Shock is defined as a condition where the circulatory system cannot provide adequate circulation to vital organs due to low blood pressure. Symptoms include increased heart rate, pale and cold clammy skin. Shock is classified as hypovolemic from blood/fluid loss, distributive from vasodilation, or cardiogenic from heart issues. The body compensates for shock through fluid shifts, decreasing fluid exiting and increasing reentry to capillaries to boost blood volume and pressure.

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The Physiology of Shock
R.E.B, 4MedStudents.com, 2003

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What is Shock?
 Shock is any condition in which the circulatory system is
unable to provide adequate circulation to the vital body
organs such as the brain,heart and lungs. As a result of a
decrease in the blood pressure.
 Shock is usually accompanied by renal failure, as a
normal compensatory mechanism, because the blood
flow to the kidney is decreased to keep enough blood for
the vital organs.

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Symptoms of shock
1. Increase heart rate as a result of the baroreflex:
• Shock will decrease the volume of blood pumped
from the heart and the blood flow to the brain. That
will activate the baroreceptors in the carotid bodies
to increase HR trying to supply enough blood to the
vital organs.
2. Pale skin:
• As a result of vasoconstriction of the peripheral
vessels, because the skin is the least priority tissue
for blood flow

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Symptoms of shock
3. Cold and clammy skin : As a result of
vasoconstriction.
• Shock decreases the skin surface temperature as a result of
vasodilatation, which will increase the internal body temperature.
Because the skin plays a major role in controlling body
temperature, as it will help in exchanging heat with the external
environment.
• There are two mechanisms to get red of the excess heat:
1. Hyperventilation ( Minimal effect in humans)
2. Vasodilatation of the vessels Flush ( Increase blood flow to the
skin) BP Real shock

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Classification of Shock
 Shock is classified according to the causes
to three classes:
 Hypovolemic shock
 Distributive shock
 Cardiogenic shock

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Causes of Shock
 Hypovolemic shock is caused by low blood volume.
 Normal blood volume is 5 L and by losing 1-2 L it can
lead to shock.
 The Decrease in blood volume is caused by:
 External blood loss: ex. Hemorrhage
 Internal blood loss: ex. Ruptured spleen caused by blunt trauma.
 Severe dehydration as a result of:
 Vomiting
 Diarrhea
This is a typical condition in cholera.
 Burns

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Causes of Shock
2. Distributive shock is caused by excess
vasodilatation (ex. Anaphylactic shock and
septicemia)
Vasodilatation Arteriole resistance increase
blood exchange from the vessels to the peripheral tissues
decrease blood return to the heart BP shock

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Causes of Shock
3. Cardiogenic shock ( heart does not pump enough
blood) is caused by:
A) Myocardial infarction weak cardiac
muscle contraction Ischemia
B) Arrhythmia ( such as ventricular fibrillation, which
will stop the heart pump and that will decrease BP
 Note: Supraventricular (Atria) fibrillation will not cause
shock because 75% of the blood transfer from the atrium to
the ventricles by passive transport.
Lead
to
As a
result of

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Causes of Shock
C) Valve problems, ex. Valvular stenosis which is
narrowing of the valves, or leakage of blood
through the valves ( Regurgitation).
D) Problems in the A-V shunt.

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Compensatory system
Blood Capillary
Venous
Blood
Hydrostatic Pressure =
30 mmHg
Hydrostatic Pressure =
20 mmHg
Colloid Osmotic Pressure= 25 mmHg
Arterial
Blood
Interstitial Fluid
Hydrostatic Pressure= 0 mmHg
NFP = +5 mmHg
NFP = - 5 mmHg
- At arterial end:
water moves out of the capillary with a NFP of +5 mmHg
Hydrostatic pressure dominates at the arterial end and net fluid flows out of the circulation.
- At venous end:
water moves into the capillary with a NFP of -5 mmHg
Oncotic pressure dominates at the venous end and net fluid will flow into the bloodstream
In the normal microcirculation

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 In shock, the hydrostatic pressure decreases and
the oncotic pressure is constant, as a result:
 The fluid exchange from the capillary to the
extracellular space decreases.
 The fluid return from the extracellular space to the
capillary increases.
That will increase the blood volume, which will increase
BP and will help to compensate shock situations.
This system is known as the “Fluid shift system”
The Compensatory system

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Blood Capillary
Venous
Blood
Hydrostatic Pressure = 20
mmHg
Hydrostatic Pressure = 10
mmHg
Colloid Osmotic Pressure= 25 mmHg
Arterial
Blood
Interstitial Fluid
Hydrostatic Pressure= 0 mmHg
Decrease outflow
Increase inflow
In shock situations