cells and debris from the lymph. When you get certain infections, the lymph nodes swell with billions of white blood cells working to clear the foreign cells causing the infection. The filtered lymph then flows back into the blood stream at certain points.

Functions of the lymphatic system: 1) To maintain the pressure and volume of the extracellular fluid by returning excess water and dissolved substances from the interstitial fluid to the circulation. 2) lymph nodes and other lymphoid tissues are the site of clonal production of immuno competent  lymphocytes and macrophages in the specific immune response.

Filtration forces water and dissolved substances from the capillaries into the interstitial fluid. Not all of this water is returned to the blood by osmosis, and excess fluid is picked up by lymph capillaries to become lymph. From lymph capillaries fluid flows into lymph veins (lymphatic vessels) which virtually parallel the circulatory veins and are structurally very similar to them.

Lymph nodes Lymph nodes lie along the lymph veins successively filtering lymph. Afferent lymph veins enter each node, efferent veins lead to the next node becoming afferent veins upon reaching it. Lympho kinetic motion (flow of the lymph) due to: 1) Lymph flows down the pressure gradient. 2) Muscular and respiratory pumps push lymph forward due to function of the semilunar valves.

Lymph nodes: are small encapsulated organs located along the pathway of lymphatic vessels. They vary from about 1 mm to 1 to 2 cm in diameter and are widely distributed throughout the body, with large concentrations occurring in the areas of convergence of lymph vessels. They serve as filters through which lymph percolates on its way to the blood.