Transcript Blood pressure 1 - Delta State University

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
• 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
• Vasoconstriction
• 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
– 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.
Supine
Standing
Decrease
Central
Blood Volume
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
– 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