Blood – a vital liquid

Blood – a vital liquid

Blood is a vital fluid that circulates continuously through the blood vessels and the heart.

This fluid serves to distribute oxygen (O2) and nutrients needed for vital processes to all tissues of the body, and to transport waste products such as carbon dioxide (CO2) or nitrogenous waste products to disposal sites (kidneys, lungs, liver, intestines). It also serves to deliver cells and molecules of the immune system to the tissues and to diffuse hormones throughout the body.

It is the bone marrow that produces blood cells in a process called hematopoiesis.
Blood is at first sight recognizable at the opening of the very first vertebrates, like the sea lamprey, petromyzon marinus, a species still living today. In the phylogenetic classification, since the Cambrian (about 500 million years ago), the Petromyzontidae already had a hemoglobin allowing the transport of oxygen to the tissues, in a closed circulation, where the blood can keep its properties. Normally inapparent, it is through bleeding that it began to be recognized and identified by its sensory characteristics (color, smell, taste, touch) before the more specific physicochemical analyses. In case of breach or effraction of the vessels, its properties of colored mobile fluid are spontaneously, rapidly and irreversibly transformed, the spilled blood signalling the attack of the integrity of an evolved living organism -its vulnerability-, and it participates since then, in the cycle of predation behaviors of many species.

The blood of vertebrates is red. It becomes light red during oxygenation in the lungs or gills. It becomes red in the arteries and then dark red when it loses oxygen to the tissues. Looking at the veins through light-colored skin, the blood appears blue but it is actually dark red, even inside the veins. It is the skin that acts as a filter, allowing only the blue to pass through.

The heart circulates blood throughout the body. It passes through the lungs to take in oxygen and remove carbon dioxide (small circulation), and then circulates through the body via the blood vessels (large circulation). It releases its oxygen and takes on carbon dioxide in the blood capillaries, which are the smallest blood vessels in the body. In its deoxygenated state, its red color is less bright (as in the case of peripheral venous blood, for example).

The blood also carries certain metabolic waste products (toxic above a certain dose), as well as certain toxins brought in by the lungs, the intestine or the transcutaneous route. The liver or kidneys extract some of these toxins, which are evacuated in the bile or urine.

As a connective tissue, blood contains cellular elements and basic substances, but it has no fibers. The color of blood comes from hemoglobin.

Erythrocytes or red blood cells (about 99%). They have no nuclei or organelles, so they are not cells per se. They contain hemoglobin (1⁄3 of the components of the cytoplasm) which allows them to transport oxygen as well as iron but also carbon dioxide or carbon monoxide. Their life span is 120 days and their destruction is carried out by the liver, spleen or bone marrow.
Leucocytes or white blood cells (0.2%), which are used in the immune system to destroy infectious agents. The leukocytes are a heterogeneous group of cells::
granulocytes or polymorphs (neutrophils, eosinophils, basophils) ;
Thrombocytes or platelets (0.6 – 1.0 %), responsible for the formation of the platelet nail that starts blood coagulation. They are not cells because they do not contain a nucleus, but fragments of cytoplasm from their precursors, the megakaryocytes (giant cells of the bone marrow).

These figurative elements constitute 45% of the whole blood, which are all the cells contained in the blood. The remaining 55% of the blood is plasma, a yellowish liquid which is the liquid phase in which the figurative elements are suspended.

Plasma is the liquid component of the blood in which the figurative elements bathe; however, it is important to understand that the figurative elements are not part of the plasma. It is made up of water, ions and various molecules that are transported throughout the body. It must also be distinguished from blood serum, a liquid resulting from a retracted blood clot, whose composition is somewhat different from that of blood plasma, as it lacks fibrinogen in particular.

Plasma is the liquid component of the blood in which figurative elements are bathed; however, it is important to understand that figurative elements are not part of plasma. It is made up of water, ions and various molecules that are transported throughout the body. It must also be distinguished from blood serum, a liquid resulting from a retracted blood clot, whose composition is somewhat different from that of blood plasma, as it lacks fibrinogen in particular.

Here are the main molecules of the plasma solute: (the solvent being obviously water)

  • Glucose ;
  • Lipids ;
  • Hormones (which can be proteins, modified amino acids, steroids, or modified lipids including prostaglandins and thromboxanes);
  • Proteins (which can be separated by electrophoresis into several peaks, albumin, α1, α2, β, γ), the main ones being:
    albumin, role, in oncotic pressure, of transporter (of bilirubin, hormones, drugs, ions…).
    immunoglobulins of the immune system
    complement proteins which have a major role in the initiation of the immune response and inflammation;
    blood coagulation proteins (coagulation factors).


A transport function: the blood (circulating liquid) ensures a double transport function, it distributes oxygen and nutrients necessary for the functioning and survival of all the cells of the body and at the same time, recovers carbon dioxide and waste (urea) which result from the activity of any living organ;
The blood is made up of an almost colorless liquid very rich in water (plasma) in which red and white blood cells and coagulants are present;
The blood is enriched with nutrients and receives a large part of the water contained in food;

Blood gets rid of collected waste (carbon dioxide, etc.) and is enriched with oxygen in the lungs;
The blood gets rid of its excess water; urine (water containing waste) is “made” by the kidneys;
Only the red blood cells, which contain hemoglobin, give the blood its red color. Their number is considerable (4,500,000 per mm3 of blood) and their essential function is the transport of oxygen and dioxygen. The latter bind to hemoglobin, facilitated by its biconcave disk shape (central region: 0.8µm, peripheral region: 2.6µm) which is the most suitable for maximum binding.


Blood can be fractionated into its different components in two ways:

Centrifugation in the presence of an anticoagulant: 2 phases are obtained, a yellowish supernatant (plasma) and a pellet of red blood cells. At the interface between these 2 phases, a whitish ring containing leukocytes can be observed. In this case, a simple agitation allows to mix all these components and to put them in suspension.
Centrifugation without anticoagulant: a yellowish supernatant (serum) is obtained and a pellet containing red blood cells trapped in a fibrin network. In this case, the red blood cells cannot be resuspended.


Blood donation; just under half a liter of blood can be collected without any significant consequences for a healthy donor

Hematology is the medical specialty in charge of the study of blood circulation disorders:

The main hematological disorders are :
Hemophilia is a genetic disease.
leukemia (or “blood cancers”);
the blood can also be “poisoned” by numerous compounds including carbon monoxide, or lead (which causes lead poisoning).

Blood and infectious diseases :

malaria: plasmodium (parasite) colonizes red blood cells.

Some diseases can be transmitted by blood transfusion, including hepatitis C and AIDS (the virus (HIV) can be transmitted by contact between a person’s blood and blood and/or semen).
For this reason, in some contexts, bloodstained objects are treated as a biohazard.


Body injuries can result in significant blood leakage (hemorrhages). Thrombocytes are used to clot blood in minor wounds, but major wounds must be repaired immediately to prevent exsanguination. Internal wounds, sometimes unnoticed, can cause severe bleeding.

Blood transfusions:

Significant blood loss, traumatic or otherwise (e.g., during surgery), or a blood disease such as anemia or thalassemia, may require blood transfusions.

Many countries have blood banks to meet the need for blood for transfusion. A transfused person must have a compatible blood type with the donor (iso-group transfusion);

it is possible to transfuse red blood cells (“leukocyte-depleted blood”: in this case, only the red blood cells are kept, in order to minimize the risk of a reaction from the recipient) or platelets alone;
the equipment used for donations is single-use, and each donation is now subject to a battery of tests to detect diseases such as HIV or hepatitis C. The risks of transmitting infectious diseases to the recipient are thus reduced to a minimum;
Since a bag of blood can only be kept for a little over a month (42 days for erythrocyte concentrates in Mannitol SAG, 5 days for platelet concentrates) and since blood cannot be manufactured artificially, donation is essential to maintain blood stocks.

Blood pressure is an important diagnostic tool.

Blood in humans represents 7 to 8% of the body mass.

The arteries carry blood from the heart to the lungs and all other organs. They are the pressure reservoir of the cardiovascular system.

The veins carry blood from the lungs or any other organ back to the heart. The volume proportion of blood in the veins is greater than in the arteries. They serve as a blood reservoir for the heart pump.

The entire blood flow passes through the lungs and then on to another organ.

The heart is a pump that pumps blood into the vessels of the circulatory system and contributes to venous return.

The blood circulates, always in the same direction, within a completely closed circuit formed by blood vessels of various sizes, spread throughout the body. The contractions of the heart ensure the circulation of the blood.

Four valves, two of which are atrio-ventricular (between the atrium and the ventricle of the heart) and two ventricular (between the ventricle of the heart and the artery) ensure the unidirectional circulation of blood in the body.

In the body of a man of 65 kilos, circulate 5 to 6 liters of blood, 4 to 5 liters in a woman (increasing to 5 to 6 liters during pregnancy)2, in that of a child, about 3 liters and 250 milliliters for a newborn.
In the red marrow of the bones, about :
25 trillion red blood cells[ref needed]; and the body must produce 2 million new ones per second to keep the same number of red blood cells.
several billion white blood cells. However, they are 600 times less numerous than red blood cells.
For one white blood cell, there are about 30 platelets and 600 red blood cells!

Because of its vital importance, blood has symbolic connotations in many religions and beliefs.

The loss of blood during menstruation is a “spectacular” physiological phenomenon, the source of many cultural beliefs and taboos; Cesare Lombroso linked it to female criminality.

Written by Rogers

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