Lecture 12 Hemostasis, Blood Clotting, and Blood Groups: Study Notes for Anatomy & Physiology

Hemostasis

Overview of Hemostasis

Hemostasis is the physiological process that stops bleeding at the site of an injury while maintaining blood in a fluid state within the vascular system. It is a rapid, localized, and highly regulated response that involves clotting factors and substances released by platelets and injured tissues.

• Hemostasis: Stopping of bleeding.

• Requires clotting factors and platelet-derived substances.

• Three main steps:

  1. Vascular spasm

  2. Platelet plug formation

  3. Coagulation (blood clotting)

• Clot retraction occurs after hemostasis to stabilize the clot.

Step 1: Vascular Spasm

Mechanism and Triggers

Vascular spasm is the immediate constriction of a blood vessel in response to injury, reducing blood flow and loss until other hemostatic mechanisms are activated.

• Vasoconstriction: Vessel narrows to limit blood loss.

• Triggered by:

  • Direct injury to vascular smooth muscle

  • Chemicals released by endothelial cells and platelets

  • Pain reflexes

• Most effective in smaller blood vessels (BVs).

• Reduces blood flow until platelet plug and coagulation occur.

Step 2: Platelet Plug Formation

Platelet Activation and Adhesion

Platelets adhere to exposed collagen fibers at the injury site, forming a temporary plug. This process is regulated by chemical signals and feedback mechanisms.

• Platelets stick to exposed collagen fibers of damaged BV.

• Prostacyclin and NO (nitric oxide) prevent unnecessary platelet sticking.

• von Willebrand Factor (VWF) stabilizes platelet-collagen adhesion.

• Activated platelets swell, become spiked and sticky, and release messengers:

  • ADP: Attracts more platelets and promotes release of their contents.

  • Serotonin and thromboxane A2: Enhance vascular spasm and platelet aggregation.

• Positive feedback: More platelets stick and release chemicals, amplifying the response (usually

  • Platelet plugs are effective for small vessel tears; larger breaks require coagulation.

Von Willebrand Factor (VWF)

Role in Hemostasis

Von Willebrand Factor is a key protein in platelet adhesion and plug formation.

• Produced by megakaryocytes (platelets) and endothelial cells.

• Monomers assemble to form a globular protein.

• Unfolds and extends in response to injury.

• Surface protein on activated platelets binds to VWF, facilitating adhesion.

Step 3: Coagulation (Blood Clotting)

Formation of Fibrin Mesh

Coagulation reinforces the platelet plug with a mesh of fibrin threads, transforming blood from liquid to gel through a cascade of reactions involving clotting factors.

• Clotting factors (pro-coagulants) are mostly plasma proteins synthesized by the liver.

• Factors are numbered I-XIII in order of discovery.

• Vitamin K is required for synthesis of four clotting factors.

• Most clotting factors circulate in inactive form.

• Coagulation occurs in three phases.

Phase 1: Two Pathways to Prothrombin Activator

Intrinsic and Extrinsic Pathways

Coagulation is initiated by either the intrinsic or extrinsic pathway, both leading to activation of Factor X and formation of prothrombin activator.

• Intrinsic pathway:

  • Clotting factors present within blood.

  • Triggered by negatively charged phospholipids on activated platelets, collagen, or glass.

• Extrinsic pathway:

  • Clotting factors located outside blood.

  • Triggered by exposure to tissue factor (TF), also known as Factor III.

  • Bypasses several steps of intrinsic pathway, so is faster.

• Both pathways converge at activation of Factor X, which complexes with Ca2+ and Factor V to form prothrombin activator.

Phases 2 & 3: Pathway to Thrombin and Fibrin

Conversion and Clot Stabilization

Prothrombin activator converts prothrombin to thrombin, which then converts fibrinogen to fibrin, forming the structural basis of the clot.

• Thrombin converts fibrinogen to fibrin.

• Fibrin mesh traps blood cells and stabilizes the clot.

• Thrombin and Ca2+ activate Factor XIII (fibrin stabilizing factor), which cross-links fibrin and strengthens the clot.

• Anticoagulants normally dominate to inhibit coagulation and prevent excessive clotting.

Blood Clotting Factors (Procoagulants)

Table: Main Blood Clotting Factors

The following table summarizes the major blood clotting factors, their nature, source, and principal function.

Factor Name	Nature	Source	Principal Function
Fibrinogen (I)	Plasma protein	Liver	Forms fibrin threads of clot
Prothrombin (II)	Plasma protein	Liver	Converted to thrombin
Tissue Factor (III)	Plasma membrane glycoprotein	Tissue cells	Activates extrinsic pathway
Calcium ions (IV)	Inorganic ion	Plasma	All pathways; needed for clotting
Proaccelerin (V)	Plasma protein	Liver, platelets	Common pathway
Antihemophilic factor (VIII)	Plasma protein	Liver, lung, endothelium	Intrinsic pathway
Fibrin stabilizing factor (XIII)	Plasma protein	Liver, bone marrow	Cross-links fibrin, stabilizes clot
Additional info:	Other factors (VII, IX, X, XI, XII) also play roles in intrinsic/extrinsic pathways.

Clot Retraction and Fibrinolysis

Retraction

After clot formation, the clot contracts to reduce its size and bring wound edges closer together.

• Clot stabilized; plasminogen trapped.

• Actin and myosin in platelets contract, pulling on fibrin and shrinking the clot.

• Serum squeezed from clot; wound edges pulled together.

• PDGF (platelet-derived growth factor) stimulates division of smooth muscle cells and fibroblasts to rebuild vessel wall.

• VEGF (vascular endothelial growth factor) stimulates endothelial cells to restore lining.

Fibrinolysis

• tPA (tissue plasminogen activator) converts plasminogen to plasmin.

• Plasmin degrades fibrin, breaking up the clot.

• Clot fragments are phagocytosed and removed.

Factors Limiting Clot Growth or Formation

Mechanisms to Prevent Excessive Clotting

• Swift removal and dilution of clotting factors by blood flow.

• Inhibition of activated clotting factors:

  • Thrombin restricted to clot by fibrin threads.

  • Antithrombin III inactivates unbound thrombin.

  • Heparin (from basophils and mast cells) enhances antithrombin III activity.

• Prevention of platelet adhesion:

  • Smooth endothelium prevents platelets from clinging.

  • Endothelium secretes anti-thrombotic substances (NO, prostacyclin).

  • Vitamin E quinone is a potent anticoagulant.

Disorders of Hemostasis

Thromboembolic Disorders

• Thrombus: Clot that develops and persists in an unbroken blood vessel; may block circulation and cause tissue death.

• Embolus: Thrombus freely floating in bloodstream.

• Embolism: Embolus obstructs a blood vessel (e.g., pulmonary, cerebral).

• Risk factors: atherosclerosis, inflammation, slow blood flow, blood stasis from immobility.

• Treatment: Anticoagulant drugs

  • Aspirin: Inhibits thromboxane A2; lowers heart attack risk.

  • Heparin: Used for cardiac care and prevention of venous thrombosis.

  • Warfarin and direct oral anticoagulants: Reduce stroke risk in atrial fibrillation; interfere with vitamin K action.

Bleeding Disorders

• Thrombocytopenia: Deficiency of circulating platelets; causes petechiae and hemorrhage.

  • Caused by suppression/destruction of red bone marrow (malignancy, radiation, drugs).

  • Platelet count <150,000/μl is diagnostic.

  • Treatment: transfusion of concentrated platelets.

• Impaired liver function: Inability to synthesize pro-coagulants due to vitamin K deficiency, hepatitis, or cirrhosis.

  • Liver disease may also prevent bile production, affecting vitamin K absorption.

• Hemophilia: Hereditary bleeding disorders.

  • Type A: Factor VIII deficiency (most common).

  • Type B: Factor IX deficiency.

  • Type C: Factor XI deficiency (milder).

  • Symptoms: Prolonged bleeding, especially into joints.

  • Treatment: Injections of genetically engineered clotting factors.

Blood Transfusions

Response to Blood Loss

• Cardiovascular system compensates by reducing vessel volume and increasing RBC production.

• Body can compensate for blood loss up to 15-30% (pallor, weakness); >30% may cause fatal shock.

Restoring Blood Volume

• Immediate replacement with normal saline or multiple-electrolyte solution (e.g., Ringer's solution) mimics plasma composition.

• Volume replacement restores circulation but not oxygen-carrying capacity.

Transfusing Red Blood Cells

Types and Safety

• Whole-blood transfusions used for rapid, substantial blood loss.

• Infusions of packed RBCs (plasma and WBCs removed) preferred to restore oxygen-carrying capacity.

• Blood banks separate blood into components; shelf life ~35 days.

• Blood groups must be typed to prevent fatal transfusion reactions.

Human Blood Groups

Antigens and Agglutinogens

• RBC plasma membrane contains many antigens (glycoproteins) that can trigger immune responses.

• Agglutinogens: RBC antigens that promote agglutination (clumping).

• Mismatched transfused blood is perceived as foreign and may be agglutinated and destroyed.

• Humans have >30 RBC antigens; presence/absence used to classify blood cells.

• Some blood groups have weak agglutinogens and are only typed for patients needing multiple transfusions.

• ABO and Rh antigens are the most clinically significant.

Antibody-Antigen Reactions

Immune Response

• Antigen: Substance that triggers antibody production by the immune system.

• Antibody: Protein that binds to and neutralizes foreign antigens.

ABO Blood Groups

Classification and Compatibility

• Based on presence or absence of two RBC agglutinogens: A and B.

• Type A: Only A antigen

• Type B: Only B antigen

• Type AB: Both A and B antigens

• Type O: Neither A nor B antigen

• Blood may contain preformed anti-A or anti-B antibodies (agglutinins), which act against transfused RBCs with ABO antigens not present on recipient's RBCs.

• Anti-A or anti-B form in blood at about 2 months of age, reaching adult levels by 8-10 years.

Blood Type	Antigens on RBC	Antibodies in Plasma
A	A	Anti-B
B	B	Anti-A
AB	A and B	None
O	None	Anti-A and Anti-B

Rh Blood Groups

Rh Factor and Clinical Significance

• 52 Rh agglutinogens (Rh factors); C, D, E most common.

• Rh+: D antigen present (Rh positive); Rh-: D antigen absent (Rh negative).

• Rh+ is the most common blood type (85% in North America).

• Anti-Rh antibodies are not spontaneously formed in Rh- individuals but can form after exposure to Rh+ blood.

• Second exposure to Rh+ blood can cause a transfusion reaction.

Erythroblastosis Fetalis

Hemolytic Disease of the Newborn

• Occurs only in Rh- mothers with Rh+ fetuses.

• First pregnancy: Rh- mother exposed to Rh+ fetal blood during delivery; mother synthesizes anti-Rh antibodies.

• Second pregnancy: Mother's anti-Rh antibodies cross placenta and destroy fetal RBCs.

• Treatment: Prebirth and exchange transfusions for baby.

• RhoGAM serum containing anti-Rh can prevent sensitization of Rh- mothers.

Transfusion Reactions

Causes and Management

• Occur if mismatched blood is infused.

• Donor RBCs attacked by recipient's plasma antibodies, causing agglutination and hemolysis.

• Symptoms: fever, chills, low blood pressure, rapid heartbeat, nausea, vomiting.

• Treatment: Prevent kidney damage with fluids and diuretics to wash out hemoglobin.

• Type O: Universal donor (no A or B antigens).

• Type AB: Universal recipient (no anti-A or anti-B antibodies).

• Autologous transfusions: Patient pre-donates own blood for future use.

Blood Typing and Cross-Matching

Ensuring Compatibility

• Donor blood is mixed with antibodies against common agglutinogens; clumping indicates presence of antigen.

• Blood is typed for ABO and Rh factor.

• Cross-matching: Mixing recipient's serum with donor RBCs and vice versa to check for compatibility.

Diagnostic Blood Tests

Clinical Applications

• Blood tests provide information on health status:

  • Low hematocrit: anemia

  • Blood glucose: diabetes

  • Leukocytosis: infection

  • Microscopic examination: variations in RBC size, shape, color (anemia)

  • Differential WBC count: proportions of each WBC type

  • Prothrombin time, platelet counts: assess hemostasis

  • CMP (comprehensive metabolic panel): blood chemistry profile for liver/kidney disorders

  • CBC (complete blood count): checks formed elements, hematocrit, hemoglobin

Blood Donations

Population Statistics

• Percentage of population that donates blood is low (often less than 10%).

• Blood donation is essential for maintaining adequate supplies for transfusions and medical emergencies.

Additional info: These notes cover key concepts from Chapter 17 (Blood) and related clinical applications, suitable for college-level Anatomy & Physiology students.