Gravity and Physiological Function
Gravity creates gradients in the lungs that influence both ventilation and perfusion. In an upright lung, gravity causes more blood flow (perfusion) at the lung bases compared to the apex due to hydrostatic pressure. |
Ventilation also increases from apex to base but less steeply than perfusion. Thus, the ventilation-perfusion ratio (VA/Q) is higher at the apex (more ventilation relative to perfusion) and lower at the base (more perfusion relative to ventilation). |
This uneven distribution of VA and Q affects gas exchange efficiency and arterial blood gases: PaO2 tends to be slightly lower at the base of the lungs due to lower VA/Q ratios, while PaCO2 is correspondingly higher. |
Oxygen (O2) content and carbon dioxide (CO2) content of the blood depend on both the amount of gas exchange occurring and the matching of ventilation and perfusion, both influenced by gravitational effects on lung zones. |
pH can be affected as CO2 retention in poorly ventilated areas leads to respiratory acidosis, while well-ventilated regions support normal pH. |
The flow of oxygen into the lungs and CO2 out of the lungs depends on adequate ventilation and perfusion matching influenced by body position and gravity. |
Effect on positioning on bronchiolar lumen
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Physical therapy implications
Gravity is the principal contributor to inhomogeneity of physiological function down the lungs. |
Body positioning significantly affects cardiovascular and pulmonary function by influencing oxygen transport, blood flow, lung expansion, and ventilation-perfusion matching. |
Gravity influences blood flow distribution and lung ventilation depending on body posture. |
Upright and moving positions promote optimal lung expansion and oxygenation by facilitating better ventilation and perfusion matching. |
The upright position can be more energetically demanding than supine positions, which are more demanding than lateral positions. Compression forces also affect the heart and cardiac output. |
Figure 1
The effect of gravity on ventilation and perfusion from the apex to the base of the lung. The net effect on the ventilation-perfusion ratio is shown by the curve
Effects of Positions on Lung Function
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Effect of arousal on cerebral activity
The more upright the patient, the greater the neurological arousal and the greater
the stimulus to breathe. Commensurate with an increase in arousal, the patient is stimulated to take deeper breaths and, hence, to increase VA . When body positioning is coupled with mobilization, vasodilation and recruitment of the pulmonary capillaries are stimulated, and this, inturn, improves the homogeneity of the distributions of VA and Q, hence augmenting V/ Q matching.
Effects of Positions on Lung Function
Monitor Responses to Position Changes
ABG Parameters |
Subjectively, the patient’s facial expression, respiratory distress, dyspnea, anxiety, peripheral edema, discomfort, and pain are assessed. |
Objectively, heart rate, blood pressure, respiratory rate, SaO2, flow rates, and bedside spirometry can be readily assessed. |
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