The "Abdominal Circulatory Pump": An Auxiliary Heart during Exercise?

Area of Science:

• Cardiovascular Physiology
• Respiratory Physiology
• Exercise Physiology

Background:

• The diaphragm's primary role is ventilation, but it also influences circulation by altering abdominal pressure and venous return.
• Diaphragmatic contractions create cyclical pressure variations affecting splanchnic venous circulation.
• Abdominal muscles may enhance the diaphragm's circulatory function during physical activity.

Purpose of the Study:

• To investigate the circulatory role of the diaphragm and abdominal muscles during exercise.
• To quantify blood volume shifts between the trunk and extremities under different breathing conditions during exercise.
• To determine if respiratory muscle action influences blood distribution during leg exercise.

Main Methods:

• Eleven healthy subjects performed plantar flexion exercise at approximately 4 METs.
• Double body plethysmography measured simultaneous changes in total body volume and trunk volume.
• Blood volume shifts between trunk and extremities (Vbs) were calculated non-invasively; three breathing types (spontaneous, rib cage, abdominal) were analyzed.

Main Results:

• During spontaneous and rib cage breathing, blood was displaced from extremities to the trunk.
• Active abdominal expiration breathing displaced blood from the trunk to the extremities.
• Exercise significantly increased Vbs swings regardless of breathing type, indicating substantial bi-directional blood shifting.

Conclusions:

• Leg exercise induces significant bi-directional blood shifts between the trunk and extremities.
• The distribution of these blood shifts is strongly dependent on the interplay of diaphragm, rib cage, and abdominal muscle activity.
• The diaphragm and abdominal muscles may function as an 'auxiliary heart' during exercise by modulating blood distribution.