BackChapter 33: Circulation – Structure and Function of Circulatory Systems
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Circulation
Major Features and Functions of Circulatory Systems
Circulatory systems are essential for transporting nutrients, gases, and wastes throughout the bodies of multicellular organisms. As organisms evolved greater complexity, specialized systems developed to ensure that all cells receive the substances they need for survival.
Key Point 1: Early cells relied on direct diffusion with their environment for nutrient and gas exchange.
Key Point 2: In multicellular organisms, most cells are too far from the external environment for diffusion alone to suffice.
Key Point 3: The evolution of circulatory systems created an "internal sea" that transports food, oxygen, and wastes close to each cell.
Example: In humans, the circulatory system delivers oxygen from the lungs to tissues and removes carbon dioxide for exhalation.
Components of Circulatory Systems
Three Major Parts
All circulatory systems share three main components:
The Heart: A muscular pump that propels blood through the system.
Blood: The liquid medium that carries nutrients, gases, and wastes.
Blood Vessels: Tubes that conduct blood throughout the body.
Types of Circulatory Systems
Open vs. Closed Circulatory Systems
Animals have evolved two main types of circulatory systems: open and closed.
Open Circulatory System: Found in many invertebrates (e.g., insects, mollusks). Blood (called hemolymph) is pumped by one or more hearts into open spaces (sinuses) where it bathes organs directly.
Closed Circulatory System: Found in vertebrates and some invertebrates (e.g., earthworms, squid). Blood is confined to vessels, allowing for more efficient transport and higher pressure.
Feature | Open System | Closed System |
|---|---|---|
Main Fluid | Hemolymph | Blood |
Vessel Structure | Open sinuses | Continuous vessels |
Efficiency | Lower | Higher |
Examples | Insects, most mollusks | Vertebrates, earthworms, squid |
Functions of the Vertebrate Circulatory System
Diverse Roles in the Body
The vertebrate circulatory system performs several vital functions:
Transport of Oxygen and Nutrients: Delivers essential substances to tissues.
Removal of Wastes: Carries carbon dioxide and metabolic wastes to excretory organs.
Distribution of Hormones: Transports signaling molecules throughout the body.
Regulation of Body Temperature: Adjusts blood flow to maintain homeostasis.
Wound Healing and Clotting: Prevents blood loss and initiates repair.
Protection: White blood cells defend against pathogens.
Additional info:
Open systems are less efficient due to lower pressure and slower circulation, suitable for smaller or less active animals.
Closed systems support higher metabolic rates and larger body sizes.
