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The Physiology of Shock

Unit 8 / week 1

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 shock1. 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 DiarrheaThis 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 mmHgNFP = +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 mmHgHydrostatic Pressure =

10 mmHg

Colloid Osmotic Pressure= 25 mmHg

Arterial Blood

Interstitial Fluid

Hydrostatic Pressure= 0 mmHg

Decrease outflowIncrease inflow

In shock situations