Endocrine System Hormone Transport * Bound hormone- hormones attached to a transport protein * Unbounded (free) hormone- one that is not attached to a transport protein * Only the free hormone can leave a blood capillary and get to a target cell. Hormone Transport in the Blood * Up to 10% of the molecules of a lipid hormone are not bound to a transport protein, i.e. free fraction * It is the free fraction that diffuses our to capillaries, binds to receptors, and triggers responses. * Transport proteins release new ones to replenish the free fraction Example of transport proteins * Thyroid hormone binds to three transport proteins * albumin * albumin-like protein called thyretin * an alpha globulin named thyroxine binding globulin (TBG) * binds most of the greatest amount * 99.8% of T3 and T4 are protein bound * bound TH serves as a long-lasting blood reservoir * Steroid hormones bind to globulins such as transcortin, the transport protein for cortisol. Mechanisms of Hormone Action
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