Blood pressure 1

Regulation of blood flow Local regulation Adjustment of blood flow by the tissue Nervous system Global implication Redistribution of blood flow Increase/decrease in heart activity (pumping) Rapid control of arterial pressure

Autonomic system Sympathetic system More important Components Vasomotor fibers Thoracic spinal nerves Lumbar spinal nerves Sympathetic nerves Innervation of internal viscera and the heart Peripheral vessels

Innervation of blood vessels Most tissues Not capillaries, precapillary sphincters, or metarterioles Increased resistance for blood flow Decreased blood flow through the tissue Decreased volume of vessels Veins Increased heart pumping

Innervation of heart Sympathetic Parasympathetic (vagus) Increased heart rate Increased strength Increased volume of pumping Parasympathetic (vagus) Decreased heart rate

Vasoconstriction Sympathetic nerve fibers Vasoconstrictor nerve fibers Very small number of vasodilators Wide distribution Tissue-dependent More powerful in some organs Kidney, GI tract, spleen, and skin

Vasomotor center Reticular substance of medulla and lower third of pons Transmission of parasympathetic impulses Vagus nerves Transmission of sympathetic impulses Spinal cord Peripheral sympathetic nerves

Area of vasomotor center Vasoconstrictor area Signals to all levels of spinal cord Excitation of preganglionic vasoconstrictive neurons Vasodilator area Inhibition of vasoconstrictor area Sensory area Receives sensory inputs from vagus and glossopharyngeal nerves Reflex control of many circulatory functions Activity of both vasoconstrictor and vasodilator

Continuous signals from vasoconstrictor area Sympathetic vasoconstrictor nerve fibers Continuous firing at 1.5 – 2 impulses per second Sympathetic vasoconstrictor tone Maintenance of partial constriction of blood vessels Vasomotor tone

Control of heart rate by vasomotor center Excitatory impulses (sympathetic) Lateral portion Increased heart contraction and heart rate Parasympathetic impulses Medial portion Sent via dorsal motor nuclei of the vagus nerves Decreased heart rate

Control of vasomotor center The CNS Reticular substances of the pons, mesencephalon, and diencephalon Hypothalamus Posteriolateral – excitation Anterior – mild excitation/inhibition Cerebral cortex Motor cortex Basal areas of brain

Role of neurotransmitter Norepinephrine Sympathetic neurotransmitter Vasoconstriction Alpha receptors Adrenal medulla Secretion of epinephrine and norepinephrine in response to sympathetic impulses Vasodilation via beta receptors (epinephrine)

Role of the nervous system in rapid control of arterial blood pressure Stimulation of sympathetic nervous system and cardioaccelerator Rapid increase in arterial pressure 2 X within 5-10 sec During exercise or stress Reciprocal inhibition of parasympathetic vagal inhibitory signals

Arterial Pressure = Cardiac Output x Total Peripheral Resistance Arterial Pressure can be increased by: Constricting almost all arterioles of the body which increases total peripheral resistance. Constricting large vessels of the circulation thereby increasing venous return and cardiac output. Directly increasing cardiac output by increasing heart rate and contractility. Figure 14-1; Guyton and Hall

Maintenance of normal arterial pressure Negative feedback reflex mechanism Majority Baroreceptor reflexes Reflex initiated by stretching of arterial walls Increased flow of blood and pressure Detected by baroreceptors/pressoreceptors Generation of inhibitory signals

Baroreceptor Spray-type nerve endings Stretch receptor Carotid sinus Hering’s nerve to glossopharyngeal nerves Aortic arch Vagus nerve Signals enter medulla Tractus solitarius

Response to pressure Net effect Very rapid Respond more rapidly to changing pressure compared to stationary pressure Inhibition of vasoconstrictor center Vasodilation Excitation of vagus nerves Decreased heart rate Net effect Decreased arterial pressure

Constrict Common Carotids Pressure at Carotid Sinuses Arterial Pressure Constrictors Figure 18-5; Guyton and Hall Figure 18-7; Guyton and Hall

Functions of the Baroreceptors Maintains relatively constant pressure despite changes in body posture. Decrease Central Blood Volume Supine Standing Sympathetic Nervous Activity Decrease Cardiac Output Vasomotor Center Sensed By Baroreceptors Decrease Arterial Pressure

Pressure buffer system Maintenance of constant pressure Reduction of variability in blood pressure Maintenance of pressure within the narrow range Baroreceptors may not be important in long-term maintenance of blood pressure Adaptation of receptors

Other mechanisms Chemoreceptors Calotid arteries and aorta Detection of oxygen concentrations Detection of carbon dioxide concentrations Detection of pH Detect changes in chemical concentrations Decreased blood pressure Excitation of vasomotor center

Activation of low-pressure receptors Atria and pulmonary arteries Detection of increase in pressure caused by increased blood flow Volume reflex Increase in glomerular pressure Increased fluid loss Decrease blood volume Secretion of atrial natriaretic peptide Maintenance of blood volume

Bainbridge reflex Increased atrial pressure Increased heart rate Caused by increased volume and stretching of sinus node Triggers increased heart rate Prevents damming of blood

CNS inschemic response Loss of blood flow to brain Cerebral ischemia Loss of nutrient Accumulation of carbon dioxide Activation of vasomotor center Excitation of vasoconstrictor and cardioaccelerator Increase in arterial blood pressure

Cerebral ischemia CNS ischemia response Cushing reaction Occlusion of blood flow to the peripheral tissues if severe CNS ischemia response Emergency pressure control system Maintenance of blood flow to the brain Cushing reaction Special type of CNS ischemia response Increased CSF pressure around the brain