Animal circulatory systems move materials through the body so cells can get what they need and remove what they produce. They transport oxygen, carbon dioxide, nutrients, hormones, immune cells, and wastes. Circulation helps maintain homeostasis, which is especially important in animals with large bodies or high activity levels.
Different animal groups solve the same transport problem with different heart structures and blood flow patterns.
In open circulatory systems, fluid leaves vessels and directly bathes organs, while in closed circulatory systems, blood stays inside vessels. Vertebrates have closed circulation, but fish, amphibians, reptiles, birds, and mammals differ in how many heart chambers they have and whether blood passes through the heart once or twice per circuit. Single circulation sends blood from the heart to gas exchange surfaces and then to the body before returning.
Double circulation separates pulmonary or lung circulation from systemic body circulation, which supports higher pressure and more efficient oxygen delivery.
Key Facts
- Circulatory systems transport O2, CO2, nutrients, hormones, immune cells, and metabolic wastes.
- Open circulation: hemolymph leaves vessels and bathes tissues directly.
- Closed circulation: blood remains inside vessels, allowing higher pressure and faster transport.
- Single circulation in fish: heart -> gills -> body -> heart.
- Double circulation: heart -> lungs or skin -> heart -> body -> heart.
- Mammals and birds have 4-chambered hearts that keep oxygen-rich and oxygen-poor blood fully separated.
Vocabulary
- Circulatory system
- An organ system that moves fluids through the body to transport gases, nutrients, signals, cells, and wastes.
- Hemolymph
- The body fluid in many open circulatory systems that bathes organs directly and performs functions similar to blood.
- Closed circulation
- A circulatory plan in which blood stays inside vessels as it travels between the heart, gas exchange organs, and body tissues.
- Single circulation
- A blood flow pattern in which blood passes through the heart once during each complete trip around the body.
- Double circulation
- A blood flow pattern in which blood passes through the heart twice during each complete circuit, once after gas exchange and once before going to the body.
Common Mistakes to Avoid
- Saying open circulatory systems have no vessels is wrong because many open systems still use vessels for part of the pathway before hemolymph enters body spaces.
- Thinking all closed circulatory systems have four-chambered hearts is wrong because fish have closed circulation with a two-chambered heart.
- Calling fish circulation double circulation is wrong because fish blood passes through the heart only once per complete circuit.
- Assuming amphibian and most reptile hearts fully separate oxygen-rich and oxygen-poor blood is wrong because their three-chambered hearts allow some mixing.
Practice Questions
- 1 A fish heart pumps blood to the gills, then to the body, and then back to the heart. How many times does a red blood cell pass through the heart in one complete circuit?
- 2 A mammal has a resting heart rate of 75 beats per minute. How many heartbeats occur in 10 minutes?
- 3 Explain why double circulation with a four-chambered heart can support more active animals than single circulation.