Overview
This lecture explains the differences between closed and open circulatory systems, focusing on their structure, function, and efficiency in animals.
Closed Circulatory System
- Blood flows entirely within blood vessels (arteries, veins, capillaries) and never leaves them.
- The heart is a muscular organ that pumps blood under pressure throughout the body.
- Arteries carry blood from the heart, capillaries facilitate exchange, and veins return blood to the heart.
- Capillaries are very fine vessels that allow nutrient and gas exchange with tissues via interstitial fluid.
- Diffusion occurs between capillaries and interstitial fluid; oxygen and nutrients enter tissues, wastes and carbon dioxide return to the blood.
- The system is highly efficient, rapidly circulating 5-6 liters of blood throughout the body in less than 20 seconds.
- High efficiency comes at a high energy cost, which is necessary for animals with high metabolic rates, like humans and other vertebrates.
Open Circulatory System
- Found in insects and other arthropods with lower metabolic rates and smaller bodies.
- The heart pumps blood (hemolymph) into open spaces where it directly bathes organs and tissues.
- Blood (hemolymph) and interstitial fluid are the same; no strict separation as in closed systems.
- Blood re-enters the heart through tiny pores called ostia, lacking a dedicated path.
- This system is less efficient but consumes less energy and suffices for small, less active animals.
Comparison: Closed vs. Open Systems
- Closed systems: blood remains in vessels, highly efficient, supports high metabolic activity, higher energy use.
- Open systems: blood leaves vessels, less efficient, suitable for small and less active animals, lower energy use.
Key Terms & Definitions
- Blood Vessels — Tubes (arteries, veins, capillaries) that carry blood throughout the body.
- Closed Circulatory System — System where blood always remains inside blood vessels.
- Open Circulatory System — System where blood leaves vessels and bathes organs directly.
- Capillaries — Smallest blood vessels allowing exchange between blood and tissues.
- Interstitial Fluid — Fluid surrounding body tissues, facilitates exchange with capillaries.
- Hemolymph — Mixture of blood and interstitial fluid found in open circulatory systems.
- Ostia — Tiny pores in the heart of animals with open circulatory systems allowing blood to re-enter.
Action Items / Next Steps
- Review the characteristics and differences between closed and open circulatory systems for upcoming assessments.
- Prepare diagrams showing the flow of blood in both circulatory systems.