Circulation

Circulatory systems vary across multicellular invertebrates based on their size, complexity, and lifestyle.
Sponges and Cnidarians: Use environmental water as circulatory fluid.
- Sponges: Water is circulated through channels.
- Hydra (Cnidarians): Employ a gastrovascular cavity for circulation and digestion; nutrients diffuse directly to tissues due to thin walls (2 cell layers).
Invertebrates with Pseudocoelom (e.g., roundworms, rotifers): Rely on body cavity fluids for circulation, allowing for nutrient exchange via body movement.
Larger Animals: Require specialized circulatory systems for oxygen and nutrient transport due to thicker tissue layers.

Found in most mollusks and arthropods.
Hemolymph: The fluid that is both circulating and extracellular fluid.
- Example (Insect): Muscular heart pumps hemolymph into body cavities, draining back into the central cavity.

Found in cephalopods, annelids (earthworms), and all vertebrates.
Blood: Always contained within vessels, allowing more efficient circulation.
- Example (Earthworm): Blood pumped from dorsal vessel goes through connecting arteries to ventral vessel, supplying tissues.

Blood = Connective tissue with:
- Plasma: 92% water; contains proteins, electrolytes, gases, nutrients, and hormones.
- Formed Elements:
- Red Blood Cells (Erythrocytes): Transport oxygen (45% of blood).
- White Blood Cells (Leukocytes): Immune response (less than 1% of blood).
- Platelets: Cell fragments critical for clotting.

92% water with:
- Plasma proteins (7%): Albumin, globulins, fibrinogen.
- Nutrients, hormones, and ions (Na+, Cl−, etc.).

Erythrocytes: Oxygen transport; typically lose nuclei during maturation in mammals.
Leukocytes: Defense against pathogens; categorized into granular (e.g., neutrophils, eosinophils) and agranular (e.g., lymphocytes, monocytes).
Platelets: Form plugs at injury sites, essential for clotting.

Upon vessel injury:
1. Vasoconstriction: Reduces blood flow.
2. Platelet plug formation: Platelets stick together and to vessel walls.
3. Clot reinforcement: Fibrin threads create a stable clot.
4. Dissolution: After healing, the clot is dissolved to prevent blockage.

Hematopoiesis: Formation of blood cells from pluripotent stem cells in bone marrow.
- Erythropoietin stimulates red blood cell production.
- Megakaryocytes produce platelets.

Chordate Ancestry: Simple tubular hearts evolved into more complex chambered hearts in vertebrates.
Fish: Possess a two-chambered heart suitable for gill circulation (inefficiency reduces pressure).
Amphibians and Reptiles: Evolved double circulation; amphibians have a three-chambered heart, while reptiles show further improvements.
Mammals and Birds: Four-chambered heart structure facilitates efficient circulation; allows for complete separation of oxygenated and deoxygenated blood, enhancing metabolic capabilities, and enabling endothermic regulation.

Fish: Simple heart with sinus venosus.
Amphibians: Three-chambered heart with some mixing of blood, specialized features to reduce mixing.
Reptiles: Partially divided ventricles; variations among species (crocodiles have complete separation).
Mammals and Birds: Fully divided four-chambered heart; efficient pumping system ensuring separate pulmonary and systemic circuits.

Open Circulatory Systems:
- Found in most mollusks and arthropods.
- Examples: Insects, lobsters.
- Structure:
- Hemolymph circulates through open cavities, not confined to vessels.
- Lower pressure system; body movement aids circulation.
- Function:
- Facilitates nutrient exchange through diffusion; less efficient in oxygen transport due to lower pressure.
Closed Circulatory Systems:
- Found in cephalopods, annelids (earthworms), and vertebrates.
- Examples: Humans, octopuses.
- Structure:
- Blood contained within vessels; allows for higher pressure and efficient oxygen transport.
- Function:
- Rapid transport of nutrients and gases; better suited for larger organisms.
No Circulatory System:
- Examples: Sponges and cnidarians (like the Hydra).
- Structure & Function: Circulation relies on environmental water and simple diffusion; efficient for small, simple organisms.

Fish:
- Two-chambered heart (1 atrium, 1 ventricle).
- Advantage: Efficient for gill circulation but limits metabolic rate due to single-loop system.
Amphibians:
- Three-chambered heart (2 atria, 1 ventricle).
- Advantage: Partial separation of oxygenated and deoxygenated blood, improving efficiency.
Reptiles:
- Three-chambered heart with partially divided ventricles (fourth chamber in crocodiles).
- Advantage: Controls blood flow better, reducing mixing.
Mammals and Birds:
- Four-chambered heart (2 atria, 2 ventricles).
- Advantage: Complete separation of oxygenated and deoxygenated blood, allowing high metabolic rates and sustained activity.

Mechanism: Gases diffuse across membranes based on partial pressure gradients.
- Higher partial pressure of oxygen (O2) outside compared to inside promotes diffusion into the organism; vice versa for carbon dioxide (CO2).
Media Types:
- Gases diffuse more readily in water than in air due to the solubility of gases.
Evolutionary Strategies:
- Larger surface area, thinner membranes, and strategies like countercurrent exchanges in gills maximize diffusion rates.

Structure: Gills composed of thin filaments and lamellae increase surface area.
Advantages:
- Highly efficient at gas exchange due to the large surface area and proximity to blood supply.
- Supports respiration in aquatic environments where oxygen is less abundant than in air.
Disadvantages:
- Vulnerable to damage and obstruction.
- Requires constant flow of water for gas exchange to be effective.

Countercurrent Flow:
- Blood flows in the opposite direction of water movement over the gills.
- Advantage: Maintains a gradient for oxygen diffusion throughout the entire length of the gill.
Concurrent Flow:
- Blood and water flow in the same direction.
- Disadvantage: Results in rapid equilibrium; reduces efficiency of gas exchange as less oxygen is absorbed compared to countercurrent flow.