Blood Plasma Composition and Properties

Blood Plasma

• Plasma is a fluid of the blood that is a solution containing ions, inorganic molecules, and organic molecules.
• It aids in the supply or removal of substances to or from various parts of the body.
• Normal volume of plasma is 5% of body weight (approximately 3500ml in a 70kg person).
• Plasma clots when outside the body, but remains fluid if an anticoagulant is added.
• Serum is plasma without fibrinogen, clotting factors II, V, and VIII, and has a higher serotonin content due to platelets broken down during clotting.

Plasma Proteins

• Plasma proteins include albumins, globulins, and fibrinogen.
• These proteins are large and do not pass through capillary pores.
• They exert a capillary osmotic pressure of approximately 25mmHg, pulling water into the blood.
• They are responsible for about 15% of blood buffering power.
• In plasma, these proteins are in an anionic form (COO-) at pH 7.4.
• They have specific functions such as antibodies and clotting factors, and nonspecific functions as carriers for hormones, solutes, and drugs.

Hemostasis

• Hemostasis is the process of stopping bleeding while maintaining blood in a fluid form.
• It involves a balance between coagulation and anticoagulation.
• Vascular injury leads to bleeding, and the response involves vasoconstriction, platelet aggregation, and clot formation.
• Endothelial damage exposes collagen, leading to platelet binding, serotonin release, and vascular constriction.
• Platelets aggregate and form a hemostatic plug, covering small punctures.
• Tissue releases thromboplastin, activating the clotting mechanism and forming fibrin.

Clotting Mechanism

• Fibrinogen is converted to fibrin monomers by thrombin, and then polymerizes to form fibrin.
• Fibrin is initially a loose mesh that is stabilized by factor XIII.
• Most steps in clot formation require Ca2+.
• Thrombin is a serine protease that converts fibrinogen to fibrin, and activates platelets, endothelial cells, and WBCs.

Clotting vs Anticlotting

• Coagulants include TXA2, which aggregates platelets.
• Anticoagulants include PGI2, which is anti-aggregating, and antithrombin III, which binds and inactivates thrombin.
• Fibrinolytics, such as plasmin, break down fibrin and fibrinogen.

Thrombomodulin

• Thrombomodulin binds thrombin, forming a thrombin-thrombomodulin complex.
• The complex becomes an anticoagulant.

Plasmin (Fibrinolysin)

• Plasmin is the active form of plasminogen, a plasma protein.
• It breaks down fibrin and fibrinogen, forming FDP, which inhibits thrombin.

Lymph

• Lymph is tissue fluid that enters the lymphatic vessels.
• It drains into the venous blood via the thoracic and right lymphatic ducts.
• Lymph contains clotting factors and clots on standing in vitro.
• It has a lower protein content than plasma (approximately 7g/dL).
• Lymph protein content varies with the region from which it drains.
• Water-insoluble fats are absorbed from the intestine into the lymphatics, and the lymph in the thoracic duct after a meal is milky due to its high fat content.

Structural Features of the Circulation

• Two major cell types make up the blood vessels: endothelium and vascular smooth muscle.
• Endothelial cells respond to flow changes, stretch, and various circulating substances, and secrete growth regulators and vasoactive substances.
• Vascular smooth muscle cells have K+, Ca2+, and Cl- channels, and contraction is produced primarily by the myosin light chain mechanism.
• They undergo prolonged contractions that determine vascular tone.