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Over 120 years ago, Ernest Starling demonstrated that fluids and other molecules move back and forward from blood to the extravascular space, and that this movement is determined by the balance between hydrostatic and oncotic pressures acting within capillaries and interstitial spaces.1 The first measurement of the effect of these forces on blood volumes (plasma and cells) was published in 1928 in a 32-page long paper in the Journal of Clinical Investigation. This paper elegantly demonstrated that total plasma volume decreases on average 11% after 30 min standing still, compared to the recumbent position, and that hematocrit increased proportionally, indicating no significant changes in total blood cell volume.2 The authors interpreted their findings as the result of the passage of water and other diffusible solutes through the capillary wall, due to an increase in hydrostatic pressure within blood vessels. Nearly thirty years later, using radionuclidelabeled red blood cells and a color dye that binds to albumin, Fawcett and Wynn evaluated the impact of these movements on parameters of the complete blood count (CBC) in healthy individuals and in patients with clinical edema and/or low albumin concentrations.3 The standard experimental procedure consisted in subjects remaining in the horizontal position for 12 h, and then standing up for 1 h (slow walking was allowed during this period). Blood was collected 15 and 60 min after rising. The horizontal position was then resumed, and additional samples were obtained after the same timepoints to evaluate whether changes were reversible. The study confirmed a mean decrease in plasma volume of 10.8% (equivalent to around 500 mL in a 70 kg adult) after 15 min, which

Tides within ourselves: how posture can affect blood volume, blood cells and clinical reasoning