Definition: Acute failure to perfuse adequately the organ and the tissues of the body .
The diagram shows the
cross section through the vascular system. At the beginning, in the aorta, its cross-section is very small. As one goes further and more towards the capillaries, then the vessels do become narrower, but the number of vessels increases much more. Therefore, the cross-sections of all these vessels (the "sum") increases a lot.
The cross section of all the capillaries
together is a total of 5000 cm2, compared to only 2.5 cm2 for the aorta!
Therefore, because there are so many more smaller vessels and capillaries in the body, this may cause a problem.
If they were all
wide open, their total cross section would be much more than the cross section of the aorta.
Therefore, if all the blood vessels were open, then the blood volume (approx. 5 litres) would easily fit in the small vessels and the blood pressure would drop to
In order to avoid that, the arterial and venous vessels must always show some degree of
Shock is the condition when the
blood pressure is too low
To be precise: this is called
cardiovascular shock, to distinguish this from other types of shock such as psychological shock.
Problems with the
heart: cardiogenic shock Problems with the blood
volume: hypovolaemic shock Problems with the
vessel wall tension: anaphylactic shock or septic shock
Heart: Cardiogenic Shock
myocardial infarction: a (large) part of the left ventricle is no longer working
myocarditis: inflammation/infection of the heart makes the muscle weaker
arrhythmias (disturbance in the rhythm): leaving not enough time in the diastole for filling the ventricles.
other cardiac causes (valvular, tamponade, etc)
Blood Volume: Hypovolaemic Shock
burns (> 20%)
internal bleeding (ruptured spleen)
Blood Vessel: Anaphylactic Shock and Septic Shock
Extreme vasodilation due to an intense allergic reaction (insect bite, allergic to medicine such as penicilline, etc).
Extreme vasodilation due to bacterial infections and the entry of bacteria and their toxic products (=endotoxins) inside the blood.
Compensated Phase: Immediate Response
Decrease in parasympathetic and increase in sympathetic activity.
Increases in Angiotensin II, adrenaline and vasopressin (=ADH)
increased chronotropy (tachycardia)
increased inotropy (contraction force)
Vasoconstriction in muscles, gut, skin and kidney -> increase in the Peripheral Resistance
Reduced perfusion in these organs also leads to acidosis, weakness, oliguria (=decreased urine output) and pallor
The skin becomes wet, cold and pale due to the increased sympathetic stimulation.
Compensated Phase: Intermediate Response
Up to 500 ml can be transfused back into the vascular system.
This will produce a temporary anaemia (= less oxygen transported) but also reduce viscosity (=less work for the heart)
Compensated Phase: Long Term Response
Reduction in renal excretion and increase in fluid intake (thirst)
increase in liver glycolysis (induced by adrenal and sympathetic stimulation) -> more blood proteins -> increase in oncotic pressure
Increased production in red blood cells.
skin is pale, cold and sweaty
pulse is rapid and weak
breathing is rapid and shallow
urine output is decreased or even stopped
general muscle weakness
reduced mental awareness or confusion
mean arterial pressure may be normal or reduced (last sign to be affected)
Decompensated Phase (= irreversible)
If the blood loss is too high and /or fluid replacement is started too late:
then irreversible damage to:
tubular necrosis (kidneys)
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