What Is Lymphatic Circulation?

Lymph circulation is an important auxiliary part of the circulatory system, which can be considered as a complement to the (blood) circulatory system.

Lymphatic circulation

Lymph circulation is an important auxiliary part of the circulatory system, which can be considered as a complement to the (blood) circulatory system.

Chinese name

Lymphatic circulation

Existing species

mammal

Lymphogenesis

Interstitial fluid enters the capillary lymphatics

Meaning

Maintain normal life activities

In mammals, the lymphatic network and lymph organs (lymph nodes, spleen, etc.) are widely distributed throughout the body. The thinnest lymphatic vessels are called capillary lymphatic vessels. Except for cartilage, cornea, lens, inner ear, and placenta, the human body has capillary lymphatic vessels, the number of which is similar to that of capillaries. Capillary lymphatics in the small intestine are called chyle ducts. Capillary lymphatic vessels are assembled into a lymphatic network and then merged into lymphatic vessels. According to its location, it can be divided into deep and superficial lymphatic vessels: superficial lymphatic vessels collect the lymph fluid of the skin and subcutaneous tissue (referred to as lymphatics); deep lymphatic vessels accompany deep blood vessels to collect the lymph in muscles, internal organs, etc. All lymphatic vessels converge into the two largest lymphatic ducts in the whole body, namely the left thoracic duct and the right right duct, and enter the left and right subclavian veins respectively (see picture). The thoracic duct is the thickest and longest lymphatic tube in the whole body, which is formed by the left and right lumbar lymphatic and intestinal lymphatics. In the lower section there is an enlarged chyle pond. Thoracic ducts also collected lymph in the upper left and lower body, which accounted for about 3/4 of the total lymphatic body. The right lymphatic duct is composed of the right cervical lymphatic stem, right subclavian lymphatic stem, and right bronchial mediastinal lymphatic stem. Lymphatic upper right lymphatics are collected, which accounts for about 1/4 of the total lymphatic body. An important feature of the lymphatic circulation is one-way flow without forming a true circulation.

In lower vertebrates, the lymphatic system of some teleosts and amphibians has a pulsating lymph heart, which can be one of the motive forces for lymph flow. The lymphatic heart of higher animals, such as mammals, has no smooth muscle layer and cannot contract with the exception of larger lymph vessels. Therefore, lymph flow is mainly driven by external forces. The main driving force is the contraction of skeletal muscle in the site of lymphatic vessels. Hydrostatic pressure gradients between active and different parts of the lymphatic vessels, in addition, there are some minor auxiliary forces.

Skeletal muscle contraction: Squeezing the lymphatic vessels to promote lymphatic flow. The medium-sized lymphatic vessels in the human body have a lymphatic flow rate of about 1.5 milliliters per minute when skeletal muscles perform moderate motions. The effect of skeletal muscle contraction is so great that the intense activity of skeletal muscles during sleep can maintain normal lymphatic flow. Conversely, standing for a long time can make lymphatic reflux of the lower limbs difficult, resulting in lower limb edema. This is due to the lack of motility of the lymph to form lymph and stagnation of interstitial fluid.

The role of hydrostatic pressure gradient: from capillary lymphatic vessels to general lymphatic vessels, and finally to the left and right lymph ducts, the hydrostatic pressure of lymph gradually decreases, forming a pressure gradient. Negative pressure can be reduced near the subclavian vein. This pressure gradient provides another motive force for lymphatic flow. For example, the hydrostatic pressure of capillary lymph vessels in mice and rabbits is about 1.9 cm H2O. Generally, the hydrostatic pressure of lymphatic vessels is about 1.4 cm H.O., which can be reduced to 0.58 cm H.O. When inhaling, the chest cavity expands, the intrathoracic pressure decreases, and negative pressure is generated. The hydrostatic pressure of the lymphatic duct also decreases, which further increases the pressure gradient. The lymphatic duct passively expands, acting like a sucker. Lymphatic aspiration lymphatic duct.

Other factors that promote lymphatic flow: The larger lymphatic wall has a smooth muscle layer, which is subject to sympathetic innervation. When the sympathetic nerve is excited, it can contract the smooth muscle layer and promote lymphatic flow. In 1976, Gayton found that small lymphatic vessels were also often in a continuous rhythmic contraction state, which produced suction, leading to a weak negative pressure, which was beneficial for aspiration of interstitial fluid, indicating that lymph flow was not completely passive, but had its weak Active factors.

The unidirectional flow of lymph, from capillary lymphatic vessels to general lymphatic vessels and even lymphatic vessels will not flow back, mainly due to the pressure gradient of valves and lymphatic vessels in thicker lymphatic vessels. It can be seen from Figure 3a that the capillary lymphatic vessel is a closed blind end with a large and irregular lumen. Although it is also composed of a layer of endothelial cells like capillaries, its endothelial cells are relatively flat. The cells are not completely closed, but cover each other like fish scales, loosely joined and form many small holes. There are overlaps between adjacent cells, where the small holes open. This structural feature is conducive to the penetration of interstitial fluid into the capillary lymphatics to form lymphatics. When the hydrostatic pressure in the lymphatics increases and there is a tendency to flow back, the overlapping parts will be squeezed to close the small holes, thereby preventing the lymph from leaking out of the vessel. outer. A unidirectional flow that only enters and exits is formed. In addition, the capillary wall has no basement membrane, so it is highly permeable and also helps the interstitial fluid to enter the capillary lymphatics.

The main structure that maintains the unidirectional flow of lymph is the large number of valves in the large lymphatic vessels. The general structure of the valve is a pair of overhanging sheets. When the lymphatic reflux occurs, the small bag formed by the valve will be quickly filled first, so that the free edge of the valve is squeezed. Together, the lymphatic vessels are closed, thereby preventing the development of reflux. However, this valve cannot resist excessive reflux pressure, and when the reflux pressure is too large, lymphatic reflux can occur.

Mammalian lymphatic vessels with a diameter of 100 to 200 microns have dispersed smooth muscle cells (endothelial layers), which can contract and promote lymph flow.

The lymphatic flow of the heart varies with the heart rate and the strength of the myocardial contraction. After injecting smooth muscle contraction drugs such as pilocarpine, muscarin, or posterior pituitary, the lymphatic flow in the small intestine area is greatly increased, and the lymph flow in the chyle duct is increased by the contraction of the small intestinal villi.

Anything that increases the effective filtration pressure of the capillaries will increase lymphatic flow. Increasing venous pressure and thus capillary pressure are particularly effective for increasing lymphatic flow. If the inferior vena cava is blocked above the entrance of the hepatic vein, the lymphatic flow in the thoracic duct will increase significantly due to the increase in the capillary pressure of the liver. The flow increases by 4 to 5 times. The increase in arterial pressure has little effect on lymph flow, but a significant decrease in arterial pressure will greatly reduce lymph flow and even stop the flow. High temperature (to 45 ° C) and low temperature (to 5 ° C) increase the effective filtration pressure of capillaries, resulting in increased lymphatic flow.

Some lymphotropic agents are toxic to capillaries, which have a certain damaging effect on the capillary wall, can increase the permeability of the capillary wall, increase the formation of interstitial fluid and lymph, and cause increased lymphatic flow. Saponin and snake venom belong to this. Another type of lymphotropic agent is non-toxic, but it can also promote lymphogenesis and increased flow, such as hypertonic sodium chloride and hypertonic glucose solution. When injected into the blood, it will quickly pass through the blood vessel wall and enter the interstitial space, improving interstitial fluid The osmotic pressure increases the water content in the interstitial fluid, resulting in increased lymphogenesis and significantly increased lymphatic flow.

Under normal physiological conditions, lymphatic flow and flow velocity are not large. The lymphatic flow rate in a dog's thoracic duct is about 0.06 ml per minute. In a fasting and quiet situation, about 1.0 to 1.5 ml per minute. In normal adults, the total amount of lymph that enters the subclavian vein through two major lymphatic ducts per hour is about 120 ml, of which about 100 ml per hour flows through the thoracic duct, and the rest flows from the right lymph duct. The difference is the area of the two ducts Caused by the size. The speed of lymph flow has nothing to do with the weight of the animal or its heart, but it is closely related to the functional status of the measured organs. For example, when the dog's limbs are at rest and walking, the lymph flow velocity is significantly different. The flow rate can be increased to 0.06 ml per minute.

Using a cannula to connect a water manometer can record the lymph pressure in each part, and found that the lymph pressure in different parts is very different. The internal pressure of the thoracic duct and the beating heart's lymphatic vessels is 15 cm of water column, while the internal pressure of the chylo canal in the small intestine can reach 40 cm of water column. The higher the pressure, the faster the flow rate. Lymphatic fish and amphibians are driven by lymphatic heartbeats to move lymph more quickly.

Capillary lymphatic vessels are blind-ended ducts closed at one end. The tube wall is composed of a single layer of flat endothelial cells. The endothelial cells are not directly connected to each other, but they cover each other to form a one-way valve that opens in the tube. The tissue fluid can only flow in. But it cannot be reversed. The pressure difference between the interstitial fluid and the capillary lymphatics is the driving force for the interstitial fluid to enter the lymphatics. Proteins and their metabolites in the tissue fluid, leaked red blood cells, invading bacteria, and small fat droplets that are digested and absorbed easily enter the capillary lymphocytes through the intercellular space.

The lymph fluid flows into the collective lymphatic vessels after the formation of capillary lymphatic vessels, and the collective lymphatic vessels of the whole body finally merge into two major trunks, namely the thoracic duct and the right lymph duct, which enter the blood circulation at the confluence of the subclavian vein and the internal jugular vein on both sides, respectively. . Therefore, the lymphatic system is an important auxiliary system for the return of tissue fluid to the blood circulation.

Lymph circulation has important physiological significance for maintaining normal life activities of the body:

(1) Recovered protein: About 75-200g of protein in tissue fluid is recovered from the lymph fluid into the blood each day, keeping the colloid osmotic pressure of tissue fluid at a low level, which is beneficial to the reabsorption of tissue fluid by capillaries.

(2) Transporting fat: 80%-90% of fat absorbed by the small intestine is absorbed by capillary lymph vessels of the small intestine

(3) Adjust the liquid balance between plasma and interstitial fluid: the total amount of fluid filtered at the capillary artery end every day is about 2 ~ 4L, of which about 3L is returned to the blood through lymph circulation. That is, the amount of lymph fluid returned in one day is approximately equivalent to the total amount of plasma in the whole body.

(4) Elimination of red blood cells, bacteria and other particles in tissues: This body defense and barrier function is mainly related to the phagocytosis of macrophages in lymph nodes and the immune response produced by lymphocytes.