The Lymphatic System
Is primarily responsible for two functions:
The lymphatic vessels act as drains that create a huge network for collecting any excess fluid that surrounds tissue cells – they feed this back to the blood
Transport of Nutrients
Within the lymphatic system large fat and protein molecules may be carried to be either returned to the general circulatory system or broken down in the lymphatic glands and organs. The lymph flows slowly and surely around the body, maintaining a constant balance of fluids that bathe the cells. It maintains flow by the same use of pumping mechanism that are to be found in the venous return system; the respiratory pump and the musco-skeletal pump.
Without the adequate drainage facilities that the lymphatic system offers and the equally efficient filtration and cleansing functions of its glands and organs, the blood and then ultimately the cells would find themselves quite literally in deep water.
The Structure of lymphatic vessels and how they differ from blood vessels.
Although it serves a unique transport function by returning tissue fluid, proteins, fats and other substances to the general circulation; lymph flow differs from the true ‘circulation’ of blood seen in the cardiovascular system. The lymphatic vessels do not, like vessels in the blood vascular system, form a closed circuit or ring, but instead begin blindly as microscopic dead end lymphatic capillaries positioned between the intercellular or interstitial space of the soft tissues where they act like drains collecting excess tissue fluid.
The lymphatic capillary is very similar to blood capillaries, except that it is formed by a very large and flat single layer of endothelial cells which overlap one another leaving rather large clefts or openings.
This structural difference allows a greater movement of fluids, and even larger molecules dissolved or carried within the fluids, to pass into or out of the lymph vessels.
Generally you will find lymphatic capillary and blood capillary networks lying side by side each other.
These lymphatic vessels then join together to form larger vessels and finally merge into two main lymphatic trunks:
The Thoracic Duct: empties its lymph from most of the body into the left subclavian vein.
The Right Lymphatic Duct: empties lymph just from the right upper quadrant of the body into the right subclavian vein.
In this way they return the excess interstitial fluid to the venous blood supply.
The lymphatic capillary contains semi lunar valves, lining the interior walls – this is similar to veins. This maintains lymph movement in one direction. Like the venous system it contains no pump so is subject to similar reverse flow pressures.
However, the venous system maintains a regular pressure to drive the blood upwards which the lymphatic system does not. The possibility of reverse flow is greater. the lymphatic vessels contain more lock gate like semi lunar valves.
A typical lymphatic vessel is very similar to a typical vein with a thick connective tissue outer covering called the tunica aventitia, a smooth muscular layer called the tunica media and an endothelium layer the tunica intima where the true cells of the vessel are found.
- lymphatics have thinner walls
- lymphatics contain more semi lunar valves
- lymphatics contain lymph nodes, located at certain intervals along their course.
At various points there are clusters of lymph nodes, which are bean shaped filters that several lymph vessels will feed into and then out again.
There are sinus channels within these nodes that slow down the flow of the lymph so that the fluid can be cleansed and acted on by lymphocytes and particles eaten by phagocytes.
The nodes are also he site of hematopoisis, or maturation for some types of lymphocytes and monocytes that begin life in the bone marrow.
The Lymphatic Pump Mechanisms
Although it has no pump of its own, the lymphatic system takes full advantage of other pump mechanisms:
The Respiratory pump: This allows for the maximum influx of lymph into the thoracic duct, to occur at the moment of maximum inspiration. ‘a fluid will move from a high to a low pressure zone’
As a result of the diaphragm descending during inspiration and compressing the abdominal organs, intra abdominal pressure rises, as it does, the pressure in the abdominal region of the thoracic duct. Conversely and simultaneously the pressure in the thoracic cavity has fallen due to the fact that the cage has been expanded. Therefore a pressure gradient has been achieved, The lymph will flow slowly but surely uphill along a lymph pressure gradient, valves adding to the movement.
The Musco-skeletal pump.This acts in the same way but depends on every contraction of skeletal muscle. The effect here is literally to milk the lymph upwards. Semi-lunar valves are needed to control the direction of the flow. The main average flow is 3 litres per day of lymph back into general circulatory system.
As physical activity increases, so does the outflow of fluid from blood capillaries into the tissue spaces of the body. The increased flow of lymph that occurs with increased physical activity helps to return this fluid to the cardiovascular system and thus serves as an important balancing homeostatic mechanism.
The structure and location of lymph nodes.
there are 6 main clusters of these glands, each is responsible for a specific area of drainage and filtration of body fluids.
- preauricular lymph nodes, are just in front of the ears, draining lateral sides of superficial tissues and skin of the head and face.
- submental and sub maxillary nodes are situated in the floor of the mouth, draining from nose, lips and teeth
- superficial cervical lymph nodes the sterno-cleidomastoid muscle runs up the side and front of neck. Secondary filters for the lymph draining he whole of the neck and head.
- superficial cubital are located just above the bend in the elbow and drain from the hands and forearm
- axillary nodes are deep under the arm in the armpit and in the upper chest, but also the thoracic wall and breasts
- inguinal nodes are in the pit of the groin and in and around the inguinal ligament, draining the legs and external genitals.
The Structure of Lymph Nodes
Lymphatic nodes or glands are the biological filters of the lymphatic system – they vary in size from a pinhead to a kidney bean.
There are several afferent lymph vessels entering the lymph node each one with a semi lunar valve. The lymph flows through the vessels into the node, where it slows down and percolates through the sinuses where foreign material is consumes by phagocytosis.
The cleaner lymph is then moved until it collects at the hilus end of the gland where it exits the node via the single efferent lymph vessel.
An artery and a vein also enter and exit at the hilus.
Each lymph node is encapsulated by a tough fibrous membrane called the capsule.
The trabeculae extends from the capsule into the gland and splits the gland into compartments. These are called corticol nodules and are packed full of lymphatic cells or lymphocytes.
During an infection germinal centres form and it is here that new lymphocytes are produced and released after reaching maturity.
The center of the node, or medulla is composed of sinuses and medullary cords lined with reticuloendothelial cells (fixed macrophages which are capable of phagocytosis, cleansing the lymph.)
Major Functions of Lymph Nodes and how they are able to perform this function.
Defence functions: Filtration and Phagocytosis
The structure of the sinus channels slows the lymph as it flows through them. This gives the reticlarendotheilial cells that line the channels time to remove the microorganisms and other injurous particles (soot for example from the lymph) and phagotise them. Lymphatic vessels can absorb more particles than blood as it its structure is more permeable; so it sucks up proteins, fats, tissue fluids etc. Sometimes such hoards of microorganisms enter the nodes that they cannot destroy enough of them to prevent injuring the node. Adentis then results – an infection of the nodes.
Cancer cells often break away from a malignant tumour and enter lymphatics. They travel to the lymph nodes where they set up new growths. This may leave too few channels for the lymph to return to the blood. For example if tumours block axillary node channels, fluid accumulates in the interstitial space of the arm, causing the arm to become markedly swollen.
The lymphatic tissue of lymph nodes serves as a site for the final stages of maturation for some types of lymphocytes and monocytes that have migrated from the bone marrow. In addition to lymph nodes and the specialised lymphatic organs, small aggregates of diffuse lymphatic tissue and other lymhatic cells are found throughout the body – especially in connective tissues and under mucous membranes.
Digestive System – Lacteal in the villi
Lymphatic vessels also play a part in digestion. Microvilli in the small intestine contain a ‘lacteal’ which is a blind ended lymph vessel that absorbs fat and other nutrients. This drains into a lymph vessel that carries the absorbed nutrients into the main lymphatic system.
How the Lymph drainage of the breast works
The breast mammary gland and surrounding tissues is drained by the following two sets of lymphatic vessels.
- Lymphatics that originate in and drain the skin over the breast (not the areola and nipple)
- Lymphatics that originate in and drain th substance of the breast itself, as well as the areola and nipple.
Superficial vessels that drain lymph from the skin and surface areas of the breast converge and form a diffuse cutaneous lymphatic plexus and large lymphatics that drain the secretory tissue and ducts of the breast occurs in the subareolar plexus located under the areolar surrounding the nipple.
Lymph nodes associated with the breast.
Over 85% of the lymph from the breast enters the lymph nodes of the axillary region. Most of the remainder enters lymph nodes along the lateral edges of the sternum.
Several very large nodes in the axillary region are in actual physical contact with an extension of breast tissue called the axillary tail of Spence. Because of the physical contact between these nodes and breast tissue, cancerous cells may be spread by both the lymphatic extension and contiguity of tissue. Other nodes in the axilla will enlarge and swell after being ‘seeded’ with malignant cells as lymph from a cancerous breast flows through them.
Masses of lymphoid tissue called tonsils are located in a protective ring under the mucus membranes of the mouth and back of the throat. They help protect against bacteria that may invade tissues in the area around the openings between the nasal and oral cavities.
The palatine tonsils are located on each side of the throat. The pharangeal tonsils (adenoids) are near the posterior opening of the nasal cavity. The linguinal tonsils are near the base of the tongue.
The tonsils serve as the first line of defence from the exterior and as such are subject to chronic infection – or tonsillitis.
Tonsillectomies have become controversial due to the critical immunological role played by the lymphoid tissue.
The thymus is a primary organ of the lymphatic system. it is an unpaired organ consisting of two pyramidal shaped lobes with delicate and finely lobulated surfaces. The thymus is located in the mediastinum extending up the neck as far as the lower edge of the thyroid gland.
Each lobule is composed of a dense cellular cortex and an inner, less dense medulla; each composed of lymphocytes in an epithelial framework quite different from the supporting connective tissue sen in other lymphoid organs. Thymic corpuscles.
This tiny organ structure plays a critical part in the body’s defences against infections, in its vital immunity mechanism.
The thymus performs two important functions:
The final site of lymphocyte development before birth. Many lymphocytes leave the thymus and move to the spleen, lymph nodes and other lymphatic tissues.
The thymus secretes a group of hormones called thymosin – these enable lymphocytes to develop into mature T-cells. Since T-cells attack foreign or abnormal cells and also serve as regulators of immune function, the thymus functions as part of the immune system mechanism.
Is located in the left hypochondrium – directly below the diaphragm, above the left kidney and descending colon and behind the fundus of the stomach.
The spleen is roughly ovaloid in shape. Like other lymphoid organs, the spleen is surrounded by a fibrous capsule with inward extensions that roughly divide the organ into compartments.
Arteries leading into each compartment are surrounded by dense masses (nodules) of developing lymphocytes. Unlike other glands of the lymphatic systems only two vessels enter the spleen – the splenic artery and splenic vein. The splenic artery enters through the hilium of the spleen and very quickly spreads out into a large dense capillary network, having engorged the spleen with blood. Several changes occur before it is collected in the venous sinuses and fed to the splenic vein.
Functions of the spleen
- Defence ‘eating foreign and damaged cells’
- Hematopoiesis ‘maturation of non-granular leukocytes’
- Red blood cell and platelet destruction – this is where iron is salvaged from haemoglobin.
- Huge blood reservoir (up to 350 ml blood).