Red Blood Cell Definition

Red blood cells (erythrocytes) are highly differentiated cells that lack organelles and the ability to proliferate. Erythrocytes are vital to the body’s function because they transfer oxygen via the circulation. Red blood cells are derived from hematopoietic stem cells in the bone marrow and have an average lifespan of 120 days. Membrane proteins enable endothelial cells, blood platelets, macrophages, and bacteria to communicate and interact with these cells.

Red Blood Cell Structure

Because red blood cells lack organelles, their structure is fairly basic compared to other cell types. These biconcave cells, the most common blood cell type, lack organelles.

An anucleate cell (one without a nucleus) is a mature red blood cell. This means it is devoid of DNA. The diameter of a red blood cell is about 8,000 nanometers (eight micrometres). It has a unique biconcave form that allows it to pass through even the tiniest capillaries while producing a vast surface area.

The cytoplasm of the cell is protected by a lipid bilayer membrane. There are no organelles in this cytoplasm, but there are a lot of haemoglobin molecules, consisting of four polypeptide chains (globins) and four carbon-based heme molecules, each of which encircles a single ferrous ion. Hemoglobin is divided into two types: HbA (adult haemoglobin) and HbF (foetal haemoglobin) (fetal hemoglobin).

Integral and peripheral proteins are found in the RBC membrane. Individuals are classified as blood types A, B, O, and AB, which are determined by integral proteins. They also provide structural stability and bind haemoglobin. On the inner side of the membrane, peripheral membrane proteins contribute to making the red blood cell extraordinarily elastic.

Red Blood Cell Development

Erythropoiesis, or erythrocyte production, begins inside the red bone marrow. The procedure creates around two million red blood cells (RBCs) per second and requires approximately two days from multipotent myeloid stem cell to red blood cell. While the red bone marrow is the most common location for red blood cell manufacturing, it may also occur in the white bone marrow. Several diseases cause extramedullary erythropoiesis to develop in the liver, thymus, and/or spleen—the same organs that create foetal red blood cells.

In the red bone marrow, multipotent hematopoietic stem cells (hemocytoblasts) differentiate into a basic myeloid progenitor cell which may become either a thrombocyte or an erythrocyte. The myeloid progenitor is prompted by chemical messengers to generate proerythroblasts, the first form of a genuine red blood cell. A proerythroblast can only develop into an RBC, but a myeloid progenitor cell is an oligopotent stem cell that can differentiate into two cell types with identical properties. As a result, it is a unipotent stem cell. The nucleus, cytoplasm, and ribosomes of this immature cell type are all big. Mitosis is a method of cell division.

The kidneys produce the cytokine hormone erythropoietin when they sense low oxygen levels. As a consequence, common myeloid progenitor cells in the bone marrow transform into proerythroblasts.

Erythropoietin and stem cell factor are both important regulators of erythropoiesis (SCF). They generate 4.2 to 6.1 million red blood cells per microliter of blood, which is considered typical. Stress erythropoiesis occurs when the kidneys detect low oxygen levels. RBC survival, differentiation, and proliferation (growth) rates are all boosted when combined with SCF. Recent research has shown that a ribonucleic acid fragment called miR-451 is involved in red blood cell maturation in mice. Micro-RNA is hypothesised to enable the maturation process to continue in humans as well.

Immature red blood cells are generated in the bone marrow. Here, previously characterised proerythrocytes transform into basophilic erythroblasts. This page contains photographs of several phases of red blood cell growth, and as you can see, the nucleus of the basophilic erythroblast is still visible, despite the fact that the cell has shrunk. During this transition, the nucleolus is gone. However, ribosomes are still tightly packed in the cytoplasm.

The basophilic erythroblast eventually becomes a polychromatophilic erythroblast, which is a smaller cell. As the cytoplasmic ribosomes begin to synthesise more haemoglobin, the hue changes. The nucleus contracts even further. The ratio of the nucleus to cytoplasm is 60–80%.

The differentiation of polychromatophilic to orthochromatophilic erythroblast, which remains in the red bone marrow, is the subsequent step in the development of red blood cells. The nucleus starts to deteriorate when the cell and nucleus contract once more.

The transition from orthochromatophilic erythroblast to reticulocyte is the ultimate stage of bone marrow development. A reticulocyte is smaller than a proerythroblast and lacks a nucleus. There aren’t many ribosomes left since the cell has enough haemoglobin protein to operate (see next heading). The reticulocyte’s reduced size permits it to leave the bone marrow and enter the circulation.

In the blood, a red blood cell develops. A developed RBC is roughly two millimetres shorter than the reticulocyte before it. It is devoid of organelles and comprises only a lipid membrane containing surface proteins, cytoplasm, and protein (hemoglobin).

Red Blood Cell Function

RBCs are responsible for transporting oxygen and (in much lower amounts) carbon dioxide between the body’s tissues and the lungs. The iron ions within haemoglobin may bond with oxygen and carbon dioxide.

Every red blood cell carries four molecules of oxygen or carbon dioxide because each haemoglobin molecule contains four iron ions. Oxyhemoglobin refers to oxygen-bound haemoglobin. Hemoglobin that has been bonded to carbon dioxide is known as carbaminohemoglobin.

Oxygen Transport

The first binding of one oxygen molecule (O2) to haemoglobin is particularly challenging due to the structure of haemoglobin. The binding of additional oxygen becomes simpler only until the initial oxygen molecule joins with the first ferrous ion and the structure of the haemoglobin has changed. The shape is no longer as efficient following the addition of a third oxygen molecule; a fourth oxygen molecule will not bond as quickly. A haemoglobin protein turns brighter red as more oxygen molecules bond to it.

The amount of oxygen delivered by a red blood cell is determined by a number of factors. The availability of oxygen in the blood and the health of the lungs are obviously critical. The capacity of RBCs to deliver adequate oxygen to the tissues is affected by Blood acidity (pH), temperature of the body and amounts of free carbon dioxide (which cause the blood highly acidic), and some hereditary illnesses. Damage to blood vessels has little effect on hemoglobin’s oxygen affinity, although it may prevent erythrocytes from reaching particular locations.

Up to 300 million haemoglobin molecules may be found in a single erythrocyte. This indicates that each adult, healthy cell has the capacity to carry over one billion oxygen molecules.

Carbon Dioxide Transport

While our blood transports 98 percent of our oxygen, 10 percent or less of carbon dioxide (CO2) combines to haemoglobin. The remainder is changed to carbonic acid inside the red blood cells. Carbonic anhydrase, a converting enzyme found in the cytoplasm of red blood cells, causes this. Carbonic acid is very volatile, forming both positively and negatively charged hydrogen ions and bicarbonate ions. The bicarbonate buffer system, which regulates body acidity, is made up of these ions (pH).

Carbon Monoxide Transport

Poisoning by carbon monoxide is more prevalent in rooms containing exposed gas heating systems. Carbon monoxide (CO) is seldom found in large amounts in the human body. However, if oxygen is available, it is more apt to bind to haemoglobin than oxygen. Carbon monoxide hinders oxygen transfer, causing a person to suffocate slowly if they stay in a poisonous environment. The fast infusion of 100% oxygen is the treatment.

The Red Blood Cell Test

The number of red blood cells is a component of the complete blood count (CBC). This provides a quick indicator of a person’s overall health to any clinician.

What constitutes an increased RBC count, a normal RBC count, and a lower RBC count depends on gender and age.

• 7 to 6.1 million red blood cells per microliter (cells/mcL) in men.
• Normal – 4.2 to 5.4 million cells/mcL in women
• Normal in children—4.0 to 5.5 million cells/mcL

Low red blood cell counts are caused by blood loss, anaemia, specific vitamin deficiencies that limit RBC proliferation rates, kidney disease (low levels of erythropoietin), and genetic illnesses that change protein production or red blood cell development and lifetime regulation.

Chronic low oxygen levels (smoking, residing at high altitudes, cardiac disease, and pulmonary disease), dehydration (which changes the proportion of blood cells to blood plasma), and hereditary or genetic illnesses including familial erythrocytosis and polycythemia are all causes of elevated red blood cell counts.

Other RBC tests include:

• Hematocrit is a volume percentage that quantifies the volume of RBCs in total blood (cells and plasma).
• HbA1C: Hemoglobin binds to more than only O2, CO2, and CO2. Glucose may also accomplish this. The glycated haemoglobin test measures the amount of glucose bound to haemoglobin. Because glucose does not bind to iron ions, it has no effect on oxygen transport.
• The ABO type examines the accessibility of integral red blood cell proteins in order to assure the safety of typed blood transfusions.
• The ESR (erythrocyte sedimentation rate) is a measurement of how rapidly RBCs travel through plasma. This may aid in the diagnosis of inflammatory diseases.
• Hematuria, or the presence of blood in the urine, may be visible or microscopic (only visible under a microscope). This might indicate an infection of the urinary system, kidney stones, or polycystic kidney disease, malignancies of the urinary system, or trauma. Some blood thinners induce RBCs to appear in the urine.

References

• Turgeon, ML. (2018). Clinical Hematology: Theory & Procedures, Enhanced Edition. Burlington (MA), Jones & Bartlett Learning.
• Levitzky MG. (2020). Clinical Physiology in Anesthetic Practice. New York, McGraw Hill Professional.
• Basten, Dr G. (2019). Blood Results in Clinical Practice: A practical guide to interpreting blood test results. Keswick (UK), M&K Publishing.