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.