Overview
This lecture covers the structure, types, and evolutionary variations of circulatory systems in animals, emphasizing differences between open and closed systems and their adaptations in vertebrates.
Functions and Role of the Circulatory System
- Most animals use a circulatory system to transport nutrients and gases throughout the body.
- Simple diffusion suffices for gas and nutrient exchange in small, primitive animals.
- Bulk flow via circulatory systems is necessary for larger, complex organisms.
Open vs. Closed Circulatory Systems
- A closed circulatory system keeps blood within vessels, circulating it unidirectionally from the heart.
- Vertebrates and some invertebrates (e.g., earthworms) have closed circulatory systems.
- Open circulatory systems (in arthropods and most mollusks) pump hemolymph into a body cavity (hemocoel), mixing with interstitial fluid.
- Hemolymph reenters the heart through ostia (openings) in open systems.
- Open systems consume less energy but are less efficient in oxygen delivery to active tissues.
Circulatory System Variations in Animals
- Sponges and rotifers rely on diffusion and do not need circulatory systems.
- Cnidarians and ctenophores use diffusion through both external and internal aqueous environments.
- Most arthropods and many mollusks utilize open systems; some active mollusks (e.g., squids) evolved closed systems for speed.
- Circulatory systems in vertebrates became more complex due to evolutionary adaptations.
Vertebrate Circulatory System Evolution
- Fish: Two-chambered heart; single circuit of blood flow through gills (gill circulation) and body (systemic circulation).
- Amphibians: Three-chambered heart; double circulation with some blood mixing, aided by a ventricle ridge.
- Reptiles: Three-chambered heart with a partial septum; less blood mixing, some have four chambers (e.g., crocodilians).
- Crocodilians have adaptations such as the foramen of Panizza for blood shunting.
- Mammals and birds: Four-chambered hearts; complete separation of oxygenated and deoxygenated blood for efficient double circulation.
- Four-chambered hearts in birds and mammals evolved independently via convergent evolution.
Key Terms & Definitions
- Closed circulatory system — blood is enclosed in vessels and circulates in a loop.
- Open circulatory system — hemolymph is pumped into a cavity, mixing with interstitial fluid.
- Hemocoel — body cavity in open circulatory systems where hemolymph flows.
- Hemolymph — fluid that serves as blood and interstitial fluid in open systems.
- Interstitial fluid — fluid surrounding body cells.
- Ostia — heart openings allowing hemolymph entry in open systems.
- Double circulation — separation of pulmonary (lung) and systemic (body) circuits.
- Convergent evolution — independent evolution of similar features in different lineages.
Action Items / Next Steps
- Review diagrams of vertebrate and invertebrate circulatory systems.
- Compare the circulatory systems of fish, amphibians, reptiles, birds, and mammals.
- Understand the functional trade-offs between open and closed systems.