11.10.08 organ systems: vascular regulation

this unit talks about the two types of vascular regulation: intrinsic and extrinsic. intrinsic regulation has several mechanisms which either vasodilate or vasoconstrict. metabolic vasodilators are generated in response to low flow or O2, and induce the vessel to dilate, which increases flow and washes the metabolite away. myogenic constriction is contraction of smooth muscle around vascular walls in response to increased transmural pressure, a sort of stabilizing counterforce. nitric oxide is another vasodilator that is derived from arginine and released from endothelial walls. finally, endothelin is a vasoconstrictor which is upregulated in hypertension. all of these mechanisms represent the vessel being stimulated by local events and auto-adjusting vessel diameter so as to increase or decrease local pressure.

contrast this with extrinsic control, which is more of a top down, executive level approach which monitors and maintains blood flow to individual organs or body parts, ensuring that increased blood flow to one does not take away from another. it "senses" mean arterial pressure by way of baroreceptors, which are essentially stretch transducers in the aortic arch and carotid sinuses. these baroreceptors then route out to the medulla, in the solitary nucleus (or NTS-- nucleus, the solitary?), and then to the nucleus ambiguus and the caudal ventrallateral medulla. the nucleus ambiguus innervates the heart parasympathetically via the vagus nerve, while the CVLM continues to the RVLM, the rostral ventrallateral medulla, which is the pacemaker of sympathetic activity and projects down to sympathetic preganglionic neurons in the spinal cord, which project out to the heart. this network of sensory and efferent neurons are influenced by the following brain structures: hypothalamus and pituitary, and the motor and limbic systems.

baroreceptors are the eyes and ears of the extrinsic regulatory system, while the autonomic nervous system are the hands that effect change in the vessels to maintain homeostasis. the sympathetic nervous system has a major role in the extrinsic regulation of blood vessels: the rate at which the post-ganglionic sympathetic neurons fire directly influences the contraction of the smooth muscle around the vessels and thus vasodilation or constriction can be adjusted simply by firing rate of neurons. norepinephrine binding to alpha receptors has the effect of vasoconstriction, and vasodilating when binding to beta receptors. the parasympathetic nervous system vasodilates when muscarinic receptors are innervated.

the last idea that this lecture talked about it is that of heart rate variability, or of the larger patterns of heart rate frequencies. when plotted out over time and analyzed by frequency groups, heart rate can be broken down into three frequencies: high frequency heart rate, which is mainly controlled by parasympathetic ("beat by beat control") and represents rapid changes due to arrhythmia or increased pulmonary circulation during inspiration. the "low" frequency band, called mayer waves, is caused by feedback rhythms created by baroreceptors, as well as sympathetic / vagal activity. the "very low" frequency waves are due to very slow changes in TPR and thermoregulation.