One hypothesis is that blood pressure is vital and defining in the storing or releasing of oxygen to/from the spleen.
en.wikipedia.org Specifically, the spleen forms within, and from, the dorsal mesentery.
en.wikipedia.org While telling the team this, they notice bleeding from his eyes and hands, blood in his urine, and an enlarged spleen.
en.wikipedia.org Signs and symptoms include fever, weight loss, fatigue, anemia, and substantial swelling of the liver and spleen.
en.wikipedia.org The serial analyses of -thalassemic mice indicate hemoglobin levels decreases over time, while the concentration of iron in the liver, spleen, and kidneys markedly increases.
en.wikipedia.org