Quick answer: Lymph (also called tissue fluid) forms when some plasma, proteins and blood cells escape from blood capillaries into the spaces between cells. It is similar to plasma but is colourless and contains less protein. Lymph drains into lymphatic capillaries, which join to form larger vessels that empty into veins. Its main jobs are to carry digested and absorbed fat from the intestine (through lacteals in the villi) and to drain excess fluid from the spaces between cells back into the blood.
Blood flows through tiny, thin-walled capillaries that pass between the cells of the body. Some of the fluid in blood passes out through the capillary walls into the spaces between cells. If this fluid were not collected and returned, it would build up in the tissues and cause swelling.
The fluid is collected by a second system of vessels, the lymphatic system, and the fluid itself is called lymph.
In CBSE Class 10 Science, Chapter 5 (Life Processes), what lymph is, how it differs from blood and its functions are frequently asked board exam questions.
What You Will Learn
- How lymph (tissue fluid) is formed
- How lymph differs from blood
- The functions of lymph and the lymphatic system
- What lacteals are and how they absorb fat
- Which way lymph flows
1. How Is Lymph Formed?
Step 1: Blood flows through capillaries under pressure.
Step 2: Some plasma, proteins and blood cells escape through tiny pores in the capillary walls into the spaces between cells. This fluid is called tissue fluid or lymph.
Step 3: The tissue fluid bathes the cells, supplying them with substances and picking up wastes.
Step 4: It drains into lymphatic capillaries, which join to form larger lymph vessels.
Step 5: The lymph vessels finally open into large veins, returning the fluid to the blood.
2. Blood vs Lymph
Common board question: How is lymph different from blood?
| Feature | Blood | Lymph |
|---|---|---|
| Colour | Red (due to haemoglobin in red blood cells) | Colourless or pale yellow |
| Red blood cells | Present | Absent |
| Proteins | More protein in plasma | Less protein |
| White blood cells | Present | Present (mainly lymphocytes) |
| Flow | Pumped around the body by the heart | Flows in one direction, from tissues to veins |
Key point: Lymph is similar to blood plasma, but it is colourless and contains less protein.
3. Functions of Lymph
1. Carries Digested Fat
Step 1: In the small intestine, digested food is absorbed through finger-like projections called villi.
Step 2: Each villus contains blood capillaries and a lymph vessel called a lacteal.
Step 3: Digested fats are absorbed into the lacteals and carried by the lymph, which later empties into the blood.
2. Drains Excess Fluid
Lymph vessels collect the extra fluid from the spaces between cells and return it to the blood. This prevents fluid from building up in the tissues.
3. Helps Fight Infection (Extra)
Along the lymph vessels are small structures called lymph nodes. They contain white blood cells called lymphocytes, which help fight infection. This goes slightly beyond the Class 10 textbook.
4. Which Way Does Lymph Flow?
Lymph flows in one direction only, from the tissues towards the heart. There is no separate pump: it is moved along mainly by the squeezing of nearby muscles, and valves in the lymph vessels stop it flowing backwards. It finally empties into large veins near the neck.
5. Summary Table
| Part | Role |
|---|---|
| Tissue fluid (lymph) | Fluid that escapes from blood capillaries into the spaces between cells |
| Lymphatic capillaries and vessels | Collect lymph and carry it towards the heart |
| Lacteals | Lymph vessels in the villi that absorb digested fat |
| Lymph nodes | Contain lymphocytes that help fight infection |
| Veins | Where lymph finally rejoins the blood |
6. Exam Tips and Common Mistakes
Exam Tip: For the functions of lymph, give the two NCERT points: it carries digested and absorbed fat from the intestine, and it drains excess fluid from the spaces between cells back into the blood.
Exam Tip: If asked which vessels absorb fat in the small intestine, answer lacteals.
Common Mistake: Saying lymph is red. Lymph is colourless, because it has no red blood cells.
Common Mistake: Saying lymph is pumped by the heart. Lymph flows in one direction, helped by muscle movements and valves.
Frequently Asked Questions
What is lymph?
Lymph is a colourless fluid formed when plasma, proteins and blood cells escape from capillaries into the spaces between cells. It is also called tissue fluid.
How is lymph different from blood?
Lymph is colourless, has no red blood cells and contains less protein than blood plasma.
What are the functions of lymph?
It carries digested and absorbed fat from the intestine and drains excess fluid from the spaces between cells back into the blood.
What are lacteals?
Lacteals are lymph vessels inside the villi of the small intestine that absorb digested fat.
Which direction does lymph flow?
In one direction only, from the tissues towards the heart, where it empties into large veins.