Transport in Animals: Circulatory Systems and Capillary Exchange

Constraints of Multicellularity

• Diffusion Limits: Large multicellular organisms have a small Surface Area to Volume ($SA:V$) ratio and large distances between internal cells and the external environment.

• Consequence: Diffusion alone cannot meet exchange needs for nutrients, $O_2$, $CO_2$, and waste removal.

• Solution: Circulatory systems utilize bulk flow of extracellular fluids to deliver substances and maintain high concentration gradients necessary for diffusion.

Components of Circulatory Systems

• Core Elements: A pump (e.g., heart), circulatory fluid, exchange surfaces (e.g., capillary beds), and tubular vessels.

• Mechanisms: Systems include structures to direct flow (one-way valves) and mechanisms to return fluid to the pump (e.g., veins and lymphatics).

• Active vs. Passive: Animal transport involves active mechanisms (pumps), whereas plant transport relies primarily on passive processes.

Variation in Circulatory Systems

• Organisms Without Systems: Some animals, such as Sponges, Cnidarians, Flatworms, and Tardigrades, lack dedicated circulatory systems.

• Open Circulatory Systems:

  • The transport fluid and extracellular fluid are the same (hemolymph).

  • Fluid flows through body cavities; found in arthropods (insects, arachnids, crustaceans) and non-cephalopod molluscs.

  • Characterized by lower pressures and lower flow rates but higher energy efficiency for the pump.

• Closed Circulatory Systems:

  • Fluid (blood) is enclosed in vessels and is separate from interstitial fluid ($IF$).

  • Allows for more rapid flow, higher pressure, and the ability to selectively alter blood flow to specific tissues.

  • Found in Annelid worms and vertebrates.

Vertebrate Evolution and Specialization

• Circuit Evolution: Systems evolved from single-circuit hearts (found in Fish) to double-circuit systems (found in Birds and Mammals).

• Double Circuit Benefits: Supports higher metabolic demands by separating pulmonary and systemic circulation.

• Respiratory decouple: Some highly active organisms with open systems (like insects) do not depend on their circulatory system for gas exchange.

Capillary Exchange and Starling Forces

• Filtration and Reabsorption: Controlled by the balance of hydrostatic and osmotic pressures.

• Starling Forces:

  • Capillary Hydrostatic Pressure ($CHP$): Fluid pressure pushing against capillary walls; highest at the arteriole end.

  • Blood Osmotic Pressure ($BOP$) / Blood Colloid Osmotic Pressure ($BCOP$): Pressure from plasma proteins pulling water back into the vessel via osmosis.

  • Interstitial Fluid Osmotic Pressure ($IFOP$): Generally negligible.

  • Interstitial Fluid Hydrostatic Pressure ($IFHP$): Generally negligible.

• Dynamic Balance:

  • Filtration occurs when pressure out ($CHP + IFOP$) is greater than pressure in ($BOP + IFHP$).

  • Reabsorption occurs when pressure in is greater.

  • Homeostasis: Maintaining this balance prevents Oedema (excess fluid in tissues); excess fluid is returned via the lymphatic system.