9.2 cardiac output

how to vary stroke volume

vary the force of contraction, which will thus vary how much blood is pumped out

how to vary force of contraction

• amt of blood pumped out is proportional to how much blood returns to the heart

• as more blood comes back to the heart, the blood that is coming back stretches the heart muscle to a greater extent than normal (preload)

• this increases the force of contraction and increases the stroke volume, the volume of blood pumped per beat

• thus: increasing the stretch increases the force of contraction

preload

degree of increased stretch in the ventricular walls

Frank-Starling mechanism

increased stretch → increased force

describe skeletal muscle’s optimal length

optimal length has maximal cross bridge activity and maximal pulling and maximal force

in the heart at rest you (are/aren’t) at the optimal length

aren’t

increasing the force of contraction will (increase/decrease) the volume of blood leaving per beat

increase (black dot is the normal resting volume that you pump out at every beat)

two types of activity that can affect the force of contraction

sympathetic activity, parasympathetic activity

how do parasympathetic and sympathetic nerve fibers affect the heart?

influence the pumping action of the heart by affecting both the heart rate and stroke volume, keeping the blood pressure, blood O2 and CO2 levels, and blood pH in the appropriate range (homeostasis)

parasympathetic activity

• vagus nerve carries parasympathetic nerve fibers to the heart (primarily)

• vagus nerve innervates the SA node (cluster of cells in the right atria that generate electrical impulses that initiate the heart beat. is the heart’s pacemaker)

• parasympathetic nerve stimulation has an inhibitory effect → decreases heart rate

mechanism for parasympathetic nerves to decrease heart rate

• parasympathetic neurons produce the neurotransmitter, acetylcholine (ACh)

• ACh binds to ligand gated channels on cardiac cell membrane which causes K+ to go out → hyperpolarizes the cell + decreases permeability of Na+ and Ca2+

  • hyperpolarized membrane takes longer to depolarize (and thereby causes an AP) → heart rate decreases

ionotropic receptors

form an ion channel pore

metabotropic receptors

indirectly linked w ion-channels on the plasma membrane of the cell thru signal transduction mechanisms (e.g., G proteins, neurotransmitters are ligands)

what is the acetylcholine receptor in the heart?

M2

• the M2 muscarinic receptors are located in the heart, where they act to slow the heart rate down to normal sinus rhythm after stimulatory actions of the sympathetic nervous system by slowing the speed of depolarization

nicotinic acetylcholine receptors (nAChR)

ionotropic acetylcholine receptors, responsive to nicotine, Na+ and K+ ion channel

muscarinic acetylcholine receptors (mAChR)

metabotropic acetylcholine receptors, responsive to muscarine

sympathetic activity

• sympathetic nerves from the thoracic region of spinal cord project to heart as cardiac nerves

• sympathetic nerves innervate the SA and AV nodes, the coronary vessels, and the atrial and ventricular myocardium (heart muscle)

• sympathetic stimulation increases heart rate and force of muscular contraction

nervous system regulation of the heart

1. sensory (green) neurons carry APs from baroreceptors to the cardioregulatory center. chemoreceptors in the medulla oblongata influence the cardioregulatory center

2. the cardioregulatory center controls the frequency of APs in the parasympathetic (red) neurons extending to the heart through the vcagus nerves. the parasympathetic neurons decrease the heart rate

3. the cardioregulatory center controls the frequency of APs in the sympathetic (blue) neurons. the sympathetic neurons extend thru the cardiac nerves and increase the heart rate and stroke volume

4. the cardioregulatory center influences the frequency of APs in the sympathetic (blue) neurons extending to the adrenal medulla. the sympathetic neurons increase the secretion of epinephrine and some norepinephrine into the general circulation. epinephrine and norepinephrine increase the heart rate and stroke volume

catecholamines + which effect does it carry out?

an amine derived from the amino acid tyrosine

• act as hormones or neurotransmitters (norepinephrine and epinephrine)

• carries out sympathetic effect on heart function

• bind to 2 different classes of receptors termed the alpha and beta adrenergic receptors

• also known as adrenergic neurotransmitters; neurons that secrete them are adrenergic neurons

epinephrine is __ from norepinephrine

derived/synthesized

norepinephrine-secreting neurons are

noradrenergic

norepinephrine

• a neurotransmitter from sympathetic neurons (specifically, the postganglionic sympathetic neurons)

• increases the rate and degree of cardiac muscle depolarization, which causes an increase in the frequency of the AP and the force and velocity of the contraction

• agonist for cell surface beta adrenergic receptors

• causes G-protein mediated synthesis and accumulation of cAMP in cardiac cells

• opens Ca2+ slow channels, increases cell’s ability to depolarize, also helps open Na+ channels

epinephrine (aka adrenaline)

• a hormone released from the adrenal medulla

• similar in structure to norepinephrine

• has essentially the same effect on the heart as norepinephrine

• nerve signals to the adrenal gland activate the conversion of stores of norepinephrine to epinephrine and cause its release into the bloodstream

• increases cardiac output and blood glucose levels

epinephrine and norepinephrine stimulation together

• increased physical activity, emotional excitement, or stressful situations cause sympathetic stimulation of the cardioregulatory center in the brainstem (medulla)

• this causes the sympathetic stimulation to the heart and causes the adrenal gland to secrete epinephrine and norepinephrine into the bloodstream

• epi and norepi are transported thru the blood to the cardiac muscle cells where they bind beta adrenergic receptors

effects of norepinephrine and epinephrine

• prep the body for fight or flight

• increase blood pressure, heart rate, upregulate the blood and oxygen supply to muscle tissue

what does a faster rise and fall indicate, and how does this occur?

indicates a shorter, stronger heartbeat. caused by increased rate of calcium release/entry + increased rate of calcium uptake

what parts of the cell do norepinephrine and epinephrine affect?

1. the channels that bring calcium into the cell

2. the channels that allow calcium to leave the sarcoplasmic reticulum

3. the calcium troponin interaction

4. the reuptake of calcium into the sarcoplasmic reticulum

if the interaction btwn the troponin and the calcium is __, heart contracts __. if calcium is taken up by the sarcoplasmic reticulum __, heart relaxes __.

faster, faster, faster, faster

sympathetic effect on contractility

for the same volume, there is a stronger contraction bc the sympathetic stimulation enables the heart to pump out more of whatever is there. you more completely empty the heart

interpret this graph

greater contraction for same end diastolic volume

for any amount of blood coming back as shown by the yellow line, you pump out more, as shown by the green line

effects of autonomic nerves on SA node

sympathetic nerves (norepinephrine on beta-adrenergic receptors): increased heart rate

parasympathetic nerves (ACh on muscarinic receptors): decreased heart rate

effects of autonomic nerves on AV node

sympathetic nerves (norepinephrine on beta-adrenergic receptors): increased conduction rate

parasympathetic nerves (ACh on muscarinic receptors): decreased conduction rate

effects of autonomic nerves on atrial muscle

sympathetic nerves (norepinephrine on beta-adrenergic receptors): increased contractility

parasympathetic nerves (ACh on muscarinic receptors): decreased contractility

effects of autonomic nerves on ventricular muscle

sympathetic nerves (norepinephrine on beta-adrenergic receptors): increased contractility

parasympathetic nerves (ACh on muscarinic receptors): no significant effect

higher end-diastolic ventricular volume, higher activity of sympathetic nerves to heart, high plasma epinephrine, lower activity of parasympathetic nerves to heart, higher stroke volume in cardiac muscle, higher heart rate in SA node, higher cardiac output