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[Physiology](https://durgical.com/en/physiology/) · [Cardiovascular](https://durgical.com/en/physiology/#s-cardio)

# 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](https://durgical.com/en/anatomy/left-ventricle/) · [Pericardium](https://durgical.com/en/anatomy/pericardium/) · [Inferior vena cava](https://durgical.com/en/anatomy/inferior-vena-cava/)

## Key points

- Hypovolemic and cardiogenic: low CO, high SVR, cold clammy skin. Hypovolemic has low filling pressures; cardiogenic has high ones.
- Distributive (sepsis, anaphylaxis, neurogenic): low SVR; CO often normal or high early; warm skin. Neurogenic: bradycardia instead of tachycardia.
- Obstructive (tamponade, tension pneumothorax, massive pulmonary embolism): low CO, high venous pressure, distended neck veins.
- Lactate rises as cells shift to anaerobic glycolysis; a high lactate marks tissue hypoperfusion.
- Compensation: baroreflex sympathetic surge, RAAS, ADH, fluid shift from interstitium into capillaries (lower capillary pressure).

## 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 mL **ESV** 127 mL **SV** 53 mL **EF** 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 | 2 mmol/L abnormal | ≥ 4 mmol/L marks severe hypoperfusion |
| Shock index (HR / SBP) | > 0.9–1 suggests shock | |
| Urine output |

| Type | Cardiac output | SVR | Filling pressure (CVP / wedge) |
| --- | --- | --- | --- |
| [Hypovolemic](https://durgical.com/en/conditions/hypovolaemic-shock/) | ↓ | ↑ | ↓ |
| [Cardiogenic](https://durgical.com/en/conditions/myocardial-infarction/) | ↓ | ↑ | ↑ |
| [Obstructive (tamponade)](https://durgical.com/en/conditions/cardiac-tamponade/) | ↓ | ↑ | ↑ (equalized) |
| [Septic (early)](https://durgical.com/en/conditions/sepsis-and-septic-shock/) | ↑ or normal | ↓ | ↓ or normal |
| [Anaphylactic](https://durgical.com/en/conditions/anaphylaxis/) | ↓ 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.

## Memory aid

Shock has four doors: the tank is empty (hypovolemic), the pump is broken (cardiogenic), the pipe is kinked (obstructive), or the pipes are too wide (distributive).

## 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.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/physiology/shock-physiology/)

## See it in 3D

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

- [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)
- [Pericardium](https://durgical.com/en/anatomy/pericardium/)
- [Inferior vena cava](https://durgical.com/en/anatomy/inferior-vena-cava/)

## Related topics

- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [The baroreceptor reflex](https://durgical.com/en/physiology/the-baroreceptor-reflex/)
- [Venous return and the Guyton curves](https://durgical.com/en/physiology/venous-return-and-the-guyton-curves/)
- [Starling forces at the capillary](https://durgical.com/en/physiology/starling-forces-at-the-capillary/)
- [Nervous control of the circulation and the CNS ischemic response](https://durgical.com/en/physiology/nervous-control-of-the-circulation-and-the-cns-ischemic-response/)

## Related conditions

- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Anaphylaxis](https://durgical.com/en/conditions/anaphylaxis/)
- [Cardiac tamponade](https://durgical.com/en/conditions/cardiac-tamponade/)
- [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/)
- [Dengue](https://durgical.com/en/conditions/dengue/)

[← All of physiology](https://durgical.com/en/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](https://openstax.org/books/anatomy-and-physiology-2e/pages/20-4-homeostatic-regulation-of-the-vascular-system) (OpenStax (Rice University), 2022); [Hypovolemia and Hypovolemic Shock](https://www.ncbi.nlm.nih.gov/books/NBK513297/) (StatPearls (NCBI Bookshelf), 2025); [Bacterial Sepsis](https://www.ncbi.nlm.nih.gov/books/NBK537054/) (StatPearls (NCBI Bookshelf), 2025); [Anaphylaxis](https://www.ncbi.nlm.nih.gov/books/NBK482124/) (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.

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Source: https://durgical.com/en/physiology/shock-physiology/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/shock-physiology/