Lymphatic System and Its Functions

The lymphatic system is a crucial part of the circulatory and immune systems.
- It plays a key role in fluid balance, immune responses, and the absorption of dietary fats.

Osmotic Pressure and Hydrostatic Pressure
- Key forces involved in fluid movement across capillary walls.
Fluid Exchange
- Approximately 3-4 liters of fluid left in the tissues by capillaries.
- This fluid, known as interstitial fluid, is collected by the lymphatic system and returned to the circulatory system.
Definitions
- Interstitial Fluid: Fluid between tissues, derived from plasma but devoid of plasma proteins.
- Lymph: This interstitial fluid once it enters the lymphatic vessels.

Lymphatic Vessels
- Collect interstitial fluid and transport it through the lymphatic system.
- Function to reclaim fluids and maintain blood volume, transport hormones and nutrients, and contribute to the immune response.
Cells in the Lymphatic System
- Primarily lymphocytes.
- Additional cells include macrophages and other immune cells.

Stroma: Structural component of lymphatic tissue, comprised of microfibers, fibrocytes, and fibroblasts.
Parenchyma: Functional cells of lymphatic tissue, primarily lymphocytes.
Analogy:
- Stroma = structural elements (walls, doors, windows of a building)
- Parenchyma = functional occupants of the building.

Reclaims fluid left behind by capillaries to prevent edema (swelling).
Helps maintain blood volume and transports various substances.
Functions in the immune response by exposing lymphocytes to pathogens, priming them for action against infections.

Structure of Lymphatic Capillaries:
- Made of simple squamous epithelium and characterized by thinner walls than blood capillaries.
- Begin as blind ends (dead ends), unlike blood capillaries which are open at both ends.
- Features overlapping endothelial cells that act like one-way valves to prevent backflow of lymph.
- Anchoring Filaments: Fibers that connect lymphatic capillary edges to the surrounding tissue, assisting in fluid absorption during edema.
Lacteals: Specialized lymphatic capillaries located in intestinal villi that transport chyle (lipid-rich lymph from dietary fat absorption).

Lipid digestion results in larger molecules like chylomicrons that cannot enter blood capillaries but can be absorbed by lacteals.
Chylomicron: A larger structure formed by combining triglycerides combined with proteins, allowing transport of fats.
Chyle: The whitish, milky fluid containing fat molecules found in lymphatic capillaries after digestion.

Formed when lymphatic capillaries merge.
Introduce the three layers of connective tissues similar to blood vessels: tunica intima, tunica media, and tunica externa.
Lymphatic vessels also contain valves to maintain unidirectional flow towards the heart.
Lymph Nodes: Small, bean-shaped structures that filter lymph, remove pathogens, and activate the immune response.
- Packaged between collecting vessels, where they can become inflamed (as seen with infections).

Lymphatic trunks collect lymph from collecting vessels and drain into lymphatic ducts.
- Major trunks include left and right lumbar, intestinal trunk, subclavian trunks, and jugular trunks.
The Thoracic Duct:
- Main duct that drains approximately 75% of the body’s lymph.
- It originates from cisterna chyli at the L1-L2 vertebral level.
- Drains into the left subclavian vein, joining with the left jugular vein.
The Right Lymphatic Duct:
- Drains lymph from the right side of the body’s upper half and is significantly shorter than the thoracic duct.
- It is formed by the union of right subclavian, jugular, and bronchomediastinal trunks.

Both lymphatic ducts drain into subclavian veins, eventually feeding into the superior vena cava, leading to the right atrium of the heart.

Factors Influencing Lymph Flow:
- Lymph flow relies on valves, muscular, and respiratory pumps, given the lack of a heart-like structure to generate pressure gradients.
- Adjacent blood vessels also assist in lymph movement through compression mechanics.
Lymphedema: A condition characterized by swelling due to blockage of lymphatic flow, which can be caused by non-functional vessels or nodes.

The predominant cells are lymphocytes, including:
- T Lymphocytes: Developed in the thymus, responsible for specific immune responses.
- B Lymphocytes: Also involved in specific immunity but different from T cells.
- Natural Killer (NK) Cells: Participate in non-specific immunity.

Lymphatic tissue can be arranged in two forms:
- Diffuse: Lymphocytes distributed widely without clear borders.
- Nodular (Follicular): Concentrated lymphocyte arrangements forming observable nodules, including a germinal center indicating rapid cell division.
Key lymphoid organs distinguished by their complete encapsulation:
- Lymph Nodes: Filters lymph and activates the immune response.
- Spleen: Filters blood, removes old red blood cells, and aids immune activation.
- Thymus: Where T lymphocytes mature and gain immunocompetence, largest during childhood and adolescence, and diminishes with age.

Refers to lymphoid tissue associated with mucosal surfaces in organs that open to the exterior, such as the intestines and respiratory tract.
Includes tonsils, Peyer’s patches, and lymphoid follicles embedded in the mucosa, contributing to immune surveillance and responses.