Video Transcript
This video is all about the human
heartbeat, how it is regulated by electrical activity, what is meant by the term
systole and diastole, and what causes the typical sounds of a heartbeat that we’re
familiar with. We’ll also consider why the blood
pressure is different in different blood vessels and find out how our own blood
pressure can be measured.
Here is a simple diagram of a human
heart. The heart is responsible for
pumping blood all around the body, and it does this by beating in a regular
rhythm. A healthy adult heart beats around
70 times every minute. Over the course of a lifetime, this
amounts to over 2.5 billion heartbeats. So how is the heartbeat
controlled? You may recall that the heart
muscle, or cardiac muscle, is myogenic. This means that it contains
specialized tissues which generate their own electrical impulses. These impulses are conducted
throughout the heart and give rise to a repeating pattern of muscle contraction and
relaxation, which is known as the cardiac cycle. Because the heartbeat is initiated
by the cardiac muscle itself, even if you remove the heart from the body, as long as
it had a sufficient supply of oxygen and glucose, it would just keep on beating.
Before we look at the electrical
activity that controls the heartbeat, let’s briefly remind ourselves of the anatomy
of the heart. The heart can be split into the
left side and the right side. Whenever you look at an image of
the heart, imagine that it belongs to someone facing you. The right side of the image would
be the left side of their heart, and the left side of the image would be the right
side of their heart. We can also split the heart top and
bottom. Each one of the chambers at the top
of the heart is an atrium. Together, they are the right and
left atria. Each one of the chambers at the
bottom of the heart is a ventricle. Together, they’re the right and
left ventricles.
Now, let’s look at the blood
vessels. These are the arteries, which carry
blood away from the heart, and these are the veins, which carry blood into the
heart. The word pulmonary refers to the
lungs. So, the pulmonary vein brings
oxygenated blood from the lungs into the left atrium, and the pulmonary artery
carries deoxygenated blood from the right ventricle to the lungs. The vena cava brings deoxygenated
blood into the right atrium, while the aorta carries oxygenated blood from the left
ventricle to all the other parts of the body.
During the cardiac cycle,
electrical impulses are first generated in the sinoatrial node, or SAN for
short. This is a group of cells which are
located in the wall of the right atrium. The SAN is also known as the
pacemaker of the heart because it’s responsible for setting the rhythm of the
heartbeat. Once the atria have filled with
blood, the SAN generates electrical impulses which spread across the walls of the
atria as a wave of electrical activity. This causes the atria to contract,
forcing blood out of them and into the ventricles. Electrical impulses from the SAN
are then transmitted to the atrioventricular node, or AVN. This is another group of cells
located between the right atrium and the right ventricle.
Upon receiving these electrical
impulses, the AVN then passes them on to the bundle of His. This introduces a delay of about
0.1 seconds, which allows the ventricles to fully fill with blood before they
contract. The bundle of His is a collection
of specialized muscle fibers found along the septum separating the right and left
sides of the heart. The bundle of His facilitates the
rapid conduction of electrical impulses from the atria down to the base of the
ventricles. Here, the bundle of His splits off
into two branches of muscle fibers called the Purkinje fibers. The Purkinje fibers facilitate the
conduction of electrical impulses up the outer walls of the ventricles. This causes the ventricles to
contract, pumping the blood up and away into the arteries and out of the heart.
It’s really important to remember
the correct order of events here. So to quickly summarize, electrical
impulses are generated by the SAN, then transmitted to the AVN. Next, they are conducted down the
bundle of His before spreading out across the Purkinje fibers. The Purkinje fibers can also be
referred to as Purkyne fibers, so don’t be confused if you see them written this
way. Now, let’s have a look at another
key feature of the heart, the valves.
Heart valves, as shown in blue on
this diagram, are flaps of connective tissue which open and close to prevent blood
flowing in the wrong direction. These two are known as
atrioventricular valves because they are found between the atria and the
ventricles. This is called the pulmonary valve
because it’s found inside the pulmonary artery, and this is the aortic valve because
it’s found inside the aorta.
During the cardiac cycle, when
blood is entering the atria through the vena cava and the pulmonary vein, the
atrioventricular valves are open, meaning most of the blood flows from the atria
into the ventricles. The cardiac muscle is relaxed while
the chambers are filling. This phase is known as
diastole. At the end of diastole, the atria
contract, forcing any remaining blood into the ventricles. Once the ventricles are fully
filled with blood, the cardiac muscle surrounding them contracts. This forces blood up and out of the
heart through the pulmonary artery and the aorta. This phase is known as systole.
During systole, the
atrioventricular valves close and the pulmonary and aortic valves open. Blood is still able to enter the
atria through the veins during systole too. Lubb dupp, lubb dupp, lubb
dupp. You may recognize that as being the
sound the heart makes when it beats. People often think that these
sounds are caused by the contractions and relaxations of the cardiac muscle that we
just talked about, but that’s not actually the case. So where do the sounds of the heart
come from? When we hear the distinctive
lubb-dupp sound of the heartbeat, we are actually hearing the sound of the heart
valves closing. The closing of the atrioventricular
valves during systole produces the lubb sound, and the closing of the pulmonary and
aortic valves during diastole produces the dupp sound.
Now, let’s turn our attention to
blood pressure. When blood leaves the heart through
the arteries, it either travels through the pulmonary artery to the lungs or through
the aorta to all the other parts of the body, right up to the top of your head and
down to the tips of your toes. In order to reach these places, the
blood needs to be flowing at a high pressure. This is achieved by the
ventricles. When they contract during systole,
they exert a large force on the blood, causing it to be pumped out of the heart at a
high pressure. Let’s fill in this table to compare
the pressure of the blood in the different types of blood vessel.
As we’ve already talked about, the
blood in the arteries coming directly out of the heart is under high pressure. As the blood gets further away from
the heart, it branches off into smaller arteries and arterioles where up to 70
percent of its pressure is lost. This means that when the blood
reaches the capillaries, which is where gases and nutrients are exchanged with the
tissues, the pressure is low. The blood is also under low
pressure as it returns to the heart through the veins. This is why veins contain valves to
make sure blood only flows in the right direction. Skeletal muscles also help to push
blood in the veins back up to the heart when they contract.
Because we have a good
understanding of what the blood pressure should be in a healthy circulatory system,
blood pressure measurements are a useful way for medical professionals to monitor a
person’s heart and blood vessel function. If their blood pressure is
significantly higher or lower than it should be, it can be a sign that the person is
at risk of an illness such as heart disease or stroke. So how do we measure blood
pressure? Blood pressure is measured using a
piece of equipment called a sphygmomanometer. As this is quite a complicated
word, it is often more simply referred to as a blood pressure monitor.
Blood pressure monitors consist of
an inflatable cuff and a pressure gauge, which can either be digital or manual. There are two types of manual
gauges, one which uses a column of mercury to measure pressure and one which uses an
aneroid dial. Although manual blood pressure
monitors are still used when the greatest accuracy is required, for day-to-day use,
digital ones are most common. Let’s see how a blood pressure
monitor works. In this example, we’re using a
digital one. First, the cuff is secured around
the upper arm before being inflated. Readings are usually taken from the
arm because it’s closer to the heart and therefore gives a more accurate reflection
of the force the heart is applying to the blood.
Just as blood flow to the lower arm
is completely cut off, air is steadily released from the cuff again and it begins to
deflate. When this happens, blood rushes
into the lower arm at the highest possible pressure, in other words, the pressure
that the ventricles apply to the blood during systole. This is measured by the cuff and
displayed as a digital readout on the screen. In a healthy adult, the systolic
pressure is usually around 120. Over the next few seconds, the
blood flow returns to normal and eventually reaches the lowest pressure. This is the pressure the ventricles
apply to the blood when they’re relaxed during diastole. Again, this is measured by the cuff
and displayed on the screen.
The diastolic pressure in a healthy
adult is usually around 80. The final blood pressure reading
has given us the systolic pressure followed by the diastolic pressure, in this case,
120 over 80. And the units are millimeters of
mercury because they’re derived from the traditional mercury pressure gauge. Now, let’s test our knowledge of
this topic by having a go at a practice question.
The cardiac cycle has two distinct
phases, systole and diastole. Complete the statement: The systole
phase occurs when the ventricles something and close the atrioventricular
valves.
First, let’s remind ourselves where
everything is located inside the heart. The atria are the two chambers at
the top of the heart, and the ventricles are the two chambers at the bottom of the
heart. The valves located in the pulmonary
artery and the aorta are known as the pulmonary valve and the aortic valve,
respectively, while the atrioventricular valves are found between the atria and the
ventricles. Just before systole, during the end
of the diastole phase of the cardiac cycle, the ventricles are relaxed and the
atrioventricular valves are open, meaning the ventricles fill with the blood that
enters the heart through the veins. Once the ventricles have fully
filled, they contract, forcing blood up and out of the heart through the
arteries.
The pressure in the ventricles is
now higher than the pressure in the atria. So, the atrioventricular valves
close. This is the systole phase of the
cardiac cycle. Now, we can answer the
question. The completed statement would be
“The systole phase occurs when the ventricles contract and close the
atrioventricular valves.”
Let’s summarize what we’ve learned
in this video by reviewing the key points. We found out that the heart muscle,
or cardiac muscle, is myogenic because it can regulate its own electrical
activity. The sinoatrial node, or SAN, is the
pacemaker of the heart because it sets the rhythm of the heartbeat. During the cardiac cycle,
electrical impulses travel from the SAN where they’re generated to the AVN, then to
the bundle of His, and finally to the Purkinje fibers.
The cardiac cycle has two phases:
systole, where the ventricles contract to pump blood out of the heart, and diastole,
where the ventricles relax as blood enters the heart. The lubb-dupp sounds of the
heartbeat are caused by the heart valves closing. Blood pressure is lower in the capillaries and the veins than it is in the arteries. Lastly, we said that blood pressure
can be measured using a sphygmomanometer, which is more simply known as a blood
pressure monitor.
