Detailed Physiology of Blood and Hemostasis
Blood is a vital, liquid connective tissue composed of 45% formed elements (red cells, white cells, platelets) and 55% plasma. Its primary functions include transporting respiratory gases, nutrients, hormones, and waste products, while also playing crucial roles in immune defense, temperature regulation, and maintaining hemostasis through complex coagulation and fibrinolysis mechanisms.
Key Takeaways
Blood is a liquid connective tissue, making up 7-8% of total body mass.
Hematopoiesis, the production of blood cells, occurs exclusively in the bone marrow.
Red blood cells transport oxygen; white cells manage immune defense and inflammation.
Hemostasis involves primary platelet aggregation and secondary fibrin coagulation.
Serum is plasma that has been naturally deprived of all coagulation factors.
What is the fundamental definition and role of blood in the body?
Blood is defined as a vital, liquid connective tissue that circulates throughout the body, irrigating all tissues and facilitating systemic communication. It constitutes approximately 5 liters in total volume for an adult, representing 7 to 8% of the body mass. Structurally, blood is composed of 45% formed cellular elements and 55% plasma, the fundamental liquid substance. Functionally, blood is essential for maintaining life, performing critical transport, defense, and regulatory roles across multiple systems, including temperature control and self-repair.
- It is a living, liquid connective tissue composed of free cellular elements and fundamental plasma substances.
- Total volume is about 5 liters, with a composition of 45% formed elements and 55% plasma.
- Functional roles include transporting respiratory gases (O2/CO2), nutrients (glucose, vitamins), and informative molecules (hormones).
- It is crucial for defense (white blood cells), heat transport, waste removal (urea), and tissue maintenance.
How are the formed elements of blood produced and what are their specific functions?
The formed elements—red blood cells, white blood cells, and platelets—are continuously produced through hematopoiesis, originating from pluripotent stem cells within the bone marrow. Red blood cells (erythrocytes) are biconcave discs primarily responsible for gas transport via hemoglobin, lasting about 120 days before enzymatic exhaustion. White blood cells (leukocytes) manage immune defense, with types like neutrophils performing phagocytosis and lymphocytes maturing in lymphoid organs. Platelets initiate primary hemostasis by forming a fragile plug at the site of vascular injury, a process crucial for immediate wound sealing.
- Hematopoiesis originates in the bone marrow from pluripotent stem cells.
- Red blood cells are biconcave discs containing hemoglobin (92% of dry weight) for O2 and CO2 transport, exhibiting plasticity for capillary passage.
- Key hemogram constants include Hematids (4.5-6.2 x 10^12/L for men), Hemoglobin (13-18 g/dl for men), and Hematocrit (42-55% for men).
- White blood cells (4,000–10,000 / mm3) include granulocytes (neutrophils, eosinophils, basophils) and monocytes, which differentiate into macrophages for immune response.
- Platelets, derived from megakaryocyte fragmentation, function in primary hemostasis through adhesion (via von Willebrand Factor) and aggregation, forming a fragile thrombus.
What are the key differences between blood plasma and serum?
Plasma is the liquid matrix of blood, making up 55% of the total volume, and is primarily composed of water, 8% proteins, and 0.8% essential minerals like sodium, potassium, and calcium. Crucially, plasma contains all the necessary coagulation factors, including fibrinogen and agglutinins, which are vital for secondary hemostasis. Serum, conversely, is derived from plasma but is specifically defined as the liquid remaining after the blood has clotted, meaning it has been naturally deprived of fibrinogen and other coagulation factors during the clotting process.
- Plasma composition includes 8% proteins and 0.8% minerals (Na, K, Ca).
- Plasma contains active coagulation factors, such as fibrinogen and agglutinins.
- Serum is defined as plasma that is specifically lacking fibrinogen and other coagulation factors.
How does the body achieve hemostasis through coagulation and fibrinolysis?
Hemostasis is the complex process that stops bleeding following vascular injury, involving two main stages. Primary hemostasis begins immediately with reflex vasoconstriction and the formation of a fragile platelet plug. Secondary hemostasis, or plasmic coagulation, consolidates this plug by activating coagulation factors, synthesized in the liver, through intrinsic and extrinsic pathways that converge to activate Factor Xa. This leads to the formation of thrombin, which converts soluble fibrinogen into a stable, insoluble fibrin network, forming the definitive red thrombus, which is later dissolved by fibrinolysis.
- Primary hemostasis is triggered by vascular lesion, resulting in reflex vasoconstriction and the formation of a platelet plug.
- Secondary hemostasis aims to consolidate the aggregate through fibrin formation, utilizing hepatic glycoproteins known as coagulation factors (e.g., K-dependent factors II, VII, IX, X).
- Activation pathways (Intrinsic via KHPM and Extrinsic via FT) converge to form the Prothrombinase Complex, activating Thrombin (IIa).
- Thrombin cleaves soluble Fibrinogen (I) into unstable Fibrin, which is stabilized by Factor XIII into an insoluble network.
- Coagulation is regulated by inhibitors like Anti-thrombin III (inhibiting serine-proteases) and the Protein C/S System (inactivating cofactors Va and VIIIa).
- Fibrinolysis, involving the activation of plasminogen into plasmin, is responsible for the temporary dissolution and degradation of the fibrin clot after healing.
Frequently Asked Questions
Where does the production of blood cells (hematopoiesis) primarily occur?
Hematopoiesis, the process of producing formed blood elements, originates from pluripotent stem cells located exclusively within the bone marrow.
What is the main functional difference between plasma and serum?
Plasma is the liquid component containing all coagulation factors, including fibrinogen. Serum is the liquid remaining after blood has clotted, meaning it lacks fibrinogen and other clotting factors.
How is the final fibrin clot stabilized during secondary hemostasis?
Thrombin first converts soluble fibrinogen into unstable fibrin. This fibrin is then stabilized by Factor XIII (FSF), creating the insoluble fibrin network that forms the definitive red thrombus.
