MS_Week 1_Function of the Heart

Cardiac conduction system

cardiac conduction system

generates and transmits electrical impulses that stimulate contraction of the myocardium

Automaticity
Excitability
Conductivity

three physiologic characteristics of two types of specialized electrical cells, the nodal cells and the Purkinje cells, provide this synchronization:

Electrophysiologic Properties of the Heart

Automaticity
Excitability (depolarization)
Conductivity
Contractility
Refractoriness

Automaticity

ability to initiate an electrical impulse spontaneously and repetitively

Excitability (depolarization)

ability to respond to an electrical impulse

Conductivity

ability to transmit an electrical impulse from one cell to another

Contractility

Contract

Refractoriness

inability to respond until repolarization

Conduction system of the heart

SA Node
AV node
Bundle of His
R & L bundle branches
Purkinje fibers

Conduction system of the heart (mnemonic)

Sending Angel Brings L&R Prosperity

composition of both the sinoatrial (SA) node and the atrioventricular (AV) node

nodal cells

sinoatrial (SA) node

primary pacemaker of the heart, is located at the junction of the superior vena cava and the right atrium

inherent firing rate of sinoatrial (SA) node in a normal resting adult heart

60 to 100 impulses/min

inherent firing rate of atrioventricular (AV) node in a normal resting adult heart

40 to 60 beats/min

inherent firing rate of purkinje fibers in a normal resting adult heart

20 to 40 beats/min

Cardiac cycle

refers to the events that occur in the heart from one heartbeat to the next. These events cause blood to flow through the heart due to changes in chamber pressures and valvular function during atrial and ventricular diastole and systole.

atrial and ventricular diastole

heart chambers are relaxed

Diastole

relaxation - filling of ventricles

Systole

Contraction - emptying

Cardiac Output

the amount of blood pumped by each ventricle during a given period.

The cardiac output in a resting adult

about 5 L/min but varies greatly depending on the metabolic needs of the body

Cardiac output formula

Cardiac Output = Heart Rate x Stroke Volume

Stroke volume

the amount of blood ejected per heartbeat

average resting stroke volume

70 mL

normal heart rate

60 to 80 bpm

Changes in heart rate are accomplished by

reflex controls mediated by the autonomic nervous system, including its sympathetic and parasympathetic divisions.

parasympathetic impulses,

travel to the heart through the vagus nerve, can slow the cardiac rate,

sympathetic impulses

increase cardiac rate

Baroreceptors

specialized nerve cells located in the aortic arch and in both right and left internal carotid arteries (at the point of bifurcation from the common carotid arteries).

Are sensitive to changes in blood pressure (BP).

three factors that primarily determine stroke volume

Preload
Afterload
contractility

Preload

refers to the degree of stretch of the ventricular cardiac muscle fibers at the end of diastole.