Cardiovascular and pulmonary responses to breath-hold diving (breath h
olding, submersion, and compression) were investigated. In addition, t
he effects of transitions between dry conditions and head-out immersio
n during eupnea were studied. Surface breath holds at a large lung vol
ume with relaxed respiratory muscles resulted in a positive esophageal
(transthoracic) pressure and a reduced cardiac output. In contrast, t
he esophageal pressure (relative to ambient pressure) was decreased, a
nd cardiac output was at least partially restored, when lung gas volum
e was reduced by compression during breath-hold diving. The increased
cardiac output that accompanied eupneic transition from dry to immerse
d conditions was associated with a shortlasting increase of alveolar g
as exchange, whereas the decreased cardiac output during immersion-to-
dry transition was associated with a long-lasting decrease of alveolar
gas exchange, both reflecting changes in the tissue gas stores of the
body. Surface breath holds were associated with a decreased O-2 uptak
e from the lung to the blood, and breath-hold dives were associated wi
th a large transient increase of O-2 uptake at depth which resulted in
a restoration of the time-averaged O-2 uptake to the eupneic control
level: these changes reflected changes in tissue O-2 stores. Compared
to surface breath holds, breath-hold dives were associated with larger
tissue retention of CO2 during breath holds, and prolonged recovery f
or CO2 elimination after breath holds. The distribution of pulmonary p
erfusion, as indicated by expirograms obtained immediately after breat
h holds, was made more homogeneous by submersion and the distribution
was further improved by compression during breath-hold dives. All of t
hese different effects on the gas exchange in breath-hold diving and i
n eupneic head-out immersion can to a large extent be explained by ass
ociated changes in cardiac output in combination with redistributions
of peripheral blood flow and venous blood volume. Thus, the different
components of breath-hold diving have profound cardiovascular and pulm
onary effects. Changes in the intrathoracic pressure and in the distri
bution of venous blood volume induce changes in cardiac output. All of
these changes affect the temporal and spatial distributions of pulmon
ary perfusion and peripheral blood flow. Also, the circulatory changes
affect the temporal and spatial distributions of alveolar gas exchang
e and of tissue gas stores of the body.