Which one of the following statements about lymph in the human body is correct?

Introduction / Context:
Lymph is a clear, slightly yellowish fluid that circulates in the lymphatic system. It plays important roles in immunity, fat absorption, and returning excess tissue fluid to the bloodstream. Exam questions often test basic facts about how lymph is formed and how it differs from blood. This question asks you to identify the correct statement about lymph from four possibilities.

Given Data / Assumptions:

• The options describe the origin, composition, and circulation of lymph.
• We assume standard human anatomy and physiology.
• The key is to distinguish between lymph and whole blood.

Concept / Approach:
Lymph forms when some of the fluid component of blood plasma filters out of capillaries into the tissue spaces, becoming tissue fluid. Most of this fluid is reabsorbed into capillaries, but a portion enters lymphatic capillaries and becomes lymph. Lymph is similar to plasma but contains far fewer proteins and no red blood cells under normal conditions; it is rich in lymphocytes and other white blood cells. It is not pumped directly by the heart; instead, skeletal muscle contractions and valves in lymph vessels help move lymph. Lymph has several functions, including immune defense and fat transport, not only hormone transport. Therefore, the correct statement must describe lymph formation from capillary filtrate.

Step-by-Step Solution:
Step 1: Evaluate option A. It states that lymph is formed from tissue fluid that originates by filtration and leakage of plasma through capillary walls. This correctly describes how tissue fluid is formed and how part of it enters lymphatic vessels to become lymph.Step 2: Evaluate option B. It claims lymph contains red blood cells similar to whole blood. In reality, lymph normally lacks RBCs and is composed mainly of plasma like fluid and white blood cells.Step 3: Evaluate option C. It says lymph is circulated by the blood circulating heart, but lymph circulation does not depend on a dedicated pump like the heart; it relies on vessel valves and muscle contractions.Step 4: Evaluate option D. It restricts lymph function solely to transporting hormones, which is incorrect because lymph also returns excess tissue fluid to blood and plays critical roles in immunity and fat absorption.Step 5: Conclude that option A is the only fully correct statement.

Verification / Alternative check:
You can verify by recalling that swelling (edema) occurs when lymph drainage is impaired, showing that lymph arises from excess tissue fluid. Also note that red blood cells remain in blood vessels in normal conditions; their presence in lymph would indicate injury or abnormal leakage. Textbook diagrams of the lymphatic system always show lymphatic capillaries picking up fluid from around tissue cells, which confirms option A.

Why Other Options Are Wrong:
Option B is wrong because red blood cells are absent from normal lymph; lymph mainly carries lymphocytes. Option C is wrong because the heart does not directly pump lymph; the lymphatic system has no central pump. Option D is wrong because lymph performs multiple functions, including immune surveillance and transport of absorbed fats from the intestine (as chyle), not just hormone transport.

Common Pitfalls:
Students sometimes equate lymph with blood plasma or think the same heart pump drives both blood and lymph. Another pitfall is focusing on a single function of lymph and ignoring others. To avoid confusion, remember that lymph is filtered fluid returning to the blood via lymphatic vessels, that it is rich in immune cells, and that it depends on valves and body movements for flow.

Final Answer:
Lymph is formed from tissue fluid that originates by filtration and leakage of plasma through capillary walls.