Q2-CB11/Cardiac Output 1 Flashcards
What is cardiac output?
the volume of blood ejected from a ventricle per minute (L/min)
equation of cardiac output
cardiac output = heart rate x number of beats per minute
CO = HR x number of bpm
What is a cardiac output of a textbook person?
5 L/min
What is cardiac output controlled by?
either heart rate and stroke volume
what controls cardiac output?
it is regulated by the automatic nervous system
What does the ANS do?
it controls multiple systems that maintains normal homeostasis
is the ANS subconscious or involuntary?
it is involuntary
The only rigorous definition for these is from the anatomy and where on the spine the fibres emerge. What are these places?
SNS: thoraco-lumbar (segments T1 to L2 )
PNS: cranio-sacral (cranial nerves III, VII, IX & X, sacral segments 2,3,4)
The Sympathetic NS primarily acts via __________
catecholamines
what are catacholamines?
Catecholamines are a group of similar substances released into the blood in response to physical or emotional stress.
The primary catecholamines are dopamine, epinephrine (adrenaline), and norepinephrine.
What is released from nerve endings in the Sympathetic NS?
noradrenaline
Sympathetic NS action in the heart is mediated via ______ and _______
alpha and beta adrenoceptors
What is the most important adrenoceptor for the heart?
beta1-adrenoceptor
Where are beta1-adrenoceptors located?
all throughout the heart (SA node, AV node, atria, and ventricles)
What is a beta-adrenoceptor?
a G-protein coupled receptor linked to an adenylate cyclase
what does a beta-adrenoceptor do?
it increases cAMP which then turns on protein kinase A
Where does the main depolarizing curent in the SA node come from?
it comes from the Na+/K+ channel, current through this channel is increased by the binding of cAMP
What causes the increase of Ca2+ into myocytes?
cAMP/PKA, when Ca2+ levels increase, so does the force of contraction
What ion is crucial for muscle contraction?
calcium
how is calcium crucial to muscle contraction in the heart?
it binds to troponin on the thin filament, allowing the thick filament to interact.
explain the SA node action potential.
What can be said about sympathetic agonists?
they are both positive chronotropes and positive inotropes
positive chronotropes increase the rate and positive inotropes increase the force
What are some types of beta-blockers?
atenolol and propranolol
What can be said about beta-blockers?
they are negative chronotropes and negative inotropes
negative chronotopes decrease the rate
negative inotropes decrease the force
What is the primary neurotransmitter in the parasympathetic NS?
acetylcholine (ACh)
At what receptors does ACh work at?
muscarinic and nicotinic cholinergic receptors
What are muscarinic cholinergic receptors?
they are G-protein coupled receptors
What are nicotinic cholinergic receptors?
they are ion channels found in nerves and skeletal muscle
Parasympathetic innervation of the heart is via the ____________
vagus nerve (cranial nerve X)
What is the main cholinergic receptor in the heart?
M2 (muscarinic type 2)
what does the vagus nerve innervate specifically?
the SA node, AV node, and parts of the atria (there is little to none innervation in the ventricles)
What does the M2 receptor do?
it inhibits adenylate cyclase, which decreases the amt of the cAMP
What happens to sodium influx when the amt of cAMP decreases?
Na+ influx is decreased so the rate of depolarization is decreased.
since it decreases the rate of SA node firing, heart rate decreases as well
what effect does the decrease in cAMP have on atrial and ventricular contraction?
it has a small effect on the force of atrial contraction but no effect on the ventricles
(the atrial effect has little if any effect on cardiac output)
explain SA node action potential in cholinergic action.
what 2 divisions of the nervous system innervate the SA node?
sympathetic and parasympathetic
What is controlled by the balance of the sympathetic and parasympathetic nervous systems?
heart rate
what is a sign of sympathetic activation?
tachycardia
(atropine, the muscurinic antagonist, can cause tachycardia)
the natural rate of the SA node is abt 100bpm, but ______ stimulation drops it down to abt 70 bpm
vagal
what is equation for mean arterial pressure?
mean arterial pressure = cardiac output x total peripheral resistance
Explain the baroreceptor reflex.
it is a negative feedback loop
sensory fibers in the aortic arch and carotid sinus detect a stretch in blood vessel walls(the firing is directly proportional to the stretch)
these fibers then go to the medulla oblongata on the brain stem
the medulla regulates the sympathetic/parasympathetic NS outflow to the heart
Explain cardiac changes in exercise.
exercise is related to an increase of sympathetic NS activity. Normal feedback via the baroreceptor would be expected to prevent this, BUT instead this looks to be reset in exercise; the effect is the BP is perceived to be too low!
What 2 changes does exercise cause in the heart?
Increases heart rate and stroke volume
– heart rate can increase approx. 300% to ~200 bpm
– stroke volume can increase approx. 175% to ~125 ml
– overall cardiac output can increase by approx. 500% to 25 L
heart rate cannot increase over ______ bpm
200
why cant heart rate increase over 200 bpm?
due to the length in action potential and high rates also decrease perfusion as this occurs in diastole
explain cardiac changes in athletes.
Athletes have low rates but large stroke volumes
– <60 bpm (bradycardia)
