Lymph is a clear-to-yellowish fluid that circulates through the lymphatic system, a network of vessels and organs separate from the main circulatory system. Under normal conditions, lymph does not contain red blood cells (RBCs). Its primary role is to maintain fluid balance and support the immune system by collecting fluid that has leaked from blood capillaries and returning it to the bloodstream.
What Lymph is and How it is Formed
Lymph fluid originates directly from blood plasma pushed out of the circulatory system. As blood flows through the capillaries, hydrostatic pressure causes the watery component of the plasma to filter through the capillary walls into the surrounding tissues. This filtered fluid is called interstitial fluid, which bathes the body’s cells, delivering nutrients and collecting waste products.
Roughly 90% of this interstitial fluid flows back into the blood capillaries, but the remaining 10% must be collected by the lymphatic system to prevent tissue swelling. The smallest lymphatic vessels, called lymphatic capillaries, are blind-ended tubes present throughout most tissues. These capillaries are specifically structured to allow fluid to enter easily.
The walls of lymphatic capillaries are made of a single layer of overlapping endothelial cells. These cells create one-way mini-valves that open inward when interstitial pressure is higher than the pressure inside the lymphatic capillary. Crucially, the pores and gaps in this system are too small to admit large cellular components like red blood cells (7 to 8 micrometers in diameter). This structural design filters the fluid, ensuring RBCs remain within the blood vessels.
The Standard Components of Lymph Fluid
The composition of lymph is similar to blood plasma, consisting mainly of water, electrolytes, and cellular waste products. It contains significantly fewer proteins than plasma, typically one-half to one-third the concentration, because most large proteins are retained by blood vessel walls. The fluid becomes progressively richer in proteins as it moves through the lymphatic system.
The most notable cellular component of lymph is the white blood cell, specifically lymphocytes. These immune cells circulate through the lymph nodes, filtering the fluid and mounting immune responses against pathogens. Lymph also collects absorbed fats from the small intestine, forming a milky fluid called chyle, which transports lipids back to the bloodstream.
When Red Blood Cells Appear in Lymph
The presence of red blood cells in lymph is an abnormal finding, indicating a breach in the integrity of the circulatory or lymphatic vessels. This condition, sometimes referred to as sinus erythrocytosis in lymph nodes, is a sign of pathology or trauma. The most common cause is a severe physical injury that directly damages both a blood vessel and a nearby lymphatic vessel.
This damage creates an unnatural pathway for high-pressure blood to leak into the lower-pressure lymphatic circulation. Surgical procedures that involve extensive tissue dissection can also inadvertently sever lymphatic ducts, leading to internal bleeding that drains into the lymphatics. Finding RBCs in lymph nodes suggests hemorrhage has occurred in the tissue area drained by that specific node.
The clinical finding of blood mixed with lymph, such as in the chest cavity, is a serious condition. This is known as hemothorax, or chylothorax if fat is also present, requiring immediate medical attention.