B3.2.13 (HL)—Lymph ducts

Lymph ducts return excess tissue fluid to blood, using valves, body movement and lymph nodes to maintain fluid balance and immune surveillance.

Excess tissue fluid enters blind-ended lymph capillaries and is returned as lymph to the blood circulation.

Lymph capillaries have very thin walls with gaps that open as tissue pressure rises, allowing fluid to enter. Overlapping wall flaps and valves prevent reverse movement.

Larger lymph ducts use one-way valves, smooth-muscle contraction and compression by body movement to move lymph at low pressure. The ducts eventually empty into large veins near the heart.

When a leg muscle contracts it compresses a lymph vessel; the valve behind closes and the valve ahead opens, moving lymph toward its return to venous blood.

Lymph ducts return fluid rather than pumping it in an arterial circuit. If drainage is blocked, excess tissue fluid accumulates and causes oedema.

HL transport adds pressure and route systems. Tissue fluid forms by capillary pressure and returns by osmotic pull or lymph. Double circulation separates pulmonary and systemic routes. The heart creates directional pressure with chambers, septum, valves, and cycle timing. Plants add root pressure and phloem pressure-flow translocation.

Tissue fluid: hydrostatic pressure out, osmotic pull back, lymph drains excess.
Heart: double circulation, one-way valves, thick left ventricle, cardiac cycle sequence.
Plant HL: root pressure by active ion loading and osmosis; phloem by source-to-sink pressure flow.

Concept essentials

Lymph capillaries collect excess tissue fluid that does not return directly to blood capillaries.
Lymph vessels use valves to maintain one-way flow at low pressure.
Body movement and smooth muscle contraction help move lymph through ducts.
Lymph nodes filter lymph and contain immune cells.

B3.2.13 (HL)—Lymph ducts

Lymph Returns Excess Tissue Fluid

Pressure, Heart, Lymph, And Phloem

Concept essentials