This thesis deals with cardiovascular and hematological responses to voluntary apnea in

humans, with a special focus on O2 usage and storage. Humans, and many other air‐breathing

animals, respond to apnea (breath holding) with a collection of interacting cardiovascular

reflexes, which are collectively called the diving response. In humans, the main characteristics of

the diving response are a reduction in heart rate (bradycardia), decreased cardiac output,

peripheral vasoconstriction and increased arterial blood pressure. Another response during

apnea in mammals, more recently also observed in man, is a transient increase in hemoglobin

concentration across a series of apneas, probably due a reduction in spleen size. There may also

be long‐term effects on erythropoiesis in the apneic diver, as suggested by high hemoglobin

levels observed in divers. The focus of the included studies are the short transient diving

response (I), the more slowly occurring transient hematological changes to apnea, most likely

related to a reduction in spleen size (II), and the possible effects of repeated apnea on serum

erythropoietin concentration (III).

I) The aim was to study the effects of body immersion on the O2‐conserving effect of the

human diving response. The results showed that, regardless of body immersion, apnea with face

immersion causes a stronger cardiovascular diving response compared to during apnea alone,

leading to a smaller reduction in arterial oxygen saturation levels. Thus the diving response is

triggered and conserves O2 even during whole‐body immersion, which has previously only been

observed during apnea without whole‐body immersion.

II) The aim was to study hematological responses to voluntary repeated maximal‐duration

apneas in divers and non‐divers. Increases in hemoglobin concentration were found across a

series of 3 apneas in elite breath‐hold divers, elite cross‐country skiers and untrained subjects.

However a larger increase in hemoglobin was found in divers compared to non‐divers, which

suggests a possible training effect of their extensive apnea‐specific training. In contrast, physical

endurance training does not appear to affect the hematological response to apnea.

III) The aim was to study the effects of serial voluntary apnea on the serum erythropoietin

concentration. In a comparison between elite breath‐hold divers and subjects untrained in apnea,

divers were found to have a 5% higher resting hemoglobin concentration. An average maximum

increase in erythropoietin of 24 % was found in untrained subjects after 15 maximal duration

apneas, preceded by 1 min of hyperventilation. This suggests a possible erythropoietic effect of

apnea‐induced hypoxia, which may connect the increased resting hemoglobin found in divers to

their apnea‐specific training.

It was concluded from these studies that man responds to apnea with a series of different

adjustments in order to limit O2 usage and increase O2 storage: The classical diving response is

effectively restricting O2‐consumption also during full immersion, the spleen related hemoglobin

increase occurs in both divers and non‐divers with different levels of physiological training, but

is more prominent in divers, and finally, the observed high levels of hemoglobin concentration in

divers may be related to enhanced erythropoiesis during dive training.