Question
What is lymph? How is it different from blood in composition and function?
Solution — Step by Step
Lymph is a colourless or pale yellow fluid that flows through the lymphatic system. It originates from blood plasma that has leaked out of blood capillaries into the surrounding tissues — this leaked fluid is called tissue fluid or interstitial fluid.
When tissue fluid enters the lymph capillaries, it becomes lymph. Think of it as blood plasma that has “escaped” into the tissues and is now being returned to the bloodstream.
Lymph contains:
- Water, dissolved salts, glucose, amino acids, fatty acids (similar to plasma)
- Lymphocytes — white blood cells that give lymph its immune function
- No red blood cells (RBCs) — this is the key difference from blood
- No platelets — so lymph does not clot the same way blood does
- High fat content — lymph from the intestine (lacteals) is milky white and called chyle due to absorbed fat
| Property | Blood | Lymph |
|---|---|---|
| Colour | Red (due to haemoglobin) | Colourless / pale yellow |
| Red Blood Cells | Present | Absent |
| Platelets | Present | Absent (or very few) |
| Proteins | Albumin, globulin, fibrinogen | Fewer proteins |
| Direction of flow | Circulates continuously | One-way: tissues → blood |
| Main cells | RBCs, WBCs, platelets | Mainly lymphocytes |
Lymph serves three major roles:
- Returns excess tissue fluid back to the blood, preventing oedema (swelling)
- Transports dietary fats absorbed from the small intestine via lacteals
- Immune defence — lymph carries lymphocytes to infection sites; lymph nodes filter pathogens from lymph
Why This Works
Blood plasma constantly leaks out of capillaries due to the hydrostatic pressure of the heart’s pumping action. If this fluid were not collected and returned, our tissues would swell dangerously. The lymphatic system is essentially the body’s drainage and return network.
The absence of RBCs explains why lymph is not red — haemoglobin stays inside blood vessels. Lymphocytes, however, can cross into the lymph and are the immune sentinels of the lymphatic system.
CBSE Class 10 and NEET frequently ask: “Why is lymph sometimes called tissue fluid?” Answer: lymph IS tissue fluid once it enters lymph capillaries. Before that, it is called interstitial fluid or tissue fluid.
Alternative Method
You can remember the differences using the mnemonic “Lymph has NO RaPs” — No RBCs, No Clots (no adequate platelets), and flows One way. Blood has all of these.
Common Mistake
Many students write that “lymph is just plasma.” While lymph derives from plasma, it has a distinct composition — fewer proteins, no RBCs, but more lymphocytes. Also, the direction of flow is different: lymph flows only toward the heart, whereas blood circulates in a closed loop.