The lymphatic system

The lymphatic system is a network of vessels, lymph nodes and lymphoid organs that returns tissue fluid to the bloodstream and provides sites for immune cell development and activity. Its vessels carry lymph towards veins at the root of the neck, while its organs include the spleen, thymus and tonsils.

What is lymph, and where does it come from?

Lymph is fluid within lymphatic vessels. It originates as interstitial fluid, the fluid occupying spaces between tissue cells. Some fluid leaves blood capillaries, the smallest blood vessels, and enters these spaces; lymphatic vessels collect fluid and proteins that are not returned directly to the blood.

Once interstitial fluid enters a lymphatic vessel, it is called lymph. Its contents vary with the tissue it drains, but include water, dissolved substances and lymphocytes, a class of white blood cells involved in immune responses. Lymph from the small intestine also carries absorbed dietary fats.

The return of lymph connects the lymphatic system to the cardiovascular system, the heart and blood vessels that circulate blood. Unlike blood circulation, lymphatic flow does not form a closed circuit driven by a central pump. It begins in tissues and proceeds towards the veins.

How do lymphatic vessels carry lymph?

Lymphatic capillaries are small, blind-ended vessels that begin within tissues. Their thin walls allow tissue fluid to enter through overlapping endothelial cells, the cells lining blood and lymphatic vessels. Anchoring fibres connect these capillaries to surrounding tissue and help prevent their collapse.

Lymph passes from capillaries into collecting lymphatic vessels. These have valves that limit backward flow, and larger collecting vessels have smooth muscle in their walls. Contraction of this muscle, movement of surrounding skeletal muscles and pressure changes during breathing help propel lymph.

The general direction of flow is:

• From tissue spaces into lymphatic capillaries.
• Through collecting vessels and regional lymph nodes.
• Into lymphatic trunks, larger vessels formed by the convergence of collecting vessels.
• Through the main lymphatic ducts, or directly from lymphatic trunks, into veins at the root of the neck.

In the small intestine, specialised lymphatic capillaries called lacteals lie within villi, the finger-like projections of the intestinal lining. Lacteals receive absorbed fats, making intestinal lymph appear milky during fat absorption. This fat-rich lymph is called chyle.

Where are the main lymph node groups?

Lymph nodes are small, encapsulated organs positioned along lymphatic vessels. An outer capsule of connective tissue surrounds each node. Afferent vessels bring lymph into the node, while efferent vessels carry it away; these terms mean travelling towards and away from a structure, respectively.

Within a node, lymph passes through sinuses, channels containing cells that encounter material carried from tissues. The cortex, or outer region, contains lymphoid follicles, rounded collections of lymphocytes. The medulla, or inner region, contains cellular cords separated by lymph-filled sinuses.

Major regional groups include:

• Cervical lymph nodes: located in the neck, these receive lymph from the head and neck. Superficial pathways feed deeper nodes, including those associated with the internal jugular vein, a major vein returning blood from the head.
• Axillary lymph nodes: located in the axilla, or armpit, these receive lymph from most of the upper limb, much of the breast and adjacent areas of the trunk.
• Inguinal lymph nodes: located in the groin, these receive lymph from much of the lower limb, the superficial lower abdominal wall and parts of the external genital and perineal regions. The perineum is the region below the pelvic floor between the thighs.

These groups are connected to deeper pathways rather than being isolated endpoints. For example, lymph leaving the inguinal nodes continues towards nodes associated with pelvic blood vessels.

What do the thoracic duct and right lymphatic duct drain?

The thoracic duct is the main lymphatic channel returning lymph from most of the body. It drains the lower limbs, pelvis and abdomen, together with the left side of the thorax, the left upper limb and the left side of the head and neck. The thorax is the chest region between the neck and abdomen.

The thoracic duct commonly begins at the cisterna chyli, a dilated lymphatic collecting sac in the upper abdomen. It enters the thorax through the aortic hiatus, the opening in the diaphragm traversed by the aorta, the body's main artery. The diaphragm is the muscular partition separating the thorax from the abdomen.

The duct ascends through the posterior mediastinum, the compartment between the lungs behind the heart. It then continues into the neck and usually enters the venous circulation near the junction of the left internal jugular and left subclavian veins. The subclavian vein carries blood from the upper limb towards the heart.

The right lymphatic duct drains the right side of the head and neck, the right upper limb and the right side of the thorax. It usually opens near the corresponding venous junction on the right. The arrangement varies: its contributing trunks may enter the veins separately rather than forming a single duct.

What are primary and secondary lymphoid organs?

Lymphoid organs contain organised populations of lymphocytes. Primary lymphoid organs are sites where lymphocytes develop and mature. Secondary lymphoid organs provide sites where mature lymphocytes encounter antigens, substances recognised by immune cells.

Bone marrow is the blood-cell-forming tissue within bones. B lymphocytes and T lymphocytes are distinct classes of lymphocyte: B cells mature in bone marrow, while T cells mature in the thymus. The primary–secondary distinction describes their role in lymphocyte development, not their size or importance.

Where is the spleen, and how does it differ from a lymph node?

The spleen lies in the upper left abdomen, beneath the diaphragm and behind the stomach. Its outer surface is covered by a connective tissue capsule. Unlike a lymph node, it filters blood rather than lymph and does not receive afferent lymphatic vessels.

Its tissue contains white pulp and red pulp. White pulp consists of lymphoid tissue arranged around arterial branches. Red pulp contains blood-filled channels and cellular cords where ageing blood cells are removed from circulation.

The spleen therefore belongs to the lymphatic system without lying along the usual pathway of lymph through nodes. Its position and blood supply connect its immune role directly to the bloodstream.

Where is the thymus, and how is it organised?

The thymus lies mainly behind the sternum, or breastbone, in the superior mediastinum and the anterior division of the inferior mediastinum. It may extend into the lower neck. It is prominent in childhood and becomes increasingly replaced by fatty tissue after puberty.

The left lobe of thymus and right lobe of thymus lie beside one another. Each lobe of thymus is divided by connective tissue partitions into smaller compartments called lobules. Each lobule has an outer cortex and an inner medulla, although the medullary tissue is continuous between adjacent lobules.

The thymus provides the environment in which T lymphocytes mature. In the 3D model beside this article, select the thymus, isolate it and rotate the view to examine its position from different directions.

Where are the tonsils?

Tonsils are collections of lymphoid tissue around the pharynx, the passage behind the nose and mouth. Together, they form an incomplete ring around the entrance to the respiratory and digestive pathways.

The palatine tonsils lie on either side of the passage behind the mouth. The pharyngeal tonsil lies in the roof and posterior wall of the nasopharynx, the part of the pharynx behind the nose. The lingual tonsil lies at the base of the tongue, and the tubal tonsils lie near the pharyngeal openings of the auditory tubes, which connect the middle ears to the nasopharynx.