Cardiac Physiology

what are the two primary roles of cardiovascular system?

distribution of essential substance to tissues and remove by-products of metabolism

what are the three secondary roles of the cardiovascular system?

circulation of hormones and NTs, heat dissipation, mediation of defense responses

3 functional parts of cardiovascular system

heart (pump), blood (liquid), vessels (distributing tubes)

what does systemic (peripheral ) circulation involve?

O2 taken up by tissues and CO2/metabolic wastes taken up by blood

what does pulmonary circulation involve?

to and from the lungs, O2 added to blood, with CO2 removed from blood and exhaled

what is the job of the right heart?

propels oxygen poor blood through the lungs

what is the job of the left heart?

propels oxygen rich blood to all other tissues

how is blood flow kept unidirectional in the heart?

one-way valves

what are the two chambers of the heart?

atria and ventricles (one on each side of the heart)

characteristics of the atrium

thin walled, receiving chamber and primer pump

characteristics of the ventricle

thick walled, main pumping chamber

what are the two types of cardiac myocytes?

contractile and myoconductive cells

what type of myocytes make up the atria and ventricles?

contractile

what are myoconductive cells?

nodal and purkinje cells; specialized excitatory/conductive cells that spontaneously generate or conduct electrical signals through the heart so that it beats rhythmically, contracrt weakly

what makes cardiac muscle striated?

sarcomeres - contractile protein arrays

how is cardiac muscle arranged?

in a lattice that behaves like a syncticum, short branches that are interconnected.

how are cardiac cells connected end to end?

by intercalated discs that are composed of desmosomes for a physical connection and gap junctions for an electrical connection

why is the interconnectedness of myocytes important?

so they can be activated instantaneously by a wave of electrical stimulation, contract as one to generate a coordinated heart beat, behave in synchrony

what are the two muscular functional syncytia?

atrial syncytium and ventricle syncytium

what is the fibrous skeleton?

separates atrial and ventricular muscle physically and electrically, tough connective tissue sheet that encircles valves, lies along plane of atrioventricular groove (coronary groove)

what is the function of the fibrous skeleton?

blocks electrical communication between atria and ventricles, except where penetrated by AV bundle (bundle of His), allows the different syncytia to be activated separately and sequentially with a time delay between .

what are the two valves of the right heart?

tricuspid ( right AV valve) and pulmonary/pulmonic (right semilunar valve)

what are the two valves of the left heart?

mitral (left AV or bicuspid valve) and aortic (left semilunar valve)

what two valves separate the ventricle from the artery connected to it?

aortic and pulmonary valve

what are the two septa that separate the right and left sides of the heart?

interventricular and interatrial septa

where do no valves exist?

between the cranial and caudal vena cavae and right atrium and the pulmonary veins and left atrium

what are the fibrous layers of the heart?

are atrial walls thinner than ventricle walls?

yes

how are valves moved?

it is passive, the pressure exerted by flowing blood changes it

when are the atrioventricular valves opened (tricuspid and mitral)?

when ventricles relax during the filling phase, to allow blood to drop from the atria above

when are AV valves closed?

when ventricles contract during the ejection phase, to prevent back flow of blood into the atria

what is the chordae tendineae?

connective tissue strands that tether edges if sheet like valve to ventricular papillary muscles below

what is the valve annulus?

the circumferential fibrous ring that the 3 cusps of the semilunar valves (pulmonary and aortic) attach to

when are semilunar valves open?

when ventricles contract, ejection phase to allow blood to exit the ventricle

when are semilunar valves closed?

when ventricles relax during the filling phase to prevent back flow into the ventricle

what are coronary arteries?

arise from out pockets of aortic root. branch over the hearts epicardium (outer surface) to carry oxygen rich blood to myocardium and capillaries penetrate wall to reach endocardium

what are coronary veins?

blood moves from capillaries to coronary veins, coalesce to form coronary sinus which delivers oxygen poor blood back to the right atrium

path of blood flow through the heart

right atrium -> tricuspid valve -> right ventricle -> pulmonary valve -> pulmonary artery -> pulmonary capillaries -> pulmonary veins -> left atrium -> mitral valve -> left ventricle -> aortic valve -> aorta

what are three structures that empty into the right atrium?

coronary sinus
cranial vena cava
caudal vena cava

what is the purpose of veins?

deliver blood back to heart and serve as major reservoir for blood

what do venules do?

collect blood from capillaries

what do capillaries do?

exchange substances between blood and systemic tissues

what do small arteries/arterioles do?

smooth arterial pulsations and regulate flow into capillaries

what is the purpose of arteries?

distribute blood under high pressure

in order for the heart to work what must the electrical system accomplish?

generate an impulse that signals the heart to beat
must rapidly communicate that signal to all cells of the ventricular syncytium
signal must result in ventricular contraction by increasing intracellular Ca2+

what is voltage or potential difference?

the work required to prevent charged particles from joining another with opposite charge, created when opposite chargers are separated by a distance/barrier

what is an electrical current?

created when particles of opposite charge flow toward one another

why is it important that there is a voltage (membrane potential) in the heart?

once polarized , using transmembrane proteins ion are allowed to flow towards one another creating electrical currents

why are electrical currents in the heart important?

how cells rapidly communicate to coordinate contraction
these currents create the signals detected on ECGs
disorders of electrical activities can lead to disease

what is required to create a membrane potentional?

semipermeable membrane

what is the resting membrane potential made up of?

weighted average of equilibrium potentials of contributing ions

what is the resting membrane potential of cardiac myocytes?

-80 to -90 mV (relative to the inside of the cell)

at rest what ion is the cell membrane potential most permeable to?

K+, equilibrium potential of -90mV

how is the -90mV resting membrane potential of myocytes established?

Na/K ATPase pumps 3 Na+ ions out of the cell for 2 K+ ions in, then K+ moves out along its concentration gradient leaving a negative membrane potential

what does it mean that cardiac tissue is excitable?

responds to electrical signals by opening/closing different channels

what happens to to membrane polarity when cardiac tissue is stimulated?

it is depolarized (membrane polarity is reversed) (cell membrane becomes less negative on the inside)

what is the action potential?

the wave of depolarization that spreads over the heart as a wave, (extracellular currents associated with this are what is detected by ECGs), cells must repolarize so this can happen again, so the AP is changes in membrane potential over time

what causes a cell to move out of phase 4 (purple) of the AP?

stimulation by adjacent cells reduces RMP to a critical level (threshold potential -70mV)

what happens when the AP is in phase 0 (green) or rapid depolarization?

at threshold, voltage gated fast Na+ channels suddenly open and Na+ flow inward rapidly, which causes a depolarization

what happens when the AP is in phase 1 (orange) or early repolarization?

Na+ channels close, voltage gated K+ channels open and K+ moves out, so the MP becomes more negative which causes voltage gated, L-type Ca2+ channels to open slowly

what happens when the AP is in phase 2 (blue) (unique to cardiac myocytes)

volage gated, L-type Ca2+ channels open and Ca2+ moves into the cell which induces more Ca2+ release from sarcoplasmic reticulum, K+ still flowing out at the same rate Ca2+ flows in so there is no change in membrane potential causing a plateau, L-type Ca2+ channels close slowly, responsible for contraction and relaxation

what happens when an AP is in phase 3 (yellow) or final repolarization?

L-type Ca2+ channels close and K+ moves out of the cell and the membrane completely repolarizes

what happens when a cell is in phase 4 (purple), resting MP?

Na+/K+ ATPase pump restores concentration gradients necessary for the next beat (K+ in, Na+ out ), Ca2+ pumped out of the cell

what is the effective refractory period?

inactive Na+ channels can't reopen so another AP can't be generated, allows heart to relax by preventing sustained, tetanic contraction, which facilitates intermittent pumping

what is the relative refractory period?

Na+ channels transitioning to ready state, cell can be stimulated to generate a new AP< but a larger than normal stimulus is required.

what are the initiators of depolarization?

cardiac pacemakers

what cells are considered non-automatic and what does that mean?

atrial/ventricular myocytes (contractile cells) means they depolarize only in response to external stimulation, the stable flat phase 4 of AP

what are the automatic (pacemaker) cells?

sinoatrial (SA) and atrioventricular (AV) nodes and Purkinje fibers (myoconductive cells)

why are pacemaker cells considered automatic?

they depolarize independently of external stimulation, unstable phase 4 from inward leak of Na+ and Ca2+ that causes spontaneous depolarization, confers automaticity and rhythmicity of the heart
phase 4 can tell you whether or not a cell has automaticity

where is the SA node found?

sits at the junction of cranial vena cava and right atrium, primary pacemaker site

where is the AV node found?

on the floor of the right atrium, atop the interventricular septum, can take over pacemaker duty of SA node fails

where is the bundle of His/Purkinje fibers found and when is it used?

make up the ventricular conduction system, can take over if SA/AV nodes fail

what does the pacemaker (SA/AV node) AP look like?

resting MP is less negative (-40), phase 4 is unstable, threshold potential is less negative, phase 0 has a slower upstroke, no plateau, repolarization is more gradual

why is phase 4 of the SA/AV nodes AP unqiue?

cells are less permeable to K+ so resting MP is less negative, gradual repolarization is due to inward leak of Na+ and Ca2+ ions via HCN (funny) channel (Na+ channel) T-type Ca2+ channels contribute

how is phase 0 (spontaneous depolarization) unique in the SA/AV node AP?

at threshold voltage gated L-type Ca2+ channels open to generate an AP, Ca2+ moves into the cell causing depolarization, PACEMAKER CELLS DO NOT CONTAIN FAST NA+ CHANNELS

how is repolarization (phase 3) caused in AV/SA node AP?

K+ channels open and K+ moves out of the cell

how is the purkinje cell AP unique?

slow rate of spontaneous phase 4 depolarization, caused by slow inward leak of Na+ through HCN (funny) channels and a rapid phase 0 like non-automatic cells

what is the rate of impulse formation?

heart rate

what are two main mechanisms that can modify the rate of impulse formation or heart rate?

change rate (slope) of phase 4 depolarization, change the phase 4 starting point (max negative potential, hyperpolarize), the autonomic nervous system influences these greatly

what would be an example of how sympathetic activation can increase heart rate?

increase Na+ and Ca2+ leak to increase slope of phase 4, via norepinephrine and epinephrine binding to beta 1 adrenergic receptors

how can parasympathetic activation decrease heart rate?

acetylcholine via muscarinic receptors decrease the Na+/Ca2+ leak to decrease phase 4 slope and increase K+ permeability to hyperpolarize

what is the hierarchy of pacemakers?

SA, AV, purkinje

what are ectopic foci/ectopic pacemakers and why would they be used?

regions other than the SA node that can initiate beats in special circumstances, used if activity of SA node is depressed, if communication between the SA node and rest of the heart is blocked, and if automaticity becomes enhanced or if a non-automatic cell develops abnormal automaticity and fires while normal pacemaking is functional

what is the purpose of the fibrous skeleton?

blocks electrical communication between atria and ventricles except where penetrated by AV bundle (bundle of His)

why is it important that there is a fibrous skeleton to block communication?

so the atrial and ventricular syncytia can be activated separately and sequentially with a time delay in between

what are considered rapid conductors (fast response) of the AP and why?

atrial/ventricular myocytes and Purkinje fibers, due to rapid phase 0, have more gap junctions and cells are larger which means decreased resistance and faster conduction

what are considered slow conductors (slow response) of the AP and why?

SA and AV node cells due to the slower up-stroke of phase 0, cells are smaller in diameter and have fewer gap junctions

what phase of the AP determines who has the greatest generation impulse?

phase 4

what phase of the AP determines the fastest conductor?

phase 0

what cells are the fastest conductors and then slowest conductors?

purkinje fibers and SA node

what cells generate impulses the fastest and then the slowest?

SA node and purkinje fibers

what is the first thing that happens in the initiation of the cardiac impulse?

impulse originates in the sinoatrial node via spontaneous phase 4 depolarization, rate of this depolarization determines the heart rate, always SA node in normal heart but can be AV or purkinje if needed

once the SA node is depolarized what happens in the normal propagation of the cardiac impulse?

depolarization of the atrial syncytium (right to left, cranial to caudal), once right upper atrium is depolarized there is a rapid conduction of impulse cell to cell through the atrial syncytium

once the atrial syncytium is depolarized what happens in the normal propagation of the cardiac impulse?

depolarization of atrioventricular node (AV), area of slowest conduction to delay transmission of impulse from atria to ventricles which allows the atria to push blood into the ventricles before ventricular contraction, AV node has a long refractory period

once the wave of depolarization has spread to the AV node what happens next in the normal propagation of the cardiac impulse?

rapid conduction down the specialized ventricular conduction (His-Purkinje) system (subendocardial network of rapidly conducting cells), this activates the large ventricular myocardium simultaneously which provides efficiency to ventricular contraction

how does the signal travel through the ventricular myocardium once it reaches the end of the purkinje fibers?

endocardial to epicardial

what are the four divisions of the His Purkinje system?

bundle of His
left and right bundle branches
left anterior and left posterior fascicles of the left bundle branch
purkinje fibers - terminal gibers of the system that penetrate the inner 1/3 of the myocardium

what are the four steps for the NORMAL intiation/propagation of the cardiac impulse?

impulse originates in SA node
depolarization of atrial syncytium
depolarization of AV node
rapid conduction down the specialized ventricular conduction (His-Purkinje) system

what does an ECG record?

the sum of all simultaneously occurring electrical activity, records extracellular signals produced by waves of depolarization through cardiac myocytes, changes in voltage are recorded as waves (PQRST), does not give info on the hearts ability to contract and relax

basics of dipole theory?

opposite and equal charges separated by a distance that generate an electrical force that has direction and magnitude and is represented by a vector called a dipole moment from negative to positive charge