10.4 Lymphatic System & Fluid Balance

A Third Circulation

Section 10.2 ended with a debt: capillary filtration exceeds reabsorption by roughly 10–15% of filtered volume, and plasma proteins that leak into the interstitium cannot re-enter the blood capillary against the oncotic gradient. Without a recovery route, interstitial pressure would rise, oncotic pressure inside the capillary would fall, and circulating volume would collapse within about a day. The lymphatic system is that recovery route — a one-way drainage network, not a circuit, running in parallel with the venous system.

The AAMC lists four functions under this topic, and each is worth being able to state independently.

1. Equalization of fluid distribution — returning roughly 3 L/day of excess interstitial fluid to the blood.
2. Transport of proteins and large glycerides — recovering escaped plasma proteins, and carrying dietary lipids as chylomicrons.
3. Production of lymphocytes involved in immune reactions — housing the tissue in which adaptive immunity is generated.
4. Return of materials to the blood — delivering all of the above into the great veins.

Structural Organization

Lymphatic Capillaries

Lymphatic capillaries are blind-ended tubes woven among blood capillaries in nearly every vascularized tissue. Their endothelial cells overlap loosely rather than forming tight junctions, creating flap-like mini-valves:

• Rising interstitial fluid pressure pushes the flaps inward, opening the vessel.
• Fluid inside the vessel pushes the flaps closed, preventing escape.

Anchoring filaments tether the endothelium to surrounding collagen so that swelling of the tissue actually pulls the vessel open, widening it exactly when drainage is most needed. This architecture is highly permeable — proteins, cell debris, bacteria and metastasizing tumor cells all enter easily, which is why cancer staging tracks lymph-node involvement.

Once inside, the fluid is called lymph. It is essentially interstitial fluid: similar electrolytes to plasma but lower protein concentration.

Drainage Pathway

Capillaries feed collecting vessels, which resemble veins with three tunics, thin walls and abundant one-way valves. Collecting vessels pass through lymph nodes, merge into regional trunks, and empty into one of two terminal ducts:

Propulsion Without a Pump

There is no lymphatic heart. Flow depends on the same mechanisms that assist venous return:

• Skeletal muscle pump — contracting muscle squeezes vessels; valves enforce one-way flow.
• Respiratory pump — thoracic pressure falling during inspiration draws lymph centrally.
• Intrinsic smooth muscle in the walls of larger collecting vessels contracts rhythmically.
• Arterial pulsation of adjacent vessels.

Immobility therefore slows lymph flow, which is why prolonged sitting produces dependent swelling.

Lymphoid Organs and Immune Function

Lymphoid tissue is the structural home of the cells covered in Section 10.5.

Primary (where lymphocytes are made and mature):

• Red bone marrow — origin of all lymphocytes; site of B cell maturation.
• Thymus — site of T cell maturation via positive and negative selection; largest in childhood and progressively involutes after puberty.

Secondary (where lymphocytes encounter antigen):

• Lymph nodes — bean-shaped filters along collecting vessels, densely packed with macrophages and lymphocytes. Afferent vessels enter the convex surface; a single efferent vessel leaves the hilum, and because outflow capacity is smaller than inflow, lymph slows and dwells long enough for antigen sampling. Node enlargement (lymphadenopathy) during infection reflects clonal proliferation.
• Spleen — the largest lymphoid organ, filtering blood rather than lymph. White pulp performs immune surveillance; red pulp removes senescent erythrocytes and recycles iron (see 10.2). Splenectomy raises the risk of overwhelming infection by encapsulated bacteria.
• Tonsils, Peyer's patches and the appendix — mucosa-associated lymphoid tissue (MALT) guarding the entrances to the respiratory and digestive tracts.

Lipid Transport: Lacteals and Chyle

Each intestinal villus contains a central lymphatic capillary called a lacteal (see 11.1). Absorbed long-chain fatty acids and monoacylglycerols are re-esterified in the enterocyte, packaged with apolipoprotein B-48 into chylomicrons, and exocytosed. Chylomicrons are far too large to cross the fenestrated blood capillary endothelium, but they pass easily between the overlapping flaps of the lacteal. The milky lipid-laden lymph is called chyle.

Two consequences the MCAT rewards:

• Dietary long-chain fat bypasses the hepatic portal vein and reaches the systemic circulation at the subclavian vein, so it is not subject to hepatic first-pass extraction. Short- and medium-chain fatty acids, which are water-soluble enough to enter the portal blood directly, are the exception.
• Drugs formulated to partition into chylomicrons likewise escape first-pass metabolism.

When Drainage Fails

Edema is clinically visible interstitial fluid accumulation. Mapping causes onto the Starling equation of Section 10.2 makes the mechanisms predictable:

Lymphedema specifically follows lymphatic blockage — after surgical node dissection, radiation, or infection by the filarial nematode Wuchereria bancrofti, which causes the massive limb swelling of elephantiasis. Because protein as well as fluid accumulates, lymphedema is characteristically firm and does not resolve with elevation alone, unlike the soft pitting edema of raised hydrostatic pressure.