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In the power company they keep the voltage in your house constant (110 V) and you vary the resistance of what you plug in to determine how much power you want to use. .
In the power company they keep the voltage in your house constant (110 V) and you vary the resistance of what you plug in to determine how much power you want to use.
The body works the same way. 100 mmHg is maintained in the aorta and autoregulation controls blood flow in each organ in the periphery.
Overall aim of the system is to keep aortic pressure constant and let the organs regulate their own flow through autoregulation.
Sympathetic nerves: Spinal cord to sympathetic chain. Innervate heart (increase rate and force), cause adrenal gland to secrete both epinephrine and norepinephrine. They innervate peripheral vessels and are constrictor. sympathetic nerves are tonic.
Parasympathetic nerves: Vagus n. only. Go to the heart’s atria and the viscera.
Peripheral vasodilation or decreases in contractility must be done by withdrawing sympathetic tone.
The vasomotor center in the brainstem receives input from hypothalamus, cranial nerves and higher centers.
The Baroreceptors are in the carotid sinus (glossopharyngeal n.) and aortic arch (vagus n.)
A. Rate of discharge increases with blood pressure.
B. Afferent activity inhibits sympathetics and activates parasympathetics.
C. Negative feedback results in moment-to-moment arterial pressure control.
D. The system is designed so that 100 mmHg is maintained in the arterial system. Each organ adjusts its resistance (autoregulation) so that its nutritional needs are met.
E. Only pressure is regulated. There is no cardiac output sensor.
vagus nerve (efferent)
cardiac sympathetic nerves
sympathetic constrictor nerves
Heart & Per. Res.
Autoregulation always has priority over sympathetic constriction. Only those vessels not participating in an active hyperemia will constrict.
Why would cutting the vagus nerve exaggerate the response?
Blood pressure changes due to:
Note that mean pressure is not changed by denervation.
The long-term control of blood pressure is controlled by blood volume
In heavy exercise blood pressure will actually increase. That is because the CNS increases the set point during heavy exercise to deliver more flow to the periphery.
Peripheral chemoreceptors Primarily control pulmonary function but their effects spill over into the CV system
Carotid and Aortic Bodies
The Cushing’s reflex: Cerebral ischemia causes a massive discharge of both sympathetic and parasympathetic nerves. The net result is hypertension with bradycardia.
Seen in head trauma with intracranial bleed.
The vagus is dominant over the sympathetics at the SA node
These effects act to lower blood volume
Venous pressure is a major determinant of cardiac output in both ventricles.
Blood volume determines venous pressure.
At the same time cardiac output determines arterial pressure
AOP = CO • TPR
The blood pressure then adjusts to maintain a balance between salt intake and loss
Because the curve is very steep a large increase in salt intake causes only a small increase in arterial pressure
The rate at which the kidney loses sodium is determined by the blood pressure
Hypertension can result from a failure of the kidney to regulate blood volume.
Either the set point can be raised e.g. occurs in renal artery stenosis
or the gain of the system can be decreased as occurs in loss of renal mass.