CV System Technical Lecture – English -Part 2

CV System Technical Lecture – English -Part 2


So to review the pacemaker, the atrioventricular
node, sets the tone. Impulses are collected by the atrioventricular bundle and delivered
down through the Purkinje fibers. Now, by having two separate events, atrial contraction,
followed by ventricular contraction, we essentially allow blood to move in a very effective one
way circuit. In fact, this is so effective that your heart pumps about 2000 gallons of
blood in this way every day. From the heart, blood is going to enter a series of arteries. Arteries are defined as structures that carry
blood away from the heart. That “A” in artery and “A” in away are great clues. As blood
flows away from the heart through the series of arteries, specific tissues are supplied
by smaller and smaller branches. In order for the heart to operate most effectively
there has to be pressure in these vessels at all times. Your cardiovascular system is
a closed loop. At no time are you losing large amounts of fluid, or at no time are you gaining
large amounts of fluid. Lets talk for just a moment about vessels again. As we said,
arteries carry the blood away from the heart, and there is a difference in size between
many of these vessels. The largest tubes, and you can think of them as simply hollow
tubes, are known as arteries. Arteries branch into smaller hollow tubes, just like the branches
of a tree, as you move out from the trunk you get consecutively smaller and smaller.
The smaller branches are known as arterioles. Arterioles, finally, branch into the smallest
pathways in the system. The smallest pathways are known as capillaries, and they will be
special for several reasons. First of all, capillaries are going to be found at sites
of exchange in the body. When a tissues needs oxygen, or when a tissue needs to give off
carbon dioxide, in order to facilitate that exchange, we must rely on small tubes that
things move easily into, and that things move easily out of.

So there are many different sites of exchange in the body. The list of capillary networks
is literally endless. After exchange occurs, usually the good things in blood, and I use
that term loosely, the good things like glucose and oxygen have been delivered by the cardiovascular
system to those tis- sues. Although most of the work is done at this point, wastes build
up in every single cell of your body at every moment of the day. In addition to delivering
the nutrients, and oxygen, your cardiovascular system also specializes in delivering wastes
away. So if we start at a site of exchange car- bon dioxide, a gas, is dissolved in blood,
and it continues to go back up to the heart. So this is kind of the second half of the
loop. From a capillary network we’re going to have… excuse me. Capillaries will emerge
to form venules, which are smaller tubes that carry blood toward the heart, and venules
will merge to form larger structures called veins, which lead back to the heart. So I think to review, away from the heart
we have arteries and arterioles. Arterioles then branch to form capillaries. After exchange
occurs, capillaries will merge to form venules, venules will grow into larger structures called
veins, and veins will ultimately be responsible for delivering blood back to the heart. One of the unique attributes of blood vessels,
most of them by the way, is that they do contain a little bit of muscle. So we’ve got miles
and miles of tubes in our heart, and because they contain muscles, they have the ability
to change size. Well you ask why would a blood vessel want to change size? It is often advantageous
to send more blood through an artery for some emergency, and it become necessary in the
case of an injury to send less blood, for example, through an artery. Smooth muscle,
the type of muscle that’s found in these tissues, have a couple of things that are sort of similar
or in common with cardiac cells. First of all, the smooth muscle in your blood vessels
is non-voluntary. That is, you cannot consciously think about smooth muscles and cause them
to contract. Secondly, smooth muscle cells are also autorhythmic, that means they can
generate some of their own action potentials, and most importantly, they can carry those
potentials through tissue to adjacent cells. So, we’ve got this closed system, and we can
increase and decrease the size of the tubes that carry blood around from the heart…
or excuse me, from the heart back to the heart. Last but not least, we have to focus just
a moment on capillaries. We always find capillaries at sites of exchange, because they are extremely
small and delicate. Cardiac tissue is fairly robust in many cases, except when we are talking
about capillaries. These are going to be small microscopic tubes, that consist of only one
thin layer of cells. Because they are so small and delicate they’re potentially damaged very
easily and we have to be careful what we do with them. But because they are so small and
thin, they readily facilitate a process called diffusion. Diffusion will be defined as the
movement of material from areas of high concentration towards areas of low concentration. For example
when a tissue’s oxygen arrives, oxygen diffuses readily out of these small tissues. When a
tissue builds up carbon dioxide and needs to get rid of it, carbon dioxide readily diffuses,
moves from areas of high concentration to areas of low concentration, into blood vessels
and then carbon dioxide can begin its journey back towards the heart. So what we see is, to wrap things up, we’ve
got two basic components. We’ve got a heart that’s basically a series of four chambers
that work in a coordinated fashion to pump blood out of the body. When blood leaves the
body, we’ve got arteries and arterioles that carry blood towards tissues. Capillaries are
unique because they are always found at sites of nutrient and waste exchange. And last,
but not least, we’ve got venules, which become veins that carry blood back up to the heart.