A 32-year-old man is brought to the trauma bay following a motor vehicle collision with severe pelvic fracture and active hemorrhage. Regarding the physiological response in hemorrhagic shock, all of the following statements are correct EXCEPT:

Explanation

## Physiological Compensation in Hemorrhagic Shock ### Correct Mechanisms of Compensation **Key Point:** The body's response to hemorrhagic shock involves multiple overlapping neuroendocrine mechanisms designed to restore circulating volume and maintain perfusion pressure. | Mechanism | Effect | Mediator | | --- | --- | --- | | Baroreceptor reflex | ↑ Heart rate, ↑ Contractility, ↑ Vasoconstriction | Sympathetic nervous system | | Catecholamine release | ↑ SVR, ↑ Cardiac output | Epinephrine, norepinephrine | | Renin-angiotensin-aldosterone system (RAAS) | ↑ Vasoconstriction, ↑ Sodium/water reabsorption | Angiotensin II, aldosterone | | Antidiuretic hormone (ADH) | ↑ Water reabsorption | Vasopressin | ### Why the Incorrect Option Is Wrong **High-Yield:** When renal perfusion pressure drops in hemorrhagic shock, the juxtaglomerular apparatus detects this and releases **renin**. Renin converts angiotensinogen to angiotensin I, which is then converted to **angiotensin II**. Angiotensin II causes **vasoconstriction** (especially of efferent arterioles), NOT vasodilation of afferent arterioles. This maintains glomerular filtration pressure despite low systemic blood pressure. **Clinical Pearl:** The afferent arteriole vasodilation seen in some physiological states (e.g., prostaglandin-mediated) is the opposite of what occurs in hemorrhagic shock. In shock, angiotensin II preferentially constricts the efferent arteriole to preserve GFR — it does not dilate the afferent arteriole. ### Phases of Hemorrhagic Shock Response 1. **Immediate (seconds):** Baroreceptor reflex → sympathetic activation 2. **Early (minutes):** Catecholamine surge → ↑ HR, ↑ SVR, ↑ contractility 3. **Sustained (minutes to hours):** RAAS and ADH activation → fluid retention and vasoconstriction 4. **Late (hours):** Decompensation if volume not restored → cellular hypoxia, metabolic acidosis, organ dysfunction **Warning:** Confusion between afferent and efferent arteriole responses is a common trap. Remember: **angiotensin II constricts the efferent arteriole** to maintain GFR in low-flow states; it does not dilate the afferent arteriole.