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Author: Matthieu Thomas • Examiner: Chafor Nkemetia Anslem
Last revised: October 8, 2024
Reading time: 11 minutes
The sang constitutes approximately 8% of the average human’s body mass. It contains erythrocytes (red blood cells), leukocytes (white blood cells), thrombocytes (platelets) and plasma.
The volume percentage of all blood cells compared to the volume of whole blood is around 45% in adults (hematocrit). The remainder consists of organic fluid, plasma (i.e. water, plasma proteins, electrolytes, etc.).
Blood is composed of:
- Cells
- Cell fragments
- Aqueous solution (plasma)
Function
Transport of substances and disposal of waste
Blood is the main conveyor of the human body. He carries gas (oxygen, carbon dioxide, nitrogen, etc.), nutrients (metabolism) and the products of cellular metabolism. The blood therefore has the function of ensuring the exchange of substances. It supplies tissues with blood gases and nutrients and, in exchange, transports metabolic waste (e.g. carbon dioxide, urea, uric acid, creatinine, etc.) to the organs dedicated to their elimination (lungs, liver, kidneys). ). It also carries chemical messengers (hormones) to their target organs.
Acid-base balance
The acid-base balance is regulated in the blood thanks to the diffusion of gases between the alveoli and the blood in the lungs (alveolar diffusion). L’oxygen diffuses from the alveoli to the blood due to the concentration gradient. It is picked up by the transport protein, hemoglobin (heme = iron-containing, globin = protein). Conversely, the carbon dioxide diffuses from the blood to the alveoli, due to its higher concentration in the blood, where it is eventually exhaled.
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Oxygen intake and carbon dioxide removal
Blood carries oxygen from the pulmonary alveoli to the cells furthest away from the body through the bloodstream. Thanks to a higher oxygen partial pressure in the plasma (compared to the cells), oxygen diffuses towards the tissues.
Carbon dioxide diffuses from cells into the blood due to higher partial pressure in the tissues. At this point it undergoes a chemical reaction which formscarbonic acid (CO2 + H2O → H2CO3), which then dissociates into a hydrogen ion (H+) and bicarbonate (HCO3-). Carbon dioxide, a metabolic waste, is therefore transported in the form of carbonic acid (more precisely in the form of a hydrogen ion and bicarbonate). In the lungs, the chemical reaction mentioned above reverses and carbon dioxide is exhaled.
To summarize, blood regulates acid-base balance through gas exchange. Blood is also responsible forhomeostasisfor example by balancing the aqueous volumes between the blood capillaries on the one hand, and the intracellular and extracellular space on the other hand. It also helps maintain a constant body temperature.
Coagulation
Clotting factors (proteins) are present in the blood and stop bleeding after a complex (cascade) activation of clotting factors following damage to blood vessels, ultimately leading to thrombus formation (thrombogenèse). At the same time, fibrinogen/fibrin prevents the pathological development of blood clots in blood vessels. Blood coagulation and fibrinolysis influence each other and maintain a sensitive balance.
Cellular components of blood
Erythrocytes
The function of erythrocytes is to transport oxygen from the lungs to peripheral tissues by attaching it to the ferrous heme group of hemoglobin. Erythrocytes are round and biconcave in shape because they do not have a nucleus. An erythrocyte measures between 8 and 10µm in diameter. A healthy adult has approximately 5 million erythrocytes per µL of blood. Furthermore, it is on the surface of the erythrocyte membrane that the blood antigens responsible for blood groups are expressed.
Erythrocytes (histological slide)
Leucocytes
Unlike mature erythrocytes, leukocytes have a nucleus. Different types of leukocytes can be found in the blood:
- Neutrophilic granulocytes (banded nucleus and segmented nucleus)
- Granulocytes œsinophiles
- Granulocytes basophiles
- Lymphocytes
- Monocytes
The normal leukocyte concentration ranges from 4000 to 10,000 leukocytes/µL, depending on the age and health of the patient. Both leukocytes and erythrocytes are descendants of pluripotent stem cells in the bone marrow.
The primary function of leukocytes is immune defense. THE lymphocytes (25 to 40% of leukocytes) are particularly responsible for the acquired immune response, the specific defense against pathogenic germs. THE lymphocytes B produce antibodies, while lymphocytes T modulate their production as well as the direct cellular immune response.
THE monocytes (4 to 8% of leukocytes) have the role of phagocytosis (i.e. the elimination of foreign bodies, bacteria, etc.) by producing extremely reactive oxygen free radicals capable of penetrating and then destroying the bacterial wall. Monocytes can differentiate into fixed macrophages in the connective tissue (histiocytes or spongiocytes) or into free macrophages.
Platelets
Platelets (thrombocytes) are another type of blood cell. They come from megakaryocytesgiant cells of the bone marrow. Their role is to ensure hemostasis in the event of damage to a blood vessel.
Platelets adhere to the vascular walls of damaged blood vessels and react with fibrin to form a solid clot within two to three minutes (bleeding time). The physiological norm for platelets is 150,000 to 400,000/µL.
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Clinical notes
As a liquid tissue, blood can be subject to pathologies – like all other tissues in the human body. These are disorders of the hematopoietic system. Of the wide range of hematological pathologies, anemia and leukemia are mentioned and explained here as examples.
Anemia
Definition
Anemia is a drop in blood hemoglobin concentration below values considered normal.
The normal value for hemoglobin concentration in men is 14 to 18 g/dL and 12 to 16 g/dL in women. Anemia goes hand in hand with a reduction in the level of erythrocytes in the blood (erythropenia). The main symptoms include mild fatigue, shortness of breath (dyspnea), and headache. Anemia is either acquired or hereditary. Acquired anemia can be related to blood loss (hemorrhage), hemolysis, disorders of the hematopoietic system, kidney pathologies, tumors, etc. Hereditary anemia can be the cause of abnormal hemoglobin (hemoglobinopathies).
Therapy
Iron supplementation, vitamin B12/B6, erythrocyte transfusion in the case of severe hemorrhage.
Leukemia
Definition of Leukemia
Leukemia, known as “blood cancer,” is a disorder of the hematopoietic system. It is characterized by an overproduction of immature leukocytes (myelocytes and myeloblasts). These cells proliferate in the bone marrow and thus prevent the production of healthy blood cells. Peripheral blood has a high level of peripheral blasts, meaning that leukocyte precursor cells are detectable there. These cells are not normally detectable in the peripheral blood of healthy individuals.
Due to hematopoietic disorders, the blood also shows a reduction in the level of erythrocytes (anemia) in addition to the increase in leukocytes.
Symptoms of leukemia include fatigue, feeling unwell, increased susceptibility to infections, enlarged lymph nodes, etc.
Therapy
Cytostatic drugs, allogeneic stem cell transplantation (bone marrow donation), radiotherapy.
Sources
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References:
- Rouvière, H., & Delmas, A. (2002). Human anatomy: Descriptive, topographic and functional (15th ed., Volumes 1-5). France: Editions Masson.
- Kamina, P. (2006). Clinical Anatomy (4th ed., Volumes IV). France: Editions Maloine.
- Moore, K.L., Agur, A.M.R., Dalley, A.F., & Dhem, A. (2011). Medical anatomy: Fundamental aspects and clinical applications (3rd ed.). Belgium: DE BOECK SUP.
- Drake, RL, Vogl, AW, Mitchell, A., Duparc, F., & Duparc, J. (2020). Gray’s Anatomy – The Handbook for Students (4th ed.). France: Elsevier Masson.
- Stevens, A., & Lowe, J. (2006). Human histology (3rd ed.). France: Elsevier Masson.
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