Lymph

Lympha

Also known as: lymphatic fluid, chyle

The clear, straw-coloured fluid inside lymphatic vessels: interstitial fluid that has entered lymphatic capillaries, carrying leaked plasma protein, lymphocytes, antigen and — from the gut — fat as milky chyle.

Show on body map Lymph Node — Sectioned

Overview

Daily lymph return≈ 2–4 L (classically ≈ 3 L)

Resting lymph flow≈ 120 mL/h (≈ 100 mL/h via thoracic duct)

Protein concentrationLimbs ≈ 20 g/L; liver ≈ 60 g/L; thoracic duct ≈ 30–50 g/L

Lymph begins as interstitial fluid. When it crosses into a lymphatic capillary it is called lymph; its composition then changes as it passes through lymph nodes (which concentrate it, add lymphocytes and remove particles and microbes). Peripheral lymph is a clear, watery ultrafiltrate of plasma with a lower protein concentration (≈ 20 g/L from most tissues) than plasma (≈ 70 g/L); hepatic lymph is protein-rich (≈ 60 g/L) because liver sinusoids are discontinuous, and the mixture in the thoracic duct contains ≈ 30–50 g/L.

Lymph returns to the blood the ≈ 3 L of fluid per day that is not reabsorbed by blood capillaries, together with a large fraction of the plasma protein that leaks out of the circulation each day — protein that has no other route back. After a fatty meal, lymph from the intestine is milky white chyle, because enterocytes package dietary long-chain fatty acids into chylomicrons, which are too large to enter blood capillaries but pass freely into lacteals.

Afferent lymph (flowing into a node) carries antigen, dendritic cells and some lymphocytes; efferent lymph (leaving a node) is rich in lymphocytes, especially recirculating T cells, and in antibody during an immune response.

Location: Within lymphatic capillaries, collecting vessels, nodes, trunks and ducts of almost all vascularised tissues.

Parts & subdivisions

Water and electrolytes
Composition similar to interstitial fluid and plasma (Na⁺ ≈ 140 mmol/L), with the same small solutes such as glucose and urea.
Proteins
Albumin, globulins and fibrinogen that have leaked from capillaries; lymph can clot slowly. Concentration varies by tissue (liver > intestine > limbs).
Cells
Mainly lymphocytes (efferent lymph); afferent lymph also carries migrating dendritic cells and macrophages. Red cells are normally absent.
Chyle
Milky, triglyceride-rich intestinal lymph containing chylomicrons absorbed by lacteals; also carries fat-soluble vitamins A, D, E and K.
Antigens and debris
Microbes, cell debris and cancer cells from the tissues — filtered by the lymph nodes they pass through.

Function

Returns excess interstitial fluid and plasma protein to the blood.
Transports dietary fat (chylomicrons) and fat-soluble vitamins from the gut to the venous blood.
Carries antigen and antigen-presenting cells to lymph nodes for immune surveillance.
Recirculates lymphocytes from lymphoid tissue back to the blood.

Physiology: Resting total lymph flow is about 120 mL/h (≈ 2–3 L/day), ≈ 100 mL/h of it via the thoracic duct. Flow increases with raised capillary pressure, reduced plasma oncotic pressure, raised interstitial protein and increased capillary permeability, and with activity (muscle pump). As interstitial pressure rises from about −6 towards 0 mmHg, lymph flow can increase roughly 20-fold — an important safety factor against oedema. About two-thirds of thoracic-duct lymph comes from the liver and intestines.

Clinical significance

Oedema

Oedema develops when filtration exceeds the maximum lymph flow — e.g. heart failure (raised venous pressure), nephrotic syndrome or liver failure (low albumin), inflammation (leaky capillaries) or lymphatic obstruction (lymphoedema, where the fluid is protein-rich and eventually non-pitting).

Chylothorax and chylous ascites

Leakage of chyle into the pleural or peritoneal cavity after thoracic-duct injury (oesophagectomy, cardiothoracic surgery, trauma) or obstruction by tumour (lymphoma). The fluid is milky with triglycerides > 1.24 mmol/L (110 mg/dL) and chylomicrons. Treated with drainage, a low-fat/medium-chain-triglyceride diet, somatostatin analogues, and duct ligation or embolisation.

Test yourself

Lymph from which region does NOT normally enter the thoracic duct?

Left lower limb
Small intestine
Right upper limb
Left side of the head and neck

Answer: Right upper limb

The thoracic duct drains everything below the diaphragm plus the left side above it. The right upper limb, right head and neck and right thorax drain via the right jugular, subclavian and bronchomediastinal trunks to the right lymphatic duct.

Which lymphatic trunks drain into the cisterna chyli?

Jugular and subclavian trunks
Bronchomediastinal trunks
Lumbar and intestinal trunks
Right and left subclavian trunks

Answer: Lumbar and intestinal trunks

The paired lumbar trunks (lower limbs, pelvis, kidneys, posterior abdominal wall) and the single intestinal trunk (GI tract) converge on the cisterna chyli at ≈ L1–L2, the origin of the thoracic duct.

Interstitial fluid enters lymphatic capillaries mainly because…

Their endothelium is fenestrated and actively pumps fluid inwards
Overlapping endothelial cell edges act as one-way flaps, held open by anchoring filaments when tissue pressure rises
Their thick basement membrane filters protein from the interstitium
Smooth muscle in their walls generates negative pressure

Answer: Overlapping endothelial cell edges act as one-way flaps, held open by anchoring filaments when tissue pressure rises

Lymphatic capillaries have loosely overlapping "oak-leaf" endothelial cells with button-like junctions and an incomplete basement membrane. Anchoring filaments pull the vessel open when interstitial pressure rises, and the overlapping edges behave as primary valves, letting fluid, protein and cells in but not out.

More lymphatic & immune system questions

Compiled from standard anatomy and physiology references — see the Lymphatic & Immune System sources. For education only; not medical advice.