Hemorrhagic shock: compensation, vicious cycles and irreversibility
- Mechanism
- Exam priority: medium
After bleeding, the body squeezes vessels and holds fluid. If too much is lost, falling flow damages the heart and vessels and shock feeds itself.
Also called: Progressive shock, Irreversible shock, Plasma-loss shock
What it is
Shock is inadequate tissue blood flow. After hemorrhage, reflexes and hormones compensate for losses up to about a fifth to a third of blood volume. Beyond that, low flow damages the heart, the vasomotor center, capillaries and cells, starting positive-feedback loops that worsen flow even without more bleeding. Late on, cells lose their high-energy phosphates and the shock becomes irreversible despite restored volume.
Key facts
- Head-down position helps venous return mainly in neurogenic shock; its benefit after hemorrhage is small and short.
- Oxygen has limited effect because the problem is transport (flow), not oxygenation of arterial blood.
- Glucocorticoids were proposed to stabilize cells and support vessels in shock; a clear benefit after hemorrhage has not been shown.
- Traumatic shock combines hemorrhage, plasma loss into damaged tissue and pain-related reflexes.
How it works, step by step
- Blood loss lowers mean systemic filling pressure and venous return, so cardiac output and arterial pressure fall.
- Baroreceptors, then chemoreceptors and CNS ischemia drive sympathetic activity: tachycardia, arteriolar and venous constriction.
- Angiotensin II, ADH and fluid absorption from the interstitium help over minutes to hours; thirst and renal retention over days.
- If diastolic pressure falls too low, coronary flow cannot meet demand: the heart weakens, output falls more (cardiac depression loop).
- Low brain flow eventually depresses the vasomotor center; sympathetic tone falls and vessels dilate (vasomotor failure).
- Slow flow lets blood sludge in small vessels; hypoxic capillaries leak protein and fluid, lowering volume further.
- Lactic acidosis, gut toxins and cell damage depress the heart and vessels: each loop lowers flow and strengthens the others.
- With ATP broken down to adenosine and lost from cells, even full volume replacement fails to restore function: irreversible shock.
Blood loss and response (healthy adult, illustrative)
| Parameter | Value | Note |
|---|---|---|
| Total blood volume | ≈ 7% of body weight | ≈ 5 L in a 70-kg adult |
| Loss well tolerated | ≈ 10% (≈ 500 mL) | e.g. blood donation |
| Loss with compensated shock | ≈ 15–30% | |
| Loss that is often lethal if rapid | ≈ 35–45% |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +Sympathetic reflexes: Can keep arterial pressure near normal after up to ≈ 30–40% loss in the first minutes; output falls earlier.
- +Angiotensin II and ADH: Constrict arterioles and retain water over minutes to hours.
- +Capillary fluid absorption: Lower capillary pressure draws interstitial fluid into the blood over hours.
- −Cardiac depression and vasomotor failure: Positive feedback that lowers output further once pressure is too low.
Clinical notes
- Postpartum hemorrhage and polytrauma are leading causes of hemorrhagic shock; early volume and bleeding control prevent the vicious cycles.
- Burns lose plasma through damaged capillaries; hematocrit rises and blood viscosity worsens flow.
- A normal blood pressure early after injury does not exclude major blood loss, because vasoconstriction holds pressure while output is low.
Test yourself
Think of the answer, then tap the question to see it.
Why does arterial pressure stay nearly normal early after a moderate hemorrhage while cardiac output has already fallen?
Sympathetic reflexes constrict arterioles and veins, raising resistance; pressure is held up while flow is still low.
Which three positive-feedback loops make shock progressive?
Cardiac depression from low coronary flow, vasomotor center failure, sludging of blood, capillary leak, acidosis and toxins from ischemic gut (any three).
What is the cellular basis of irreversible shock?
ATP is degraded to ADP, AMP and adenosine, which diffuses out of cells; rebuilding the high-energy phosphate pool takes hours, so cells fail even when flow returns.
How does shock from severe burns differ in the blood from hemorrhagic shock?
Burns lose plasma rather than whole blood, so hematocrit and viscosity rise, which slows flow further.
Connections
In Biochemistry2
In Pharmacology1
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: Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 2026); Hypovolemic shock: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Postpartum Hemorrhage (StatPearls (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.