Blood. Part 2.

Summary

This lecture explains the defense functions of blood, focusing on hemostasis and immune defense. Hemostasis is the process of stopping bleeding, with two mechanisms: platelet-mediated hemostasis for small vessels and blood clotting for larger ones. The detailed clotting cascade involves various factors like fibrinogen, thrombin, and calcium. The role of anticoagulation systems, including heparin, is discussed to prevent unwanted clots. Additionally, it covers blood groups (ABO system) and the Rh factor, emphasizing their importance in safe blood transfusion and preventing Rh incompatibility during pregnancy.

Takeaways

• 🩸 Hemostasis involves two mechanisms: platelet-mediated hemostasis and blood clotting, which help stop bleeding.
• 🛑 Platelet-mediated hemostasis works in small blood vessels by causing vasoconstriction and platelet aggregation at the site of injury.
• 🧩 Blood clotting primarily occurs in larger blood vessels where platelets alone cannot stop the bleeding, involving a cascade of plasma factors.
• 🧬 Fibrinogen is a key factor in blood clotting, converting to fibrin to form a mesh that stabilizes the blood clot.
• 🔗 The clotting process involves a cascade of factors, including thrombin, calcium (Factor IV), and other clotting factors like Factor V, VII, X, and XIII.
• 💉 Calcium is essential for blood clotting; anticoagulants like sodium citrate can prevent clotting by binding calcium.
• 🛡️ The body has an anticoagulation system (e.g., heparin, antithrombin III) to prevent excessive clot formation and protect against strokes or heart attacks.
• 🧪 Fibrinolysis is the process that dissolves unnecessary clots through plasmin, breaking fibrin into soluble peptides once healing is complete.
• 🧫 Blood groups are primarily classified under the ABO system and Rh factor; mismatches can cause severe immune reactions during blood transfusions.
• 👶 Rh incompatibility between a mother and fetus can lead to complications in future pregnancies, but this can be mitigated by administering antibodies after the first birth.

Q & A

What is hemostasis?

Hemostasis is the process of stopping bleeding. It can occur through two mechanisms: platelet-mediated hemostasis and blood clotting. Platelet-mediated hemostasis typically happens in small blood vessels, while blood clotting occurs in larger vessels.

What role do platelets play in hemostasis?

Platelets aggregate at the site of a blood vessel injury, get activated, release biologically active substances like serotonin and thromboxane A2, and initiate vasoconstriction to reduce blood flow and blood loss. They also change shape to help form a stable clot.

How does blood clotting differ from platelet-mediated hemostasis?

Blood clotting, also known as coagulation, occurs in larger blood vessels where vasoconstriction is not effective. It involves a cascade of reactions leading to the conversion of fibrinogen to fibrin, which forms a mesh to stop bleeding.

What is fibrinogen and what role does it play in blood clotting?

Fibrinogen is a plasma clotting factor (Factor I) that gets converted into fibrin, which forms strands that create a mesh to stop bleeding. This process is triggered by the enzyme thrombin.

What are the intrinsic and extrinsic pathways of blood clotting?

The intrinsic pathway is a 'working horse' that sustains blood clotting by amplifying the clotting process. The extrinsic pathway acts as a 'spark' to initiate clotting quickly when tissue factors enter the blood. Both pathways work together to form a stable clot.

What is the role of calcium in blood clotting?

Calcium (Factor IV) is crucial for blood clotting. It helps activate certain clotting factors, such as Factor X. Without calcium, the clotting process cannot proceed, as demonstrated when substances like sodium citrate bind calcium and prevent coagulation.

What is the role of anti-coagulation systems in the body?

Anti-coagulation systems, such as heparin and antithrombin III, prevent excessive clot formation by inhibiting clotting factors. This is important to avoid conditions like strokes or heart attacks caused by unwanted clots in blood vessels.

What is fibrinolysis and why is it important?

Fibrinolysis is the process of breaking down blood clots that are no longer needed. Plasmin, an enzyme formed from plasminogen, breaks fibrin into soluble peptides, which are then washed out of the bloodstream.

What are agglutinogens and agglutinins, and how do they determine blood groups?

Agglutinogens are antigens found on the surface of red blood cells (A and B), while agglutinins are antibodies found in plasma (alpha and beta). The combination of agglutinogens and agglutinins determines a person's blood group.

What is Rh factor and how can it cause complications during pregnancy?

The Rh factor is an antigen (often D antigen) on red blood cells. If a mother is Rh-negative and the fetus is Rh-positive, the mother's immune system may produce antibodies against the Rh antigen, potentially causing problems in future pregnancies. To prevent this, anti-D antibodies are given to the mother after the first pregnancy.