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Components of blood

The different components that make up blood. Plasma, white blood cells, red blood cells, platelets.


If you prick your finger or scrape your knee, you'll see some droplets of blood form. Just by eye, these droplets may seem to be made of uniform red liquid, similar to food coloring or paint. However, if you were to look under a microscope, you would see that your blood is actually a mixture of liquid and cells. And if you could zoom in even further, you would see that there are also many macromolecules (such as proteins) and ions (such as sodium) floating in the liquid. All of these components are important to the roles blood plays in the body.

What is blood?

Blood, by definition, is a fluid that moves through the vessels of a circulatory system. In humans, it includes plasma (the liquid portion), blood cells (which come in both red and white varieties), and cell fragments called platelets.
  • Plasma is the main component of blood and consists mostly of water, with proteins, ions, nutrients, and wastes mixed in.
  • Red blood cells are responsible for carrying oxygen and carbon dioxide.
  • Platelets are responsible for blood clotting.
  • White blood cells are part of the immune system and function in immune response.
Cells and platelets make up about 45% percent of human blood, while plasma makes up the other 55% percent. The diagram below shows red blood cells, white blood cells of different types (large, purple cells), and platelets.


Plasma, the liquid component of blood, can be isolated by spinning a tube of whole blood at high speeds in a centrifuge. The denser cells and platelets move to the bottom of the tube, forming red and white layers, while the plasma remains at the top, forming a yellow layer.
The plasma is about 90% water, with the remaining 10% made up of ions, proteins, nutrients, wastes, and dissolved gases. The ions, proteins, and other molecules found in plasma are important for maintaining blood pH and osmotic balance, with albumin (the main protein in human plasma) playing a particularly important role.
Some of the molecules found in the plasma have more specialized functions. For example, hormones act as long-distance signals, antibodies recognize and neutralize pathogens, and clotting factors promote blood clot formation at the site of wounds. (Plasma that’s been stripped of its clotting factors is called serum.) Lipids, such as cholesterol, are also carried in plasma, but must travel with escort proteins because they don’t dissolve in water.

Red blood cells

Red blood cells, or erythrocytes, are specialized cells that circulate through the body and deliver oxygen to tissues. In humans, red blood cells are small and biconcave (thinnest in the center, just 7 - 8 μm in size), and do not contain mitochondria or a nucleus when mature.
These characteristics allow red blood cells to effectively perform their task of oxygen transport. Small size and biconcave shape increase the surface area-to-volume ratio, improving gas exchange, while lack of a nucleus makes additional space for hemoglobin, a key protein used in oxygen transport. Lack of mitochondria keeps red blood cells from using any of the oxygen they’re carrying, maximizing the amount delivered to tissues of the body.
_Image modified from "Modified sickle cell," by NHLBI (public domain)._
In the lungs, red blood cells take up oxygen, and as they circulate through the rest the body, they release the oxygen to the surrounding tissues. Red blood cells also play an important role in transport of carbon dioxide, a waste product, from the tissues back to the lungs. Some of the carbon dioxide binds directly to hemoglobin, and red blood cells also carry an enzyme that converts carbon dioxide into bicarbonate. The bicarbonate dissolves in plasma and is transported to the lungs, where it's converted back into carbon dioxide and released.
Red blood cells have an average life span of 120 days. Old or damaged red blood cells are broken down in the liver and spleen, and new ones are produced in the bone marrow. Red blood cell production is controlled by the hormone erythropoietin, which is released by the kidneys in response to low oxygen levels. This negative feedback loop ensures that the number of red blood cells in the body remains relatively constant over time.

Platelets and clotting

Platelets, also called thrombocytes, are cell fragments involved in blood clotting. They are produced when large cells called megakaryocytes break into pieces, each one making 2000 - 3000 platelets as it comes apart. Platelets are roughly disc-shaped and small, about 2 - 4 μm in diameter.
When the lining of a blood vessel is damaged (for instance, if you cut your finger deeply enough for it to bleed), platelets are attracted to the wound site, where they form a sticky plug. The platelets release signals, which not only attract other platelets and make them become sticky, but also activate a signaling cascade that ultimately converts fibrinogen, a water-soluble protein present in blood plasma, into fibrin (a non-water soluble protein). The fibrin forms threads that reinforce the platelet plug, making a clot that prevents further loss of blood.
_Image modified from Components of the blood: Figure 4, by OpenStax College, Biology (CC BY 4.0)._

White blood cells

White blood cells, also called leukocytes, are much less common than red blood cells and make up less than 1% of the cells in blood. Their role is also very different from that of red blood cells: they are primarily involved in immune responses, recognizing and neutralizing invaders such as bacteria and viruses.
White blood cells are larger than red blood cells, and unlike red blood cells, they have a normal nucleus and mitochondria. White blood cells come in five major types, and these are divided into two different groups, named for their appearance under a microscope.
  • One group, the granulocytes, includes neutrophils, eosinophils, and basophils, all of which have granules in their cytoplasm when stained and viewed on a microscope.
  • The other group, the agranulocytes, includes monocytes and lymphocytes, which do not have granules in the cytoplasm.
    _Image modified from "Components of the blood: Figure 3," by OpenStax College, Biology (CC BY 4.0)._
Each type of white blood cell plays a specific role in defense. For example, some white blood cells are involved in engulfing and breaking down pathogens, while others recognize specific microorganisms and launch immune responses against them. Different types of white blood cells have different lifetimes, ranging from hours to years, and new cells are produced primarily in the bone marrow (although some are made or mature in the thymus, lymph nodes, and spleen).

Stem cells and blood cell production

Red blood cells, white blood cells, and platelet-producing cells are all descended from a common precursor: a hematopoietic stem cell.
A hallmark of stem cells is that they divide asymmetrically. That is, one daughter cell remains a stem cell of the same type, while the other daughter cell acquires a new identity. For hematopoietic stem cells, which are found in the bone marrow, one daughter cell remains a hematopoietic stem cell, while the other goes on to become a different type of stem cell: either a myeloid stem cell or a lymphoid stem cell.
Image modified from "Hematopoietic system of bone marrow," by OpenStax College, Anatomy & Physiology (CC BY 3.0).
The myeloid stem cells and lymphoid stem cells also divide asymmetrically, with their non-stem cell daughters generating the mature cell types of the blood. Myeloid stem cells give rise to red blood cells, platelets, and some types of white blood cells, while lymphoid stem cells give rise to the types of white blood cells classified as lymphocytes.
Image modified from "Hematopoietic system of bone marrow," by OpenStax College, Anatomy & Physiology (CC BY 3.0).
Hematopoietic, myeloid, and lymphoid stem cells divide throughout a person's lifetime, generating new blood cells to replace old and worn-out ones.

Want to join the conversation?

  • winston baby style avatar for user Kelly
    If there is bacteria in our blood and there is only 1% of white blood cells, wouldn't that take a long time to dispose of the bacteria?
    (9 votes)
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    • piceratops ultimate style avatar for user Ian Cox
      Most bacteria is good and makes our body function better. If we didn't have any bacteria in our body, we would die. A lot of the bacteria which gets into your body will actually help fight off bad bacteria, viruses, etc! A very small percentage of the bacteria in our body is bad, so when there is one, the white blood cells in addition to some of the good bacteria will help fight it off. 1% is actually plenty, because if the body can get to it before it multiplies constantly, then it can get rid of the bad thing quickly and we won't even know we were sick.
      (15 votes)
  • blobby green style avatar for user 😊
    what is nucleus
    (0 votes)
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    • aqualine seedling style avatar for user SpinosaurusRex
      The nucleus is a membrane bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction. It is commonly the most prominent organelle in the cell.

      The nucleus is surrounded by a structure called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm. The cell's chromosomes are also housed within the nucleus. Chromosomes contain DNA which provides the genetic information necessary for the production of other cell components and for the reproduction of life.
      (25 votes)
  • aqualine ultimate style avatar for user Rosse
    how have we evolved if we have such a complicatid biology?
    (5 votes)
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    • starky tree style avatar for user Wugee
      Think of single-celled organisms. They are among the first of all life-forms, and they are incredibly simplistic. Evolution is the process of becoming better and adapting, therefore advanced beings have advanced biology. It would definitely be hard for an amoeba to formulate the answer to a differential equation with simple flagellate motions. However, our brains are capable of doing so thanks to the multiple support systems, which are complicated biology.

      Hope this helped!
      (12 votes)
  • male robot johnny style avatar for user Seye.Solabomi
    What is the amount of time platelets use to clot where a person injured his or herself?
    (4 votes)
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  • starky ultimate style avatar for user Ev
    I thought red blood cells are already larger in diameter than the smallest capillaries, so that they have to squeeze through. but now white blood cells are even larger? Don't they also have to travel through the same capillaries?
    (5 votes)
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    • leafers seed style avatar for user Jonathan Widarsa
      Firstly, there are three types of capillaries in our body - continuous, fenestrated and sinusoid. The major difference n these three types are that continuous capillaries are less permeable than fenestrated, having sinusoid to be the most permeable and big-sized capillaries in our body. Apparently, the main functions of these types vary accordingly with the permeablity of the vessels and for sinusoid, as it is the largest capillary, allows larger molecules to flow through it, including white blood cells of any kind.

      Another idea unrelated to this also explains that the role of leukocytes are to fend off external breaches by activating immune system, phagocytosis, releasing substances, etc. Thus, leukocytes are able to escape through any vessel walls to arrive at the needed regions of the body ASAP. So they do not have to pass through the capillaries.

      I've checked some biology sites and these are all informations I could gather, hope it helps :).
      (8 votes)
  • aqualine ultimate style avatar for user YEBrandy
    Are erythrocytes (after maturation) still considered cells of the body? (No nucleus, etc)
    (6 votes)
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  • leaf blue style avatar for user Silvana
    The text says: ''Red blood cells, white blood cells, and platelet-producing cells are all descended from a common precursor: a hematopoietic stem cell.''
    My question is: Where are hematopoietic stem cells produced?
    I'm referring to the section: Stem cells and blood cell production.
    (3 votes)
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  • duskpin ultimate style avatar for user keilahvillasana
    How many red blood cells are produced every day? And how come our bodies don't OVERproduce? Can you have too many red blood cells?
    (2 votes)
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    • female robot grace style avatar for user rune
      I got this from the Internet, but I think it will help.
      Approximately 2.4 million new erythrocytes are produced per second in human adults. In a normal adult about half a liter are produced by the bone marrow every week.
      High red blood cell count -
      high red cells count is caused by a disorder called Polycythemia Vera. This is a genetic condition that causes the bone marrow to produce too many blood cells. People with polycythemia have an increase in hematocrit, hemoglobin, or red blood cell count above the normal limits.
      for more information visit : -
      I know this page has a lot of ads and Spam, but it also has some valuable information
      (4 votes)
  • aqualine seed style avatar for user Madina Roshan
    would the components of blood be effected by your blood type?
    (2 votes)
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    • male robot donald style avatar for user Tybalt
      Yes. Platelets and especially plasma have their own traits that are influenced by one's blood type. Plasma, for instance, can carry anti-A antibodies, anti-B antibodies, both antibodies, or none depending on blood type. This is why blood type still must be considered for plasma and platelet transfusions.

      Does that help?
      (3 votes)
  • blobby green style avatar for user gianina carletti
    Why is a stem cell an ethical issue?
    (2 votes)
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    • male robot donald style avatar for user Tybalt
      While everybody has stem cells, the cells used for research primarily come from human embryos. Here's the problem: when you harvest stem cells from an embryo, you destroy it. Killing a human being is typically viewed as murder; however, this being an unborn child raises several questions. Would it still be classified as murder if the child was never born? Can it even be classified as murder if the human being was very early in development, to the point where it was still only a mass of cells? Does it feel anything at that point? Is it part of the woman's body, or is it its own being?

      Much of the questions asked about the matter tie closely to the issue of abortion. It is an ethical issue because when exactly something becomes its own being is unclear, and there is a clear and significant pro (forwarding science for the better) and a clear and significant con (the death of a potential child) to be seen. There is no definite answer to this question, and it is ultimately up to your morals and views to give you the answer.

      Does this help?
      (3 votes)