Describe the process of blood circulation. What are the various factors that influence coagulation process in animal system?

Blood Circulation: A Detailed Process

Blood circulation is broadly divided into two circuits: the pulmonary circuit and the systemic circuit. The heart acts as the central pump driving this circulation.

1. Pulmonary Circulation

1. Deoxygenated Blood Entry: Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava.
2. Right Ventricle: It passes into the right ventricle.
3. Pulmonary Artery: The right ventricle pumps the blood through the pulmonary artery to the lungs.
4. Gas Exchange: In the lungs, gas exchange occurs – carbon dioxide is released, and oxygen is absorbed.
5. Pulmonary Veins: Oxygenated blood then returns to the left atrium via the pulmonary veins.

2. Systemic Circulation

1. Left Atrium: Oxygenated blood enters the left atrium.
2. Left Ventricle: It passes into the left ventricle, which is the most powerful chamber of the heart.
3. Aorta: The left ventricle pumps the oxygenated blood into the aorta, the largest artery in the body.
4. Distribution: The aorta branches into smaller arteries, which carry blood to all parts of the body (organs, tissues, and cells).
5. Capillary Exchange: In capillaries, exchange of oxygen, nutrients, and waste products occurs between blood and tissues.
6. Veins: Deoxygenated blood then flows into veins, which converge to form the superior and inferior vena cava, returning the blood to the right atrium, completing the cycle.

Factors Influencing Coagulation

Coagulation, also known as blood clotting, is a complex process that prevents excessive blood loss following injury. It involves a cascade of enzymatic reactions. These reactions can be broadly categorized as intrinsic, extrinsic, and local factors.

1. Extrinsic Pathway

This pathway is initiated by tissue factor (TF), a glycoprotein released from damaged tissues. It is the faster of the two pathways.

• Tissue Factor (TF): When blood vessels are damaged, TF binds to Factor VII.
• TF-VIIa Complex: This complex activates Factor X, leading to the formation of prothrombinase.

2. Intrinsic Pathway

This pathway is triggered by factors within the blood itself, typically when blood comes into contact with a foreign surface.

• Contact Activation: Exposure to collagen or other negatively charged surfaces activates Factor XII.
• Cascade of Activations: A series of activations occurs involving Factors XII, XI, IX, and VIII, ultimately leading to Factor X activation and prothrombinase formation.

3. Common Pathway

Both the intrinsic and extrinsic pathways converge on the common pathway.

• Prothrombinase: This complex converts prothrombin (Factor II) into thrombin.
• Thrombin: Thrombin converts fibrinogen (Factor I) into fibrin.
• Fibrin Polymerization: Fibrin monomers polymerize to form a fibrin mesh, which traps blood cells and platelets, forming a clot.
• Clot Stabilization: Factor XIII, activated by thrombin, cross-links fibrin strands, stabilizing the clot.

Local Factors Influencing Coagulation

• Calcium (Ca²⁺): Essential for many steps in the coagulation cascade.
• Vitamin K: Required for the synthesis of several clotting factors (II, VII, IX, X).
• Platelets: Play a critical role in clot formation by adhering to damaged vessel walls and releasing factors that promote coagulation.
• Von Willebrand Factor (vWF): Mediates platelet adhesion to the vessel wall.
• Antithrombin III: A natural anticoagulant that inhibits thrombin and other clotting factors.
• Protein C and Protein S: Vitamin K-dependent proteins that contribute to the regulation of coagulation.

Pathway	Initiation	Key Factors
Extrinsic	Tissue Factor (TF) release	TF, Factor VII, Factor X
Intrinsic	Contact with foreign surface	Factor XII, Factor XI, Factor IX, Factor VIII
Common	Prothrombinase formation	Prothrombin, Thrombin, Fibrinogen, Fibrin

Disruptions in any of these factors can lead to bleeding disorders (e.g., hemophilia) or thrombotic conditions (e.g., deep vein thrombosis).