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Open Circulatory System

Blood empties into an open body cavity to supply the tissues with nutrients

closed circulatory system

Blood flows through connected blood vessels, pumped by the muscular hearts.

  How does type of circulatory system relate to organism size and activity level?

• Open systems → good for small, low‑metabolic animals

• Closed systems → support high metabolic rates, larger bodies, and active lifestyles

·       What are the three major kinds of blood vessels and how do they differ?

Arteries

large lumen, muscular wall… carry blood away from heart.”

Veins

larger lumen, elastic wall… carries blood back to heart.”

Capillaries

thin‑walled (single‑cell layer). Ideal for exchange.”

·       Why is a decrease in pressure and velocity (speed) in the capillaries beneficial?

allows time for diffusion of O₂, CO₂, nutrients, and wastes

Prevents damage to capillary walls

Heart structure (atria & ventricles) fish, mammals , amphibians

Fish

• 1 atrium, 1 ventricle

Amphibians

• 2 atria, 1 ventricle

Mammals

• 2 atria, 2 ventricles

Path of blood through each heart (fish, mammals, amphibians)

Fish (single circulation)

• Deoxygenated blood is pumped from heart … to gill capillaries… then systemic capillaries.

Amphibians (mixed blood)

“Mixed oxygenated and deoxygenated blood is pumped out of the common ventricle…to the lungs and body

Mammals (double circulation)

Pulmonary circulation → lungs… systemic circulation → body.”

Where oxygen is picked up & dropped off

• Picked up:

  • Fish → gills

  • Amphibians → lungs + skin

  • Mammals → lungs

• Dropped off:

  • All groups → systemic capillaries

Does blood return to the heart between pulmonary & systemic circuits?

Fish:

❌ No — single circuit
→ Less efficient

Amphibians:

⚠ Partially — mixed blood in one ventricle

Mammals:

✅ Yes — fully separate circuits
Benefit:

• Higher pressure to body

• More efficient O₂ delivery

• Supports high metabolism

How does the structure of the circulatory system relate to efficiency of the system and metabolic rates?

More structural separation + higher pressure = more efficient oxygen delivery = higher metabolic rates.

o What are the chambers and the difference in function between the R and L sides?

Right side

• Receives deoxygenated blood

• Pumps to lungs (low pressure)

Left side

• Receives oxygenated blood

• Pumps to body (high pressure)

  What is the function of heart valves?

• Prevent backflow

• Ensure one‑way movement

Why left ventricle wall is thicker

• Pumps blood to entire body

• Needs more force

What sets the heart rhythm?

SA node (pacemaker)

What modifies the rhythm?

• Sympathetic → increases rate

• Parasympathetic → decreases rate

How electrical signals spread

Across muscle cells

• Through gap junctions

Across heart regions

“Atria are stimulated first… then ventricles.”