Red Blood Cells (RBC)

The basic biological component of blood is red blood cells (RBCs). They play a role in oxygen delivery throughout the body and have characteristics that set them apart from other types of human cells. At any given moment, adult people contain over 20-30 trillion RBCs, accounting for nearly one-quarter of the total amount of human cells. By far the most frequently produced constituent is the erythrocyte, also known as a red blood cell (or RBC). The principal activities of erythrocytes are to carry breathed oxygen from the lungs to the body's tissues, as well as to transfer some (approximately 24 per cent) carbon dioxide waste from the tissues to the lungs for exhalation. Erythrocytes stay in the circulatory system.

The article discusses what is RBC, the functions of RBC. Since the composition and shape of RBC is a major aspect of the overall physiological importance of the cell, haemoglobin, the primary functional protein of the RBC is discussed in the article. The function of haemoglobin is described below in the article.

Shape of RBC

The shape of RBC or the anatomy can be broadly discussed by categorizing it into two categories: the external and internal RBC structure. RBCs have a flat, concave core and are disc-shaped. The cells can move freely even through the smallest blood arteries because of the biconcave form. Gas exchange with tissues takes place in capillaries, which are small blood vessels that are only one cell diameter. Although many RBCs are broader than capillaries, their structure allows them to slip through. A normal human RBC is substantially smaller than most other human cells, with a disc diameter of 6–8 micrometres and a thickness of 2 micrometres. They may expand to the size of a spherical shape without rupturing their cell membrane. When their form changes, they are unable to transport oxygen or participate in gas exchange. People with sphere-shaped RBC are known to have anaemia or sickle-cell anaemia.

RBCs have no nucleus, or most organelles, such as the endoplasmic reticulum and mitochondria, even though they are called cells. RBCs in the body are unable to proliferate or multiply. They also lack the necessary components for gene expression and protein synthesis. While most cells travel through the body via chemotaxis, RBCs are transported through the body only by blood flow and pressure. The most significant component of RBCs is haemoglobin molecules. Haemoglobin is a specific protein that provides a binding site for oxygen and other molecules to be transported.

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What is Haemoglobin?

Since we have understood the basic concept of what are red blood cells, it is important to understand what haemoglobin is as it plays a critical role in the function of RBC. Red blood cells (RBCs) contain haemoglobin, a globular protein that transports oxygen throughout the body via circulation. It's a respiratory pigment that helps oxyhaemoglobin transfer oxygen from the lungs to other parts of the body. Carbon dioxide is also transported back in the form of carbaminohemoglobin by haemoglobin. The structure of haemoglobin is approximately spherical, with a diameter of about 5.5 nm (Mr 64,500; abbreviated Hb). Immature RBCs manufacture Hb heme in their mitochondria and cytoplasm. The globin protein is produced in the cytoplasm by ribosomes. Even after the nucleus is destroyed, the remaining rRNA in adult mammalian RBCs continues to generate Hb until the reticulocytes enter the vasculature.

Structure of Haemoglobin

Since we have understood what is haemoglobin let us look into the structure of haemoglobin, it is important to understand as the structure of the protein is critical to the function of haemoglobin which in turn affects the red blood cells function. Max Perutz deduced the molecular structure of haemoglobin in 1959. Two polypeptide chains, each with two subunits, constitute the main species of haemoglobin. Haemoglobin is a multimeric globular protein that serves as the human body's oxygen carrier. Haemoglobin is a tetrameric protein, which means that it is made up of four subunits. Each subunit is linked with a prosthetic heme group (\[Fe^{2+}\]). At the centre of the porphyrin ring is iron. Two alpha polypeptide chains and two beta polypeptide chains make up the four subunits.

• \[Subunit -\alpha\] – A 141-amino-acid-residue long alpha polypeptide chain makes up this subunit.

• \[Subunit - \beta\] – A 146-amino-acid-residue long beta polypeptide chain makes up this subunit.

In the quaternary structure, there is significant contact between subunits. After a brief urea treatment, haemoglobin partly dissociates, although dimers remain intact. The subunits are held together by hydrophobic interactions, hydrogen bonds, and a few ion pairs or salt bridges. According to X-ray research, haemoglobin has two major conformations: the R state and the T state. Although both forms of oxygen bind to haemoglobin, the R state has a far higher affinity for haemoglobin.

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What is the Function of Red Blood Cells? 

As we have all the important anatomical aspects of red blood corpuscles including the shape of RBC, let us look into the red blood cells function. The primary function of RBC is the gaseous exchange. The RBC also plays an important part in regulating the pH of the blood. Let us discuss the functions individually.

Gaseous Exchange

RBCs need a protein called haemoglobin to help in gas exchange. Oxygen binding is a cooperative process. The oxygen-binding affinity of haemoglobin increases gradually until all binding sites on the haemoglobin molecule are filled. As a result, haemoglobin’s oxygen-binding curve, also known as the oxygen saturation or dissociation curve, is sigmoidal rather than the conventional hyperbolic curve associated with noncooperative binding. Haemoglobin must efficiently bind oxygen in the lungs, where the \[pO_{2}\] is approximately 13.3 kPa, and release oxygen in the tissues, where the \[pO_{2}\] is around 4 kPa. A protein with a high affinity for oxygen would bind it well in the lungs but release very little in the tissues. The cooperative binding of a ligand to a multimeric protein, such as the binding of oxygen to haemoglobin, is known as allosteric binding. \[O_{2}\] can operate as a ligand as well as an activating homotropic modulator. Because each subunit has just one \[O_{2}\] binding site, conformational changes are passed from one subunit to the next via subunit-subunit interactions.

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pH control

pH control is one of the essential red blood cell functions. RBCs regulate blood pH by altering the carbon dioxide form in the blood. Blood acidity is linked to carbon dioxide. Because most carbon dioxide enters the bloodstream as a bicarbonate ion, which is a dissociated form of carbonic acid in solution, this is the case. The respiratory system controls blood pH by altering the rate at which carbon dioxide is expelled from the body, a process that requires RBC molecular activity. RBCs affect blood pH in a variety of ways. Carbonic anhydrase is a secreted enzyme that catalyzes the conversion of carbon dioxide and water to carbonic acid. The driving force of pH in the blood is bicarbonate and hydrogen ions, which dissociate in solution to form bicarbonate and hydrogen ions. 

Carbonic anhydrase also takes water from carbonic acid, converting it back to \[CO_{2}\] and water. This mechanism is required for carbon dioxide to exist as a gas during alveolar-capillary gas exchange. Blood pH decreases as carbon dioxide is transformed from its dissolved acid form and expelled via the lungs. This process can take place without RBCs or carbonic anhydrase, but it will take significantly longer. This reaction is one of the quickest in the human body because of the catalytic activity of carbonic anhydrase.

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Factors Affecting the Function of RBC

Since we have understood the function of RBC let us look into the factors affecting the function. It is important to understand most of the factors that affect the overall efficiency of the process by disrupting the function of haemoglobin. The main factors are mentioned below. 

• pH- As the pH rises, so does the quantity of \[H^{+}\], which reacts with the globin amino acid, stabilizing deoxy Hb. As a result, haemoglobin's binding affinity and carrying capacity are diminished.

• Ionic Charge- As the ionic charge in the system rises, it interacts with globin amino acid residues, stabilizing deoxy Hb and resulting in low oxygen binding affinity.

• BPG- The negative charge on BPG stabilizes the beat chain of globin, lowering oxygen binding affinity. BPG bis phosphoglycerate is connected with RBC.

• Altitude- As altitude rises, the partial pressure difference reduces, resulting in a lower concentration of oxygen being released into tissue via oxy Hb.

• The Concentration of Carbon Dioxide- As the concentration of carbon dioxide rises, Hb deprotonates, lowering the oxygen binding affinity.

The factors affecting the function of RBC can be better understood by the oxygen-haemoglobin dissociation graph. 

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Conclusion

In conclusion of the article, we have learnt what is RBC, what is haemoglobin, we have learnt about the shape of RBC. the function of RBC dependent on the function of haemoglobin is also studied in the article.