Final answer:
Fluids from the blood capillaries enter the interstitial spaces, and some of this becomes lymph in the lymphatic vessels. Lymph carries large solutes and is filtered by the lymph nodes before returning to the venous circulation. The lymphatic system, not pressurized by the heart, relies on muscle action and one-way valves to move fluid.
Step-by-step explanation:
The circulation of fluids between blood capillaries, lymphatics, and venous circulation is a critical aspect of human physiology. Blood capillaries are small blood vessels where the exchange of gases, nutrients, and waste occurs between the blood and tissues. Some of the fluid and small molecules forced out of the capillaries by blood pressure collect in the interstitial space, which is the fluid-filled space between cells.
Not all of this fluid is reabsorbed into the blood capillaries; a portion enters the lymphatic capillaries and becomes lymph. This lymph is similar in composition to the interstitial fluid and contains proteins, which cannot easily re-enter the capillaries due to their size. As lymph flows through the lymphatic system, it passes through lymph nodes where immune cells filter out pathogens and debris. Eventually, the lymph returns to the bloodstream through the thoracic duct and right lymphatic duct, which drain into the venous system at the subclavian veins. This circulation is crucial for maintaining fluid balance and for immune defense. Unlike the circulatory system, the lymphatic system is not a closed, pressurized circuit, and relies on other means to move fluid, such as the action of smooth muscles, skeletal muscles, and one-way valves. It exists as an open system, culminating in drainage points into veins near the heart. This demarcates the key differences and interaction between these essential systems in our body.