Red blood cells or RBCs are also called erythrocytes. They act as active components in the blood responsible for transporting gases and nutrients throughout the human body.
Red blood cells (RBCs)
They play an important role in oxygen transport as they contain hemoglobin.
Hemoglobin is a protein that binds to oxygen and carries it from the lungs to the tissues of the body.
The normal blood erythrocyte count is about 3.9-5.5 million per microliter in women and 4.1-6 million per microliter in men.
They appear red due to the presence of an iron-containing pigment called hemoglobin. Oxygen is absorbed by hemoglobin and delivered to different parts of the body.
RBCs are made in the bone marrow, which is located in the ends of long, flat, and irregular bones.
Structure of Red Blood Cells (RBCs)
RBCs are unique in that they lack a nucleus and mitochondria, allowing them to contain more hemoglobin and perform their primary function more efficiently.
Red blood cells or erythrocytes are red cells with a biconcave shape.
They are flattened discs that lack nuclei and cytoplasmic organelles, containing the red protein hemoglobin for gas transport, mainly oxygen and some carbon dioxide.
It is 7.5 µm in diameter, 2.6 µm thick at the edge and 0.8 µm thick at the center.
The structure of RBCs can be described as follows:
- Biconcave shape: They are small, round and biconcave, which enables them to move through tiny blood vessels and reach the body’s tissue.
- Membrane: The cell membrane of an RBC is composed of proteins and lipids, providing structural integrity and malleability to the cell.
- Cytoskeleton: The cytoskeleton of an RBC is composed of proteins such as actin, spectrin, band 3, protein 4.1, ankyrin, which helps to maintain cellular structural integrity and flexibility.
- Hemoglobin: Each RBC contains approximately 270 hemoglobin molecules, which are responsible for oxygen transport.
Formation of hemoglobin
- Hemoglobin is a complex protein comprising of globin and iron containing haem, produced within developing erythrocytes in the red bone marrow.
- In mature erythrocytes, hemoglobin binds with oxygen to create oxyhemoglobin, imparting the characteristic red color to blood.
- Additionally, hemoglobin plays a crucial role in excretion. It help transport CO2 from body cells to the lungs.
- Each hemoglobin molecule contains four iron atoms, and each of these atoms can carry one molecule of oxygen.
- Consequently, a single hemoglobin molecule can transport up to four oxygen molecules.
Life cycle of Red Blood Cells (RBCs) (Erythropoiesis)
The lifespan of RBCs in circulation is approximately 120 days.
Erythrocytes originate in red bone marrow before entering the bloodstream and undergo multiple developmental stages.
Erythropoiesis is the process of RBC development from stem cells that takes around seven days.
The life cycle of erythrocytes comprises three phases:
- Production
- Maturation and
- Destruction
1. Production
- Erythropoiesis primarily occurs in the bone marrow. It begins with hematopoietic stem cells (HSCs), giving rise to multipotent progenitors, which then develop into erythroid-committed precursors and mature RBCs.
- Erythropoietin is the primary regulator of erythropoiesis, driving RBC precursor proliferation and differentiation and preventing erythroblast apoptosis.
2. Maturation
The maturation process involves several morphological changes to produce highly functional specialized cells. This process includes the expulsion of the nucleus and the loss of organelles, such as the smooth and rough endoplasmic reticulum, Golgi apparatus, and ribosomes, leading to the formation of mature biconcave RBCs.
3. Destruction
Mature RBCs have a lifespan of about 120 days. At the end of their lifespan, they are removed from circulation and destroyed, primarily by macrophages in the spleen and liver.
Functions of red blood cells (RBCs).
Respiratory Gas Transport: The primary role is the transport of respiratory gases.
Oxygen transport: Hemoglobin in RBC combines with oxygen to form oxyhemoglobin. About 97% of the oxygen is transported as oxyhemoglobin.
Transport of carbon dioxide: Hemoglobin combines with carbon dioxide to form carboxyhemoglobin and about 30% of the CO2 is transported. RBCs contain carbonic anhydrase, which is important for converting water and carbon dioxide into bicarbonate, facilitating the transport of carbon dioxide (about 63 percent in this form).
Buffering Action: Hemoglobin is an effective buffer, which regulates the concentration of hydrogen ions and helps in maintaining the acid-base balance.
Determination of blood group: RBCs contain blood group antigens (A, B, and Rh factor). It helps in blood group determination and prevents reactions in incompatible blood transfusions.
Roles of red blood cells (RBCs) in immunity
RBCs play an important role in the immune response against pathogens. They are mainly known for their function of transporting oxygen. They contribute to the innate immune response and some of the ways RBCs work against pathogens are:
Binding and scavenging: RBCs can bind and scavenge circulating chemokines, nucleic acids, and pathogens, which helps to clear these substances and minimize their potential damage to the host.
Phagocytosis: RBCs may be susceptible to phagocytosis in certain settings of phagocytosis, leading to immunity. For example, when plasma CpG DNA levels rise above homeostatic norms, such as during sepsis or infection, TLR9-dependent binding to RBCs results in anemia and innate immunity.
Production of reactive oxygen species (ROS): In erythrocytes, hemoglobin and heme stimulate reactive oxygen species (ROS) to eliminate and destroy hemolytic pathogens.
Immune recognition: Specific blood group antigens on red blood cells are present on molecules with receptors for specific microbes, resulting in hemolysis. This interaction can occur when latent antigens are exposed, when microbial toxins change the properties of antigens, or when microbes directly affect the formation of new red blood cells.
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