• Hemorrhage = escape of blood from blood vessels into surrounding tissues or outside the body.
• It occurs after blood-vessel damage and may become worse when blood clotting is defective.
• Causes include:
  • chronic congestion with capillary bleeding
  • trauma
  • atherosclerosis
  • inflammatory erosion of vessel walls
  • neoplastic erosion of vessel walls
• Bleeding may be massive when a large artery or vein is affected.
• Hemorrhagic diatheses = disorders with an increased tendency to bleed, sometimes even after minor injury.
• They may result from inherited or acquired defects involving:
  • vessel walls
  • platelets
  • coagulation factors
• All three normally work together for proper hemostasis.
• Hemorrhage may occur as:
  • external bleeding, or
  • accumulation of blood inside tissue = hematoma.
• Hematomas may range from:
  • minor → bruise
  • severe → massive internal hematoma that can be fatal.
• Large bleeding into body cavities is named according to location:
  • Hemothorax → blood in pleural cavity
  • Hemopericardium → blood in pericardial cavity
  • Hemoperitoneum → blood in peritoneal cavity
  • Hemarthrosis → blood in a joint
• Large hemorrhages may occasionally cause jaundice because macrophages break down large amounts of red cells and hemoglobin.
• Petechiae:
  • tiny hemorrhages, about 1–2 mm
  • occur in skin, mucous membranes, or serosal surfaces (Fig. 3.4A)

• Causes:
  • thrombocytopenia
  • defective platelet function
  • loss of vascular wall support, as in vitamin C deficiency
• Purpura:
  • slightly larger hemorrhages, about 3–5 mm
  • Causes include those of petechiae plus:
    • trauma
    • vasculitis
    • increased vascular fragility
• Ecchymoses:
  • larger subcutaneous hematomas, about 1–2 cm
  • commonly called bruises
  • Macrophages remove and break down extravasated red cells.
  • Color changes occur as:
    hemoglobin (red-blue) → bilirubin (blue-green) → hemosiderin (golden-brown).
• Severity of hemorrhage depends on:
  • amount of blood lost
  • speed of blood loss
  • site of bleeding
  • health of the patient
• Rapid loss of up to about 20% of blood volume may be tolerated by a healthy adult.
• The same loss may cause cardiovascular decompensation in patients with underlying heart or lung disease.
• Greater blood loss may cause hemorrhagic (hypovolemic) shock.
• Location is extremely important:
  • a small subcutaneous bleed may be harmless
  • a similar bleed in the brain may be fatal (Fig. 3.4B).
• Chronic or repeated external blood loss, such as from peptic ulcer or menstrual bleeding:
  → loss of hemoglobin iron → iron deficiency anemia.
• Internal bleeding, such as a hematoma, usually does not cause iron deficiency because iron from red cells is efficiently recycled.

KEY CONCEPT

• Hemorrhage = blood escapes from vessels.
• Petechiae = 1–2 mm, Purpura = 3–5 mm, Ecchymoses = 1–2 cm.
• Clinical danger depends mainly on volume + rate + location + patient condition.
• Large rapid blood loss → hypovolemic shock.
• Chronic external bleeding → iron deficiency anemia, while internal blood is usually recycled.

CONCEPTUAL EXAMPLES

• Low platelets → tiny skin spots → petechiae.
• Larger small hemorrhages → purpura.
• Trauma under the skin → ecchymosis/bruise with changing colors.
• Massive rapid bleeding → ↓ circulating blood volume → hypovolemic shock.
• Chronic peptic-ulcer bleeding → repeated iron loss → iron deficiency anemia.

HEMOSTASIS AND THROMBOSIS

• Hemostasis = normal formation of a blood clot after traumatic vascular injury to stop bleeding.
• Thrombosis = pathologic formation of a thrombus inside a vessel damaged by disease.
• Thus:
  Normal vessel injury → hemostasis
  Disease-related vessel injury → thrombosis

Hemostasis

• Hemostasis is a carefully controlled process involving:
  • platelets
  • clotting factors
  • endothelium
• It occurs at the site of vascular injury and forms a clot that prevents or limits bleeding.
• The overall sequence is shown in Fig. 3.5.
• Immediately after injury:
  Arteriolar vasoconstriction → ↓ blood flow to injured area.
• Vasoconstriction is produced by:
  • neurogenic reflexes
  • local release of endothelin, a strong vasoconstrictor from endothelium.
• This effect is temporary; permanent control requires platelets + coagulation factors.
• Primary hemostasis = platelet plug formation:
  • Endothelial injury → exposes subendothelial collagen.
  • Collagen binds von Willebrand factor (vWF).
  • vWF promotes platelet adhesion and activation.
  • Activated platelets change from small discs into flat, spiky cells → ↑ surface area.
  • Platelets release granules → recruit more platelets.
  • Platelets aggregate → form the primary hemostatic plug.
• Secondary hemostasis = fibrin deposition:
  • Vascular injury exposes tissue factor.
  • Tissue factor is normally present on subendothelial cells such as smooth muscle cells and fibroblasts.
  • Tissue factor binds and activates factor VII.
  • This starts the coagulation cascade → thrombin generation.
  • Thrombin converts fibrinogen → insoluble fibrin.
  • Fibrin forms a mesh around the platelet plug.
  • Thrombin also activates more platelets → more aggregation.
  • Result → platelet plug becomes stronger and consolidated.
• Clot stabilization:
  • Fibrin is cross-linked by factor XIII.
  • Platelet aggregates contract.
  • Together → form a solid, permanent plug that prevents further bleeding.
  • Counterregulatory mechanisms limit clotting to the injury site and later promote clot removal and tissue repair.
• The endothelium controls whether a clot forms, grows, or dissolves.
• Healthy endothelial cells:
  • inhibit platelet aggregation
  • inhibit coagulation
  • promote fibrinolysis
• After endothelial injury or activation → balance shifts toward clot formation.
• Endothelium may be activated by:
  • microbial pathogens
  • abnormal hemodynamic forces
  • proinflammatory mediators
• These changes can increase the risk of thrombosis.

KEY CONCEPT

• Hemostasis occurs in a clear sequence:

Vessel injury → vasoconstriction → platelet adhesion/activation → primary platelet plug → tissue factor + factor VII → thrombin → fibrin → factor XIII cross-linking → stable clot

• Primary hemostasis = platelets.
• Secondary hemostasis = fibrin formation.
• Factor XIII = stabilizes fibrin.
• Healthy endothelium is mainly anticoagulant, while injured/activated endothelium favors clotting.

CONCEPTUAL EXAMPLES

• Small vessel cut → vessel constricts first → reduces blood loss.
• Exposed collagen + vWF → platelets stick → temporary platelet plug.
• Tissue factor activates coagulation → thrombin forms fibrin → stronger clot.
• Factor XIII cross-links fibrin → stable permanent plug.

FIG. 3.5 — NORMAL HEMOSTASIS

Easiest concept: Blood vessel injury → stop bleeding → make stable clot

Whole figure in one line

Vasoconstriction → platelet plug → fibrin clot → clot stabilization

A. VASOCONSTRICTION

🔴 Red vessel lumen = blood inside arteriole
🩷 Pink inner lining = endothelium
🟤 Outer muscular layer = arteriole smooth muscle
🟣 Thin supporting layer = basement membrane
🧵 ECM collagen becomes exposed at injury.

At the injury:

Vessel injury
→ reflex contraction + endothelin release
→ smooth muscle contracts
→ vessel becomes narrower
→ blood flow decreases

🟨 Yellow arrows = contraction toward the injured area.

Memory:

First response = SQUEEZE the vessel.

B. PRIMARY HEMOSTASIS = PLATELET PLUG

🟢/yellow cells = platelets

① Platelet adhesion

Injury exposes vWF + collagen.

vWF = von Willebrand factor

vWF acts like glue:

Damaged vessel → vWF exposed → platelets stick

② Shape change

Platelets become activated and change from smooth discs into sticky irregular cells.

③ Granule release

Activated platelets release:

• ADP
• TXA₂ = thromboxane A₂

These signals call and activate more platelets.

④ Recruitment

More platelets arrive.

⑤ Aggregation

Platelets stick to each other.

➡️ Result = temporary platelet plug

Memory:

Adhere → Activate → Release → Recruit → Aggregate

C. SECONDARY HEMOSTASIS = FIBRIN

Now the platelet plug must become stronger.

① Tissue factor

Damaged tissue exposes/releases tissue factor.

→ starts the coagulation cascade.

② Phospholipid complex

Activated platelet surfaces provide a platform for clotting reactions.

③ Thrombin activation

Coagulation cascade produces thrombin.

Thrombin is the key enzyme that converts:

Fibrinogen → fibrin

④ Fibrin polymerization

🟦/green mesh around platelets = fibrin strands

Fibrin forms a net around the platelet plug.

➡️ Temporary plug becomes a strong clot.

Memory:

Tissue factor → Thrombin → Fibrin

D. CLOT STABILIZATION

Now the clot is tightened and reinforced.

① Platelet contraction

Platelets contract.

→ clot becomes smaller, tighter, and stronger.

② Factor XIIIa

Factor XIIIa makes covalent cross-links between fibrin strands.

Think:

Loose fibrin net → XIIIa → locked strong fibrin net

🔴 Trapped red cell = RBC caught in clot
🟣 Trapped neutrophil = inflammatory cell caught in clot

🎨 COLOR / STRUCTURE MAP

• 🔴 Red discs = red blood cells
• 🟢/yellow cells = platelets
• 🟦/green mesh = fibrin
• 🩷 vessel lining = endothelium
• 🟤 vessel wall = smooth muscle
• 🟨 arrows = direction of each hemostatic step

🧠 Fastest exam recall

1. Vasoconstriction

Endothelin → vessel narrows

2. Primary hemostasis

vWF → platelet adhesion → platelet plug

3. Secondary hemostasis

Tissue factor → thrombin → fibrin

4. Stabilization

Factor XIIIa → cross-links fibrin

⭐ Ultimate memory:

Squeeze → Stick → Strengthen → Stabilize

Platelets

• Platelets are essential for hemostasis because they:
  • form the primary platelet plug that initially seals vessel injury.
  • provide a surface where activated coagulation factors gather and work.
• Platelets are disc-shaped, anucleate cell fragments released from bone-marrow megakaryocytes into blood.
• Their function depends on:
  • surface glycoprotein receptors
  • a contractile cytoskeleton
  • α-granules
  • dense (δ) granules
• α-Granules contain:
  • P-selectin
  • fibrinogen
  • factor V
  • vWF
  • fibronectin
  • platelet factor 4
  • PDGF
  • TGF-β
• Dense granules contain:
  • ADP
  • ATP
  • polyphosphate
  • Ca²⁺
  • serotonin
  • epinephrine
• After vascular injury, platelets contact exposed collagen + vWF and begin forming the platelet plug (Fig. 3.5B).
• Platelet adhesion:
  • vWF acts as a bridge:
  • exposed collagen ← vWF → platelet GpIb
  • Thus, platelets stick to the injured vessel wall (Fig. 3.6).
  • vWF deficiency → von Willebrand disease.
  • GpIb deficiency → Bernard-Soulier syndrome.
  • Both can cause bleeding.
• After adhesion, platelets rapidly change from smooth discs into spiky “sea-urchin” shapes → greatly ↑ surface area.
• Platelet activation also changes GpIIb/IIIa, increasing its ability to bind fibrinogen.
• Negatively charged phospholipids, especially phosphatidylserine, move to the platelet surface.
  • They bind Ca²⁺.
  • This surface helps assemble coagulation factor complexes.
• Platelets then release their granules.
• Shape change + granule release = platelet activation.
• Important activators include:
  • thrombin
  • ADP
• Thrombin activates platelets through protease-activated receptors (PARs).
• ADP released from dense granules activates more platelets:
  platelet activation → ADP release → more platelet activation
  = recruitment.
• Activated platelets also produce thromboxane A₂ (TxA₂) → strongly promotes platelet aggregation.
• Aspirin inhibits cyclooxygenase → ↓ TxA₂ formation → ↓ platelet aggregation → mild bleeding tendency.
• Platelet growth factors such as PDGF contribute to repair of the damaged vessel wall.
• Platelet aggregation occurs after activation:
  • activated GpIIb/IIIa binds fibrinogen.
  • fibrinogen bridges neighboring platelets.
  • platelets therefore stick to one another → aggregation.
• GpIIb/IIIa deficiency → Glanzmann thrombasthenia → bleeding disorder.
• Early platelet aggregation is reversible.
• Thrombin then strengthens the plug by:
  • further platelet activation
  • further aggregation
  • irreversible platelet contraction
• Platelet contraction depends on the cytoskeleton and tightens the platelet mass.
• At the same time:
  thrombin → fibrinogen → insoluble fibrin.
• Thrombin also activates factor XIIIa → cross-links fibrin → firmly locks platelets in place.
• This produces the definitive secondary hemostatic plug.
• Red cells and leukocytes may become trapped in the plug.
• Leukocytes can also adhere to P-selectin on activated platelets.

KEY CONCEPT

Vessel injury → collagen exposed → vWF binds GpIb → platelet adhesion → platelet activation → ADP + TxA₂ recruit more platelets → GpIIb/IIIa binds fibrinogen → platelet aggregation → thrombin makes fibrin → factor XIIIa cross-links fibrin → stable clot

• GpIb = adhesion
• GpIIb/IIIa = aggregation
• vWF = bridge to collagen
• ADP + TxA₂ = recruit/aggregate platelets
• Thrombin + fibrin = stabilize the plug

CONCEPTUAL EXAMPLES

• vWF deficiency → platelets cannot adhere properly → bleeding.
• GpIb deficiency → defective adhesion → Bernard-Soulier syndrome.
• GpIIb/IIIa deficiency → defective aggregation → Glanzmann thrombasthenia.
• Aspirin → ↓ TxA₂ → ↓ platelet aggregation.
• Thrombin + factor XIIIa → fibrin becomes strong and cross-linked → stable hemostatic plug.

FIG. 3.6 — Platelet Adhesion & Aggregation

Easiest concept: Platelets first STICK to the injured vessel, then STICK to each other

Whole figure in one line

Collagen exposed → vWF binds collagen → platelet GpIb binds vWF → platelet activated by ADP → GpIIb/IIIa activated → fibrinogen bridges platelets → platelet aggregation

1️⃣ Vessel injury exposes subendothelial collagen

🩷 Pink vessel wall = endothelium/subendothelium
🔴 Red rod-like structures = exposed collagen

Normally collagen is hidden beneath intact endothelium.

Injury → endothelium breaks → collagen becomes exposed

2️⃣ von Willebrand factor (vWF) attaches to collagen

vWF works like a bridge/glue.

Think:

Collagen — vWF — Platelet

So vWF connects the injured vessel wall to the platelet.

🟦 Label pointing to the bridge = von Willebrand factor

❌ If vWF is deficient:

➡️ von Willebrand disease

3️⃣ Platelet GpIb binds vWF

🟠 Orange circles = platelets
🟢 receptor on platelet = GpIb

GpIb is the platelet receptor that grabs vWF.

Therefore:

Collagen → vWF → GpIb → platelet adhesion

❌ GpIb deficiency:

➡️ Bernard-Soulier syndrome

🧠 Memory:

Bernard = “B” = GpIb

4️⃣ Platelet activation → ADP released

Once the platelet sticks, it becomes activated.

ADP acts on platelets and causes a:

Conformational change

= change in receptor shape so it can work properly.

The black arrows in the platelet show this activation/change in shape.

5️⃣ ADP activates GpIIb/IIIa

🟣 U-shaped receptor = GpIIb/IIIa

Before activation, it does not bind fibrinogen efficiently.

ADP → changes GpIIb/IIIa shape → receptor becomes active

6️⃣ Fibrinogen bridges two platelets

🔵 Blue bead-like chain = fibrinogen

Fibrinogen binds:

GpIIb/IIIa on platelet 1
↔️ fibrinogen
↔️ GpIIb/IIIa on platelet 2

This joins platelets together.

This is called:

Platelet aggregation

❌ If GpIIb/IIIa is deficient

Platelets can stick to the vessel, but they cannot aggregate properly.

➡️ Glanzmann thrombasthenia

🧠 Memory:

Glanzmann = GpIIb/IIIa problem

🎨 Color/structure map

• 🟠 Orange spheres = platelets
• 🟢 Green receptor = GpIb
• 🟣 Purple receptor = GpIIb/IIIa
• 🔵 Blue beads = fibrinogen
• 🔴 Red structures in wall = collagen
• 🩷 Pink wall = endothelium/subendothelium

⭐ Most important distinction

ADHESION

Collagen → vWF → GpIb → platelet sticks to vessel

AGGREGATION

GpIIb/IIIa → fibrinogen → platelet sticks to platelet

🧠 Fast exam memory

vWF + GpIb = ADHESION
Fibrinogen + GpIIb/IIIa = AGGREGATION

Diseases:

• vWF deficiency → von Willebrand disease
• GpIb deficiency → Bernard-Soulier syndrome
• GpIIb/IIIa deficiency → Glanzmann thrombasthenia

⭐ One-line recall

GpIb grabs vWF; GpIIb/IIIa grabs fibrinogen.

Coagulation Factors

• Coagulation factors work in an amplifying enzyme cascade → finally produce an insoluble fibrin clot.
• The importance of individual factors differs between laboratory testing and clotting inside the body (Fig. 3.7), but the basic principles are similar.
• The coagulation cascade can be imagined as a “dance”, where activation passes from one coagulation factor to the next (Fig. 3.8).

• Each reaction needs:
  • Enzyme = activated coagulation factor
  • Substrate = inactive coagulation factor waiting to be activated
  • Cofactor = accelerates the reaction
• These components assemble on the negatively charged phospholipid surface of activated platelets.
• Calcium (Ca²⁺) is also required for assembly of coagulation-factor complexes.
• Ca²⁺ binds to specially modified glutamic acid residues in factors II, VII, IX, and X.
• Formation of these modified residues requires vitamin K.
• Warfarin interferes with vitamin K metabolism → reduces the normal function of these vitamin K–dependent factors.
• In laboratory testing, coagulation is divided into extrinsic and intrinsic pathways.
• Prothrombin time (PT) evaluates the extrinsic pathway:
  • factors X, VII, V, II (prothrombin), and fibrinogen
  • Tissue factor + phospholipids + Ca²⁺ are added to plasma.
  • The time required to form a fibrin clot is measured.
• Partial thromboplastin time (PTT) evaluates the intrinsic pathway:
  • factors XII, XI, X, IX, VIII, V, II, and fibrinogen
  • Negatively charged particles + phospholipids + Ca²⁺ are added.
  • Factor XII is activated, and the time to fibrin-clot formation is measured.
• PT and PTT are very useful clinically, but they do not exactly reproduce coagulation inside blood vessels.
• Factor deficiencies show this difference:
  • Deficiency of V, VII, VIII, IX, or X → moderate to severe bleeding.
  • Prothrombin deficiency → incompatible with life.
  • Factor XI deficiency → usually mild bleeding.
  • Factor XII deficiency → no bleeding disorder.
• The normal physiologic role of factor XII is uncertain.
• Excess factor XII activity can predispose to angioedema because factor XII can promote bradykinin generation by cleaving high-molecular-weight kininogen.
• In the body:
  • Factor VIIa + tissue factor is considered the major activator of factor IX.
  • Factor IXa + factor VIIIa is the major activator of factor X.
• Thrombin can also activate factor XI → provides positive feedback and amplifies coagulation.
• Thrombin is the most important coagulation factor because it controls several parts of hemostasis and links clotting with inflammation and repair.
• Thrombin converts:
  soluble fibrinogen → fibrin monomers → insoluble fibrin clot.
• Thrombin further increases fibrin formation by activating:
  • factor V
  • factor VIII
  • factor XI
• Thrombin activates factor XIII → factor XIII cross-links fibrin → makes the clot more stable.
• Thrombin also activates platelets through PARs → platelet activation + aggregation + contraction.
• PARs are also present on inflammatory cells, endothelial cells, and other cells (Fig. 3.9).
• Thrombin activation of these receptors contributes to tissue repair.
• Thrombin can also have an anticoagulant effect.
• When thrombin encounters normal endothelium, it can change from procoagulant to anticoagulant activity → helps prevent the clot from spreading beyond the injured area.

KEY CONCEPT

• Coagulation cascade → thrombin → fibrin → stable clot.
• PT = extrinsic pathway.
• PTT = intrinsic pathway.
• Vitamin K is required for factors II, VII, IX, X.
• Warfarin interferes with vitamin K.
• Thrombin is central:
  fibrin formation + factor XIII activation + platelet activation + cascade amplification.
• Normal endothelium can change thrombin toward an anticoagulant role, limiting clot extension.

CONCEPTUAL EXAMPLES

• Vessel injury → tissue factor + VIIa → activates IX → IXa + VIIIa → activates X → more coagulation.
• Thrombin → fibrinogen becomes fibrin → clot forms.
• Thrombin → factor XIII activation → fibrin is cross-linked → clot becomes stronger.
• Warfarin → interferes with vitamin K → factors II, VII, IX, X function less effectively.
• Factor XII deficiency → abnormal laboratory pathway but no clinical bleeding disorder.

FIG. 3.9 — THROMBIN: The Central Player in Hemostasis

🧠 Easiest idea: Thrombin makes the clot AND activates cells around the clot

Whole figure in one flow

Thrombin → fibrin clot + stronger platelets + activated endothelium/leukocytes + tissue repair

1️⃣ Thrombin → Fibrinogen → Fibrin

🔵 Fibrinogen = soluble protein floating in blood.

⬇️ Thrombin cuts it

🔵 Fibrin = insoluble threads.

Fibrinogen → Fibrin → fibrin mesh → clot

Memory:

Thrombin = fibrin maker

2️⃣ Thrombin → Factor XIII → Factor XIIIa

🟢 Factor XIII = inactive form
🔴 Factor XIIIa = active form

Thrombin activates:

Factor XIII → XIIIa

Then XIIIa cross-links fibrin strands.

➡️ Loose fibrin → strong, stable fibrin clot

Exam:

Factor XIIIa = fibrin cross-linker

3️⃣ Thrombin → 🟡 Platelet activation

🟡/green clump = activated platelets.

Thrombin activates platelets through PAR
(protease-activated receptors).

This causes:

Platelet activation
→ aggregation
→ degranulation
→ more TXA₂

TXA₂

Thromboxane A₂

→ recruits/activates more platelets
→ promotes vasoconstriction

➡️ Clot becomes bigger and stronger.

4️⃣ Platelets release PDGF

PDGF = platelet-derived growth factor

⬇️ Black arrow toward vessel wall

PDGF stimulates:

Smooth muscle cells / fibroblast-like repair cells
→ proliferation
→ production of ECM

ECM

= extracellular matrix, the supporting material used in tissue repair.

So:

Platelet → PDGF → repair of damaged vessel wall

5️⃣ Thrombin → Endothelial activation

🩷 Thin cells lining the vessel = endothelial cells.

Thrombin activates them.

Activated endothelium produces more leukocyte adhesion molecules.

⬇️

White cells can now stick to the vessel wall.

6️⃣ 🟣 Neutrophil adhesion

The purple cell attached to the vessel wall = neutrophil.

Thrombin → endothelial activation → adhesion molecules ↑ → neutrophil sticks

➡️ Links clotting with inflammation.

7️⃣ 🟠 Monocyte activation

Thrombin binds PAR on monocytes.

Thrombin → PAR → monocyte activation

➡️ Monocytes participate in inflammation and tissue repair.

8️⃣ 🩷 Lymphocyte activation

Yellow arrow from thrombin toward the lymphocyte:

Thrombin → lymphocyte activation

Again, this shows that thrombin is not only a clotting enzyme—it also promotes cellular/inflammatory responses.

🎨 COLOR / ARROW MAP

• 🔵 Blue curved strand = fibrin/fibrinogen
• 🟢 Factor XIII = inactive
• 🔴 Factor XIIIa / red dots = activated cross-linking factor
• 🟡 Green-yellow clump = platelets
• 🩷 vessel lining = endothelium
• 🟠 cell = monocyte
• 🟣 round leukocytes = neutrophil / lymphocyte
• 🟨 Yellow arrows = effects produced by thrombin
• ⚫ Black arrows = downstream clot/repair effects

⭐ Fastest exam recall

THROMBIN does 4 major things:

1. Fibrinogen → Fibrin
2. Factor XIII → XIIIa → cross-links fibrin
3. Activates platelets → aggregation + degranulation + TXA₂
4. Activates endothelium + leukocytes → inflammation

🧠 One-line memory

THROMBIN = “Clot + Platelets + Inflammation + Repair.”

Factors That Limit Coagulation

• Once coagulation begins, it must stay limited to the injured vessel site; otherwise, harmful widespread clotting may occur.
• Several mechanisms prevent the clot from spreading:
  • Flowing blood dilutes and washes away activated coagulation factors.
  • These activated factors are then rapidly removed by the liver.
  • Coagulation also requires negatively charged phospholipids, mainly supplied by activated platelets.
  • Activated platelets are concentrated at the injury site, so coagulation is limited mainly to that area.
  • The most important control mechanisms come from the intact endothelium surrounding the injured site.
• Activation of coagulation also activates the fibrinolytic system → limits clot size and later helps dissolve the clot (Fig. 3.10).
• Fibrinolysis = breakdown of fibrin within a clot.
• The main enzyme responsible is plasmin.
  • Plasmin → breaks down fibrin.
  • It also interferes with further fibrin polymerization.
• Breakdown of fibrin produces fibrin split products.
• An important fibrin breakdown product is D-dimer.
• Increased D-dimer levels can be useful clinical markers of several thrombotic conditions.
• Plasmin is formed from an inactive circulating precursor:
  plasminogen → plasmin.
• The most important activator of plasminogen is tissue plasminogen activator (t-PA).
• t-PA is produced mainly by endothelial cells.
• t-PA works most effectively when it is bound to fibrin.
• Therefore, its fibrinolytic action is mainly concentrated at the site of the clot.
• Once plasmin is formed, it must also be controlled.
• α₂-plasmin inhibitor binds free plasmin → rapidly inhibits it → prevents excessive fibrin breakdown.

KEY CONCEPT

• Clotting is limited by:
  blood flow washing away factors + liver removal + localized platelet phospholipids + intact endothelium.
• Clot removal occurs through:

t-PA → plasminogen → plasmin → fibrin breakdown → fibrin split products/D-dimers

• t-PA starts fibrinolysis.
• Plasmin breaks fibrin.
• D-dimer indicates fibrin breakdown.
• α₂-plasmin inhibitor stops free plasmin.

CONCEPTUAL EXAMPLES

• Vessel injury → clot forms only at the damaged area because activated factors are washed away from surrounding normal vessels.
• After the clot has done its job → t-PA activates plasmin → fibrin is broken down → clot gradually dissolves.
• Fibrin breakdown → D-dimers appear in blood.
• Plasmin escapes away from the clot → α₂-plasmin inhibitor quickly neutralizes it.

FIG. 3.10 — FIBRINOLYTIC SYSTEM

🧠 Easiest concept: After a clot has done its job, plasmin cuts the fibrin clot and removes it.

Whole figure in one flow

Plasminogen → t-PA/urokinase → PLASMIN → fibrin breakdown → fibrin degradation products

1️⃣ 🟡/🔵 Central mass = Fibrin clot

The clot contains:

• 🟡 Platelets
• 🔵 tangled strands = fibrin
• Gray structures = trapped cellular material
• Plasminogen becomes attached to the fibrin clot.

Think:

Plasminogen = inactive clot-dissolving enzyme

2️⃣ Endothelium releases t-PA and urokinase

🩷 Flat cells lining the vessel = endothelium.

They produce:

t-PA = tissue plasminogen activator

and urokinase

These convert:

Plasminogen → PLASMIN

⬇️

🔴 Red curved structures = active plasmin

🧠 Memory:

t-PA turns plasminogen into plasmin.

3️⃣ Plasmin cuts fibrin

⚫ Black arrow:

Plasmin → attacks fibrin clot

The fibrin mesh is chopped into small pieces:

🔵 Small blue fragments = fibrin degradation products

So:

Fibrin clot → Plasmin → Fibrin degradation products

➡️ The clot gradually dissolves.

⭐ This process = FIBRINOLYSIS

4️⃣ 🟣 α₂-Antiplasmin stops free plasmin

Plasmin must mainly act at the clot, not everywhere in the blood.

🟣 Purple molecules = α₂-antiplasmin

It binds any free plasmin:

Free plasmin + α₂-antiplasmin
→ inactive α₂-antiplasmin/plasmin complex

➡️ Prevents excessive fibrin breakdown elsewhere.

Memory:

Antiplasmin = anti-plasmin → STOP plasmin

5️⃣ PAI = Plasminogen Activator Inhibitor

🔴 Red dotted arrows = inhibition.

PAI blocks:

t-PA / plasminogen activators

Therefore:

PAI ↓ plasmin formation → ↓ fibrinolysis

So PAI protects against too much clot dissolution.

🎨 ARROW / COLOR MAP

• 🔵 tangled fibers = fibrin
• 🟡 cells in clot = platelets
• 🔴 curved pieces = plasmin
• 🟣 blobs = α₂-antiplasmin
• ⚫ Black arrows = activation/breakdown pathway
• 🟨 Yellow arrows = important regulatory actions
• 🔴 Dotted arrow + ❌ = PAI inhibition
• 🩷 flat vessel cells = endothelium

⭐ Two opposite forces

🟢 Clot removal

t-PA → plasmin → fibrin breakdown

🛑 Clot removal controlled

PAI blocks t-PA
α₂-antiplasmin blocks free plasmin🧠 Fastest exam recall

t-PA activates plasminogen → plasmin.
Plasmin destroys fibrin → fibrin degradation products (including D-dimer from cross-linked fibrin).

🔥 One-line memory:

t-PA STARTS plasmin; plasmin CUTS fibrin; α₂-antiplasmin STOPS plasmin; PAI STOPS t-PA.

Endothelium

• The endothelium controls the balance between clot formation and clot breakdown.
• This balance determines whether a clot forms, grows, stays limited, or dissolves (Fig. 3.11).
• Normal endothelial cells are mainly antithrombotic.
• They:
  • inhibit platelets
  • inhibit coagulation factors
  • promote fibrinolysis
• Together, these actions prevent thrombosis and keep clotting limited to the site of vascular injury.
• When endothelium is injured or activated by proinflammatory factors, it loses many of these antithrombotic properties.
• Antithrombotic actions of normal endothelium can be grouped into three major effects:
• Platelet inhibitory effects:
  • Intact endothelium acts as a physical barrier → prevents platelets from contacting subendothelial vWF and collagen.
  • It releases prostacyclin (PGI₂) and nitric oxide (NO) → inhibit platelet activation and aggregation.
  • Adenosine diphosphatase breaks down ADP → decreases platelet activation.
  • PGI₂ and NO also cause vasodilation → helps wash away coagulation factors.
  • Endothelial cells also bind thrombin → reduce thrombin-induced platelet activation.
• Anticoagulant effects:
  • Normal endothelium separates circulating coagulation factors from tissue factor in the vessel wall.
  • Important anticoagulant molecules include:
    • thrombomodulin
    • endothelial protein C receptor
    • heparin-like molecules
    • tissue factor pathway inhibitor (TFPI)
• Thrombomodulin pathway:
  • Thrombomodulin binds thrombin.
  • Once bound, thrombin loses much of its normal procoagulant activity.
  • Instead, thrombin activates protein C.
  • Protein C is vitamin K–dependent and works with protein S.
  • Activated protein C + protein S → inhibit factors Va and VIIIa.
  • Result → less coagulation.
• Heparin-like molecules:
  • Bind and activate antithrombin III.
  • Antithrombin III inhibits:
    • thrombin
    • factor IXa
    • factor Xa
    • factor XIa
    • factor XIIa
  • The clinical effect of heparin depends on increasing antithrombin activity.
• TFPI:
  • Works with protein S.
  • Inhibits the tissue factor–factor VIIa complex.
  • Result → reduces activation of the coagulation pathway.
• Fibrinolytic effects:
  • Normal endothelial cells produce t-PA.
  • t-PA promotes fibrinolysis by helping generate plasmin.

KEY CONCEPT

• Normal endothelium is strongly antithrombotic.
• It protects against clotting by:

↓ platelet activation + ↓ coagulation + ↑ fibrinolysis

• Important pathways:
  • PGI₂ + NO + ADP breakdown → inhibit platelets
  • Thrombomodulin → protein C + protein S → inhibit Va and VIIIa
  • Heparin-like molecules → antithrombin III → inhibit thrombin and IXa, Xa, XIa, XIIa
  • TFPI → inhibits tissue factor–VIIa
  • t-PA → promotes fibrinolysis

CONCEPTUAL EXAMPLES

• Healthy endothelium → platelets cannot contact collagen → no unnecessary platelet plug.
• Thrombomodulin binds thrombin → thrombin switches from clot-promoting to protein C activation.
• Protein C + protein S → shut down Va and VIIIa → coagulation slows.
• Heparin-like molecules → activate antithrombin III → major clotting enzymes are inhibited.
• Endothelium releases t-PA → fibrin clot is gradually broken down.

FIG. 3.11 — ANTITHROMBOTIC EFFECTS OF NORMAL ENDOTHELIUM

🧠 Easiest idea: Healthy endothelium prevents unwanted clotting in 3 ways

Whole figure in one line

Healthy endothelium → blocks coagulation + blocks platelets + promotes fibrinolysis → prevents thrombosis

1️⃣ LEFT — Heparin-like molecules + Antithrombin

🟡 Antithrombin = natural anticoagulant in blood.

🔻 Heparin-like molecule on endothelium greatly increases antithrombin activity.

Then:

Heparin-like molecule → activates antithrombin → inhibits thrombin + other coagulation factors

Result:

❌ Less fibrin formation
❌ Less clotting

Memory:

Heparin helps Antithrombin STOP thrombin.

2️⃣ Tissue Factor Pathway Inhibitor (TFPI)

TFPI is made by normal endothelium.

⬇️ Black dotted arrow

It inactivates the Tissue factor–VIIa complex.

So:

TFPI → blocks tissue factor pathway → ↓ coagulation

Memory:

TFPI = Tissue Factor Pathway OFF

3️⃣ CENTER — Thrombomodulin + Protein C

🔵 Thrombin binds to thrombomodulin on the endothelial surface.

Normally thrombin promotes clotting, but when it binds thrombomodulin:

It changes job:

Thrombin + thrombomodulin → activates Protein C

The endothelial protein C receptor also helps this process.

Active Protein C needs Protein S

Protein C → Active Protein C

Then:

Active Protein C + Protein S
→ inactivates Factor Va and Factor VIIIa

Result:

↓ thrombin generation
↓ coagulation
↓ thrombosis

Exam memory:

Protein C/S cut off 5 and 8

➡️ Va + VIIIa inhibited

4️⃣ RIGHT — PGI₂, NO, and ADPase

Normal endothelium produces:

PGI₂ = Prostacyclin

NO = Nitric oxide

Adenosine diphosphatase

These all reduce platelet activity.

⬇️

Inhibit platelet aggregation

Simple concept:

They tell platelets:

“Do NOT stick together.”

5️⃣ t-PA → Fibrinolysis

Normal endothelium releases:

t-PA = tissue plasminogen activator

t-PA converts:

Plasminogen → plasmin

Plasmin breaks down fibrin.

So:

t-PA → fibrinolysis → clot breakdown

🎨 COLOR / ARROW MAP

• 🩷 Flat purple-pink cells = endothelium
• 🟡 Yellow molecules = antithrombin
• 🔵 Dark blue structure = thrombin
• 🟢 Green structures = Protein C system
• ⬛ Black dotted arrows = downstream inhibitory effects
• 🔵 Blue arrows = endothelial protective actions

⭐ THREE BIG ANTITHROMBOTIC ACTIONS

1. Anticoagulant

• Heparin-like molecules → antithrombin
• TFPI
• Protein C + Protein S

2. Antiplatelet

• PGI₂
• NO
• ADPase

3. Fibrinolytic

• t-PA → plasmin → fibrin breakdown

🧠 Fastest exam recall

Healthy endothelium = anti-clot surface

Antithrombin stops thrombin
Protein C/S stop Va & VIIIa
PGI₂ + NO stop platelets
t-PA breaks the clot

⭐ One-line memory

Normal endothelium = STOP coagulation + STOP platelets + DISSOLVE fibrin.