It’s Professor Dave, let’s look at the heart.
Now that we’ve learned about the composition and function of blood, it’s time to learn
about the system that circulates blood around the body, which is rather appropriately called
the circulatory system, also known as the cardiovascular system.
This is comprised of the heart, and an incredible number of arteries and veins and capillaries
that carry blood to every extremity of your body.
There is a lot to discuss here, so let’s start by taking a close look at the heart.
We have a tendency to romanticize the heart, and in the past we have even assigned it as
a center of thought and emotion.
We now know that this is not true, as the brain is where all of that activity takes
place, while the heart, which sits within the medial cavity of the thorax known as the
mediastinum, serves simply as a pump.
But this doesn’t make it any less remarkable, as we shall see.
The first thing we must point out is that the heart participates in two circuits.
Blood enters the right side of the heart and is sent to the lungs, where it becomes oxygenated,
when the oxygen we breath in from the air around us diffuses through the lungs and enters
the bloodstream to bind to hemoglobin.
Then this returns to the left side of the heart, to complete the pulmonary circuit.
Then the oxygenated blood leaves the left side of the heart and is sent throughout the
body to deliver oxygen and nutrients to all the various tissues that need oxygen to perform
cellular respiration.
It unloads all that oxygen and eventually makes it back to the right side of the heart,
thus completing the systemic circuit.
So we have two circuits, and two receiving chambers in the heart where these cycles finish,
those being the right atrium and left atrium.
Along with these, there are two main pumping chambers, those being the right ventricle
and left ventricle.
The heart is covered by something called the pericardium, which is made of dense connective
tissue that protects the heart and maintains its position.
The two walls of the pericardium are the fibrous pericardium and the serous pericardium, the
latter of which also has two layers, the parietal layer and the visceral layer, also known as
the epicardium.
The epicardium is considered the outermost layer of the heart wall, the others being
the myocardium and endocardium.
The myocardium makes up most of the heart, and it is made largely of cardiac muscle.
The rest is made of connective tissue fibers that form a dense network called the cardiac
skeleton, providing structural support and insulating the electrical activity.
The endocardium is a white sheet of endothelium that sits on some more connective tissue,
and this lines the heart chambers.
Speaking of chambers, the heart has four of them.
These are the two atria and the two ventricles we talked about.
The atria are separated by the interatrial septum, and the ventricles are separated by
the interventricular septum.
The atria are the receiving chambers where blood arrives, which is then pushed down into
the ventricles.
Oxygen-poor blood enters the right atrium through three different veins.
The superior vena cava delivers blood from upper parts of the body, the inferior vena
cava delivers blood from lower parts of the body, and the coronary sinus collects blood
draining from the myocardium.
For the left atrium, there are four entryways, and these are the pulmonary veins, which deliver
blood from the lungs back to the heart.
The ventricles make up much more of the volume of the heart, and these are the actual pumps.
When the ventricles contract, that’s what propels the blood out of the heart and into
circulation.
The right ventricle sends blood into the pulmonary trunk which goes to the lungs, and the left
ventricle sends blood into the aorta, which is an artery, the largest one in the body.
We will talk about arteries and other blood vessels later, for now let’s stick to the
anatomy of the heart.
Another important feature is the heart valves.
These ensure the unidirectional flow of blood, meaning they keep the blood going in the correct
direction.
There are two atrioventricular valves, or AV valves, which connect each atrium to its
corresponding ventricle.
The right one is tricuspid, meaning it has three little flaps, whereas the left one is
mitral, or bicuspid, meaning two flaps.
When the heart is relaxed, blood flows through, but when a ventricle contracts, the valve
will close, due to the change in pressure.
There are two other valves, called semilunar valves, and these are the aortic valve and
the pulmonary valve.
These connect the ventricles and the arteries that stem from them, preventing blood that
is leaving from flowing back into the ventricle.
Now let’s go a little deeper.
When we learned about the types of muscle earlier in the series, we saw that the heart
has its very own type, cardiac muscle, which is not found anywhere else in the body.
Cardiac muscle fibers are similar to skeletal muscle fibers in that they are striated, and
the mechanism of contraction with the sliding filaments is the same.
But cardiac muscle fibers are not long and multinucleate.
Instead, they are short and fat, each with one or two nuclei, and they are branched and
interlocked with one another.
The junctions between the cells are called intercalated discs, and these contain desmosomes
which hold things together, and gap junctions which allow for ions to pass through.
The sarcomeres look similar to those found in skeletal muscle fibers, but there is more
variance in the diameters of the myofibrils, and there is more branching amongst them.
Also, the sarcoplasmic reticulum is a bit simpler, and the T tubules enter twice per sarcomere.
The mechanism by which the heart pumps is similar to what we already know about skeletal
muscle cells and the action potential, but there are some key differences.
First, a small percentage of cardiac muscle cells can excite themselves, they do not need
a nerve impulse.
This is called automaticity.
Also, contraction is highly coordinated, all the fibers in the heart contract as a unit,
and this is because of the gap junctions that tie everything together, allowing ions and
therefore depolarization to spread throughout the heart one cell at a time.
And lastly, the refractory period is much longer in the heart, needing more time before
another contraction can occur, which ensures the heart functions properly.
One of the most remarkable things about the heart is its intrinsic cardiac conduction system.
There are certain specialized cells whose job is not to contract, but rather to distribute
impulses throughout the heart, to initiate contraction from within, and also to ensure
that contraction is perfectly coordinated.
These are called pacemaker cells, and they have an unstable resting potential that continuously
depolarizes until the threshold is reached.
Problems with this system are what lead to arrhythmias and fibrillation, which are irregular
heart rhythms or rapid contractions.
The heart also has coronary arteries and coronary veins, because the heart needs to be supplied
with blood just like any other organ, and we will get a closer look at these later.
So that’s a basic introduction to the heart.
There is plenty more to discuss regarding heart rate and other phenomena, but that will
have to wait for another time.
For now, let’s continue by looking at other aspects of the circulatory system.