Shock physiology !

Mechanism
Exam priority: high

Shock is when the circulation cannot deliver enough oxygen to the organs; cells switch to inefficient energy and start to fail.

Updated 2026-11-01

Also called: Circulatory shock, Hypovolemic, cardiogenic, distributive, obstructive

See it in 3D: Left ventricle · Pericardium · Inferior vena cava

What it is

Shock is generalized inadequate tissue perfusion relative to oxygen demand. It is grouped by cause: hypovolemic (lost volume), cardiogenic (pump failure), obstructive (blocked filling or outflow) and distributive (vasodilation and leak, as in sepsis or anaphylaxis). Compensation first protects the brain and heart; if perfusion is not restored, a vicious cycle makes shock irreversible.

How it works, step by step

A primary problem lowers cardiac output or vascular resistance, and arterial pressure falls.
Baroreceptors trigger sympathetic activation: tachycardia, vasoconstriction of skin, gut and kidney, venoconstriction.
RAAS and ADH retain Na⁺ and water; low capillary pressure draws interstitial fluid into the blood.
If perfusion stays low, cells switch to anaerobic glycolysis: lactate and metabolic acidosis.
Acidosis and ischemia depress the heart and relax vascular smooth muscle; capillaries leak.
Low coronary perfusion weakens the heart further: a positive feedback loop toward irreversible shock and multi-organ failure.

Left-ventricular pressure–volume loop

EDV 180 mLESV 127 mLSV 53 mLEF 30%ESP 85 mmHg

HF with reduced EF

Heart failure with reduced EF: a flat ESPVR (↓ contractility) and a dilated LV: high EDV and ESV, low SV and EF, and a raised end-diastolic pressure.

Tap a corner of the loop (MC, AO, AC, MO) to see the valve event.

Current loop
Normal loop
ESPVR (contractility, Ees)
EDPVR (passive filling)
Ea (arterial afterload)
Stroke volume (SV)

Cardiogenic shock in a loop: compare with lowering preload to see the hypovolemic pattern.

Markers often used

Parameter	Value	Note
Mean arterial pressure	< 65 mmHg suggests hypoperfusion
Serum lactate	> 2 mmol/L abnormal	≥ 4 mmol/L marks severe hypoperfusion
Shock index (HR / SBP)	> 0.9–1 suggests shock
Urine output	< 0.5 mL per kg per hour is oliguria

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Hemodynamic patterns of shock

Type	Cardiac output	SVR	Filling pressure (CVP / wedge)
Hypovolemic	↓	↑	↓
Cardiogenic	↓	↑	↑
Obstructive (tamponade)	↓	↑	↑ (equalized)
Septic (early)	↑ or normal	↓	↓ or normal
Anaphylactic	↓ or normal	↓	↓
Neurogenic	↓	↓	↓

Clinical notes

Classes of hemorrhage: blood pressure may stay normal until about 30% of blood volume is lost; tachycardia and narrow pulse pressure appear first.
In Colombia, severe dengue causes shock by plasma leakage: rising hematocrit with falling platelets and a narrowing pulse pressure.

Test yourself

Think of the answer, then tap the question to see it.

A patient is hypotensive with warm skin, bounding pulses and fever. Which type of shock, and what are CO and SVR?

Early septic (distributive) shock: low systemic vascular resistance with normal or high cardiac output.

How can you tell cardiogenic from hypovolemic shock using filling pressures?

Both have low output and high SVR; cardiogenic has high CVP and pulmonary wedge pressure (congested lungs), hypovolemic has low filling pressures (flat neck veins).

Why does lactate rise in shock?

Low O₂ delivery forces cells to rely on anaerobic glycolysis, which regenerates NAD⁺ by turning pyruvate into lactate; the liver also clears less lactate.

Why is neurogenic shock associated with bradycardia rather than tachycardia?

A high spinal cord injury interrupts sympathetic outflow, leaving vagal tone unopposed: vasodilation with a slow heart rate.

How does progressive shock become self-sustaining?

Low pressure reduces coronary flow and acidosis depresses the heart, which lowers output further; vascular tone fails and capillaries leak, closing a positive feedback loop.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 20.4 Homeostatic Regulation of the Vascular System (OpenStax (Rice University), 2022); Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 2025); Bacterial Sepsis (StatPearls (NCBI Bookshelf), 2025); Anaphylaxis (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 24). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.