Shock as a medical term is a sudden loss of blood flow. So your blood pressure drops, oxygen levels on the body/brain reduce and your heart rate picks up to try and compensate. Ultimately it will lead to unconsciousness and then death.
The sudden loss of blood flow can be from many things. Internal or external bleeding, such as trauma. So being hit by a car, or an artery or large vein being cut.
Internal organs can also rupture, either due to trauma or say an internal tumour rupturing.
Anaphylactic shock also causes shock, as fluid will come out of the blood and into your capillary beds. So plasma leaks out of your blood where it’s needed to help the blood flow, into your tissues. So you swell up like a balloon, rather than keeping all that lovely blood flowing in your system.
Severe full thickness burns also can do this, in a similar way to anaphylaxis. But plasma will leak out of the severe burns area.
There’s also heatstroke and dehydration. Both resulting in loss of fluids in the body, which in extreme cases causes loss of blood plasma volume.
That’s why people in shock are put on drips initially, to help raise the blood volume. Nasty thing shock!
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