Open and closed circulatory systems | Body fluids and circulation | Biology | Khan Academy

Blood is a life-giving liquid that circulates throughout the body within blood vessels, bringing in food and oxygen and taking away waste products. A 'closed circulatory system' means blood always stays within these vessels, never leaving them.

In a closed system, the heart pumps blood into arteries, which branch into tiny capillaries that reach all tissues. Exchange of materials (oxygen, nutrients, waste) happens between capillaries and interstitial fluid surrounding tissues. Blood then returns to the heart through veins. This system is incredibly efficient and fast, covering vast distances daily due to the powerful heart, but it requires significant energy.

In an open circulatory system, like in insects, the heart pumps blood into vessels, but the blood then leaves these vessels and directly bathes the organs and tissues. This is possible because these animals have lower metabolic rates, are cold-blooded, and are smaller, so diffusion is sufficient. The blood and interstitial fluid are essentially the same, called hemolymph.

In open systems, blood returns to the heart through tiny pores called ostia. This process is less efficient and slower as there's no directed path, but it consumes less energy. While less efficient, it suffices for smaller, simpler animals with lower energy demands. Closed systems are highly efficient and energy-intensive, suitable for active animals like humans, while open systems are less energy-consuming but also less efficient.