Haemorrhage Flashcards
Excessive blood loss is a medical emergency. What can it lead to?
Decreased blood volume results in a fall in BP and impaired delivery of nutrients to cells. This state is called shock.
What are the 4 physiological responses to blood loss?
- Immediate - stop the bleeding
- Short term - restore blood pressure
- Medium term - restore fluid volume
- Long term - replace blood constituents
Describe the immediate physiological response to blood loss
Stopping blood flow - haemostasis
Several components involved: vascular response, platelet response, plasma response (coagulation)
Describe the vascular response of the immediate response to blood loss
Smooth muscle: spasm due to trauma, myogenic response, humoral factors (vasoconstrictors)
Endothelium: platelet adhesion and aggregation, anticlotting and fibrinolysis
What does haemostasis mean?
Stopping the flow of blood
Describe the platelet response of the immediate response to blood loss
Damage to blood vessel -> turbulent blood flow -> platelets come into contact with blood vessel wall (collagen)
Platelets aggregate
Platelets release chemicals that cause further aggregation
Formation of platelet plug
Effective in sealing small blood vessels
Describe the process of coagulation in the immediate response to blood loss
Fibrinogen is converted to fibrin
Blood clots are made of fibrin
Numerous clotting factors are involved
The synthesis of several clotting factors requires Vitamin K
Clotting factors are activated in an enzymatic cascade
Which clotting factor converts prothrombin to thrombin?
Factor Xa (Ca2+, Factor V, and phospholipids also involved)
What does thrombin do?
Converts fibrinogen to soluble fibrin
Converts Factor XIII to Factor XIIIa
What does factor XIIIa do?
Converts soluble fibrin to insoluble fibrin
Describe the instrinsic pathway in coagulation
Vascular damage, contact activation, involving factors XII, XI, IX and VIII, phospholipid and calcium ions
Convert Factor X to Factor Xa
Describe the intrinsic pathway in coagulation
Tissue damage, tissue factors (tissue thromboplastin, factor VII), phospholipid , calcium ions
Convert Factor X to Factor Xa
What does Factor Xa do?
Converts prothrombin to thrombin in coagulation cascade (immediate response to blood loss)
Describe the relationship between the intrinsic and extrinsic pathways in coagulation (immediate response to blood loss)
The 2 pathways interact in a complex manner
Extrinsic pathway = important in initiating clotting after an injury
Intrinsic pathway = maintains the process once it has started
Intrinsic system may have a role in thrombosis
What is fibrinolysis?
The enzymatic breakdown of fibrin in blood clots
Once the blood clot has served its purpose, it is dissolved
Which enzyme is responsible for fibrinolysis?
Plasmin
What does plasminogen activator do?
Converts plasminogen to plasmin
Describe the short term response in blood loss
Fall in blood pressure -> compensatory mechanisms triggered by arterial baroreceptors
Aim to restore blood pressure
What are baroreceptor reflexes mediated by?
Sympathetic nerves
Hormones: adrenaline, angiotensin II, vasopressin
Describe the baroreceptor firing response when blood volume decreases and the effect this has on blood pressure
Decreased blood volume -> decreased blood pressure
Decreased baroreceptor firing
This is detected by CVS centres in the brainstem
Leads to increased sympathetic nervous system activity: increased heart rate, increased cardiac output, increased ventricular contraction, increased venous constriction, increased arteriole constriction (increased peripheral resistance)
All of this leads to increased mean arterial blood pressure
Describe the medium response to blood loss
Restoring blood volume by:
- Shifting interstitial fluid back into blood vessels
- Deceasing fluid loss in kidney
- Increasing fluid intake
What does oncotic pressure arise from?
Plasma proteins
Describe the effects that baroreceptor reflexes have on on capillary blood pressure in response to a decrease in arterial blood pressure
Vasoconstriction of arterioles: increased total peripheral resistance, decreased capillary blood pressure
This decreases the hydrostatic blood pressure pushing fluid out of the capillary
More fluid is drawn back into the capillaries by the oncotic pressure
Describe the process of decreasing fluid loss in the kidneys
Decreased glomerular filtration Increased reabsorption of Na+ and water by stimulating release of: - renin-angiotensin-aldosterone - ADH Both of these are vasoconstrictors
