This study clearly shows that the spleen acts as a dynamic red blood cell reservoir at high altitude. Recruitment of part of the reservoir occurs already during rest at high altitude, and during exercise, the spleen has a capacity to expel a supplementary volume of red blood cells into circulation, to cope with the aggravated hypoxia. It is evident from the transient elevation of Hb that spleen contraction has an effect on circulating erythrocyte volume. The close association between Hb and spleen volume supports this conclusion. This is in line with previous observations of simultaneous spleen contraction and Hb elevation during apnea (Baković et al. 2005) and during simulated altitude by eupneic hypoxia during rest (Richardson et al. 2008), but it has never been observed at high altitude.
This is the first clear evidence from a field observation that the human spleen has an important role in adjusting circulating Hb to fit the demands to varying degrees of hypoxia at high altitude. In this way, the un-acclimatized climber will have a means to cope with hypoxia much earlier than if only the Hb elevation caused by enhanced erythropoiesis would be available, which takes at least a week to develop (West 2012). The main functional effect would be to enhance the capacity for blood oxygen storage and transportation (Schagatay et al 2001), with known benefits at high altitude (West 2012).
The maximal capacity for the spleen to contract seems to be reached already during exercise at 3700 m, when the smallest volume of 166 mL is reached, and no further reduction is seen with exercise at the higher altitude 4200 m, thus this volume may be the smallest possible volume for the spleen to reach when “empty”. The largest volume observed, 273 mL, suggests that an average volume of about 100 mL of concentrated blood can be expelled from the spleen, and with a Hb about double that of the circulating blood (Laub et al., 1993; Stewart and McKenzie 2002) this would add the equivalent of the oxygen carrying capacity of an additional 200 mL of circulating blood. Based on an estimated blood volume in our group (Nadler et al. 1962), that would lead to an addition of approximately 4 g/L of circulating Hb. However, the maximum increase of Hb observed was about the double. There have been similar observations in other studies, e.g., by Richardson et al (2008) which concluded that the spleen contributed to 60% of the observed increase in hematocrit during hypoxic exposure. This suggests that other stagnant pools of erythrocytes may also be recruited, or that there was a shift in plasma volume induced, or possibly that the timing of the spleen volume and blood sampling was not synchronized with the minimum and maximum values, respectively.
The only previous observation of similar changes of this dynamic elevation of Hb with altitude was in a small study by Richardson et al. (2007) where it was discovered that the transient elevation in Hb during apnea at 1230 m was attenuated at 3840 m and eventually disappeared at 5100 m. This attenuation of the Hb response when individuals traveled to progressively higher altitudes suggests that the spleen may eventually reach its maximally contracted state during rest at high altitude, which means it cannot produce any further Hb elevation despite an additional hypoxic stimulus (Richardson et al. 2007). On the descent from altitude, the Hb responses during apnea returned, and were greater in magnitude than that on the same altitude during the ascent, suggesting that acclimatization had abolished the need of a tonic contraction during rest (Richardson et al. 2007). Thus, the tonic spleen contraction during rest on ascent may be a means to cope with the chronic hypoxia before acclimatization leads to elevated Hb and makes other protective mechanisms available. After this initial stage, only the severe hypoxia related to exercise will lead to blood-boosting for transiently increasing the oxygen carrying capacity, a response that would likely be very beneficial for fine-tuning oxygen delivery across a longer exposure to high altitude hypoxia when the oxygen transport system is under stress.
An enhancement of spleen function was found to be part of the acclimatization to hypoxia in rats (Kuwahira et al. 1999). Earlier studies on climbers found enhanced spleen contraction during exercise after long-term high-altitude exposure during an expedition summiting Mount Everest (Engan et al. 2014). In another group, trekking to less extreme altitude, it was observed that both spleen volume and contraction during exercise were enhanced on return to low altitude (Rodríguez-Zamora et al. 2015). The difference between the results in these two similar studies could possibly be caused by a general catabolism in the “Death Zone” above 8000 m, not allowing spleen enlargement. It was also recently observed that top climbers had larger spleens and spleen contraction during apnea, compared to recreational high altitude trekkers (Schagatay et al. 2020). Taken together these studies suggest that also the spleen itself may be subject to altitude acclimatization, which is in line with the current study’s observations of a significant spleen function in elevating Hb at altitude. Thus, the spleen may have two distinctive effects at high altitude: (1) to cope with the acute hypoxia in the un-acclimatized lowlander, before erythropoiesis has elevated Hb sufficiently to counteract it, and (2) a long-term effect increasing spleen volume and contractility with altitude exposure, as part of the acclimatization process, so that a larger dynamic red cell volume can be recruited by spleen contraction during exercise and stored away in the spleen during rest, to reduce blood viscosity.
Both our present study and the results cited above seem to be conflicting with a study by Sonmez et al (2007), in which it was reported that long-term exposure to high altitude in lowlanders resulted in reduced splenic volume and increased Hb across 6 months. In that time, substantial altitude acclimatization has elevated Hb via increased erythropoiesis, and it is not clear how this relates to the relatively short-term adjustments seen in the current study.
Our present results do not support the conclusions from the recent study by Purdy et al (2018) that the spleen does not mobilize erythrocytes during ascent to high altitude. In that study, baseline spleen volume was not found to differ between the altitudes 1045 m, 3440 m, and 4240 m, neither did the spleen contract as a stress response to handgrip exercise at high altitude (Purdy et al. 2018). In the same study, spleen contraction was seen at all altitudes on injection of phenylephrine hydrochloride, and the authors concluded that the spleen does not contribute to acclimatization to high-altitude hypoxia, due to alterations in spleen reactivity to increased sympathetic activation at altitude (Purdy et al. 2018). The spleen is mainly innervated by sympathetic nerves with both α- and β-adrenoceptors (Ayers et al. 1972) and spleen contraction can probably be induced by both neural input and catecholamine release (Stewart and McKenzie 2002). While resting spleen volumes observed by Purdy et al (2018) at 1045 m (273 mL), were similar to what we observed at 1370 m (252 mL), their handgrip exercise at 1045 m only caused a spleen contraction of 8%, while in our study, the step test caused a 21% spleen contraction. Likewise, the Hct in their study increased from baseline by 4%, while in our study, by 6%. This suggests that the handgrip exercise is not as powerful a stimulus as the exercise with large muscle groups causing aggravated hypoxia during step test in our study. Furthermore, in the study by Purdy et al (2018), the shrinkage of the spleen observed after phenylephrine infusion could be caused by eliciting spleen vasoconstriction, thus by a different mechanism than the physiological neural stimulation. There could also be other vascular responses from injection of the phenylephrine bolus, e.g., overstimulation of the baroreceptors, which could have blunted the endogenous sympathetic activity. Therefore, we consider the study by Purdy et al. (2018) inconclusive of whether spleen contraction occurs during exercise at high altitude. Their lack of effect of altitude on resting spleen volume remains contrasting to our observations. Purdy et al (2018) thus suggested that the constant spleen volume at different altitudes is a result of a decreased reactivity to sympathetic activity but while there may be a long-term downregulation of beta-adrenoreceptors with chronic hypoxia (Richalet et al 1988), this may not likely have developed in these relatively un-acclimatized subjects. While both the studies by Richardson et al. (2007) and Purdy et al (2018) may suggest that splenic reactivity was reduced at high altitude, the present study involving whole-body exercise induced a sufficient stressor to manifest as a powerful response also at high altitude. However, we suggest that the spleen response will likely reach a “roof” due to its anatomical properties, which appears to be the case at the highest altitudes.
The input responsible for spleen contraction in our study is most likely the hypoxia during rest and the aggravated hypoxia during whole-body exercise, in accordance with earlier findings (Richardson et al. 2008). Hypercapnia has also been found to trigger spleen contraction during apnea (Richardson et al. 2012) but would not likely be involved at high altitude due to the increased ventilation. In our study, the RR increased with increasing altitude and baseline ETCO2 was reduced, with further changes with exercise. Exercise in itself may have contributed as it has been found to directly induce spleen contraction (Laub et al. 1993; Stewart and McKenzie 2002).
An important role of the spleen as a dynamic red cell reservoir for predicting performance in apneic divers has been reported in several studies (Schagatay et al 2001; Bakovic et al. 2003; Schagatay et al 2012, Ilardo et al 2018) and we interpret from our current results that it could have the same role in humans exposed to high altitude. We recently reported a negative correlation between individual spleen volume during rest in Kathmandu at 1370 m and the incidence of symptoms of acute mountain sickness (AMS) in trekkers going to Mt Everest Base Camp (P < 0.05; Holmström et al. 2019). However, in that study, the spleen contraction during apnea at 1370 m was not significantly associated with AMS symptoms (P = 0.121; Holmström et al. 2019). It was however recently found that spleen baseline volume was greater in experienced mountaineers going to climb Mt Everest, than in Mt Everest Base Camp trekkers (Schagatay et al. 2020), suggesting that spleen function is important for successful climbing at high altitude. It was also recently reported that larger spleens and more powerful contractions were present in the Sherpa population of high altitude origin, compared to Nepali lowlanders (Holmström et al. 2020). A difference between Sherpas currently residing high and low, respectively, was also observed, suggesting that not only genetic factors but also exposure to hypoxia may determine spleen size (Holmström et al. 2020).
As an important step to understand spleen function at high altitude better, the current study shows that the spleen was more contracted during rest with higher altitudes, which was reflected by a progressively higher baseline Hb. We, therefore, speculate that spleen contraction during rest could be responsible for at least part of the transient early elevation of Hb described previously, which has been attributed to a reduced plasma concentration due either to dehydration or hormonal changes at high altitude (reviewed by West 2012). We thus interpret our result as showing an alternative explanation for the phenomenon of early elevation of Hb in newcomers to altitude, at least in strong responders. The obvious benefit of the observed mechanism would be to fine-tune circulating Hb between the oxygen demands and the aim to reduce viscosity. Thus, Hb can be optimized between—on the one hand—enhanced oxygen demands with the more severe hypoxia with exercise and—on the other—that part of the red cell supply is stored in the spleen between bouts of exercise to reduce viscosity and thereby limit the strain on the cardiovascular system related to the polycythemia. This spleen-derived regulation of circulating Hb to meet the short-term demands could possibly allow more efficient use of the limited resources during the hypoxic stress at high altitude. This original finding suggests a possible mechanism whereby a large and contractile spleen could enhance high-altitude performance which should be further studied.
We suggest that the observed transient Hb elevation with spleen contraction during work at high altitude could help explain the association between individual spleen volume and AMS symptoms, as reported by Holmström et al (2019), although with our limited LLQ data we could not support this association. The correlation between LLQ score and SaO2 during exercise in our study, despite a limited sample, supports that people with high SaO2 may likely suffer less from AMS, in accord with several other studies (West 2012).
By necessity, a field study like this has restrictions on which laboratory equipment can be used, and has to rely on smaller monitors and often simpler methods than in a stationary laboratory, as all equipment has to be carried on the back to location, and the laboratory has to be rapidly mounted before tests can begin at every location. The data from the equipment in this study have, however, been compared to results obtained with more advanced monitors in stationary laboratories at high altitude (e.g., Holmström et al 2019) and been found to be in agreement.
Full measurements of minute ventilation and cardiac output would have been valuable. Measuring RR is not sufficient to determine ventilation, just as HR is not enough to determine cardiac output. However, these variables can give an indication of the effects of the regulatory influence of combined hypoxia and hypocapnia during exercise at high altitude. The ETCO2 was recorded in %, while measurements using mmHg would have been more useful with respect to determining effects on cerebral blood flow.
We recruited 12 subjects from a trekking group consisting of 14 lowlanders, but drop-out is common in straining field conditions. After 1 of the subjects did not complete all tests, the number of subjects was 11, which was just sufficient according to our power calculations prior to the expedition, and only 8 subjects filled out the LLQ self-assessment form. We believe that this limited sample does not allow negative conclusion on the correlation between spleen volume and physiological variables or LLQ. The small group studied is a limitation of this study.