About 100 milliliters per hour of lymph flows through the thoracic duct of a resting human, and approximately another 20 milliliters flows into the circulation each hour through other channels, making a total estimated lymph f low of about 120 ml/hr or 2 to 3 liters per day.

Effect of Interstitial Fluid Pressure on Lymph Flow. Figure 1 shows the effect of different levels of interstitial fluid pressure on lymph flow as measured in animals. Note that normal lymph flow is very little at interstitial fluid pressures more negative than the normal value of −6 mm Hg. Then, as the pressure rises to 0 mm Hg (atmospheric pressure), flow increases more than 20-fold. Therefore, any factor that increases interstitial fluid pressure also increases lymph flow if the lymph vessels are functioning normally. Such factors include the following:

• Elevated capillary hydrostatic pressure

• Decreased plasma colloid osmotic pressure

• Increased interstitial fluid colloid osmotic pressure

• Increased permeability of the capillaries

Fig1. Relation between interstitial fluid pressure and lymph  f low in the leg of a dog. Note that lymph flow reaches a maximum  when the interstitial pressure, PT, rises slightly above atmospheric  pressure (0 mm Hg). (Courtesy Drs. Harry Gibson and Aubrey Taylor.)

All of these factors cause a balance of fluid exchange at the blood capillary membrane to favor fluid movement into the interstitium, thus increasing interstitial fluid volume, interstitial fluid pressure, and lymph flow all at the same time.

However, note in Figure 1 that when the interstitial fluid pressure becomes 1 or 2 mm Hg greater than atmospheric pressure (>0 mm Hg), lymph flow fails to rise any further at still higher pressures. This results from the fact that the increasing tissue pressure not only increases entry of fluid into the lymphatic capillaries but also compresses the outside surfaces of the larger lymphatics, thus impeding lymph flow. At the higher pressures, these two factors balance each other almost exactly, so lymph flow reaches a maximum flow rate. This maximum flow rate is illustrated by the upper level plateau in Figure 1.

Lymphatic Pump Increases Lymph Flow. Valves exist in all lymph channels. Figure 2 shows typical valves in collecting lymphatics into which the lymphatic capillaries empty.

Fig2. Structure of lymphatic capillaries and a collecting lymphatic, with the lymphatic valves also shown. 

Motion pictures of exposed lymph vessels in animals and in humans show that when a collecting lymphatic or larger lymph vessel becomes stretched with fluid, the smooth muscle in the wall of the vessel automatically contracts. Furthermore, each segment of the lymph vessel between successive valves functions as a separate automatic pump. That is, even slight filling of a segment causes it to contract, and the fluid is pumped through the next valve into the next lymphatic segment. This fluid fills the subsequent segment and a few seconds later it, too, contracts, the process continuing all along the lymph vessel until the fluid is finally emptied into the blood circulation. In a very large lymph vessel such as the thoracic duct, this lymphatic pump can generate pressures as great as 50 to 100 mm Hg.

Pumping Caused by External Intermittent Com pression of the Lymphatics. In addition to the pumping caused by intrinsic intermittent contraction of the lymph vessel walls, any external factor that intermittently com presses the lymph vessel also can cause pumping. In order of their importance, such factors are as follows:

• Contraction of surrounding skeletal muscles

 • Movement of the parts of the body

• Pulsations of arteries adjacent to the lymphatics

• Compression of the tissues by objects outside the body

The lymphatic pump becomes very active during exercise, often increasing lymph flow 10- to 30-fold. Conversely, during periods of rest, lymph flow is sluggish (almost zero).

Lymphatic Capillary Pump. The terminal lymphatic capillary is also capable of pumping lymph, in addition to the pumping by the larger lymph vessels. As explained earlier in the chapter, the walls of the lymphatic capillaries are tightly adherent to the surrounding tissue cells by means of their anchoring filaments. Therefore, each time excess fluid enters the tissue and causes the tissue to swell, the anchoring filaments pull on the wall of the lymphatic capillary and fluid flows into the terminal lymphatic capillary through the junctions between the endothelial cells. Then, when the tissue is compressed, the pressure inside the capillary increases and causes the overlapping edges of the endothelial cells to close like valves. Therefore, the pressure pushes the lymph forward into the collecting lymphatic instead of backward through the cell junctions.

The lymphatic capillary endothelial cells also contain a few contractile actomyosin filaments. In some animal tissues (e.g., the bat’s wing), these filaments have been observed to cause rhythmical contraction of the lymphatic capillaries in the same rhythmic way that many of the small blood vessels and larger lymphatic vessels contract. Therefore, it is probable that at least part of lymph pumping results from lymph capillary endothelial cell contraction in addition to contraction of the larger muscular lymphatics.

Summary of Factors That Determine Lymph Flow. From the previous discussion, one can see that the two primary factors that determine lymph flow are (1) the interstitial fluid pressure and (2) the activity of the lymphatic pump. Therefore, one can state that, roughly, the rate of lymph flow is determined by the product of interstitial fluid pressure times the activity of the lymphatic pump.

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مواضيع ذات صلة

Role of the Nervous System in Rapid Control of Arterial Pressure

Special Features of Nervous Control of Arterial Pressure

The Lymphatic System Plays A Key Role in Controlling Interstitial Fluid Protein Concentration, Volume, and Pressure

Formation of Lymph

Exchange of Water, Nutrients, and Other Substances Between the Blood and Interstitial Fluid

Flow of Blood in the Capillaries—Vasomotion

Structure of the Microcirculation and Capillary System

Basic Principles of Circulatory Function

Physical Characteristics of the Circulation

Abnormal Rhythms That Result From Block of Heart Signals Within the Intracardiac Conduction Pathways Sinoatrial Block

Abnormal Sinus Rhythms Tachycardia

Flow of Current Around the Heart During the Cardiac Cycle