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Hemostasis is vital for _______ and is achieved by a delicate balance between _______ and _______.
Hemostasis is vital for life and is achieved by a delicate balance between procoagulants and anticoagulants.
Hemostasis is blood clotting or coagulation as a mechanism to prevent significant blood loss from the circulation.
Hemostasis is the prevention of blood loss when a vessel is severed or ruptured.
Hemostasis utilizes several mechanisms to limit blood loss until the vessel is repaired including vascular constriction
, formation of a platelet plug
, formation of a blood clot
, clot retraction
and finally removal of the clot
(dissolved) after repair.
Vascular constriction attempts to _______ blood loss _______.
Vascular constriction attempts to limit blood loss immediately.
● Immediately after a blood vessel is severed or ruptured, the trauma (and possibly the stretch: unitary smooth muscle in blood vessel walls) causes the vessel to constrict to limit blood loss
● This vasoconstriction
is primarily caused by local factors released from the damaged tissue, Thromboxane A2
released from platelets
synthesized from arachidonic acid formed by PLA2), and nervous reflexes
initiated principally by pain receptors
adhere to the damaged endothelium (simple squamous epithelial cells lining/forming all blood vessels) and release TXA2
whereas adjacent undamaged endothelium
releases Prostaglandin I2
: also called Prostacyclin
and also synthesized from arachidonic acid formed by PLA2) which inhibits platelet
aggregation from spreading inappropriately.
Platelets or _______ are very important _______ _______ vital to _______.
Platelets or thrombocytes are very important cell fragments vital to hemostasis.
_______ _______ may be sufficient for _______ leaks.
Platelet plugs may be sufficient for minor leaks.
Platelets and Formation of the Platelet Plug
● Leaks occur frequently in the smaller vessels and a platelet plug
is often sufficient to seal them: blood coagulation is not necessary.
- ● Platelets (also called Thrombocytes) are cell fragments ~1 - 4m in diameter formed from Megakaryocytes (see previously) in the
- bone marrow or blood especially as they squeeze through the smaller capillaries.
● Normally blood contains ~150,000 - 300,000 platelets/mm3
have no nucleus and therefore like RBCs cannot replicate
contain contractile proteins
including actin and myosin
(as found in muscle) and thrombosthenin
which allow the platelets
to rapidly release intracellular granules
- ● Platelets also contain remnants of the ER and Golgi apparatus in order to: produce enzymes store Ca2+ produce mitochondria and the enzymes necessary to produce ATP
- produce prostaglandins
- produce two important proteins (namely Fibrin-stabilizing factor and a growth factor which stimulates endothelial cells, vascular smooth muscle cells and fibroblasts to multiply and grow...all of which are necessary for repair)
● On the external face of the platelet
cell membrane are glycoproteins
which prevent them from adhering to the normal undamaged endothelial surface but does adhere to damaged endothelium and even more so to exposed collagen fibers from the underlying connective tissue.
also contain high concentrations of phospholipids
that are involved in blood clotting.
have a short ½-life
(~8 - 12 days) and are removed by macrophages in the spleen (phagocytosis).
● Contact between platelets
and damaged endothelium or exposed collagen stimulates the platelets
to swell and assume irregular shapes and radiating pseudopodia
: contractile proteins cause the release of intracellular granules containing important factors. The platelets
become "sticky" and adhere to collagen
and a protein called von Willlebrand factor
from blood. In addition they secrete large amounts of ADP
(blocked by "Plavix") and TXA2
which activate additional nearby platelets
which themselves then adhere to the original ones in a + feedback loop: this is the platelet plug
● The plug
is loose at first and this is often adequate to plug the leak but subsequently during blood clotting the fibrin fibers
formed further strengthen it.
Blood Clotting (Coagulation)
● A blood clot (as distinct from a platelet plug) starts to form after ~15 - 20 seconds if trauma is significant
and after ~1 - 2 minutes if minor
. This process is initiated by substances released from the damaged endothelium, platelets and blood proteins adhering to the damaged blood vessel lining.
- ● After ~3 - 6 minutes most reasonable holes are plugged by the clot. After ~20 - 60 minutes the clot
- retracts due to platelets and closes the hole even more and more tightly.
- ● Once a clot has formed there are two options:
- 1) the clot can be invaded by fibroblasts (stimulated by the platelet growth factor) which form fibrous connective tissue throughout the clot; or
- 2) the clot can be dissolved if appropriate.
- ● If invaded by fibroblasts the repair is usually complete (reorganized into fibrous connective
- tissue) after ~1 - 2 weeks.
● Blood clotting is a complex process primarily involving two overlapping enzyme cascades
that usually occur in the blood but that can also occur in tissues if necessary.
● There are >50 substances
found to affect or cause clotting. Some promote clotting
") while others inhibit clotting
"). There is a balance between these two groups of substances. Normally anticoagulants
dominate but when there is damage, procoagulants
dominate at the site.
- ● The process can be simplified to three essential steps:
- 1) trauma stimulating either of two enzyme
- cascades resulting in the formation of a complex called Prothrombin activator;
- 2) The subsequent conversion of Prothrombin into the active enzyme Thrombin;
- 3) The conversion of the soluble plasma protein Fibrinogen into insoluble Fibrin.
● Fibrin fibers
enmesh platelets, RBC's and plasma to form the clot.
Clotting Factor: Factor I
Clotting Factor: Factor II
Clotting Factor: Factor III
- Tissue Factor
- Tissue Thromboplastin
Clotting Factor: Factor IV
Clotting Factor: Factor V
- Labile Factor
- Ac-globulin (Ac-G)
Clotting Factor: Factor VII
- Serum prothrombin conversion accelerator (SPCA)
- Stable Factor
Clotting Factor: Factor VIII
- Antihemophilic factor (AHF)
- Antihemophilic globulin (AHG)
- Antihemophilic Factor A
Clotting Factor: Factor IX
- Plasma thromboplastin component (PTC)
- Christmas Factor
- Antihemophilic Factor B
Clotting Factor: Factor X
- Stuart Factor
- Stuart-Prower Factor
Clotting Factor: Factor XI
- Plasma thromboplastin antecedent (PTA)
- antihemophilic factor C
Clotting Factor: XII
Clotting Factor: Factor XIII
Clotting Factor: Prekallikrein
Clotting Factor: High molecular weight Kininogen (HMWK)
Clotting Factor: platelets
Release Thromboxane A2 (TXA2)
Initiation of Coagulation: Formation of Prothrombin Activator
● Prothrombin activator
is formed by two different enzyme cascades: the "Extrinsic
" pathway is principally initiated by trauma to the endothelium and surrounding/supporting tissue while the "Intrinsic
" pathway originates in the blood itself.
● Both pathways utilize plasma proteins
called "clotting factors" which are mostly proteolytic enzymes
which act in a cascade
activating subsequent members similarly to the MAPKKK signal transduction pathway (those were kinases however). The factors are designated by Roman numerals (like the cranial nerves) and the "active" form is distinguished by an "a."
Blood Clotting/Coagulation: The Extrinsic Pathway
● The Extrinsic pathway is initiated by the release of Tissue factor
: also called Tissue Thromboplastin
) which is composed primarily of phospholipids
from the membranes of damaged cells.
● This complexes with and activates factor VII
(→VIIa) and together with Ca2+
activates factor X
● Factor Xa
combines with phospholipids
from Tissue factor
or platelets plus factor V
to form prothrombin activator;
initially however factor V
is inactive in this complex.
- ● Again in the presence of Ca2+ within a few seconds prothrombin is converted to active
- thrombin: factor V is now activated by thrombin in a + feedback loop which in turn increases the activity of factor Xa, etc..
Blood Clotting/Coagulation: The Intrinsic Pathway
- ● The Intrinsic pathway is initiated by trauma to blood itself or exposure to collagen: platelets are
- activated and release phospholipids (including the lipoprotein Platelet factor III [not Tissue factor III]) and factor XII is activated (→XIIa).
● This starts a cascade whereby XIIa
together with High Molecular Weight Kininogen
(and accelerated by Prekallikrein) activates XI
(→XIa) which in turn activates IX
(→IXa) in the presence of Ca2+
together with activated factor VIII
(→VIIIa: increasingly activated by thrombin
so forming another + feedback loop analogous to that seen previously in the Extrinsic pathway with factor V), Ca2+
and platelet phospholipids (including Platelet factor III) activate factor X
- ● This pathway is blocked if deficient in either factor VIII or platelets: a deficiency in factor VIII is
- the prime cause of “classic” hemophilia while if due to ↓no. platelets it is termed Thrombocytopenia.
● The remainder of this pathway is the same as for the Extrinsic pathway with the formation of Prothrombin activator
Blood Clotting/Coagulation: Integration of both the Extrinsic and Intrinsic Pathways and the summary of involvement of Ca2+
● As can be seen Ca2+
is involved in several places in both pathways. The absence of Ca2+ therefore prevents blood clotting
by either pathway and its removal can prevent clotting in blood samples (deionizing, combining with citrate or oxalate [precipitates Ca2+
- ● Rupturing a blood vessel will activate both
- pathways simultaneously: Tissue factor (factor III) released from damaged tissue will activate the Extrinsic pathway while exposed collagen from the
- underlying connective tissue will activate the Intrinsic pathway.
● However the Extrinsic pathway
can proceed very rapidly
especially with the release of amounts of Tissue factor ( response with damage): clotting can begin within ~15 sec!
● The Intrinsic pathway
is generally much slower
typically taking ~1-6 minutes to form a clot.
Blood clotting is initiated by a variety of stimuli and proceeds via two pathways (the _______ and the _______) that share common factors and an end point: the formation of _______.
Blood clotting is initiated by a variety of stimuli and proceeds via two pathways (the intrinsic and the extrinsic) that share common factors and an end point: the formation of fibrin.
_______ is vital to initiating the pathways that lead to _______ _______.
Ca2+ is vital to initiating the pathways that lead to blood coagulation.
Prevention of Clotting & Intravascular Anticoagulants
● Any leak of blood from the vasculature must be prevented, but inappropriate clotting must also be prevented. A delicate balance must be achieved whereby clotting is initiated rapidly
and effectively where it is needed (Procoagulants prevail
) but not elsewhere (anticoagulants prevail
- ● Endothelial Surface Factors are probably the most important in preventing inappropriate clotting. For example, these include:
- 1) the “smoothness” of the endothelial surface which prevents activation of the Intrinsic pathway;
- 2) the glycocalyx covering which repels clotting factors and platelet attachment/aggregation and a protein expressed on the endothelial surface
- called Thrombomodulin which binds to Thrombin and inhibits its function.
● In addition a protein activated by this complex (Thrombomodulin + Thrombin) called Protein C
acts as an anticoagulant
by inactivating factors Va and VIIIa.
● These inhibiting factors (anticoagulants) are all lost or reduced when the endothelial surface is compromised as occurs when an athersclerotic plaque
Normally, clotting is prevented by endogenous _______.
If repair does occur, then clots are normally removed by _______.
Normally, clotting is prevented by endogenous anticoagulants.
If repair does occur, then clots are normally removed by plasmin.
Endogenous Anti-Thrombin Actions
- ● Some important anticoagulants are found in the blood itself (endogenous) and act by inactivating or removing Thrombin. The most powerful of these are the fibrin fibers themselves (the “end product” of the
- clotting mechanisms) and an a-globulin called Antithrombin III (also called Antithrombin-heparin Cofactor).
● As a clot forms ~85-90% of the active Thrombin
formed is adsorbed onto fibrin fibers
so inactivating it which helps limit the spread of the clotting process. Any remaining Thrombin
combines with Antithrombin III
which also inactivates over ~12-20 minutes.
Anticoagulants in Clinical Use: Heparin
● Heparin is a powerful endogenous anticoagulant that is normally present in the blood and tissues but in amounts ↓level that it is ineffective. It is administered in doses clinically to prevent intravascular clotting
● Heparin itself is a virtually inactive conjugated polysaccharide
that when combined in Antithrombin III
increases its activity ~x100-1000.
● This complex (Heparin
+ Antithrombin III
) also removes activated factors IXa, Xa, XIa and XIIa.
● Heparin is secreted by many cell types but especially by Mast cells
in connective tissue
● Many Mast cells
are located near the lungs and liver where the slow “percolation
” [Pharmacology definition of percolation: the extraction of the soluble principles of a crude drug by the passage of a suitable liquid through it] of blood can cause the formation of many potentially dangerous small emboli
(unattached blood clots circulating which can block blood vessels).
Removal of Clots: Lysis by Plasmin
- ● Plasma contains a protein called Plasminogen (also called Profibrinolysin) which when activated
- forms Plasmin (or Fibrinolysin) which is a proteolytic enzyme similar to the digestive
- enzyme Trypsin: when active it “digests” fibrin fibers, Fibrinogen, Prothrombin and factors V, VIII and XII.
- ● When a clot forms, amounts of inactive Plasminogen are trapped and require activation by Tissue Plasminogen Activator (t-PA)
- which is released very slowly (over several days) from the damaged tissue. This therefore delays the onset of clot removal until the necessary repairs have occurred.
What three conditions are most significant in causing excessive bleeding?
● Excessive bleeding can result from a deficiency of any of the previously discussed Procoagulants
- ● There are three conditions that are particularly significant in being able to cause excessive bleeding, and they are:
- 1) vitamin K deficiency
- 2) hemophilia
- 3) thrombocytopenia
What are three conditions that can cause excessive bleeding?
- 1) Vitamin K Deficiency
- 2) Hemophilia and Factor VIII
- 3) Thrombocytopenia (platelet deficiencies)
Vitamin K has a role in producing adequate blood clotting factors
● Almost all the clotting factors (Procoagulants) are synthesized in the liver
. Therefore, any liver disease can affect the production of procoagulants
resulting in an tendency to bleed.
- ● Vitamin K is necessary for the synthesis of four important clotting factors including Prothrombin and
- factors VII, IX and X. Production of the anticoagulant Protein C is also affected.
● Vitamin K is normally synthesized by symbiotic gut bacteria
and is therefore not a limiting factor (neonates are still establishing normal gut “fauna and flora”
and therefore are an exception to the rule...they frequently receive Vitamin K injections after birth to make up for this lack of production for the first week till their guts kick in).
● Because Vitamin K is fat soluble
, anything interfering with this (this what, Dr. Wright?) can result in a deficiency (absence/↓amounts of bile can cause this). Therefore, any patient scheduled for surgery with liver problems is given vitamin K 4-8 hours prior to surgery to ensure adequate production of clotting factors
● Vitamin K1
is also known as phylloquinone
(also called phytonadione
). Vitamin K2
) is normally produced by bacteria in the large intestine, and dietary deficiency is extremely rare
unless the intestines are heavily damaged, are unable to absorb the molecule, or are subject to decreased production by normal flora, as seen in broad spectrum antibiotic use. [Wikipedia 1/13/2011]
Hemophilia and Factor VIII
- ● Hemophilia is primarily genetic/inherited. It is a “sex-linked” disease (X-chromosome: ♀ carriers vs. ♂ expressed) is almost exclusively
- found in males. ~85% of cases are due to a deficiency or abnormality of Factor VIII and occurs at a frequency ~1 in 10,000 in U.S. males; this is “classic” hemophilia.
● The remaining ~15% of cases
are primarily due to factor IX
deficiency (also “sex-linked” so also found almost exclusively in ♂
● In some extreme cases a slight trauma, such as loosing a tooth, can result in several days of bleeding
- ● Factor VIII has two active parts:
- 1) the smaller component of Factor VIII is very important for the Intrinsic pathway, which is deficient in “classic” hemophilia;
- 2) the larger component of Factor VIII is deficient in von Willebrand’s Disease which has different characteristics.
- ● The only real treatment for the previous two conditions is to infuse human-sourced
- Factor VIII which is very expensive.
● Platelets are normally present in the blood in numbers of 150,000-300,000/mL. Problems can occur if this falls below 50,000/mL and can be lethal if <10,000/mL.
● ↓no. platelets tends to result in many tiny leaks not being repaired effectively (because a “platelet plug” is not formed). Superficially, this can result purplish blotches in the skin (called “purpura”) primarily due to the loss of clot retraction by platelets.
● The most common condition of this type is called idiopathic thrombocytopenia (meaning thrombocytopenia of unknown cause) which seems to be the result of autoimmune platelet destruction by antibodies.
● Can be treated by infusion of whole blood and/or a spleenectomy. Removing the spleen would stop the removal of platelets.
Inappropriate clot formation, also known as a _______ _______, can be prevented by _______and removed by _______.
Inappropriate clot formation, also known as a thromboembolic condition, can be prevented by anticoagulants and removed by t-PA.
● Abnormal or inappropriate clotting
in a blood vessel is called a thrombus
(multiple are called thrombi). If the thrombus or a portion of the thrombus breaks loose, it is called an embolus
(multiple are called emboli) and can move through the circulation until it blocks a narrower vessel
- ● There are two main causes for this inappropriate clotting:
- 1) any roughened endothelial surface such as that
- caused by an arteriosclerotic plaque can initiate clotting
- 2) very slowly moving blood which allows the accumulation of procoagulants which are constantly being formed.
● Current treatment involves perfusing genetically-engineered t-PA
which activates Plasmin to dissolve the clot
- ● Bed-ridden or immobile patients are particularly at risk (or those not moving for long periods of time, as in travel) as large clots can occur in leg veins from which emboli can break free and enter the right-side of the heart and onto the lungs where they can block the pulmonary arteries (pulmonary embolism). If both pulmonary arteries are blocked simultaneously
- then death will occur rapidly.
- ● If a patient has amounts of traumatized or necrotic tissue, this can result in amounts of
- tissue factor being released into the blood where it can cause widespread clotting called Disseminated
- Intravascular Coagulation.
Anticoagulants in Clinical Use
● Sometimes it is important to delay the clotting process by using anticoagulants
. The most useful of these are Heparin
and the Coumarins such as Warfarin
- ● Heparin can delay clotting time up to ~30 min. and the effect is immediate. Heparin is inactivated by an enzyme in the blood called Heparinase
- after ~1.5 to 4 hours.
● The Coumarins like Warfarin
cause ↓plasma levels of Prothrombin and factors VII, IX and X
which are all synthesized in the liver. They act by competing with vitamin K
- ● Thrombin antagonists/inhibitors are also being investigated (such as Hirudin from the medicinal leech) as are thrombin agonists selective for it's
- anticoagulant properties.
● The clotting time of patients is routinely checked in hospital laboratories if any problem is suspected.
● Prothrombin levels are estimated by measuring the clotting time in the presence of excess Ca2+ and Tissue factor. Other factors are estimated in a similar manner with all components present in excess (so not limiting) except for the factor being estimated.
Summary Question: What do platelets release to cause vasoconstriction?
Thromboxane A2 (TXA2), which is formed via PLA2/arachadonic acid pathway.
Summary Question: What else besides platelets may cause vasoconstriction?
The stretching of unitary smooth muscle in blood vessel walls and neurogenic reflexes initiated by pain receptors.
Summary Question: What "factors" form the Prothrombin Activator?
- Factor V
- Factor X
- platelet phospholipids
Summary Question: What is significant about Factor V?
Initially it is inactive, but a positive feedback loop from Thrombin increasingly activates it.
Summary Question: What does Thrombin do?
- Primarily converts fibrinogen to fibrin
- Creates positive feedback loop in Factor V, Factor VIII, and Factor XI
- Acts as an anticoagulant linking Protein C to Thrombomodulin on endothelial cells, which then degrades Factors V and Factor VIII
• Hemostasis is vital for life and is achieved by a delicate balance between pro- and anticoagulants.
• Vascular constriction attempts to limit blood loss immediately.
• Platelets (thrombocytes) are very important cell fragments vital to hemostasis.
• Platelet plugs may be sufficient for minor leaks.
• Blood clotting is initiated by a variety of stimuli and proceeds via two pathways that share common factors and an end point: the formation of fibrin.
• Without Ca2+ you won't have blood coagulation. Ca2+ is a vital part of these pathways and processes for blood clotting.
•Normally clotting is prevented by endogenous factors such as heparin (anticoagulants).
•After repair, clots are normally removed by plasmin.
•Vitamin K and platelet deficiencies plus inherited clotting factor abnormalities can all cause excessive bleeding (hemophilia).
•Inappropriate clot formation (thromboembolic conditions) can be prevented by anticoagulants and removed by t-PA.