Animal Circulatory Systems: Functions and Components

The Internal Environment

The internal environment refers to the extracellular fluid within an organism. In simple invertebrates, the interstitial fluid acts as the plasma that bathes all cells. More complex invertebrates possess a transportation system—comprising the circulatory system and haemolymph—that delivers nutrients and oxygen to cells while removing waste products. In some invertebrates and all vertebrates, both interstitial fluid and blood plasma are present, alongside lymph circulating through the lymphatic system.

Circulatory System Functions

In simpler animals, nutrients and oxygen are absorbed directly from the interstitial plasma. In more complex animals, a specialized circulatory system transports these substances. In vertebrates, this system consists of the blood and lymphatic circuits, performing the following functions:

• Carries oxygen from the lungs to cells and CO2 from cells to the lungs.
• Transports digestive nutrients to cells.
• Removes waste products from cells.
• Transports hormones from endocrine glands to target organs.
• Carries antibodies.
• Maintains constant body temperature.

Components of the Circulatory System

• Circulatory Fluid: The medium in which substances are transported, either dissolved or bound to respiratory pigments.
• Vessels: Tubes of varying sizes that circulate fluids throughout the body, including arteries, veins, capillaries, and lymph vessels.
• Heart: The organ responsible for pumping fluid through the system. Types include:
  • Tubular: The simplest form, consisting of vessels that push liquid through contractions.
  • Loculated: Features cavities called atria and ventricles, separated by valves.
  • Accessory Hearts: Usually located near the gills.

Respiratory Pigments

• Hemoglobin: Red due to iron in contact with oxygen; found in annelids and vertebrates.
• Hemocyanin: Blue due to copper in contact with oxygen; found in arachnids and mollusks.
• Hemerythrin: Red-purple due to iron in contact with oxygen; found in some polychaete annelids.
• Chlorocruorin: Green due to iron; found in some polychaete annelids.

Types of Circulatory Vessels

The circulatory system consists of a network of vessels categorized into three types:

• Arteries: Carry the circulatory fluid from the heart to the organs.
• Veins: Return the fluid back to the heart.
• Capillaries: Thin vessels that connect arteries and veins, reaching every cell in the body.