Short-term effects of normobaric hypoxia on the human spleen
Authors: Matt X. Richardson, Angelica Lodin, Jenny Reimers, Erika Schagatay
DOI / Source: 10.1007/s00421-007-0623-4
Date: 28 November 2007
Reading level: Beginner
Why This Matters for Freedivers
This paper supports a simple but important idea: low oxygen alone can trigger your spleen to squeeze out extra red blood cells, even without breath-holding. That helps explain why repeated apneas can “prime” you for later dives, and it hints that short hypoxic exposures (like some types of apnea training) may briefly boost oxygen-carrying capacity—but only for minutes, not days.
Synopsis
Freedivers often hear about the “spleen response”: do a few breath-holds, and your spleen can contract and release extra red blood cells into circulation, slightly increasing hemoglobin and hematocrit. That’s like a temporary mini “blood boost” that can help you tolerate low oxygen a bit better for the next few apneas. But here’s the big question this study asked: is it the breath-holding itself that triggers the spleen, or is it mainly the hypoxia (low oxygen) that happens during breath-holds?
To test this, the researchers removed breath-holding completely. Five healthy, fit volunteers lay resting and simply breathed low-oxygen air (normobaric hypoxia) through a mask for 20 minutes. The oxygen percentage was 12.8%, roughly simulating being at about 4100 m altitude, but at normal sea-level pressure. They measured spleen volume using ultrasound, and at the same time they took blood samples to measure hemoglobin (Hb) and hematocrit (Hct). They also recorded heart rate and oxygen saturation continuously.
The spleen responded quickly. Within just 3 minutes of starting the low-oxygen breathing, spleen volume had already dropped by about 13%. By 15–20 minutes of hypoxia, spleen volume was down by roughly 18%. At the same time, Hb and Hct rose by about 2%—small, but real—showing extra red blood cells had entered circulation.
As expected, oxygen saturation fell strongly during the hypoxic period, and heart rate increased. What’s interesting is how fast everything reversed: once they switched back to normal air, oxygen saturation recovered within a few minutes, and spleen volume plus Hb/Hct drifted back toward baseline within about 10 minutes.
The authors also estimated how much of the blood change could realistically come from the spleen itself, and their calculations suggested the spleen accounted for a large portion of the observed rise in hematocrit during hypoxia.
The takeaway is clean: hypoxia by itself is enough to trigger spleen contraction and a brief rise in circulating red blood cells, even when you’re breathing normally and not holding your breath. For freedivers, this strengthens the idea that the “priming” effect of repeated apneas is at least partly driven by oxygen drops, and it also suggests a possible role for the spleen as a rapid, short-term helper during sudden hypoxic situations (and early altitude exposure).
Abstract
Spleen contraction resulting in an increase in circulating erythrocytes has been shown to occur during apnea. This effect, however, has not previously been studied during normobaric hypoxia whilst breathing. After 20 min of horizontal rest and normoxic breathing, five subjects underwent 20-min of normobaric hypoxic breathing (12.8% oxygen) followed by 10 min of normoxic breathing. Ultrasound measurements of spleen volume and samples for venous hemoglobin concentration (Hb) and hematocrit (Hct) were taken simultaneously at short intervals from 20 min before until 10 min after the hypoxic period. Heart rate, arterial oxygen saturation (SaO2) and respiration rate were recorded continuously. During hypoxia, a reduction in SaO2 by 34% was accompanied by an 18% reduction in spleen volume and a 2.1% increase in both Hb and Hct. Heart rate increased 28% above baseline. Within 3 min after hypoxia SaO2 had returned to pre-hypoxic levels, and spleen volume, Hb and Hct had all returned to pre-hypoxic levels within 10 min. Respiratory rate remained stable throughout the protocol. This study of short-term exposure to eupneic normobaric hypoxia suggests that hypoxia plays a key role in triggering spleen contraction and subsequent release of stored erythrocytes in humans. This response could be beneficial during early altitude acclimatization.