- What is the average blood volume in women and in men? What is the normal percentage of blood occupied by erythrocytes and by plasma in women and in men? What is the hematocrit? What is the buffy coat?
- Describe the three steps of hemostasis, including a comparison of the intrinsic and extrinsic pathways by which the clotting cascade is triggered.
The correct answer and explanation is:
1. Blood Volume, Components, Hematocrit, and Buffy Coat
- Average Blood Volume:
- Women: Approximately 4–5 liters.
- Men: Approximately 5–6 liters.
- Normal Percentage of Blood Components:
- Erythrocytes (Red Blood Cells):
- Women: 37–47% (hematocrit).
- Men: 42–52% (hematocrit).
- Plasma:
- Women: 53–63%.
- Men: 48–58%.
- Erythrocytes (Red Blood Cells):
- Hematocrit: The hematocrit is the percentage of total blood volume occupied by red blood cells. It reflects the oxygen-carrying capacity of blood and is higher in men due to testosterone-stimulated erythropoiesis.
- Buffy Coat: The buffy coat is the thin, whitish layer between the plasma and erythrocytes when blood is centrifuged. It contains white blood cells (leukocytes) and platelets, representing less than 1% of blood volume.
2. Hemostasis and Clotting Pathways
Hemostasis is the process that stops bleeding when blood vessels are injured. It occurs in three steps:
- Vascular Spasm:
- Immediate vasoconstriction of the damaged vessel reduces blood flow and limits blood loss.
- Triggered by vessel injury and mediated by endothelial cells releasing factors like endothelin.
- Platelet Plug Formation:
- Platelets adhere to exposed collagen at the injury site via von Willebrand factor (vWF).
- Platelets release granules containing ADP, serotonin, and thromboxane A2, recruiting more platelets and forming a temporary plug.
- Coagulation (Clotting Cascade):
- The clotting cascade strengthens the platelet plug with fibrin, forming a stable clot. The cascade involves two pathways:
- Intrinsic Pathway:
- Triggered by damage to blood vessel walls.
- Slower and more complex; involves factors XII, XI, IX, VIII, and X.
- Extrinsic Pathway:
- Triggered by external trauma exposing tissue factor (TF).
- Faster and involves factor VII and factor X activation.
- Intrinsic Pathway:
- Both pathways converge at the common pathway, where prothrombin is converted to thrombin, which then converts fibrinogen into fibrin to form the clot.
- The clotting cascade strengthens the platelet plug with fibrin, forming a stable clot. The cascade involves two pathways:
This intricate system ensures localized clotting to stop bleeding without obstructing healthy blood flow.