The terms "closed system" in the context of the lymphatic and circulatory systems refer to how these systems have defined pathways through which fluids (lymph and blood, respectively) circulate, and the fact that these pathways are not open to the external environment.
Circulatory System
- Definition: The circulatory system is considered a closed system because blood is contained within a network of blood vessels—arteries, veins, and capillaries.
- Fluid Dynamics: Blood flows continuously through this network, which is a closed loop. The system is pressurized, and blood does not leak out into the surrounding tissues under normal circumstances.
- Confinement: The heart pumps blood, maintaining its flow through these vessels. Any exchange of substances (e.g., oxygen, nutrients, and waste) occurs across the walls of the capillaries, but the blood itself remains within the confines of the circulatory system.
Lymphatic System
- Definition: The lymphatic system is also considered a closed system because it consists of lymphatic vessels that transport lymph fluid, which originates from interstitial fluid (fluid between cells).
- One-Way Flow: The lymphatic vessels carry lymph in one direction back toward the circulatory system, particularly into the subclavian veins near the heart.
- Valves and Compartmentalization: Like veins, lymphatic vessels have valves that prevent backflow, ensuring a one-way path. While lymph can move in and out of tissues, the overall network of lymphatic vessels is enclosed and helps maintain fluid balance within the body.
In summary, both systems are termed "closed" because they involve fluid movement within defined pathways (blood vessels for the circulatory system and lymphatic vessels for the lymphatic system) that do not allow for free flow of fluid into and out of the external environment.