Fewell - Electrical properties of the heart Flashcards
How do physiologist gauge blood pressure
by the height it can drive a column of liquid
- water or mercury
- normal adult mean blood pressure is around 100mmHg
What causes tissue blood flow
-tissue blood flow is caused by the driving pressure (generated from the pumping action of the heart)
across variable resistance
pressure transducer
changes pressure into an electrical signal used to measure blood pressure
why is the variable resistance different between organs
- different organs have different blood flow rates but same driving pressure
- organs regulates its own pressure
Darcy’s Law
Change in pressure = flow x resistance
Poiseuille’s Law
resistance = (8 x viscosity x length)/(pi x radius^4)
*radius of blood vessels changes to regulate flow but not length
pressure within the systemic & pulmonary blood vessels
- losing pressure over time
- organs receive blood at low pressure contract and than send the blood out at a higher pressure
how is blood pressure described
-statolic over diastolic
driving pressure
pressure between two spots in the circulatory system (one is higher and one is lower)
transmural pressure
inside - outside
(outside is usually 0 but in the heart sometimes external pressure is greater than internal resulting in no flow)
-ex: across a wall of the heart
hydrostatic pressure
- the pressure exerted by a fluid due to the force of gravity
- gravity causes a hydrostatic pressure when there is a difference in height
giraffe neck vs human neck driving pressure
higher in giraffe because it has a longer neck and therefore a bigger heart compared to a human
hydrostatic pressure reference point
the heart is at zero height
-varies with bodies position
hydrostatic pressure when recumbent/horizontal
- no need to add or substract hydrostatic pressure to overall intravascular pressure
hydrostatic pressure when upright
- need to add (if below reference point) or substract (if above reference point) hydrostatic pressure to overall intravascular pressure
excitable heart cells
the cell of the heart, like neurons, are excitable and generate action potentials
what excites the heart
the heart initiates its own continuous succession of contractions
types of heart muscle cells
-myocardial contractile cells (99%)
-myocardial excitatory & conductive cells (1%)
>no myofibrils cannot contract
where are electrical stimulus generated in the heart
SA node
rate at which SA node depolarizes
60-100 times a minute
how is the heart electrical system modulated?
ANS modulates heart rate and contraction strength
but heart can function without it
Automaticity
the ability to generate its own heart beat
rhythmicity
the regularity of peacemaking activity
Electrical system of the heart
-hierarchy from SA node to AV node to AV bundle to right/left bundle branches and purkinje fibers (lose pressure as you go down?)
Excitation - contraction coupling
because excitation of myocytes triggers contraction, propagation of action potentials must be timed to synchronize atrial and ventricular contraction in order to move blood
***SA node has slow conduction velocity to give atrial time to contract
what is cardiac action potential based on the _ of the _
the speed of the upstroke therefore action potentials can be slow or fast
slow response action potential
-pacemaker • Sinoatrial (SA) Node • Atrioventricular (AV) Node • Lacks early repolarization phase 1 • Lacks stable phase 4
fast response action potential
-non-pacemaker • Atrial & ventricular myocytes • Common bundle & bundle branches* • Purkinje fibers of the heart* • Divided into 5 phases *can exhibit spontaneous depolarization under special conditions
Cardiac Myocyte & Nerve Cell
Action Potentials
- very different
- cardiac myocytes last 250-300ms vs nerve cells have around 5ms
fast action potential order of phases
depolarizing phase -> initial repolarizing phase -> plateau phase -> final repolarizing phase
channels responsible for fast action potential
>depolarizing phase
-depolarizing phase: fast voltage-gated Na+ channels open (-90 to +20mV)
channels responsible for fast action potential
>initial repolarizing phase
-initial repolarizing phase: fast voltage-gated Na+ channels close and fast voltage-gated K+ channels open (20-15mV)
channels responsible for fast action potential
>plateau phase
-plateau phase: L-type voltage-gated Ca 2+ channels open, fast voltage-gated K+ channels close and slow voltage-gated K+ channels partially open (15mV)
channels responsible for fast action potential
>final repolarizing phase
final repolarizing phase: L-type voltage-gated Ca2+ channels close and slow voltage-gated K+ channels fully open (+15mV–90mV)
phases responsible for slow action potential order
pacemaker potential -> depolarizing phase -> repolarizing phase
channels responsible for slow action potential
>pacemaker potential
-pacemaker potential: voltage-gated K+ channels close and F-type Na+ channels open (-60 to -40mV)
channels responsible for slow action potential
>depolarizing phase
-depolarizing phase: L-type voltage-gated Ca2+ channels open (-40mV to +20mV)
channels responsible for slow action potential
>repolarizing phase
-repolarizing phase: L-type voltage-gated Ca2+ channels close and voltage-gated k+ channels open (20mV to -60mV)
Modulation of the Heart Electrical System
Conducted by the ANS
-heart rate and contraction strenght
AUTOMATICITY
the ability to
generate its own heartbeat
RHYTHMICITY
the regularity of pace-making activity
muscarinic receptors slow action potential & ANS
increase K+ permeability, which hyperpolarizes the cell, decreases slope of phase 4 depolarization, and slows HR
β1-adrenergic receptors slow action potential & ANS
increase Na+ permeability, increases slope of phase 4 depolarization and increases HR
Relationship among action potential,
refractory period and tension developed in
Skeletal and Cardiac Muscle
Cardiac muscle needs a longer refractory period therefore plateau stage. Need to allow the muscles to relax to fill up the heart?
Natural exication of the heart
-ordered fashion because of excitation system, which is slowed at particular parts to allow ventriculus to fill before they contract and atria to fill before they contract
Atrial & Ventricular muscle act as____
act as functional syncytia
functional syncytium definiton
electrical impulses propagate freely
between cells in every direction, so that the myocardium
functions as a single contractile unit.
functional syncytium allows _____ of the myocardium
This property allows rapid,
synchronous depolarization of the myocardium
Requirements effective pumping of blood >depolarization must be propagates \_\_ >action potentials must be \_\_ >absence of \_\_ > substantial delay between _ >coordinated _ of _ >_ contraction beging at _ and _
-Depolarization propagates through cardiac
muscle very rapidly
-The action potentials of cardiac muscle are
unusually sustained
-Absence of tetany
-Substantial atrial to ventricular delay
-Coordinated contraction of ventricular cells
-Ventricular contraction begins at the apex of the heart,
progressing upwards to eject blood into the great arteries
Reason why The action potentials of cardiac muscle are unusually sustained
This prevents premature relaxation, maintaining initial
contraction until the entire myocardium has had time
to depolarize and contract.
Reason why there is an absence of tetany in the heart
After contracting, the heart must relax to fill up again.
Sustained contraction of the heart without relaxation
would be fatal, and this is prevented by a temporary
inactivation of certain ion channels.
Why does is there as substantial atrial to ventricular delay
This allows the atria to completely empty their contents into the
ventricles; simultaneous contraction would cause inefficient filling and
backflow. The atria are electrically isolated from the ventricles,
connected only via the AV node which briefly delays the signal.
Why is there a coordinated contraction of ventricular cells?
The ventricles must maximize systolic pressure to force blood through
the circulation, so all the ventricular cells must work together.
Why does the Ventricular contraction begins at the apex of the heart,
progressing upwards to eject blood into the great arteries?
Contraction that squeezes blood towards the exit is more efficient than a
simple squeeze from all directions. Although the ventricular stimulus
originates from the AV node in the wall separating the atria and
ventricles, the Bundle of His & Bundle Branches conduct the signal to
the apex.
