The Composition and Function of Blood
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The Composition and Function of Blood

October 2, 2019


Professor Dave here, let’s learn about blood. Of all the substances within the human body,
blood is one of the more familiar ones, as we’ve all had an injury that involves bleeding. For a long time it was not well-understood
exactly what blood is, or what it does, we just knew that if you lose enough of it, you die. But we now have an intimate understanding
of this fluid, as well as the circulatory system whose function it is to continuously
pump blood around the body. We will get to this system in a moment, first
let’s examine blood itself, what it’s made of, and why it is so critical for human life. First let’s mention that blood is technically
considered a connective tissue, and as such it is the only fluid tissue in the body, full
of fibrous proteins. It is comprised of formed elements, which
are blood cells, suspended in a fluid called plasma. If we place blood in a centrifuge, it will
separate into its components. The densest section is comprised of erythrocytes,
or red blood cells, and the least dense section will be the yellowish plasma. They are separated by the buffy coat, a white
layer containing platelets as well as leukocytes, otherwise known as white blood cells. As a whole, blood is responsible for distributing
various substances around the body, most notably oxygen, which we can’t survive very long without. But it also carries nutrients absorbed from
the digestive tract, and hormones secreted by endocrine organs. Blood also delivers waste products to the
organs that will dispose of them, like the carbon dioxide that we exhale. Beyond this, blood serves to regulate pH in
various tissues, maintain body temperature, and prevent infection. Let’s discuss each component of blood now,
beginning with plasma. This is a sticky fluid made mostly of water,
but also containing a variety of proteins, nutrients, ions, gases and hormones. The most abundant plasma protein is called
albumin, which contributes significantly to plasma’s osmotic pressure, and this is followed
by a variety of globulins, which bind to certain molecules for transport. Moving to the formed elements, these are erythrocytes,
or red blood cells, leukocytes, or white blood cells, and platelets. Red blood cells and platelets are interesting
in that they don’t possess all the typical organelles and they don’t divide, they are
replaced by stem cells in the bone marrow. Red blood cells are very numerous in the bloodstream,
and they are shaped like flattened discs with depressed centers. There is no nucleus, not much of anything
inside other than lots of hemoglobin. This is the protein that allows for the transport
of oxygen throughout the bloodstream, which is picked up in the lungs and then released
for tissue cells throughout the body. There are other proteins as well that have
structural or protective functions, but hemoglobin will be the focus here. This is the structure of hemoglobin, it is
made of a protein called globin, consisting of four polypeptides, two identical alpha
chains and two identical beta chains, each of which is bound to a heme group with iron
at the center. The iron in this heme is able to bind to an
oxygen molecule in reversible fashion, so that it can bind and then release when necessary,
so each hemoglobin can bind four oxygen molecules, and there are around two hundred fifty million
hemoglobins per red blood cell, so one red blood cell can transport one billion oxygen molecules. Blood cells are produced through a process
called hematopoiesis, and this occurs in the red bone marrow, which is a soft network of
connective tissue found on certain blood capillaries, and which contains hematopoietic stem cells. For erythrocytes, this is more specifically
called erythropoiesis, and billions of new red blood cells are made every day to maintain
a nearly constant number, given that red blood cells function properly for only about three
months, only to be destroyed by macrophages, which are a phagocytic type of white blood cell. Next let’s look at leukocytes, or white
blood cells, which unlike the far more abundant red blood cells, are complete cells with nuclei
and organelles. These are part of the immune system, which
we will discuss in greater detail later, but for now, we will just understand that these
help us defend against pathogens and other harmful things. These use the circulatory system to get around
the body, but they can also slip out into other connective tissues to do their work. There are two types of white blood cells,
granulocytes and agranulocytes, which differ in the presence or absence of granules. The three types of granulocytes are neutrophils,
which kill bacteria, eosinophils, which kill parasitic worms, and basophils, which contain
histamine that attracts other white blood cells to a site of inflammation. Agranulocytes, on the other hand, can be lymphocytes,
which fight viruses and tumors, and also give rise to plasma cells, which produce antibodies,
which we will discuss later, or they can be monocytes, which become macrophages that can
eat up intruders. Leukocytes are produced by leukopoiesis, which
is stimulated by certain chemical messengers. Lastly we get to the platelets. These are fragments of large cells called
megakaryocytes. These fragments are essential during blood
clotting, which happens when blood vessels are damaged, as platelets can plug up any
holes or tears to seal things off. They flow through the bloodstream in an inactive
state unless needed, dying every ten days or so and constantly regenerated. Megakaryocytes form due to repeated mitotic
cycles that do not perform cytokinesis, so the result is one huge cell with a multilobed nucleus. This then presses against a sinusoid, and
its extensions burst to release the platelets. These platelets are important during hemostasis,
which is the process by which the body will stop bleeding through vascular spasm, platelet
plug formation, and then coagulation, or blood clotting. This essentially means that where there is
damage to a vessel, smooth muscle will contract, platelets will plug the tear, and a protein
called fibrin will form a mesh to patch everything up. Once the vessel has healed, the clot is removed
through a process called fibrinolysis, so that there is no blockage in the vessel. So those are the components of blood. We can look at a diagram like this one to
see how these arise during hematopoiesis. We can also briefly mention the different
blood types that humans can exhibit. These are A, B, AB, and O. These have to do with glycoproteins and glycolipids
found in the plasma membranes of red blood cells. A and B refer to two different agglutinogens
that can be found in these membranes, so blood group A has one of them, B has the other,
AB has both, and O has neither. This is important for blood transfusions,
because if someone’s body only recognizes A and gets blood with B, the new blood cells
will be recognized by antibodies as foreign and destroyed, which can be fatal, so someone
with AB blood can receive any blood, since both A and B will be recognized, hence they
are universal recipients, and someone with O blood can give blood to anyone, since there
are no markers to be recognized, hence they are universal donors. There are also Rh blood groups which refer
to agglutinogens called Rh factors, and for these a person is either positive or negative. This is reported along with the ABO blood
group by tacking on positive or negative to the end, giving us groups like O positive,
A negative, and so on. And with that, we are familiar enough with
the structure and function of blood that we can begin to examine the circulatory system
as a whole, so let’s move forward and do just that.

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  1. Hey Professor Dave! Just wanted to thank you for the videos. It's really nice to have a single channel that explains a whole lot of stuff! I personally enjoy learning about a variety of things, so when I'm a bit tired of one specific topic I go read about geography, or biology, or something else.

    It's nice to learn by reading but it's even better hearing someone who understands the stuff explain it, so thanks!

    "But what is it exactly? What's it made of? What does it do?" that's legit me earlier today in the bathroom trying to remember what Van der Waals force is haha

  2. 4:06 One thing I emphasize to my MCAT students is that there is a difference between oxygen molecules and oxygen atoms. Oxygen is diatomic in gaseous form so hemoglobin contains 4 oxygen molecules (O2) but 8 oxygen atoms (O).

  3. 0:09 Blood
    0:45 Blood is considered a Connective Tissue
    0:57 Formed elements suspended in plasma.

    Put it in a Centrifuge, and it’s parts can be seen.

    1:10 Red Blood Cells= Erythrocytes
    Platelets And White Blood Cells= Buffy Coat
    Plasma= Plasma

    1:32 Blood transports Oxygen, nutrients, hormones, and Carbon Dioxide

    2:00 + Regulates Ph
    + Maintain body temperature
    + Prevent infection

    2:11 Plasma
    Erythrocytes

    3:10 The Red Blood Cell
    3:38 Hemoglobin

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