14.5 Blood

Created by CK-12 Foundation/Adapted by Christine Miller

Figure 14.5.1 “I want to suck your blood.”


From Bram Stoker’s famous novel about Count Dracula, to films such as Van Helsing and the Twilight Saga, fantasies featuring vampires (like the one in Figure 14.5.1) have been popular for decades. Vampires, in fact, are found in centuries-old myths from many cultures. In such myths, vampires are generally described as creatures that drink blood — preferably of the human variety — for sustenance. Dracula, for example, is based on Eastern European folklore about a human who attains immortality (and eternal damnation) by drinking the blood of others.

What Is Blood?

14.5.2 Components of Blood
Figure 14.5.2 If blood is centrifuged (spun at high speed), it separates into its major components based on density, as shown here: plasma, leukocytes (white blood cells) and platelets, and erythrocytes (red blood cells). All blood normally contains these components in about the same proportions.

The average adult body contains between 4.7 and 5.7 litres of blood. More than half of that amount is fluid. Most of the rest of that amount consists of blood cells. The relative amounts of the various components in blood are illustrated in Figure 14.5.2. The components are also described in detail below.

Blood is a fluid connective tissue that circulates throughout the body through blood vessels of the cardiovascular system. What makes blood so special that it features in widespread myths? Although blood accounts for less than 10% of human body weight, it is quite literally the elixir of life. As blood travels through the vessels of the cardiovascular system, it delivers vital substances (such as nutrients and oxygen) to all of the cells, and carries away their metabolic wastes. It is no exaggeration to say that without blood, cells could not survive. Indeed, without the oxygen carried in blood, cells of the brain start to die within a matter of minutes.

Functions of Blood

Blood performs many important functions in the body. Major functions of blood include:

  • Supplying tissues with oxygen, which is needed by all cells for aerobic cellular respiration.
  • Supplying cells with nutrients, including glucose, amino acids, and fatty acids.
  • Removing metabolic wastes from cells, including carbon dioxide, urea, and lactic acid.
  • Helping to defend the body from pathogens and other foreign substances.
  • Forming clots to seal broken blood vessels and stop bleeding.
  • Transporting hormones and other messenger molecules.
  • Regulating the pH of the body, which must be kept within a narrow range (7.35 to 7.45).
  • Helping regulate body temperature (through vasoconstriction and vasodilation).

Blood Plasma

Plasma is the liquid component of human blood. It makes up about 55% of blood by volume. It is about 92% water, and contains many dissolved substances. Most of these substances are proteins, but plasma also contains trace amounts of glucose, mineral ions, hormones, carbon dioxide, and other substances. In addition, plasma contains blood cells. When the cells are removed from plasma, as in Figure 14.5.2 above, the remaining liquid is clear but yellow in colour.

Blood Cells

The cells in blood include erythrocytes, leukocytes, and thrombocytes. These different types of blood cells are shown in the photomicrograph (Figure 14.5.3) and described in the sections that follow.

14.5.3 SEM Blood Cells
Figure 14.5.3 Highly magnified blood cells in this image include doughnut-shaped red blood cells, rough-surfaced white blood cells, and small disc-shaped platelets.


The most numerous cells in blood are red blood cells, also called erythrocytes. One microlitre of blood contains between 4.2 and 6.1 million red blood cells, and red blood cells make up about 25% of all the cells in the human body. The cytoplasm of a mature erythrocyte is almost completely filled with hemoglobin, the iron-containing protein that binds with oxygen and gives the cell its red colour. In order to provide maximum space for hemoglobin, mature erythrocytes lack a cell nucleus and most organelles. They are little more than sacks of hemoglobin.

Erythrocytes also carry proteins called antigens that determine blood type. Blood type is a genetic characteristic. The best known human blood type systems are the ABO and Rhesus systems.

  • In the ABO system, there are two common antigens, called antigen A and antigen B. There are four ABO blood types, A (only A antigen), B (only B antigen), AB (both A and B antigens), and O (neither A nor B antigen). The ABO antigens are illustrated in Figure 14.5.4.
  • In the Rhesus system, there is just one common antigen. A person may either have the antigen (Rh+) or lack the antigen (Rh-).
14.5.4 ABO Blood Group
Figure 14.5.4 Each of the ABO blood types is characterized by different glycoproteins on red blood cells.

Blood type is important for medical reasons. A person who needs a blood transfusion must receive blood of a compatible type. Blood that is compatible lacks antigens that the patient’s own blood also lacks. For example, for a person with type A blood (no B antigen), compatible types include any type of blood that lacks the B antigen. This would include type A blood or type O blood, but not type AB or type B blood. If incompatible blood is transfused, it may cause a potentially life-threatening reaction in the patient’s blood.


Leukocytes (also called white blood cells) are cells in blood that defend the body against invading microorganisms and other threats. There are far fewer leukocytes than red blood cells in blood. There are normally only about 1,000 to 11,000 white blood cells per microlitre of blood. Unlike erythrocytes, leukocytes have a nucleus. White blood cells are part of the body’s immune system. They destroy and remove old or abnormal cells and cellular debris, as well as attack pathogens and foreign substances. There are five main types of white blood cells, which are described in Table 14.5.1: neutrophils, eosinophils, basophils, lymphocytes, and monocytes. The five types differ in their specific immune functions.

Table 14.5.1: Major Types of White Blood Cells
Type of Leukocyte Per cent of All Leukocytes Main Function(s)
Neutrophil 62% Phagocytize (engulf and destroy) bacteria and fungi in blood.
Eosinophil   2% Attack and kill large parasites; carry out allergic responses.
Basophil  less than 1% Release histamines in inflammatory responses.
Lymphocyte 30% Attack and destroy virus-infected and tumor cells; create lasting immunity to specific pathogens.
Monocyte   5% Phagocytize pathogens and debris in tissues.


Thrombocytes, also called platelets, are actually cell fragments. Like erythrocytes, they lack a nucleus and are more numerous than white blood cells. There are about 150 thousand to 400 thousand thrombocytes per microlitre of blood.

The main function of thrombocytes is blood clotting, or coagulation. This is the process by which blood changes from a liquid to a gel, forming a plug in a damaged blood vessel. If blood clotting is successful, it results in hemostasis, which is the cessation of blood loss from the damaged vessel. A blood clot consists of both platelets and proteins, especially the protein fibrin. You can see a scanning electron microscope photomicrograph of a blood clot in Figure 14.5.5.


14.5.5 Blood Clot
Figure 14.5.5 Erythrocytes become trapped in a coagulating clot so they cannot escape through a break in a blood vessel.
14.5.6 Activated Thrombocytes
Figure 14.5.6 The shape of platelets (thrombocytes) after they are activated helps them to stick together and form a plug for a damaged blood vessel.

Coagulation begins almost instantly after an injury occurs to the endothelium of a blood vessel. Thrombocytes become activated and change their shape from spherical to star-shaped, as shown in Figure 14.5.6. This helps them aggregate with one another (stick together) at the site of injury to start forming a plug in the vessel wall. Activated thrombocytes also release substances into the blood that activate additional thrombocytes and start a sequence of reactions leading to fibrin formation. Strands of fibrin crisscross the platelet plug and strengthen it, much as rebar strengthens concrete.

Platelet Party
Figure 14.5.7 Image by Nick Seluk/ theAwkwardYeti.com. (c) Used with permission.

Formation and Degradation of Blood Cells

Blood is considered a connective tissue, because blood cells form inside bones. All three types of blood cells are made in red marrow within the medullary cavity of bones in a process called hematopoiesis. Formation of blood cells occurs by the proliferation of stem cells in the marrow. These stem cells are self-renewing — when they divide, some of the daughter cells remain stem cells, so the pool of stem cells is not used up. Other daughter cells follow various pathways to differentiate into the variety of blood cell types. Once the cells have differentiated, they cannot divide to form copies of themselves.

Eventually, blood cells die and must be replaced through the formation of new blood cells from proliferating stem cells. After blood cells die, the dead cells are phagocytized (engulfed and destroyed) by white blood cells, and removed from the circulation. This process most often takes place in the spleen and liver.

Blood Disorders

Many human disorders primarily affect the blood. They include cancers, genetic disorders, poisoning by toxins, infections, and nutritional deficiencies.

  • Leukemia is a group of cancers of the blood-forming tissues in the bone marrow. It is the most common type of cancer in children, although most cases occur in adults. Leukemia is generally characterized by large numbers of abnormal leukocytes. Symptoms may include excessive bleeding and bruising, fatigue, fever, and an increased risk of infections. Leukemia is thought to be caused by a combination of genetic and environmental factors.
  • Hemophilia refers to any of several genetic disorders that cause dysfunction in the blood clotting process. People with hemophilia are prone to potentially uncontrollable bleeding, even with otherwise inconsequential injuries. They also commonly suffer bleeding into the spaces between joints, which can cause crippling.
  • Carbon monoxide poisoning occurs when inhaled carbon monoxide (in fumes from a faulty home furnace or car exhaust, for example) binds irreversibly to the hemoglobin in erythrocytes. As a result, oxygen cannot bind to the red blood cells for transport throughout the body, and this can quickly lead to suffocation. Carbon monoxide is extremely dangerous, because it is colourless and odorless, so it cannot be detected in the air by human senses.
  • HIV is a virus that infects certain types of leukocytes and interferes with the body’s ability to defend itself from pathogens and other causes of illness. HIV infection may eventually lead to AIDS (acquired immunodeficiency syndrome). AIDS is characterized by rare infections and cancers that people with a healthy immune system almost never acquire.
  • Anemia is a disorder in which the blood has an inadequate volume of erythrocytes, reducing the amount of oxygen that the blood can carry, and potentially causing weakness and fatigue. These and other signs and symptoms of anemia are shown in Figure 14.5.8. Anemia has many possible causes, including excessive bleeding, inherited disorders (such as sickle cell hemoglobin), or nutritional deficiencies (iron, folate, or B12). Severe anemia may require transfusions of donated blood.
14.5.7 Symptoms of Anemia
Figure 14.5.8 Anemia has wide-ranging effects on the human body because oxygen is essential for normal functioning of cells in every organ system.

Feature: Myth vs. Reality

Donating blood saves lives. In fact, with each blood donation, as many as three lives may be saved. According to Government Canada, up to 52% of Canadians have reported that they or a family member have needed blood or blood products at some point in their lifetime. Many donors agree that the feeling that comes from knowing you have saved lives is well worth the short amount of time it takes to make a blood donation. Nonetheless, only a minority of potential donors actually donate blood. There are many myths about blood donation that may help explain the small percentage of donors. Knowing the facts may reaffirm your decision to donate if you are already a donor — and if you aren’t a donor already, getting the facts may help you decide to become one.

Myth Reality
“Your blood might become contaminated with an infection during the donation.” There is no risk of contamination because only single-use, disposable catheters, tubing, and other equipment are used to collect blood for a donation.
“You are too old (or too young) to donate blood.” There is no upper age limit on donating blood, as long as you are healthy. The minimum age is 16 years.
“You can’t donate blood if you have high blood pressure.” As long as your blood pressure is below 180/100 at the time of donation, you can give blood. Even if you take blood pressure medication to keep your blood pressure below this level, you can donate.
“You can’t give blood if you have high cholesterol.” Having high cholesterol does not affect your ability to donate blood. Taking cholesterol-lowering medication also does not disqualify you.
“You can’t donate blood if you have had a flu shot.” Having a flu shot has no effect on your ability to donate blood. You can even donate on the same day that you receive a flu shot.
“You can’t donate blood if you take medication.” As long as you are healthy, in most cases, taking medication does not preclude you from donating blood.
“Your blood isn’t needed if it’s a common blood type.” All types of blood are in constant demand.

14.5 Summary

  • Blood is a fluid connective tissue that circulates throughout the body in the cardiovascular system. Blood supplies tissues with oxygen and nutrients and removes their metabolic wastes. Blood helps defend the body from pathogens and other threats, transports hormones and other substances, and helps keep the body’s pH and temperature in homeostasis.
  • Plasma is the liquid component of blood, and it makes up more than half of blood by volume. It consists of water and many dissolved substances. It also contains blood cells, including erythrocytes, leukocytes and thrombocytes.
  • Erythrocytes, (also known as red blood cells) are the most numerous cells in blood. They consist mostly of hemoglobin, which carries oxygen. Erythrocytes also carry antigens that determine blood type.
  • Leukocytes (also referred to as white blood cells) are less numerous than erythrocytes and are part of the body’s immune system. There are several different types of leukocytes that differ in their specific immune functions. They protect the body from abnormal cells, microorganisms, and other harmful substances.
  • Thrombocytes (also called platelets) are cell fragments that play important roles in blood clotting, or coagulation. They stick together at breaks in blood vessels to form a clot and stimulate the production of fibrin, which strengthens the clot.
  • All blood cells form by proliferation of stem cells in red bone marrow in a process called hematopoiesis. When blood cells die, they are phagocytized by leukocytes and removed from the circulation.
  • Disorders of the blood include leukemia, which is cancer of the bone-forming cells; hemophilia, which is any of several genetic blood-clotting disorders; carbon monoxide poisoning, which prevents erythrocytes from binding with oxygen and causes suffocation; HIV infection, which destroys certain types of leukocytes and can cause AIDS; and anemia, in which there are not enough erythrocytes to carry adequate oxygen to body tissues.

14.5 Review Questions

  1. What is blood? Why is blood considered a connective tissue?
  2. Identify four physiological roles of blood in the body.
  3. Describe plasma and its components.

14.5 Explore More

Joe Landolina: This gel can make you stop bleeding instantly, TED, 2014.

Can Synthetic Blood Help The World’s Blood Shortage? Science Plus, 2016.

How bones make blood – Melody Smith, TED-Ed, 2020.



Figure 14.5.1

vampire_PNG32 from pngimg.com is used under a CC BY-NC 4.0 (https://creativecommons.org/licenses/by-nc/4.0/) license.

Figure 14.5.2

Blood-centrifugation-scheme by KnuteKnudsen at English Wikipedia on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 14.5.3

SEM_blood_cells by Bruce Wetzel and Harry Schaefer (Photographers)/ NCI AV-8202-3656 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).

Figure 14.5.4

ABO_blood_type.svg by InvictaHOG on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).

Figure 14.5.5

Blood_clot_in_scanning_electron_microscopy by Janice Carr from CDC/ Public Health Image LIbrary (PHIL) ID #7308 on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:Public_domain).

Figure 14.5.6

Blausen_0740_Platelets by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 14.5.7

Platelet_Party_900x by Awkward Yeti (used with permission of the author) © All Rights Reserved

Figure 14.5.8

Symptoms_of_anemia.svg by Mikael Häggström on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/en:public_domain).


Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Blood, organ and tissue donation. (2020, April 28). Government of Canada. https://www.canada.ca/en/public-health/services/healthy-living/blood-organ-tissue-donation.html#a3

Canadian Blood Services. (n.d.). There is an immediate need for blood as demand is rising. https://www.blood.ca

Science Plus. (2016, March 2). Can synthetic blood help the world’s blood shortage? https://www.youtube.com/watch?v=hgp8LtwFSBA&feature=youtu.be

TED. (2014, November 20). Joe Landolina: This gel can make you stop bleeding instantly. YouTube. https://www.youtube.com/watch?v=e-5wqwp64MM&feature=youtu.be

TED-Ed. (2020, January 27). How bones make blood – Melody Smith. YouTube. https://www.youtube.com/watch?v=1Qfmkd6C8u8&feature=youtu.be



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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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