Unit 3: Circulation

Table of Contents

1. Composition of Blood

Blood is a fluid connective tissue that circulates throughout the body, transporting substances. It consists of plasma and formed elements.

[Image of the components of blood (plasma, buffy coat, erythrocytes)]

a) Plasma (55%)

The liquid matrix of blood. It is ~90% water, and contains:

b) Formed Elements (45%)

  1. Erythrocytes (Red Blood Cells - RBCs):
    • Structure: Biconcave discs, anucleated (no nucleus) when mature.
    • Function: Contain hemoglobin to transport O₂ and CO₂.
  2. Leukocytes (White Blood Cells - WBCs):
    • Structure: Larger than RBCs, have a nucleus.
    • Function: Part of the immune system; defend against pathogens. (e.g., Neutrophils, Lymphocytes, Monocytes).
  3. Thrombocytes (Platelets):
    • Structure: Small, irregular cell fragments.
    • Function: Essential for blood clotting.

2. Mechanism of Blood Coagulation

Coagulation (clotting) is the process of forming a blood clot (thrombus) to prevent blood loss from a damaged vessel. It is a complex cascade of events involving platelets and plasma proteins called clotting factors.

Simplified Steps:

  1. Vascular Spasm: The damaged blood vessel constricts to reduce blood flow.
  2. Platelet Plug Formation: Platelets adhere to the damaged vessel wall and "activate," becoming sticky and releasing chemicals that attract more platelets to the site, forming a temporary plug.
  3. Coagulation Cascade (Clot Formation):
    • A complex series of reactions (involving >12 clotting factors) is initiated.
    • This cascade leads to the conversion of an inactive plasma protein, Prothrombin, into its active enzyme form, Thrombin. (This step requires Vitamin K and Ca²⁺).
    • Thrombin then catalyzes the conversion of another plasma protein, the soluble Fibrinogen, into insoluble Fibrin strands.
    • These Fibrin strands form a mesh that traps RBCs and platelets, creating a stable, gel-like clot that seals the wound.
[Image of the blood coagulation cascade (simplified)]
Key Reaction: Thrombin (enzyme) converts Fibrinogen (soluble) → Fibrin (insoluble mesh).

3. Single and Double Circulation

a) Single Circulation

Found in fish. The heart has two chambers (one atrium, one ventricle).
Pathway: Heart → Gills (gets oxygenated) → Body Tissues (delivers O₂) → Heart.
Blood passes through the heart only *once* per complete circuit. This is less efficient.

b) Double Circulation

Found in mammals and birds. The heart has four chambers. Blood passes through the heart *twice* per complete circuit. This is highly efficient and maintains high blood pressure.

  1. Pulmonary Circuit: Pumps deoxygenated blood to the lungs and returns oxygenated blood to the heart.
    (Right Ventricle → Lungs → Left Atrium)
  2. Systemic Circuit: Pumps oxygenated blood to the rest of the body and returns deoxygenated blood to the heart.
    (Left Ventricle → Body Tissues → Right Atrium)
[Image comparing single and double circulation]

4. Structure of Human Heart

The heart is a four-chambered muscular organ that pumps blood throughout the body.

[Image of the structure of the human heart, showing all four chambers, valves, and major vessels]

Key Structures:

5. Mechanism of Blood Circulation

Follow the path of a single red blood cell starting from the right atrium:

  1. Deoxygenated blood enters Right Atrium (from body).
  2. Passes through Tricuspid Valve into Right Ventricle.
  3. Right Ventricle pumps blood through Pulmonary Valve into Pulmonary Artery.
  4. Blood goes to the Lungs, releases CO₂, and picks up O₂.
  5. Oxygenated blood returns to Left Atrium (via Pulmonary Veins).
  6. Passes through Bicuspid (Mitral) Valve into Left Ventricle.
  7. Left Ventricle pumps blood through Aortic Valve into Aorta.
  8. Aorta branches, delivering oxygenated blood to all Body Tissues.
  9. Deoxygenated blood returns to the Right Atrium (via Vena Cavae), and the cycle repeats.

6. Cardiac Cycle

The cardiac cycle refers to the complete sequence of events in the heart from the beginning of one heartbeat to the beginning of the next. It has two main phases:

  1. Diastole (Relaxation Phase):
    • All chambers are relaxed.
    • Blood passively flows from the atria into the ventricles (AV valves are open).
  2. Systole (Contraction Phase):
    • Atrial Systole: The atria contract, pushing the last bit of blood into the ventricles.
    • Ventricular Systole: The ventricles contract.
      • Pressure rises, slamming the AV valves *shut* (produces the first heart sound, "LUB").
      • Pressure continues to rise until it forces the Semilunar valves *open*.
      • Blood is ejected into the Aorta and Pulmonary Artery.
  3. Return to Diastole: Ventricles relax, pressure drops, Semilunar valves snap *shut* (produces the second heart sound, "DUB").
[Image of the phases of the cardiac cycle]

7. ECG (Electrocardiogram)

An ECG (or EKG) is a graphical recording of the electrical activity of the heart during the cardiac cycle.

[Image of a typical ECG waveform (P-QRS-T)]

Key Waves:

8. Pacemaker

The heart's rhythm is not controlled by the brain (though the brain can change its *rate*). It is controlled by a specialized group of cells called the Sinoatrial (SA) Node, located in the wall of the right atrium.