Body Fact

As the lymphatic system lacks a pump, lymphatic vessels have to make use of contracting muscles that assist the movement of lymph. Therefore, lymphatic flow is at its greatest during exercise due to the increased contraction of muscle.

Lymphatic capillaries

Lymphatic vessels commence as lymphatic capillaries in the tissue spaces of the body as minute blind-end tubes, as lymph is a one-way circulatory pathway. The walls of the lymphatic capillaries are like those of the blood capillaries in that they are a single-cell-layer thick to make it possible for tissue fluid to enter them. However, they are permeable to substances of larger molecular size than those of the blood capillaries.

The lymphatic capillaries mirror the blood capillaries and form a network in the tissues, draining away excess fluid and waste products from the tissue spaces of the body. Once the tissue fluid enters a lymphatic capillary it becomes lymph and is gathered up into larger lymphatic vessels.

Lymphatic vessels

Lymphatic vessels are similar to veins in that they have thin, collapsible walls and their role is to transport lymph through its circulatory pathway. They have a considerable number of valves which help to keep the lymph flowing in the right direction and prevent backflow. Superficial lymphatic vessels tend to follow the course of veins by draining the skin, whereas the deeper lymphatic vessels tend to follow the course of arteries and drain the internal structures of the body. Networks or plexuses of lymphatic channels exist throughout the body. These intertwined channels are found in the following areas:

• mammary plexus - lymphatic vessels around the breasts

• palmar plexus — lymphatic vessels in the palm of the hand

• plantar plexus — lymphatic vessels in the sole of the foot.

The lymphatic vessels carry the lymph towards the heart under steady pressure and about two to four litres of lymph pass into the venous system every day. Once lymph has passed through the lymph vessels it drains into at least one lymphatic node before returning to the blood circulatory system.

Lymphatic nodes

At intervals along the lymphatic vessels lymphatic nodes occur. A lymphatic node is an oval or bean-shaped structure covered by a capsule of connective tissue. It is made up of lymphatic tissue and is divided into two regions: an outer cortex and an inner medulla.

There are more than 100 lymphatic nodes placed strategically along the course of lymphatic vessels. They vary in size between one to 25 mm in length and are massed in groups. Some are superficial and lie just under the skin, whereas others are deeply seated and are found near arteries and veins.

KEY FACT

If an area of the body becomes inflamed or otherwise diseased, the nearby lymph nodes will swell up and become tender, indicating that they are actively fighting the infection.

Afferent lymphatic vessel

Capsule

Inner medulla

Efferent lymphatic vessels

Vascular supply for lymph node

Outer cortex

Afferent lymphatic vessels

Each lymphatic node receives lymph from several afferent lymphatic vessels and blood from small arterioles and capillaries. Valves of the afferent lymphatic vessels open towards the node, therefore lymph in these vessels can only move towards the mode. Lymph flows slowly through the node moving from the cortex to the medulla, and leaves through an efferent vessel which opens away from the node. The afferent vessels enter a lymphatic node and the efferent vessels drain lymph from a node.

The function of a lymphatic node is to act as a filter of lymph to remove or trap any micro-organisms, cell debris or harmful substances which may cause infection so that when lymph enters the blood, it has been cleared of any foreign matter. When lymph enters a node, it comes into contact with two specialised types of leucocytes:

• macrophages - these are phagocytic in action. They engulf and destroy dead cells, bacteria and foreign material in the lymph

• lymphocytes - these are reproduced within the lymphatic nodes and can neutralise invading bacteria and produce chemicals and antibodies to help fight disease.

Once filtered the lymph leaves the node by one or two efferent vessels which open away from the node. Lymphatic nodes occur in chains so that the efferent vessel of one node becomes the afferent vessel of the next node in the pathway of lymph flow. Lymph drains through at least one lymphatic node before it passes into two main collecting ducts before it is returned to the blood.

Lymphatic ducts

From each chain of lymphatic nodes the efferent lymph vessels combine to form lymphatic trunks which empty into two main ducts - the thoracic and the right lymphatic ducts. These ducts collect lymph from the whole body and return it to the blood via the subclavian veins.

The thoracic duct is the main collecting duct of the lymphatic system. It is the largest lymphatic vessel in the body and extends from the second lumbar vertebra up through the thorax to the root of the neck. The thoracic duct collects lymph from the left side of the head and neck, left arm, lower limbs and abdomen, and drains into the left subclavian vein to return it to the bloodstream.

Fig 6.4 A lymphatic node

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