Introductory Human Physiology

POSITIVE FEEDBACK reinforces the stimulus rather than decreasing or removing it (Figure 5) and is therefore an unstable condition.

NEGATIVE FEEDBACK loops remove the stimulus (Figure 3). A critical consequence of negative feedback control is that it allows the system to resist deviation of a given parameter from a preset range (set point). Negative feedback is the most common form of homeostatic control in biological systems

REFLEX CONTROL involves the nervous and endocrine systems. Reflex control responds to changes that are more widespread or systemic in nature.

LOCAL CONTROL involves paracrine (between neighbors) and/or autocrine (self-to-self) responses. Proteins called cytokines mediate local control.

Fluid compartments of the body

Peptide hormones bind to cell surface receptors which activate a second messenger (Figure 14). This results in a rapid (seconds) change in function/metabolism

The lipid solubility of the hormone dictates the cellular location of its receptor. Plasma insoluble hormones bind to intracellular receptors; plasma soluble hormones bind to cell surface receptors.

Thyroid and steroid hormones bind to intracellular receptors to activate transcription. This results in synthesis of new proteins and therefore is a slow response (30 minutes or more).

Thyroid hormones (T3 and T4) contain “double tyrosines” covalently bound to iodine atoms. T4 can be converted to T3 within target tissues.

The concentration of free plus bound equals total hormone concentration in the blood

only free is active