Circulation 5 Flashcards
Basal tone
Theoretical reference point. Amount of vascular contraction found under resting conditions without neural or hormonal influences
Resting sympathetic tone
Amount of vascular constriction found under resting conditions as a result of tonic sympathetic nerve activity
Resistance during sympathetic resting tone vs. basal tone
Higher due to presence of tonically- released norepinephrine
Active vs. passive mechanisms
Active move it away from basal tone, whereas passive mechanisms move it toward basal tone
Alpha receptors
Located on vascular smooth muscle; causes vasoconstriction. (Heart and brain have very few of these)
Beta-1 receptors
Primary adrenergic receptor on cardiac muscle; stimulates heart rate and contractility
Beta-2 receptors
Secondary adrenergic receptors on cardiac muscle that stimulates heart rate and contractility
Also located on vascular smooth muscle and causes vasodilation
Cholinergic receptors
Muscarininc receptors stimulated by acetylcholine
Parasympathetic fibers on blood vessels
Limited number of blood vessels; cerebral, viscera, genitalia, bladder, and large bowel
Causes vasodilation
NOT on skeletal muscle or cutaneous vessels
Sympathetic cholinergic pathway
Postganglionic sympathetic fibers that release acetylcholine on effectors
Example: sweat glands of nonapical skin to indirectly induce vasodilation
Baroreceptor reflex
A negative feedback loop to control arterial pressure
Arterial baroreceptors play a key role in what?
Short-term adjustments of blood pressure in response to relatively abrupt changes in blood volume, cardiac output, or peripheral resistance
Where are baroreceptors located?
In nerve terminals located in the walls of carotid sinus and aortic arch
What is special about the tissue where baroreceptors are located?
The walls are relatively thinner, making them more sensitive to the changes in blood pressure
Instead of “baroreceptors,” what are they actually?
Mechanoreceptors that sense the change in stretch of the vascular walls
Where do the nerve fibers of the baroreceptors connect?
Join the glossopharygeal (carotid) and vagus (aortic) nerves to vasomotor centers in the medulla
What happens with a decrease in arterial pressure as sensed by the baroreceptors?
Increased sympathetic activity (increased peripheral resistance, increased heart rate, and increased ventricular contractility) and decrease in parasympathetic activity (cessation of inhibition of heart rate)
What happens with an increase in arterial pressure as sensed by baroreceptors?
The exact opposite of what happens during and increase in blood pressure (increased parasympathetic and decreased sympathethic)
What are baroreceptors most responsive to?
More responsive to pulsatile (phasic) pressures, but still somewhat responsive to constant (static) pressures
What happens in hypertension with baroreceptors?
Sensitivity decreases, so threshold is increased and receptors are less sensitive to changes in transmural pressure
Peripheral chemoreceptors
Small, highly vascular bodies located in the region of the aortic arch and just medial to the carotid sinus at the bifurcation of the internal and external carotid arteries
What activates peripheral chemoreceptors?
Low arterial PO2 and high arterial PCO2 as well as low pH
Chemoreceptors are involved with which autonomic process?
Respiration
Also involved with the heart during severe hypoxia
When the chemoreceptor signals go through the ninth and tenth cranial nerves to the medulla, what do they do?
Simulate efferent sympathetic or parasympathetic nerves to cause vasoconstriction or bradycardia
What effect on the heart does hypoxia have, and why?
Induces tachycardia; increase in ventilation acts via stretch receptors to centrally inhibit efferent vagal nerve activity
Hormonal control of the circulatory system
Renin-angiotensin-aldosterone (primary mechanism responsible for long-term regulation of blood pressure)
What three effects does angiotensin II have?
- Vasoconsriction of renal/systemic vessels
- Increase in sodium resorption by release of aldosterone
- Stimulates hypothalamus to release ADH and increase thirst
Sympathetic stimulation from baroreceptors causes release of what substance from the kidneys?
Renin in the juxtaglomerular apparatus
Renin has what effect in the kidneys?
Stimulates ACE to convert angiotensin I to angiotensin II
