7.4 Comparative cardiovascular systems in animals

Cardiovascular systems in animals come in various forms, from open to closed systems. The heart's structure ranges from two to four chambers, each adapted to the animal's specific needs and environment. These differences impact how blood flows and oxygen is delivered throughout the body.

Circulation pathways vary among species, with gill circulation in aquatic animals and pulmonary circulation in air-breathers. Some animals have developed specialized adaptations, like marine mammals' ability to dive deep for extended periods, showcasing the incredible diversity of cardiovascular systems in the animal kingdom.

Circulatory System Types

Open vs Closed Circulatory Systems

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• Open circulatory system pumps blood into body cavities (hemocoel) allowing direct contact with tissues and organs
• Found in arthropods (insects, spiders) and most mollusks (snails, clams)
• Closed circulatory system pumps blood through a network of vessels that transport blood to tissues without direct contact
  • Blood remains separate from interstitial fluid surrounding cells
  • Found in annelids (earthworms), cephalopod mollusks (octopus, squid), and all vertebrates

Variations in Heart Structure

• Two-chambered heart has one atrium and one ventricle
  • Found in fish
  • Blood flows in single loop, first to gills for oxygenation then to rest of body
• Three-chambered heart has two atria and one ventricle
  • Found in amphibians and most reptiles
  • Oxygenated and deoxygenated blood mix in the single ventricle
• Four-chambered heart has two atria and two ventricles
  • Found in birds, mammals, and crocodilians
  • Oxygenated and deoxygenated blood remain completely separated
  • Allows for higher blood pressure and more efficient oxygen delivery to tissues

Circulation Pathways

Gill and Pulmonary Circulation

• Gill circulation in fish and some amphibian larvae
  • Heart pumps deoxygenated blood to gills where gas exchange occurs
  • Oxygenated blood then flows to rest of body
• Pulmonary circulation in air-breathing animals
  • Heart pumps deoxygenated blood to lungs
  • Gas exchange occurs in lungs, oxygenating the blood
  • Oxygenated blood returns to heart to be pumped to body

Systemic Circulation

• Systemic circulation delivers oxygenated blood from heart to body tissues
  • In fish, blood flows from gills directly to systemic circulation
  • In animals with pulmonary circulation, oxygenated blood returns to heart first
• Deoxygenated blood from tissues returns to heart to be re-oxygenated

Countercurrent Exchange

• Countercurrent exchange is a mechanism for efficient transfer of heat or gases between fluids flowing in opposite directions
  • Examples include heat exchange in limbs of marine mammals and oxygen uptake in fish gills
• In fish gills, water and blood flow in opposite directions over gill filaments
  • Maintains a constant diffusion gradient for efficient oxygen uptake from water into blood

Specialized Adaptations

Diving Adaptations in Marine Mammals

• Marine mammals like whales and seals have adaptations for prolonged deep dives
• Increased oxygen storage in blood and muscle
  • High levels of hemoglobin in blood and myoglobin in muscles
• Reduced heart rate (bradycardia) and restricted blood flow to non-essential organs during dives
  • Maintains blood flow to heart, lungs and brain
• Collapse of lungs at depth to avoid nitrogen absorption and decompression sickness
• Efficient oxygen extraction allows some whales to dive for over 2 hours on a single breath