Circulatory Pathways

Think of the circulatory system as the delivery network of the body. There are two main designs in the animal kingdom:

A. Open Circulatory System (The "Bathtub" Method)

  • Mechanism: The heart pumps blood out, but instead of staying in pipes (vessels), it pours into open spaces called sinuses. The body tissues soak directly in this blood.
  • Pressure: Low, because fluid isn't confined.
  • Who has it? Arthropods (like insects) and Molluscs (like snails).

B. Closed Circulatory System (The "Plumbing" Method)

  • Mechanism: Blood never leaves the network of closed blood vessels (arteries, veins, capillaries). It circulates continuously.
  • Advantage: The body can precisely regulate where blood goes and how fast it flows.
  • Who has it? Annelids (earthworms) and Chordates (including humans).

Evolution of the Vertebrate Heart

As animals became more complex, their hearts evolved to separate oxygen-rich blood from oxygen-poor blood.

Group Chambers Circulation Type Key Feature
Fishes 2 (1 Atrium, 1 Ventricle) Single Circulation Heart pumps deoxygenated blood to gills for oxygenation.
Amphibians & Reptiles 3 (2 Atria, 1 Ventricle) Incomplete Double Circulation Oxygenated and deoxygenated blood mix in the single ventricle.
Crocodiles (Exception) 4 Double Circulation Unlike other reptiles, crocodiles have a fully 4-chambered heart.
Birds & Mammals 4 (2 Atria, 2 Ventricles) Double Circulation Strict separation of blood. Left side = Oxygenated; Right side = Deoxygenated.

Human Heart Anatomy

Location & Origin:

  • It sits in the thoracic cavity (chest), between the lungs, tilting slightly left.
  • It originates from the Mesoderm germ layer.
  • Protection: It is wrapped in a double-layered bag called the Pericardium. The space between layers is filled with pericardial fluid to reduce friction (shock absorber).

Section of a human heart

The Four Chambers:

  1. Atria (Top): Receiving chambers. Thin walls. Separated by the Inter-atrial septum.
  2. Ventricles (Bottom): Pumping chambers. Thick walls (to pump blood out). Separated by the Inter-ventricular septum.

The Valves (One-Way Doors): Valves prevent backflow. Blood must flow: Atria →\rightarrow Ventricles →\rightarrow Arteries.

  • Tricuspid Valve: Between Right Atrium & Right Ventricle. (Has 3 flaps).
  • Bicuspid (Mitral) Valve: Between Left Atrium & Left Ventricle. (Has 2 flaps).
  • Semilunar Valves: Guard the exits to the large arteries (Pulmonary Artery & Aorta).

The Conducting System (The Heart's Wiring)

The heart doesn't wait for the brain to tell it to beat; it is Auto-excitable. Special muscle fibers called Nodal Tissue generate and spread electricity.

The Pathway of the Impulse:

  1. Sino-atrial Node (SAN):

    • Role: The Pacemaker. It starts the beat.
    • Location: Top right of Right Atrium.
    • Speed: Fires 70–75 times/min (your heart rate).

    ↓\downarrow (Signal travels to)

  2. Atrio-ventricular Node (AVN):

    • Location: Bottom left of Right Atrium.
    • Role: Catches the signal and passes it to the ventricles.

    ↓\downarrow

  3. Bundle of His (AV Bundle):

    • Passes through the septum and splits into Right and Left branches.

    ↓\downarrow

  4. Purkinje Fibres:

    • Tiny fibers that spread the signal through the Ventricular muscles, causing them to contract.

Circulatory Pathways and Human Heart

  • "Try to be Right": The Tricuspid valve is on the Right side. (So, Bicuspid must be on the Left).
  • Chamber Evolution:
    • Fish = 2 (Simple)
    • Frog/Lizard = 3 (Mixing)
    • Croc/Bird/Human = 4 (Perfect separation)
  • The Electrical Path: SAN →\rightarrow AVN →\rightarrow Bundle →\rightarrow Purkinje.
    • (Think: Sstart At Node… then down to the muscles).
  • SAN Stats: Remember 70-75. That's the normal heart rate because the SAN sets the pace.

💡 Questions and Answers

Q1: Distinguish between the tricuspid and bicuspid (mitral) valves.

Answer: Tricuspid valve has 3 flaps and is found between the right atrium and right ventricle. Bicuspid (mitral) valve has 2 flaps and is between the left atrium and left ventricle. Both stop backflow into atria.

Key Points:

  • Tricuspid = Right side = 3 cusps (RA→RV)
  • Bicuspid/Mitral = Left side = 2 cusps (LA→LV)

Q2: Why is the SAN called the natural pacemaker?

Answer: Because SAN produces the highest number of electrical impulses (about 70–75 per minute), so it sets the heartbeat rate for the whole heart.

Key Points:

  • SAN fires fastest
  • Controls heart rhythm

Q3: What type of circulatory system do arthropods have?

Answer: Arthropods have an open circulatory system, where the blood (haemolymph) flows into open sinuses and bathes organs.

Key Points:

  • Open system = sinuses
  • Example: insects

Q4: How many chambers does a crocodile heart have?

Answer: Crocodile has a four-chambered heart, even though it is a reptile.

Key Points:

  • Crocodile = reptile exception = 4 chambers

Q5: What is the function of the pericardium?

Answer: Pericardium is a protective covering of the heart. The pericardial fluid inside reduces friction when the heart beats.

Key Points:

  • Protection
  • Friction reduction via fluid