Blood

Hemostasis

Hemostasis is the physiological process that stops bleeding at the site of an injury while maintaining blood flow elsewhere in the circulation. It involves a series of tightly regulated steps to form a stable blood clot.

• Initiation: Blood vessel injury triggers a cascade of reactions, activating clotting factors via two pathways (intrinsic and extrinsic).

• Final Step: The enzyme thrombin converts fibrinogen (soluble plasma protein) into fibrin (insoluble protein), which forms a meshwork that stabilizes the platelet plug.

• Clot Formation: The clot is a gel-like structure composed of fibrin strands, trapping red blood cells (RBCs) and giving the clot its red color.

• Clot Retraction: Platelets contract, pulling on fibrin strands to tighten the clot and bring wound edges closer together. This process squeezes out serum (fluid), leaving blood cells trapped in the clot.

• Fibrinolysis: The breakdown of the clot occurs to restore circulation. The liver secretes plasminogen (inactive enzyme), which is converted by thrombin into plasmin (active enzyme). Plasmin digests fibrin, dissolving the clot.

• Key Point: Thrombin is involved in both clot formation and breakdown.

Example: After a cut, hemostasis prevents excessive blood loss and initiates tissue repair.

Blood Groups and Blood Types

Blood is classified into groups based on the presence or absence of specific antigens on the surface of RBCs. The most clinically important systems are the ABO and Rh groups.

• ABO System: Classification is based on antigens labeled A, B, AB (both), or O (neither).

• Rh System: The Rh antigen (also called D antigen) is present in most people (Rh+); absence indicates Rh-.

• Antibodies: Individuals do not have antibodies against their own blood type. For example, type A individuals have anti-B antibodies, type B have anti-A, type AB have none, and type O have both anti-A and anti-B antibodies.

Example: A person with type A blood has A antigens on RBCs and anti-B antibodies in plasma.

Blood Typing

Blood typing is performed to determine a person's blood group before transfusion or organ transplantation.

• Procedure: A drop of blood is mixed with antiserum containing antibodies against A, B, or Rh antigens.

• Agglutination: If the blood cells have the corresponding antigen, the antibodies will cause them to clump (agglutinate).

Example: If blood agglutinates with anti-A serum but not anti-B, the person is type A.

Transfusions and Compatibility

Blood transfusions are used to treat anemia or blood loss. Compatibility between donor and recipient is crucial to prevent adverse reactions.

• Type O-: RBCs have no A or B antigens, making them universal donors for RBC transfusions.

• Type AB: Plasma lacks anti-A and anti-B antibodies, making them universal recipients for plasma.

• Cross-Matching: There are more than 24 blood groups and over 100 antigens; always cross-match to avoid reactions.

• Incompatible Transfusion: Recipient's plasma antibodies may bind to donor RBC antigens, causing agglutination and hemolysis, which can be fatal.

• RBC Transfusion: Usually involves packed RBCs (no plasma) to minimize antibody reactions.

Example: Type O- blood can be given to any recipient, but type AB+ can receive any blood type.

Hemolytic Disease of the Newborn (HDN)

HDN is a condition that can occur when an Rh- mother carries an Rh+ fetus. Exposure to fetal blood can cause the mother to develop anti-Rh antibodies, which may attack fetal RBCs in subsequent pregnancies.

• Mechanism: Fetal blood leaks into maternal circulation, stimulating antibody production if the mother is Rh- and the fetus is Rh+.

• Second Pregnancy: Maternal anti-Rh antibodies cross the placenta and attack fetal RBCs, causing agglutination and hemolysis.

• Prevention: Injection of RHOGAM (anti-Rh antibodies) prevents the mother from developing anti-Rh antibodies.

Example: An Rh- mother receives RHOGAM after delivery of an Rh+ baby to prevent HDN in future pregnancies.

Summary Table: ABO and Rh Blood Groups

Additional info: Table entries for "Can Receive From" and "Can Donate To" are based on standard transfusion compatibility. Always cross-match due to other antigens.

Key Equations

• Conversion of Fibrinogen to Fibrin:

• Conversion of Plasminogen to Plasmin: