Look at elephant seals for example. They store 24L of blood in their spleen. They do not need to do 'warm up' dives to contract their spleen. Their spleen contracts entirely, on their first dive.
Obviously their diving times are much longer than ours, allowing ample time for the spleen to contract. The elephant seals average diving time is about 20-25 min (longest dives 60-120 min). A human performing an apnea longer than perhaps 7-8 min might be able to elicit a maximal splenic contraction, but I guess that if your apneas are shorter than that, you need repeated apneas to see the maximal effect.
But I agree that the elephant seals are really astonishing divers. E.g., female elephant seals dive more or less continuously during 8 months between moult and next breeding season. On average, 89% of this 8-mo period is spent underwater. Although these seals do occasionally remain at the surface for an hour or two, in a study in which the seals were wearing time/depth-recorders, the maximum surface interval recorded for one female was only 5 min during 40 days at sea!
It follows that the idea of the spleen functioning as a scuba tank in large marine mammals such as elephant seals has to be reevaluated in the light of the above-mentioned diving behavior. Most marine mammals do not spend long periods at the surface between dives. In both northern and southern elephant seals, surface periods longer than 3 min are unusual, even when the animal is diving continuously for 24 h or more. The spleen/red blood cells need more than 10 min after a diving bout before the red blood cell concentration returns to resting levels. Hence, the red blood cell concentration will rise on the first dive of a bout and, in the case of elephant seals, could remain elevated for months. Thus the idea of the spleen serving as a scuba tank may be true for the first dive of a bout, but for the successive dives, the red blood cells are already in the circulating blood.
Spleen contraction might explain why the 2nd apnea is longer than the first. But it will not explain why the 5th is longer than the 4th, since every study I read showed that the spleen did not contract anymore after the 3rd apnea.
Yes, that is true. I can perhaps explain this (again referring to our studies mainly on non-divers, i.e. shorter apnea times than many trained freedivers).
The spleen size is getting smaller over apnea 1 to 3 in a series, coinciding with an increase in red blood cell concentration/hematocrit (but apnea 5 in a series has the same spleen volume and hematocrit as observed in apnea 3). Thus, the spleen contracts and ejects red blood cells during apneas 1-3. This response has reached its maximum in the third apnea.
At the same time the "physiological breaking point" (the time of the first involuntary breathing movement) is delayed over apnea 1-3, leading to a longer breath-holding time (short-term training effect). The increases in hematocrit and time to the physiological breaking were absent in spelectomized individuals, lending support to the notion that splenic contraction is important in explaining the delay in the physiologic breaking point in apneas 2-5 compared to the first apnea in a series.
In our studies, the physiological breaking point occurred at more or less the same time in apnea 3 as in apnea 5. No further splenic contraction in apnea 4-5, no further delay of the physiologic breaking point.
Nevertheless, the breath-holding time increased from apnea 3 to apnea 5. This was because the time with involuntary breathing movements was increased in apneas 4 and 5 (compared to apnea 3). Traditionally the time from the physiologic breaking point to the end of an apnea is called the "struggle phase", and is often referred to as being determined by psychological factors. This was the period increasing in length during apnea 4-5 explaining the continued short-term training effect even after the splenic contraction had reached its maximum.
The time from the start of the apnea to the physiological breaking point is called the "easy-going phase", determined mainly by the arterial PCO2 reaching a critical level triggering respiratory movements that can be voluntarily resisted during the struggle phase. So putting the above long reasoning in a simplistic way, mainly physiological factors (splenic contraction) contribute to the lengthening of the easy-going phase over apneas 1-3, and mainly psychological factors (better tolerance to the urge to breathe) contribute to the lengthening of the struggle phase during apneas 4-5.
Eric, I hope that my line of reasoning was possible to follow...
/Johan