Physiology Essay: The Lymphatic System

Anatomy and Physiology Assignment 2 – Essay

The Lymphatic System

The lymphatic system has three primary functions. It drains excess interstitial fluid from tissue spaces and returns it to the blood, enabling circulatory blood volume to be maintained; transports lipids and lipid-soluble vitamins A, D, E, and K absorbed through the gastrointestinal tract; and is responsible for the body’s adaptive (specific) immunity, ie. defences against microbes that breach its innate (non-specific) immunity (eg. skin, fever, inflammation): it adjusts or adapts to perform specific responses to specific microbes. In this essay, I will focus on the lymph nodes, particularly their role in adaptive immunity, and how they relate to the lymphatic system as a whole,

They are bean-shaped, approximately 1 – 25 mm long, and act as a lymph filter. They are positioned in groups in specific regions of the body and named after that region, eg. inguinal (groin) and axillary nodes (arm) (Rhoades et al 2003).

The node’s outermost layer is the capsule, dense connective tissue covering the node, which extends into it in extensions called trabeculae that section the node into compartments, providing structural support and a route for blood vessels to enter the node, via which B and T cells can also enter (Milling n. d.). Deep to the capsule is a network of reticular fibres and fibroblasts which, with the capsule and trabeculae, make up the stroma.

Lymph flow through the node is one-way, entering through several afferent vessels on the convex side of the node’s ‘bean shape’ which contain valves opening toward the node’s centre, directing lymph into the node.

Lymph then flows through three sinuses, irregular channels containing branching reticular fibres, lymphocytes, and macrophages, first the subcapsular sinus, immediately beneath the capsule, then the trabecular sinuses extending through the cortex parallel to the trabeculae, and then into the medullary sinuses. All sinuses filter the

Dendritic cells ‘present’ antigens to T cells, causing T cells to proliferate into either helper T cells, which aid B cell activation, or cytotoxic T cells, which leave the lymph node via the efferent vessels (Milling n. d.) to areas of antigenic activity in the body. Juxtaposition of B and T cells with dendritic cells is essential for their activation (Mondino et al 1996).

The medulla contains B cells, macrophages, and antibody-producing plasma cells which have migrated from the cortex, all embedded within a reticular fibre and cell network.

From the medullary sinuses, the filtered lymph drains into one or two efferent lymphatic vessels located at the hilum (depression in the concave side of the ‘bean shape’) (blood vessels also enter and exit the node here). Valves in the efferent vessels direct lymph out of the node, along with antibodies secreted by plasma cells and cytotoxic T cells. Efferent vessels are wider and fewer than afferent, slowing lymph transport through the node, allowing time for the filtration process, while the lymph’s flow path through the node ensures antigens percolate through the lymphocytes and APCs to optimise effective antigen presentation (Mondino et al