Blood components

Definitions & Key takeaways

There are four main components of blood: plasma, red blood cells, white blood cells, and platelets. Plasma is the liquid component of blood that consists of 55 % of the blood volume. It contains water, electrolytes, nutrients, hormones, and proteins. Red blood cells (RBCs) carry oxygen from the lungs to the body tissues. Red blood cells also transport carbon dioxide from the body. White blood cells (WBCs) help fight infection and disease. Platelets help with clotting.

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Introduction0:00–0:26

The word “blood” evokes lots of thoughts - from tiny paper cuts to major injuries - spilling the red liquid is almost never a good thing.
That’s because blood helps us move nutrients and waste around the body, regulate our pH level, and helps prevent infections.
In fact, some components of blood help prevent the loss of blood during an injury. The components of blood can be separated out by simply spinning the blood in a centrifuge-- a machine that whips a vial of blood in a circle over and over, really quickly.

Blood components0:26–0:58

A bit like what happens to clothes in a washing machine. When blood is centrifuged, the heaviest blood components moving to the bottom, and the lightest ones moving to the top.
Overall, three distinct layers form: the erythrocytes or red blood cells at the bottom, the buffy coat--which contains platelets and immune cells in the middle, and plasma at the top.

Erythrocytes0:58–2:21

So starting at the bottom of the tube, there’s the large layer that takes up approximately 45% of the total blood volume made up of erythrocytes.
This value is called the hematocrit. A decreased hematocrit means that there are too few erythrocytes, either because they’re not being made or because they are being destroyed.
On the other hand, an increased hematocrit can be due to dehydration, because if there’s less liquid in the blood, then the portion taken up by erythrocytes would rise.
Alternatively, there might simply be too many erythrocytes being made, which can happen in some diseases. Now, the main function of erythrocytes is to carry oxygen to tissues and bring carbon dioxide to the lungs so it can be expired.
Erythrocytes are shaped liked thin biconcave discs--meaning they have a depressed center which makes them flexible enough to fit through even the smallest blood vessels.
This shape also increases their surface area which helps them conduct gas exchange efficiently. Erythrocytes lack organelles like the nucleus, which creates even more room for hemoglobin proteins which carry oxygen.
While red blood cells are fantastic for gas exchange, the fact that they don’t have any organelles means that they only live for about 120 days.
So red blood cells are always being regenerated in the bone marrow. The thin white middle layer just above the erythrocytes is called the buffy coat, and it contains platelets and leukocytes, or white blood cells.

Buffy coat2:21–2:39

This layer generally accounts for less than 1% of the volume of whole blood--with most of the volume being taken up by the leukocytes.
Now, platelets are small pieces that split off of larger cells called megakaryocytes in the bone marrow. The main role of platelets is to clump together and form a plug that helps seal off a damaged blood vessel and prevent blood loss.

Platelets2:39–2:53

Leukocytes2:53–4:46

Then there are the leukocytes which are the only complete cells in blood--meaning they have all the usual organelles. There are lots of different types of leukocytes - and they all help to ward off pathogens like bacteria and viruses, destroy cancerous cells, and neutralize toxins.
Some leukocytes are called granulocytes because they contain tiny sacs called granules that are filled with inflammatory molecules.
Neutrophils are the most common granulocyte, making up about 60% of the leukocytes and these are usually the first to respond to an infection.
Other granulocytes include eosinophils and basophils, but these only make up about 2-5% of leukocytes. Eosinophils are largely responsible for fighting off parasitic infections while basophils are key in allergic reactions.
Leukocytes that don’t have granules, include lymphocytes and monocytes. Lymphocytes include B cells, T cells, and Natural killer cells, and make up about 35% of leukocytes.
Lymphocytes are responsible for the adaptive immune response which includes antibody production and is what allows our immune system to have “memory” so that we can effectively respond to pathogens that have caused infections in the past.
Lastly, there are the monocytes which make up about 5% of the leukocytes - these cells help gobble up bacteria or other pathogens via phagocytosis.
Unlike the other blood components, leukocytes have the ability to leave the blood and enter into tissues using a process called diapedesis, which is where they slip in between endothelial cells that line the blood vessels.
In a way leukocytes are like a mobile army--utilizing the blood as a highway to get to different areas of the body. Now, the top layer contains plasma which makes up approximately 55% of whole blood and is acellular, meaning it has no cells.

Plasma4:46–6:37

About 90% of plasma is water, and the rest is composed of proteins, electrolytes, and dissolved gases. The most abundant protein found in blood is albumin.
Albumin is made in the liver and it helps maintain oncotic pressure-- which is the force that helps to keep water in the bloodstream.
In addition, albumin is a transport protein - it shuttles fatty acids, calcium, lipid soluble hormones, and even some medications around the body.
Another group of plasma proteins are globulins, which include gamma globulins, which are antibodies that stick to pathogens and label them for destruction, as well as alpha and beta globulins which transport fats, metal ions, and fat-soluble vitamins.
Fibrinogen is another abundant plasma protein and it’s involved in clot formation for damaged vessels. Fibrinogen helps platelets attach to one another to form the initial platelet plug, and then other clotting factor proteins that are also in the plasma come along to stabilize the clot.
All of these clotting factor proteins, including fibrinogen, can be removed from a sample of plasma - and when that happens, what remains is called serum.
The electrolytes that are found in the plasma include mainly sodium, potassium, calcium, bicarbonate, and chloride. These electrolytes play vital roles in maintaining normal acid base physiology in the blood, and help regulate blood osmolarity - which is the overall concentration of blood.
Other solutes in the plasma include hormones, nutrients like glucose, and respiratory gases like oxygen and carbon dioxide that are dissolved in the blood.
Alright, as a quick recap - the blood is made up of about 45% erythrocytes, 55% plasma, which is mostly water but also contains proteins like albumin, electrolytes, and dissolved gases, and less than 1% is made up of platelets and leukocytes.

Review6:37–6:53