Spleen contraction elevates hemoglobin concentration at high altitude during rest and exercise

Abstract

Purpose Hypoxia and exercise are known to separately trigger spleen contraction, leading to release of stored erythrocytes. We studied spleen volume and hemoglobin concentration (Hb) during rest and exercise at three altitudes. Methods Eleven healthy lowlanders did a 5-min modified Harvard step test at 1370, 3700 and 4200 m altitude. Spleen volume was measured via ultrasonic imaging and capillary Hb with Hemocue during rest and after the step test, and arterial oxygen saturation (SaO2), heart rate (HR), expiratory CO2 (ETCO2) and respiratory rate (RR) across the test. Results Resting spleen volume was reduced with increasing altitude and further reduced with exercise at all altitudes. Mean (SE) baseline spleen volume at 1370 m was 252 (20) mL and after exercise, it was 199 (15) mL (P < 0.01). At 3700 m, baseline spleen volume was 231 (22) mL and after exercise 166 (12) mL (P < 0.05). At 4200 m baseline volume was 210 (23) mL and after exercise 172 (20) mL (P < 0.05). After 10 min, spleen volume increased to baseline at all altitudes (NS). Baseline Hb increased with altitude from 138.9 (6.1) g/L at 1370 m, to 141.2 (4.1) at 3700 m and 152.4 (4.0) at 4200 m (P < 0.01). At all altitudes Hb increased from baseline during exercise to 146.8 (5.7) g/L at 1370 m, 150.4 (3.8) g/L at 3700 m and 157.3 (3.8) g/L at 4200 m (all P < 0.05 from baseline). Hb had returned to baseline after 10 min rest at all altitudes (NS). The spleen-derived Hb elevation during exercise was smaller at 4200 m compared to 3700 m (P < 0.05). Cardiorespiratory variables were also affected by altitude during both rest and exercise. Conclusions The spleen contracts and mobilizes stored red blood cells during rest at high altitude and contracts further during exercise, to increase oxygen delivery to tissues during acute hypoxia. The attenuated Hb response to exercise at the highest altitude is likely due to the greater recruitment of the spleen reserve during rest, and that maximal spleen contraction is reached with exercise.

Splenic contraction is enhanced by exercise at simulated high altitude

“…Thus, the spleen responded in a step-wise manner to the progressively increased and decreased stimuli, which demonstrates that the spleen has the ability to fine tune the amount of circulating red cells in the short term to optimize between metabolic needs and cardiovascular load (sheer stress). This is the first laboratory study confirming earlier long-term field observations suggesting that, when non-acclimatized lowlanders are exposed to HA, a partial tonic splenic contraction is induced on which additional stimuli may induce yet a more powerful response (Richardson and Schagatay 2007 ; Holmström et al 2020b ; Schagatay et al 2020b ). The present study shows such a fine-tuned graded response may also be induced in the short term.…”

“…This was unexpected as previous research has displayed a 2.1% Hb elevation resulting from splenic contraction in normobaric hypoxia equivalent to 4100 m (F i O 2 12.8%) ascribed to hypoxia-induced splenic contraction (Richardson et al 2008 ) but as our simulated altitude of 3500 m was lower, this could have resulted in a too small drop in SpO 2 . In addition, in a previous study, baseline spleen contraction and Hb elevation during rest was small and the main effect was seen with exercise (Schagatay et al 2020a , b ). Another possible cause of the minor Hb effect, despite a significant spleen contraction is that we used the same time point for spleen measurements and blood samples, but there could be a circulatory delay of the Hb effect.…”

“…While splenic contraction occurs during exercise (Laub et al 1993 ; Stewart et al 2003 ), apnea (Schagatay et al 2005 ), normobaric hypoxia (Richardson et al 2008 ; Lodin-Sundström and Schagatay 2010 ) hypobaric hypoxia (Holmström et al 2020b ; Schagatay et al 2020b ) and during handgrip exercise (Frances et al 2008 ), the mechanisms behind human splenic contraction have not been fully clarified. Nevertheless, hypoxia appears to be a major factor inducing the response (Richardson et al 2008 ; Lodin-Sundström and Schagatay 2010 ).…”

“…Baseline spleen volume has also been found to decrease in size during incremental ascent at HA (Schagatay et al 2020b ; Holmström et al 2020b ), attesting to a tonic splenic contraction during ascent, with a further volume reduction in response to exercise (Schagatay et al 2020b ) or apnea-induced acute hypoxia (Holmström et al 2020b ). Collectively, these two field studies demonstrated a potentially important role of the splenic contraction at HA.…”

Sustained splenic contraction after daily cocaine administration in rats

“…Since the spleen is a relatively large organ and extremely rich in RBC, it is considered to act as a reservoir of RBCs [ 21 , 22 ]. During oxygen deficiency caused by diving [ 23 ], exercise [ 24 ], high altitude [ 25 , 26 ], or sleep apnea [ 27 ], the spleen contracts to release RBCs into the central as well as peripheral circulation for oxygenation [ 22 ]. This RBC supply through splenic contraction is considered to help cardiac output [ 21 ].…”