In humans, oxygen is effectively delivered to the tissues because of the presence of several allosteric modulators. Name three of these modulators and explain how their presence allows oxygen to be delivered to the tissues. Then explain why having higher concentrations of Mb would allow whales and seals to stay submerged under water for a long time. Also, if bicarbonate serves as an allosteric modulator of hemoglobin binding in crocodiles, what is the source of HCO3- in the crocodile tissues?