Published May 02, 2023 by

Blood - Definition, Composition, Plasma, Blood Cells and Functions


Blood is liquid connective tissue that is circulated throughout the cardiovascular system. In adult humans, there are approximately 5 liters of blood circulating through the heart and blood vessels. Like any connective tissue, the composition of blood consists of cells and extracellular matrix. Blood cells, also called formed elements, are erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). The production of blood takes place in the red bone marrow. The extracellular component of blood is a liquid and known as plasma. The functions of blood are many, but some of the most important are: 

1.The transport and delivery of oxygen, nutrients, hormones, and other humoral substances. 

2.The transport of carbon dioxide (CO2) and metabolic wastes from cells. 

3.Vital functions and reactions of the body such as coagulation, acid-base balance and thermoregulation.

Based on its approximate composition, blood is usually classified as oxygenated or deoxygeneted blood. 

1.Oxygenated blood comes from the pulmonary circulation, flows through the arteries, and is rich in oxygen and nutrients, which it carries to the tissues. Oxygen gives the light red color to this blood. 

2.Deoxygenated blood comes from the tissues, flows through the veins, and is rich in carbon dioxide and tissue waste, which it carries to the lungs for elimination. Carbon dioxide gives this blood a darker red color. This article will discuss the histological features and functions offubalance.


Plasma is a clear, slightly yellowish liquid that represents 55% of the total blood volume. The blood cells are suspended into  the plasma through red blood cells, white blood cells, and platelets. It is made up of water (upto 90%), mineral salts and a large amount of proteins that ensure the proper functioning of our body, among which the following stand out:

Plasma proteins are divided into five main groups:


makes up about 60% of plasma proteins. It is produced in the liver and its main function is to maintain the osmotic and oncotic pressure of the blood. Generally speaking, if albumin levels drop below what is needed, plasma tends to leak out of the blood vessels. This leads to decreased blood volume, which can progress to hypovolemic shock.Albumin, a protein that transports hormones and drugs and carries them to the tissues. Of the 5 liters of blood that a 70-kilo person has on average, about 3 liters are plasma.

 2.Alpha (α) and beta (β) globulins:

produced mainly in the liver. They contain various transport factors (such as transferrin, which binds and transports iron) and coagulation factors.These are responsible, along with platelets, for stopping bleeding when we cut ourselves. 

3. Gamma (γ) globulins: 

also known as antibodies or immunoglobulins. They are produced by B lymphocytes (plasma cells).It is a defense that protect us from infections. 


produced by hepatocytes. It undergoes structural changes during coagulation, allowing the formation of the clot that prevents blood loss (hemorrhage) from an injured blood vessel. 

5.Complement proteins:

a group of proteins that are activated in multiple inflammatory events and help in the elimination of microorganisms.

In addition to the specific functions described above, plasma proteins act as buffers or buffers to maintain the homeostatic value of blood pH (7.4). These proteins have the ability to bind to and neutralize acidic or alkaline substances, substances that would otherwise cause changes in blood pH. This makes plasma proteins one of the most powerful buffer systems that help maintain the acid-base balance of the body. Other buffer systems include phosphate, bicarbonate, and carbonic acid ions, as well as the complex mechanism of renal filtration.

In your physiology classes, you will probably hear the word serum, which is often confused with plasma. When blood leaves the blood vessels, clotting factors and fibrinogen react with each other to produce the clot. What remains of the plasma after clot formation is the serum. Basically this is the definition of plasma and serum: 

Plasma: liquid part of the blood that consists of water, proteins and nutrients.

Serum: part of the plasma without fibrinogen or coagulation factors.

plasma donation 

Plasma donation or "plasmapheresis" consists of extracting blood, separating the blood cells using a cell separator, retaining the plasma and returning the rest to the donor through the same route.

The donation takes about 45 minutes. Since plasma is mostly water, donor recovery is very fast and donations can be more frequent. Plasma can be donated every 15 days. 

Who can donate plasma? 

To donate plasma, the same requirements must be met as to be a blood donor. You must be between 18 and 70 years old, weigh more than 50 kilos, be in good health and, if you are a woman, not be pregnant. Also, if you have given blood, you always have to wait a month for your next donation. 

All blood groups are valid, although group AB is the universal plasma donor, compatible with all recipients. Unfortunately, only 3 out of 100 people in the world are from this group.

What is plasma used for? 

Every day patients from all over Catalonia need plasma for their treatments. Learn about their stories here.

When a person has suffered major burns or a serious accident, plasma transfusion helps stop bleeding and restore blood volume.

It is also possible that a patient only needs some of the proteins contained in the plasma. In these cases, plasma proteins are extracted and medicines are made, called "blood products", which can only be obtained from the blood of other people. 

Hemophiliacs, for example, can bleed extensively because they are missing a protein that helps blood clot.

Blood cells:

Blood cells are the formed elements suspended in the plasma. These are erythrocytes, leukocytes and thrombocytes. Their structure and appearance are often examined under the microscope in blood smears, and their relative counts are calculated in routine blood tests. The total number of blood cells in blood tests is known as CBC or complete blood count.


They are round, biconcave cells without a nucleus that carry oxygen attached to their heme groups. 

The erythrocytes are the cell in the highest percentage that does not bleed and is also called hemacy or red blood cell. 

Erythrocytes, red blood cells or red blood cells are one of the constituent cells of blood, just like platelets and white blood cells. It contains hemoglobin, a protein that binds oxygen to transport it from the lungs to the tissues and organs, in the bloodstream. Not running their functions, the organs and tissues consume oxygen and produce carbon dioxide, which is also transported by red blood cells to the lungs and, subsequently, expired.

How do you recognise the erythrocytes?

Contrary to most cells (and as happens, for example, with white blood cells), the red blood cells do not have a cell nucleus. This gives them some plasticity in the form and means that they can move through the bloodstream more easily. In general, keep a rounded shape and slightly flattened in the center.

Its main component is hemoglobin. It is this protein that is given to the vermelha and, in turn, to the blood, because the red globules represent 40% of the blood.

Like most cells in the human body, erythrocytes are produced in the bone marrow under the command of the hormone erythropoietin and with the help of essential micronutrients for this function, such as iron and vitamins B12 and B9. 

The red cells have a relatively short life span, when compared to other cells: 120 days. Over this time, they lose their cell membranes and end up being shed by sludge and liver.

What is the ideal amount of erythrocytes?

The ideal levels of red blood cells depend on sex and age. In general terms:

Children: between 4.0 and 5.5 thousand erythrocytes per microliter of blood

Men: 4.7 to 6.1 million erythrocytes per microliter of blood 

Women: 4.2 to 5.4 million erythrocytes per microliter of blood 

What tests can be done on erythrocytes?

The hemogram is to analyze the blood and it is possible to analyze the levels of red blood cells present in the blood, as well as those of other components of the blood stream. 

What risk factors can affect your erythrocyte levels? 

There are some factors more frequently related to lifestyle, such as an unbalanced diet, lack of iron and vitamins, or the fact of being a smoker, which can affect production and blood levels. The use of anabolic steroids may have the same effect. 

2.Leukocyte :

Leukocytes are consist of neutrophils, eosinophils, basophils, lymphocytes (B and T) and monocytes.


Leukocytes or as they are also called white blood cells, are a type of cell that belongs to the bloodstream, and that has functions related to the protection of the organism against infectious agents, bacteria or viruses. So, when their levels increase or decrease, they serve as indications of the establishment of some pathological process in the body.

These cells are formed by the bone marrow, so when diseases such as tumors occur, a pathology known as leukemia may appear, which is characterized by limiting the body to fight against harmful agents, as well as the appearance of the following symptoms: fatigue, tiredness , sudden weight loss, hair loss.

Classification of Leukocyte :

According to the function and structure of the cells, they can be classified into several types. Among which we can highlight monocytes (eliminate dead tissue), lymphocytes (fight viruses, cancer, infections), neutrophils (attack bacteria and fungi), basophils (act on allergies and release histamine) and eosinophils (fight against parasites). .

 How are leukocytes measured? 

To measure this type of cells, it is necessary for the person to see a doctor who will evaluate them and then order a blood test. After the blood extraction, an analysis of the amount of leukocytes in the liquid is carried out using a microscope, and thus it is determined which are the types of cells that are increased or decreased.

How do leukocytes work? 

Leukocytes act in different ways depending on the harmful agent that you want to attack, but in general what these white blood cells do is leave the bloodstream through a cellular process, then they go to the place where the substance is located and proceed to destroy it through a mechanism called phagocytes.

How to increase or decrease leukocytes?

Depending on the alteration, that is, if there is an increase or if, on the contrary, the blood values ​​are decreased, different interventions can be carried out since these deficits have characteristic symptoms. So, when you want to reduce certain blood cells, what are administered are medications, such as antibiotics in cases of bacteria, which help to attack the harmful agent, and instead to increase the body's defenses, it is recommended to consume nutritious foods rich in protein, vitamins and minerals. 

But, for example, in the case of cancer, a more specific treatment must be carried out, which may include radiotherapy and chemotherapy.


A tiny, disk-shaped piece of cell found in the blood and spleen. They are derived from megakaryocytes and are responsible for hemostasis. Thrombocytes are fragments of very large cells in the bone marrow called megakaryocytes. They help produce blood clots to slow or stop bleeding and to facilitate wound healing. There are problems when the number of thrombocytes is insufficient or excessive, or the platelets do not work as they should. Measuring the number of platelets in the blood can sometimes help diagnose certain diseases or disorders. Also called platelet. 

Functions of blood:

Courier and waste disposal 

Blood is the most important means of transport in the body. One of the major function of blood is the transportation of gases (oxygen, carbon dioxide, nitrogen, etc), nutrients (for metabolism) and final products of cellular metabolism. Hence, the blood has the task of ensuring the exchange of substances. It provides gases and nutrients to the tissues and in return transports waste products (carbon dioxide, urea, uric acid, creatinine, among others) to the organs responsible for eliminating them (lungs, liver, kidneys). Again, it carries chemical messengers ( enzymes and hormones) to its target organs.

Acid-base balance 

Homeostasis or acid-base balance is regulated in the blood by gaseous diffusion between the alveoli and the lungs (alveolar diffusion). Oxygen is diffused from the alveoli into the blood because of the concentration gradient. It is picked up by a transport protein, hemoglobin (heme=contains iron, globin=protein). Conversely, carbon dioxide diffuses from the blood into the alveoli due to its increased blood concentration and is then exhaled.

Oxygen supply and carbon dioxide removal 

The blood carries oxygen from the alveoli to the most remote cell in the body. Due to the higher pressure of oxygen in the plasma (relative to the pressure in the cells), it diffuses into the tissues.

Carbon dioxide diffuses from the cells into the blood due to the increased pressure of the gas in the tissues. Once in the blood, a chemical reaction occurs that creates carbonic acid (CO2 + H2O → H2CO3), which dissociates into hydrogen ions (H+) and bicarbonate (HCO3-). This is how carbon dioxide, the final product of metabolism, is transported as carbonic acid (or rather as hydrogen ions and bicarbonate). Once it reaches the lungs, the previous reaction is reversed and the carbon dioxide is exhaled.

In conclusion, blood regulates acid-base homeostasis through gas exchange. The blood is also responsible for homeostasis, for example, the balance of water between the blood capillaries and the intra- and extracellular spaces. In addition, it also helps maintain a constant body temperature. 


Clotting factors are dissolved in the blood and stop bleeding after a complex cascading activation, initiated by injury to a blood vessel, ultimately ending in thrombus (clot) formation. This process is called thrombogenesis. On the other hand, plasmin breaks down pathological clot formation in blood vessels by breaking down fibrin (fibrinolysis). Coagulation and fibrinolysis influence each other and maintain a sensitive balance.