11. The primary function of lymphatic system is
    (1) Acquired immunity
    (2) Innate immunity
    (3) The removal of excess fluids from body tissues.
    (4) Phagocytosis

Introduction

The lymphatic system is an essential part of the human body that works closely with the cardiovascular and immune systems. It performs several vital functions related to fluid regulation, immune defense, and nutrient absorption. Among various roles it plays, the primary function of the lymphatic system is the removal of excess fluids from body tissues to maintain fluid balance. This article explores the comprehensive functions of the lymphatic system and highlights why removal of excess interstitial fluid is its main role compared to other immune-related functions such as acquired immunity, innate immunity, or phagocytosis.

Overview of the Lymphatic System

The lymphatic system comprises a network of lymphatic vessels, lymph nodes, and organs such as the spleen, thymus, tonsils, and bone marrow. It circulates a clear fluid called lymph, which originates as interstitial fluid leaked from blood capillaries into the tissues.

Primary Function: Removal of Excess Tissue Fluid

One of the body’s constant physiological challenges is maintaining fluid homeostasis. Approximately 20 liters of plasma exit the blood vessels daily to nourish tissues as interstitial fluid. About 17 liters of this fluid reabsorbs directly back into the blood capillaries, while the balance (around 3 liters) remains in the interstitial spaces and needs to be drained to prevent tissue swelling (edema).

• The lymphatic vessels collect this excess fluid and drain it back into the bloodstream, completing the fluid cycle.

• This process is vital to prevent tissue swelling, maintain blood volume, and support efficient nutrient and gas exchange in tissues.

Secondary Functions: Immune System Support

Besides fluid balance, the lymphatic system acts as a major immune system component:

• Transport of Lymphocytes: The lymphatic organs (lymph nodes, spleen) filter lymph and trap pathogens, communicating with B and T lymphocytes to mount immune responses.

• Adaptive Immunity: The system supports acquired immunity by facilitating lymphocyte maturation and antigen presentation within lymphatic organs.

• Innate Immunity and Phagocytosis: Macrophages and dendritic cells in lymph nodes engulf pathogens (phagocytosis), serving as first-line innate immune defenders.

Why Removal of Excess Fluids Is the Primary Function

While the immune functions are critical, the lymphatic system originated evolutionarily and physiologically as a drainage network to maintain fluid homeostasis. Immune surveillance and lymphocyte trafficking are integrated functions built on this primary framework. Without fluid removal, tissues would swell excessively, leading to impaired cellular function and severe health problems.

Incorrect Options Explained

• (1) Acquired immunity: Though the lymphatic system plays a role in adaptive immunity via lymphoid organs and lymphocytes, this is a secondary function.

• (2) Innate immunity: Phagocytic activity occurs in lymph nodes, but innate immunity occurs broadly throughout the body involving multiple systems.

• (4) Phagocytosis: This is a cellular immune function mainly carried out by macrophages and neutrophils within the lymphatic system but is not the system’s primary role.

Correct answer: (3) The removal of excess fluids from body tissues