Its Formation and Movement Lymph

Return of Flui

courses are open for the return of fluid. It may go back directly into the blood ves sels or pass into the lymphatics. That substances can be ab sorbed directly into the blood vessels has been quite conclusively demonstrated. The explanation of the forces involved in their passage was supplied by Starling. He showed that the proteins of blood plasma possess an osmotic pressure of between 25 and 3o mm. Hg. It is estimated that the hydrostatic pressure of the blood at the arterial end of a capillary is above this figure, while at the venous end it is below. At the arterial end, therefore, the excess of hydrostatic pressure causes the fluid to pass from the capillary to the tissue spaces. At the venous end the excess of osmotic pressure produces the reverse phenomenon and fluid passes from the tissue spaces into the capillary.

This conception has recently been very beautifully confirmed in the frog by Landis who has directly measured the intracapil lary pressure and found, when compared with the osmotic pres sure of the proteins, to bear out the above relationship. One can thus understand that a constant interchange between blood and tissue fluid can take place without the production of a drop of lymph. Indeed this is the case in a limb at rest. In the muscular tissue of this part the blood capillaries are relatively imperme able so that a considerable colloid osmotic pressure is available for the work of resorption.

Function of the Lymphatics.

If, however, the limb is thrown into action the arterioles and capillaries of the part di late and the hydrostatic pressure in them consequently increases.

That is to say the length of capillary over which resorption can occur by virtue of the excess of osmotic over hydrostatic pres sure is lessened so that fluid tends to accumulate in the tissue spaces. As will be shown later this is not the only cause of ac cumulation of fluid, but it is under these conditions that a flow of lymph occurs.

In the liver the blood capillaries are extremely permeable, so that very little osmotic pressure is available for the resorption of fluid. This excess fluid which cannot return to the blood vessels passes into the lymphatics and is continuously drained away in that fashion. It would appear then that the lymphatics are a system for draining the tissues of excess fluid and so preventing them from becoming oedematous and waterlogged.

Metabolism and Lymph Flow.

Apart from the forgoing explanation the following considerations are of great importance.

An organ or tissue in activity is breaking down larger into a greater number of smaller molecules, that is, the metabolic processes result in the production of a fluid possessing greater osmotic pressure than the fluid of the resting tissue. This increase of osmotic pressure attracts water from the blood, and so increases the amount of fluid in the tissue.

The effects of metabolism on lymph flow can be very well seen in the case of the submaxillary gland. During the secretion of saliva, consequent on stimulation of the chorda tympani nerve there is a great increase of the lymph flow from the gland. If now the secretory apparatus is paralysed by atropine so that stimulation of the chorda tympani no longer evokes a secretion of saliva, there is no increase in lymph outflow, though the di lator action of the nerve on the blood vessels remains unim paired. Obviously the activity of the salivary secretory cells is the determining factor of the increased lymph output.

It is to be emphasized that tissue metabolism is of the utmost importance in the production of lymph not only quantitatively but qualitatively. In consequence much recent work has been an attempt to gain a clearer insight into intermediate tissue me tabolism from a study of lymph coming from various parts. From this work it has been found that hormones such as adrenalin and insulin have considerable influence not only on the quantity of lymph produced by a part, but also on its composition with regard to proteins and salts.

The tissue cells at all times do not hold the same amount of water—in different physiological conditions different quantities are found. For example it seems probable that a change in hydro gen ion concentration is capable of altering the water balance of a tissue, and in all probability alterations in the concentration of other ions also have a like influence. Such fluctuations alter the lymph flow.

Heidenhain's Researches and Theories.—The views presented are at the present time not universally accepted as expressing the whole truth. Heidenhain, in a long series of researches, endeav oured to establish the view that lymph production depended on a secretory activity on the part of the endothelial cells of the lym phatic capillaries. That is, lymph would be a secretion and would not reflect the composition of tissue fluid. Some of the main argu ments adduced by Heidenhain depended on observations of sub stances which produced an increased lymph flow—lymphagogues —when injected intravenously.