Insulin in the blood circulation

• The lungs employ an extensive absorbent surface. Their extent in humans is 50-140m2, and they contain around 500 million alveoli. The alveolar-capillary barrier is thin.

• This surface is well perfused. The perfusion flow is about 5 litres per minute, and the pulmonary capillary blood volume is around 0.25 litres.

• Many peptidases are absent, and are infact encountered in the gastrointestinal tract.

• The administered insulin does not immediately pass through the liver, unlike insulin physiologically secreted by the pancreas, which is channelled via the portal vein to the liver.

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