Disclaimer: this discussion will not make you a faster cyclist. It might even get you kicked out group rides if you discuss it. That said, exercise physiology is cool and some people might find it of interest.
I came across a paper discussion the splenic changes during acute sub-maximal (60% VO2 max) and maximal exercise (graded ramp to exhaustion). The spleen helps to remove aging RBCs out of systemic circulation, but also serves as a small blood cell “bank”. I failed to recognize the relatively large increase in circulating RBCs (along with platelets and WBCs) released from the spleen during acute exercise. Splenic volume decreases by ~ 60% during exercise and recovers fairly rapidly during recovery (within 20 minutes). The relative contribution to circulating RBCs is variable based on the estimate of the spleen volume with ranges from 1.5% to 10% increase in RBC count during exercise, but even 1.5% could be associated with an increase in exercise performance. The increase in RBC count partially contributes to the increase in hematocrit with exercise (along with decreased plasma volume). Disappointing to me, aerobically trained athletes did not have larger spleens as this would have been an interesting training adaptation. That said, no one has studied the inter-individual variability in spleen mass on exercise performance. Fun fact, some animals such as seals and horses store ~50% of their RBCs in the spleen during inactivity. Running performance is decreased in horses post-splenectomy. For any individuals with extra time that are interested in looking at this older article, it can be found here: https://www.ncbi.nlm.nih.gov/pubmed/12482773