Lymph: Formation and Functions | Body Fluids | Humans | Biology

In this article we will discuss about:- 1. Introduction to Lymph 2. Properties of Lymph 3. Composition 4. Functions 5. Rate of Flow 6. Formation 7. Circulation.

Contents:

1. Introduction to Lymph
2. Properties of Lymph
3. Composition of Lymph
4. Functions of Lymph
5. Rate of Flow in Lymph
6. Formation of Lymph
7. Circulation of Lymph

1. Introduction to Lymph:

The lymphatic vessels at the periphery are micro­scopic blind (closed) end vessels, known as lymphat­ic capillaries. These tiny vessels are situated in the intercellular spaces and their walls formed by en­dothelial cells supported by the fibrous connective tissue (Fig. 5.3).

These capillaries repeatedly join together to form bigger lymphatic vessels, which pass through the lymph nodes, receive more tribu­taries and gradually increase in size. All the lymph from the body is finally collected into two big chan­nels—the right lymphatic duct and the thoracic duct (or left lymphatic), which open respectively at the right and left subclavian veins.

The right lymphat­ic duct, about 1.25 cm long, drains from the right forelimb and the right side of the neck and chest (Fig. 5.4). The thoracic duct, being about 38—45 cm long and about 4-6 mm in diameter, emerges from the cisterna (receptaculum) chyli and also receives the left cervical duct, which collects lymph from the left forelimb, left side of the neck and chest. The cister­na chyli, being situated on the front of the body of the second lumbar vertebra, receives all the lymph coming from two hind-limbs and alimentary canal (Fig. 5.4A).

The lymphatic vessels are provided with valves which help the lymph stream to flow in the direc­tion of the chest. The primary lymphatic vessels that remain in the centre of small intestinal villi are known as lacteals and during the course of digestion lacteals are filled with milk-white fluid, chyle. The chemical composition of chyle, except for its high fat content, is similar to that of the lymph in other parts of the body. In the central nervous system there are no lymphatics.

Here, cerebrospinal fluid takes the place of lymph. Lymphatic capillar­ies are not also found in the cartilage, spleen, epidermis, internal ear and eyeball. The function of lymphatics is to carry tissue fluid from tissues to veins and the return of water and protein from the interstitial fluid to blood from which they come. And the function of lacteals is to help in the absorption of digested food materials generally fats from the intestine.

2. Properties of Lymph:

Lymph should be regarded as modified tissue fluid. Lymph is the clear watery-appearing fluid found in lymphatic vessels and is formed by the passage of substances from blood capillaries into tissue spaces. This process is known as transudation which involves the processes of diffusion and filtration. A pure sample of lymph can be obtained by inserting a canula in the thoracic duct of an animal.

Lymph, as collected from thoracic duct during fasting, is transparent, yellowish in colour, faintly alkaline in reaction and clots slowly. Its colloidal osmotic pressure is lower than that of plasma but is believed to be higher than that of the tissue fluid. Its hydrostatic pressure is very low. After a fatty food, the lymph of the thoracic duct appears milky due to the presence of minute droplets of emulsified fat absorbed from the alimentary canal.

3. Composition of Lymph:

Microscopic examination of lymph depicts that it contains a large number of leucocytes (mostly lymphocytes) ranging from 500 to 75,000 per cu. mm. No blood platelets present.

The composition of the non-cellular part of lymph (fasting) is as follows:

i. Water-94%.

ii. Solids-6%.

Solids:

i. Protein:

Total protein content is roughly half that of plasma and varies from 2.0—4.5%. It varies according to the part of the body from which it is collected and also according to the degree of activity of the region. Lymph from the liver contains three times (6%) as much proteins as that coming from the limb (2%).

Lymph from the intestine contains protein which is intermediate between these two (4%). Three varieties of proteins are found— albumin, globulin and fibrinogen. In addition to this, traces of prothrombin are also found. Fibrinogen content is very low. Probably it is due to this, that lymph coagulates very slowly. Albumin is proportionally much more than globulin, as compared with plasma.

The albumin/globulin ratio, which is about 1.5:1.0 in plasma, is much higher in the lymph. The protein content of lymph is higher than that of tissue fluid. But since lymph is derived from tissue fluid, this difference is not easily understood. It has been suggested that water is possibly removed from the lymph as it flows along the lymphatics; in this way, the proteins become concentrated. The higher amount of albumin is supposed to be due to its lesser molecular weight and size, and consequently higher rate of diffusion.

ii. Fats:

In fasting condition fat content is low but after a fatty diet it may be 5.0—15%.

iii. Carbohydrates:

Sugar, 132.2 mgm per 100 ml (Dog’s plasma contains 123.0 mgm per 100 ml on the average).

iv. Other Constituents:

(Expressed in mgm per 100 ml) urea, 23.5 mgm (plasma, 21.7 mgm); non-protein nitrog­enous substance, 34.8 mgm (plasma 32.6 mgm); creatinine, 1.4 mgm (plasma 1.37 mgm); chlorides, 711 mgm (plasma 678 mgm); Total phosphorus, 11.8 mgm (plasma 22 mgm); inorganic phosphorus 5.9 mgm (plasma5.6 mgm); calcium 9.84 mgm (plasma 11. 7 mgm). Enzymes and antibodies are also present.

From the above, the difference between plasma and lymph may be noted. Protein, calcium and total phos­phorus are lower than in plasma. Chlorides and glucose are distinctly higher. Other constituents are also higher than in plasma to some extent.

4. Functions of Lymph:

i. Nutritive:

It supplies nutrition and oxygen to those parts where blood cannot reach.

ii. Drainage:

It drains away excess tissue fluid and the metabolites and in this way tries to maintain the volume and composition of tissue fluid constant.

iii. Transmission of Proteins:

Lymph returns proteins to the blood from the tissue spaces.

iv. Absorption of Fats:

Fats from the intestine are also absorbed through the lymphatics.

v. Defensive:

The lymphocytes and monocytes of lymph act as defensive cells of the body. The lymphatics also remove bacteria from tissues.

5. Rate of Flow in Lymph:

Rate of flow of lymph along the human thoracic duct is from 1.0—1.5 ml per minute. In dogs it is much higher. Lymphatogogue is the substance that increases the rate of lymph flow.

Regulation of the rate of lymph flow depends upon:

(a) Interstitial pressure,

(b) Arterial pulsation,

(c) Intrathoracic pressure, and

(d) Muscular massage.

6. Formation of Lymph:

Since lymph is formed from tissue fluid, anything that increases the amount of tissue fluid will increase the rate of lymph formation. Lymph formation depends upon physical factors. There is no vital secretory process involved in it.

The following factors are responsible for lymph formation:

1. Capillary Pressure:

If the capillary pressure is raised, the rate of lymph formation increases. This is seen in venous obstruction. [But after some time, the rate slows down due to increased accumulation of fluid in the tissue spaces and the consequent rise of hydrostatic pressure of the tissue fluid.]

2. Permeability of the Capillary Wall:

Under any condition, where the permeability of the capillary wall is increased, more tissue fluid will be formed and consequently more lymph.

The following factors increase capillary permeability:

i. Rise in Temperature:

Increased temperature of a particular locality increases capillary permeability.

ii. Substances acting directly on the Capillary Wall:

Peptone, foreign proteins, histamine and extracts of straw­berries, crayfish, mussels, leech, etc., exert an injurious effect upon the capillaries and thereby increase their permeability.

iii. Reduced Oxygen Supply:

Under conditions of oxygen lack lymph flow increases due to higher permeabil­ity of the vessels. It acts probably by damaging the capillary endothelium. Anoxia, anaemia, stasis of blood due to vascular congestion, produces such results.

3. Substances that Alter the Osmotic Pressure:

Anything that reduces the colloidal osmotic pressure of blood will increase the formation of tissue fluid and lymph. Normal or hypotonic saline, when given intravenously, will dilute the plasma colloids and reduce the osmotic pressure. Moreover, blood pressure will be raised. Both these factors will favour formation of tissue fluid and lymph. Hypertonic solutions will exert the same effect in a better way.

Hypertonic solutions, introduced in the blood, will draw in more fluid from the tissue spaces at first and will increase the blood volume further. Blood pressure will be raised to a great extent, and plasma colloids will be further diluted. In this way hypertonic solutions will increase the formation of lymph much more than the normal or hypotonic solutions. Solutions of NaCl, glucose, Na₂SO₄, etc., may be used for this purpose.

4. Increased Metabolic Activity of an Organ:

Increased activity of a particular area increases the flow of lymph in the locality.

It is due to:

i. Formation of more metabolites which increase the osmotic pressure of the tissue fluid.

ii. Local vasodilatation and increased capillary pressure and permeability.

iii. Relative anoxia.

iv. Increased temperature of the locality.

The last two also act by increasing the capillary permeability.

5. Massage and Passive Movements:

These increase lymphatic flow to some extent just like active muscular contraction.

7. Circulation of Lymph:

In the frogs circulation is maintained by rhythmically contracting lymph hearts. But in higher animal no such pump exists and the flow is maintained by a number of physical factors only.

They are as follows:

i. Pressure Gradient:

In the tissues, the pressure of lymph (8 to 10 mm of Hg) is higher than that in the thoracic duct (0 to 4 mm Hg).

ii. Presence of Valves:

Presence of valves in the lymphatic channels helps to maintain the flow in one direction.

iii. Muscular Action:

Muscular (skeletal) contraction, active or passive, compresses the lymphatic vessels and carries the lymph onwards because the valves within lymphatics prevent backflow. Contraction of the villi helps to pump the chyle from the central lacteals into the basal lymphatics. This lymph is carried up to the cisterna chyli by the help of the intestinal movements.

iv. Respiratory Movements:

During inspiration, due to the descent of the diaphragm intrathoracic pressure falls, which thereby sucks in lymph into the thorax? Moreover, intra-abdominal pressure rises. This com­presses the cisterna chyli, so that lymph flow through the thoracic duct is increased. These pressure chang­es during inspiration are very important factors in maintaining lymphatic circulation.