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Components of Blood and their Functions

Blood is important tissue with a diverse set of functions in the body. It’s commonly thought of for its homeostatic procress: stabilizing pH, regulating temperature, and balancing osmotic pressure. However, blood also supports growth and development by distributing nutrients and hormones, while also removing waste. Blood also has a protective functions. Clotting factors  prevent blood loss and prevent pathogens from entering wounds, while pathogen-fighting white blood cells survey the circulatory system for foreign substances.


  • Plasma makes up more than half of blood and contains mostly water, proteins, salts, proteins and lipids.
  • There are three important cellular components to blood: erythrocytes, leukocytes and platelets.
  • Erythrocytes allow for gas transport; leukocytes provide immunity; and platelets prevent blood loss. 
  • Blood also aids the body in homeostatic processes like temperature regulation, osmotic balance, and balancing pH.

Blood Composition

Blood is the liquid that moves through the vessels. Separate blood into it’s components and you’ll find a layer of  plasma, a very small layer called the buffy coat, and the hematocrit. In humans, cellular components make up approximately 45 percent of the blood, with the liquid plasma making up the rest. Interestingly, blood is 20 percent of a person’s extracellular fluid and eight percent of weight.

Plasma makes up roughly 55% of blood and contains mostly water. The rest of the plasma is a combination of proteins, electrolytes, lipids, sugars and vitamins.

The buffy coat is very small component of blood containing white blood cells (leukocytes) and platlets (thrombocytes).

The hematocrit is the ratio of red blood cells to entire blood volume. Red blood cells make up 45% of the entire blood volume. 

Components of blood by percentage
Author/Source: Biomed Guide | Adapted From: OpenStax

Plasma and Serum

The liquid and cellular components of blood are separated by centrifuging the blood at high rotations. The upper layer (plasma), consists of 90 percent water with other substances required for maintaining the body’s pH, osmotic load, and for protecting the body. The plasma also contains the coagulation factors and antibodies.

Serum is plasma without the coagulation factors. There are several components that are transported by serum. Proteins that assist with maintaining pH and osmotic balance while also giving viscosity to the blood are one component. The serum also contains antibodies, which are specialized proteins that are important for defense against pathogens. Lipids, including cholesterol, are also transported in the serum, along with various other substances including nutrients, hormones, metabolic waste, plus external substances, such as, drugs, viruses, and bacteria.

Human serum albumin is the most abundant protein in human blood plasma and is synthesized in the liver. Albumin, which constitutes about half of the blood serum protein, transports hormones and fatty acids, buffers pH, and maintains osmotic pressures. Immunoglobin is a protein antibody produced in the mucosal lining and plays an important role in antibody mediated immunity.

Key Terms

  1. Hematocrit – the ratio of red blood cells to blood volume.
  2. Plasma – liquid component of blood containing water, salts and macromolecules.
  3. Coagulation factors – proteins in that control bleeding through clotting.
  4.  Serum – plasma without coagulation factors.


Cellular Components

The three cellular components that are summarized in the table below. 

Adapted From: OpenStaxLicense: CC BY 4.0



Common Name: Red blood cells.
Birthplace: Bone marrow; differentiated from myleoid stem cells.
Structure: Biconcave;  at maturity erythrocytes not contain a nucleus or mitochondria and are only 7–8 µm in size.
Function: Gas transport; delivery of  O2 to – and removal CO2 from – cells in the body
Lifespan: 100-130 days.
Notes: Contain hemoglobin; a special protein for transporting gases. 

Gas Transport

The red coloring of blood comes from the iron-containing protein hemoglobin. The principle job of this protein is to carry O2, but it also transports CO2. Hemoglobin is packed into red blood cells at a rate of about 250 million molecules of hemoglobin per cell. Each hemoglobin molecule binds four oxygen molecules so that each red blood cell carries one billion molecules of oxygen. There are approximately 25 trillion red blood cells in the five liters of blood in the human body, which could carry up to 25 sextillion (25 × 1021) molecules of oxygen in the body at any time.

In mammals, erythrocytes lack of organelles to leave more room for hemoglobin molecules. The absence of a mitochondria ensures the cell won’t use of the oxygen for metabolic respiration. Interestingly. only mammals have anucleated red blood cell, and some mammals (camels, for instance) even have nucleated red blood cells. The advantage of nucleated red blood cells is that these cells can undergo mitosis. Anucleated red blood cells metabolize anaerobically (without oxygen), making use of a primitive metabolic pathway to produce ATP and increase the efficiency of oxygen transport.

Red blood cells
Author: staff (2014)  | License: CC BY 4.0

Erythro- is derived from the Greek word ‘erythros’ (ερυθρός) meaning red. The -cyte portion is derived from ‘kytos’ which literately means ‘hollow vessel’ but is contemporary translation is taken to mean ‘cell’.

Link to Learning –

Not all organisms use hemoglobin to transport O2.  Some invertebrates use hemolymph rather than blood which use different pigments to bind O2.  One example is hemocyanin; a blue-green, copper-containing protein found in mollusks, crustaceans, and some of the arthropods. 



Common Name: White blood cells.
Birthplace: Bone marrow; differentiated from both lymphoid and myleoid stem cells.
Structure: Large and nucleated; different leukocytes had different sizes, but generally each is between 10–20 µm.
Function: Immune response; identification of foreign organisms and removal of pathogens. 
Lifespan: Hours to days; however some may live years.
Notes: There are several different white blood cells; they have different specific functions that revolve around immunity.


Structure: Contain granules in their cytoplasm.
Types of cells: Neutrophils, Eosinophils, and Basophils.


Structure: Do not contain granules in their cytoplasm.
Types of cells: Monocytes, and Lymphocytes



Lymphocytes are the primary cells of the immune system and include B-cells, T-cells, and natural killer cells.

      • → B-cells destroy bacteria and inactivate their toxins. They also produce antibodies.
      • → T-cells attack viruses, fungi, some bacteria, transplanted cells, and cancer cells. T cells attack viruses by releasing toxins that kill the viruses.
      • → Natural killer cells attack a variety of infectious microbes and certain tumor cells.

Some leukocytes become macrophages, which are phagocytic cells that can engulf and digest microbes and cellular debris. These cells can either stay at the same site or move to areas of inflammation. Chemical signals from foreign particles or damaged cells, provide signals that the macrophage used to guide itself.

White blood cells
Author: staff (2014)  | License: CC BY 4.0

Leuko- (λευκό) translated from the Greek literately means white. The -cyte portion is derived from ‘kytos’ which literately means ‘hollow vessel’ but is contemporary translation is taken to mean ‘cell’.

Link to Learning – HIV Infection

One reason that HIV is so dangerous is because it compromises immune cells. It has the ability to enter T-cells and once inside the cell, HIV replicated itself using the T-cell’s genetic machinery. After the HIV virus replicates, it is transmitted directly from the infected T cell to macrophages. The presence of HIV can remain unrecognized, and during this time, the virus is doing irreversible damage to key immune cells. 


Platelets aka Thrombocytes

Common Name
Birthplace: Bone marrow; within megakaryocytes. For each megakaryocyte, 2000–3000 platelets are formed.
Structure: Small, roughly 2–4 μm; contain many small vesicles but are anucleated (without a nucleus).
Function: Clotting; prevention of blood loss. 
Lifespan: Between 8–12 days.
Notes: There are several different white blood cells; they have different specific functions that revolve around immunity.


Blood Clotting

Blood must clot to heal wounds and prevent excess blood loss. Small cell fragments called platelets (thrombocytes) are attracted to the wound sites. Here, they adhere by extending many projections and releasing their contents. These contents activate other platelets and also interact with coagulation factors. The coagulation factors in turn convert fibrinogen (a water-soluble protein) present in blood serum into fibrin (a non-water soluble protein) causing the blood to clot. Many platelets converge and stick together at the wound site forming a platelet plug (also called a fibrin clot). The plug or clot lasts for a number of days and stops the loss of blood.

Author: staff (2014)  | License: CC BY 4.0

Technically, no. They’re cytoplasmic fragments of larger cells called megakaryocytes. Thrombocyte, literately meaning ‘clot’ ‘cell’, is somewhat of a misnomer since they are not actually cells.

  1. Clark MA, Douglas M, Choi J. “40.2 Components of the Blood.” Biology 2e. OpenStax, 2018. Houston, TX. CC BY 4.0 | License Terms: Edited & Adapted | Access for free
  2. Zedalis, J, and  Eggebrecht, J. “31.2 Components of the Blood.” Biology for AP® Courses, p. OpenStax, 2018. Houston, TX. TermsEdited & Adapted | Access for free at
  3. “Blausen 0909 – Types of white blood cells – English labels” by staff (2014), license: CC BY. Source: “Medical gallery of Blausen Medical 2014”