Cardiovascular Diseases
 |  |
| Anatomy and Physiology of the Cardiovascular System |
valve. These valves ensure that the blood flows in the right direction and are part of
the sounds we here through our stethoscopes.
the sounds we here through our stethoscopes.
We can simplify the functions of these chambers into two types, those that receive
blood from outside the heart, the atria, and those which send the blood outside of
the heart, the ventricles. It is the smooth function of both sets of chambers that
provide the circulation of blood to the rest of the body.
The heart is actually 3 layers of tissue; the "endocardium" a thin lining in the
chambers; the "myocardium" a thick layer of contractile muscle cells with specialized
fibers forming a conduction system through the muscle; and the visceral pericardium
or "epicardium" that covers the other layers. Surrounding all of this is the parietal
"pericardium" a sac like membrane that encases the heart in the chest. The two
Pericardium layers secrete fluid to lubricate the heart during its contraction, making it
easier to move in the "sac."
Conduction System
In the myocardium there are specialized fibers that are very conductive and allow the
rapid transmission of electrical impulses across the muscle, telling them to contract. In
order to maximize the force of the contraction there is uniformity in the sequence.
That is, the atria contract, then the ventricles contract. This allows both sets to fill
properly before ejecting the blood to its next destination. These two sections are
independent, yet linked to a single impulse, (in a healthy heart,) initiated by the
sinoatrial, (or sinus) node. The tissue around the valves helps to channel the impulse
from the sinus node through another collection of specialized tissue, the
atrioventricular node, that is situated between the two sets of chambers. This area
allows slightly slower transmission of the impulse to the ventricles, allowing the atria
to empty into the ventricles before they contract and force the blood to the lungs or
body. This area, the A/V Node, slows the impulse down to about 1/25th of the
original signal then passes it through to the atrioventricular bundle, or the bundle of
his. This bundle divides itself into two distinct tracts through the ventricles, the
bundle branches, and on to the Purkinje fibers, where the muscle of the ventricle is
stimulated to contract from the bottom up, maximizing the force of ejection.
Action Potential
This is the complex section of cardiac anatomy and requires some patience to
understand. In the heart are specialized tissue collections that have a unique
property, they rhythmically emit electrical impulses. The cause of this phenomena is
the "leaky membrane" that allows the regular exchange of Sodium, Potassium, and
Calcium ions and causes a change in the polarization of the cells. Sodium ions move
into the cell and start the depolarization, Calcium ions extend that depolarization.
When the Calcium ions stop entering the cell Potassium ions move in and the
repolarization of the cell begins. To simplify this, the Sodium starts the cells
stimulation, the Calcium extends that stimulation to allow the entire muscle to
contract before Potassium comes along and tells it to relax for a moment and get
ready for the next wave. The most important thing to remember about this action is
the period where the cells reset for the next wave.
This refractory period has two stages, the Absolute and Relative refractory periods.
Let's use an example I learned from a friend to simplify this concept. Think of a toilet,
(strange I know, but read on,) when you flush it you have initiated the impulse, (the
water floods the bowl and changes the pressure.) It takes a few seconds but the
action is complete when the bowl empties, (but in the heart it's a few tenths of a
second.) Now if you try to flush it again before the upper chamber has filled with
water nothing happens. This is similar to the Absolute refractory phase in that the
muscle is drained and needs a moment to recharge, so an impulse sent to it would
not cause it to contract (push the handle down and nothing really happens.) As the
chamber in the toilet fills partially we can flush it again, with weak results but it still
initiates the sequence. This is similar to the relative refractory period, where the cell is
not fully charged but will attempt to contract if it receives a signal. This is important to
remember because an impulse during the relative refractory period can cause
premature contractions leading to compromised filling and poor ejection of blood from
the heart. This can also lead to life threatening arrhythmias that so severally
compromises the hearts ability to pump
that death can occur quickly.
Like this, called the R on T Phenomena: <Picture: Text Version>
Automaticity is the direct result of a cleverly designed "leaky membrane" which
regulates the exchange of Sodium, Potassium, and Calcium ions to change the
polarization of the cells. The sequence is as follows:
1.Sodium ions enter the cell and begin the depolarization. 2.Calcium ions follow and
extend the depolarization even further. 3.Once Calcium stops moving inward,
Potassium ions move in and repolarization begins.
In a nutshell, sodium starts the cells' stimulation. Calcium extends the stimulation
thereby allowing the entire muscle to contract before Potassium finally comes along
and tells the muscle to relax for a moment and prepare for the next cycle.
The important part of this cycle is the period where the cells reset and prepare for the
next wave. This is called the refractory period because the cells are refractory to (or
unaffected by) further stimulation.
Actually, there are two portions of the refractory period:
Absolute refractory period - during this period, absolutely no stimulation can cause
another action potential. This is the first part of the refractory period. •Relative
refractory period - during this portion, it is possible to cause another action potential,
but the intensity of the contraction will be relative to the time in this period. So, the
further into the period, the better the contraction.
An impulse during the relative refractory period may cause a premature contraction. In
this situation, the chambers are not filled completely. According to the Frank-Starling
Law, this decreased preload will cause cardiac output to decrease. •Additionally,
serious and life-threatening dysrhythmias can arise if the R-wave of the next beat falls
in certain portions of the previous T-wave. It didn't take a genious to call this the "R
on T phenomena
To make this idea clear more, just follow the coming example:
Imagine, if you will, a toilet. When you pull the handle, (initiate an impulse) water
floods the bowl. This event takes a couple of seconds and you cannot stop it in the
middle. Once the bowl empties, the flush is complete. Now the upper tank is empty. If
you try pulling the handle at this point, nothing happens (absolute refractory) Wait
for the upper tank to begin refilling (Potassium moves back). You can now flush again,
but the intensity of the flushes increases as the upper tank refills
The Electrocardiogram
We can see the impulse as it crosses the heart by measuring the electrical current
with electrodes placed on the patients skin in relative and specific places. This impulse,
when filtered through a specially designed machine, produces characteristic waveforms
that we can compare to established normal waveforms and determine a few things
about the current state of the heart. The normal ECG complex consists of three key
elements.
1). P-Wave, representing the impulse across the atria to the A/V Node; 2). QRS
representing the impulse as it travels across the ventricles;
3) T-Wave, representing the repolarization of the ventricles.
By noting the shape, consistency, and the time between these waveforms we can
learn more about the conduction system, damaged areas, and areas that are not
receiving enough oxygen to meet their needs.
the sounds we here through our stethoscopes.
the sounds we here through our stethoscopes.
We can simplify the functions of these chambers into two types, those that receive
blood from outside the heart, the atria, and those which send the blood outside of
the heart, the ventricles. It is the smooth function of both sets of chambers that
provide the circulation of blood to the rest of the body.
The heart is actually 3 layers of tissue; the "endocardium" a thin lining in the
chambers; the "myocardium" a thick layer of contractile muscle cells with specialized
fibers forming a conduction system through the muscle; and the visceral pericardium
or "epicardium" that covers the other layers. Surrounding all of this is the parietal
"pericardium" a sac like membrane that encases the heart in the chest. The two
Pericardium layers secrete fluid to lubricate the heart during its contraction, making it
easier to move in the "sac."
Conduction System
In the myocardium there are specialized fibers that are very conductive and allow the
rapid transmission of electrical impulses across the muscle, telling them to contract. In
order to maximize the force of the contraction there is uniformity in the sequence.
That is, the atria contract, then the ventricles contract. This allows both sets to fill
properly before ejecting the blood to its next destination. These two sections are
independent, yet linked to a single impulse, (in a healthy heart,) initiated by the
sinoatrial, (or sinus) node. The tissue around the valves helps to channel the impulse
from the sinus node through another collection of specialized tissue, the
atrioventricular node, that is situated between the two sets of chambers. This area
allows slightly slower transmission of the impulse to the ventricles, allowing the atria
to empty into the ventricles before they contract and force the blood to the lungs or
body. This area, the A/V Node, slows the impulse down to about 1/25th of the
original signal then passes it through to the atrioventricular bundle, or the bundle of
his. This bundle divides itself into two distinct tracts through the ventricles, the
bundle branches, and on to the Purkinje fibers, where the muscle of the ventricle is
stimulated to contract from the bottom up, maximizing the force of ejection.
Action Potential
This is the complex section of cardiac anatomy and requires some patience to
understand. In the heart are specialized tissue collections that have a unique
property, they rhythmically emit electrical impulses. The cause of this phenomena is
the "leaky membrane" that allows the regular exchange of Sodium, Potassium, and
Calcium ions and causes a change in the polarization of the cells. Sodium ions move
into the cell and start the depolarization, Calcium ions extend that depolarization.
When the Calcium ions stop entering the cell Potassium ions move in and the
repolarization of the cell begins. To simplify this, the Sodium starts the cells
stimulation, the Calcium extends that stimulation to allow the entire muscle to
contract before Potassium comes along and tells it to relax for a moment and get
ready for the next wave. The most important thing to remember about this action is
the period where the cells reset for the next wave.
This refractory period has two stages, the Absolute and Relative refractory periods.
Let's use an example I learned from a friend to simplify this concept. Think of a toilet,
(strange I know, but read on,) when you flush it you have initiated the impulse, (the
water floods the bowl and changes the pressure.) It takes a few seconds but the
action is complete when the bowl empties, (but in the heart it's a few tenths of a
second.) Now if you try to flush it again before the upper chamber has filled with
water nothing happens. This is similar to the Absolute refractory phase in that the
muscle is drained and needs a moment to recharge, so an impulse sent to it would
not cause it to contract (push the handle down and nothing really happens.) As the
chamber in the toilet fills partially we can flush it again, with weak results but it still
initiates the sequence. This is similar to the relative refractory period, where the cell is
not fully charged but will attempt to contract if it receives a signal. This is important to
remember because an impulse during the relative refractory period can cause
premature contractions leading to compromised filling and poor ejection of blood from
the heart. This can also lead to life threatening arrhythmias that so severally
compromises the hearts ability to pump
that death can occur quickly.
Like this, called the R on T Phenomena: <Picture: Text Version>
Automaticity is the direct result of a cleverly designed "leaky membrane" which
regulates the exchange of Sodium, Potassium, and Calcium ions to change the
polarization of the cells. The sequence is as follows:
1.Sodium ions enter the cell and begin the depolarization. 2.Calcium ions follow and
extend the depolarization even further. 3.Once Calcium stops moving inward,
Potassium ions move in and repolarization begins.
In a nutshell, sodium starts the cells' stimulation. Calcium extends the stimulation
thereby allowing the entire muscle to contract before Potassium finally comes along
and tells the muscle to relax for a moment and prepare for the next cycle.
The important part of this cycle is the period where the cells reset and prepare for the
next wave. This is called the refractory period because the cells are refractory to (or
unaffected by) further stimulation.
Actually, there are two portions of the refractory period:
Absolute refractory period - during this period, absolutely no stimulation can cause
another action potential. This is the first part of the refractory period. •Relative
refractory period - during this portion, it is possible to cause another action potential,
but the intensity of the contraction will be relative to the time in this period. So, the
further into the period, the better the contraction.
An impulse during the relative refractory period may cause a premature contraction. In
this situation, the chambers are not filled completely. According to the Frank-Starling
Law, this decreased preload will cause cardiac output to decrease. •Additionally,
serious and life-threatening dysrhythmias can arise if the R-wave of the next beat falls
in certain portions of the previous T-wave. It didn't take a genious to call this the "R
on T phenomena
To make this idea clear more, just follow the coming example:
Imagine, if you will, a toilet. When you pull the handle, (initiate an impulse) water
floods the bowl. This event takes a couple of seconds and you cannot stop it in the
middle. Once the bowl empties, the flush is complete. Now the upper tank is empty. If
you try pulling the handle at this point, nothing happens (absolute refractory) Wait
for the upper tank to begin refilling (Potassium moves back). You can now flush again,
but the intensity of the flushes increases as the upper tank refills
The Electrocardiogram
We can see the impulse as it crosses the heart by measuring the electrical current
with electrodes placed on the patients skin in relative and specific places. This impulse,
when filtered through a specially designed machine, produces characteristic waveforms
that we can compare to established normal waveforms and determine a few things
about the current state of the heart. The normal ECG complex consists of three key
elements.
1). P-Wave, representing the impulse across the atria to the A/V Node; 2). QRS
representing the impulse as it travels across the ventricles;
3) T-Wave, representing the repolarization of the ventricles.
By noting the shape, consistency, and the time between these waveforms we can
learn more about the conduction system, damaged areas, and areas that are not
receiving enough oxygen to meet their needs.
