The Circulatory System: An Overview
The circulatory system, also known as the cardiovascular system, is a transport network that moves nutrients, gases like oxygen and carbon dioxide, and waste products throughout the body. It's composed of the heart, blood, and blood vessels, and plays a key role in maintaining homeostasis and supporting cellular function.
Evolutionary (Phylogenetic) Origins of the Circulatory System
A. Invertebrates
Invertebrate circulatory systems are simpler, with two primary types:
- Open Circulatory System: Common in arthropods (e.g., insects, crustaceans), where blood (or hemolymph) isn't confined to vessels and instead bathes the organs directly. Hemolymph transports nutrients but not oxygen.
- Closed Circulatory System: Seen in annelids (e.g., earthworms), where blood flows entirely within vessels. Their blood contains hemoglobin dissolved in plasma, allowing for better oxygen transport than in open systems.
B. Protochordates (Early Chordates)
- Tunicates (sea squirts): Possess a very simple, single-chambered tubular heart that can reverse its direction of pumping. Despite being basic, they are evolutionarily important as close relatives of vertebrates.
- Lancelets (Amphioxus): Represent a more advanced model. They don’t have a true heart, but a pulsating ventral aorta pushes blood through paired aortic arches. Blood collects in a sinus venosus and drains via cardinal veins. A hepatic portal system is already present, suggesting early specialization of circulation.