Human blood contains many different kinds of cells. The most abundant cell type in the human blood is the red blood cell (technically referred to as the erythrocyte). In fact, the red blood cells are responsible for the red color of our blood. The main function of the red blood cell is to transport oxygen from the lungs to the tissues. For this purpose, red blood cells contain the protein hemoglobin, which binds to oxygen and helps in its transport.
Any reduction in the number of red blood cells due to any cause can severely disrupt the oxygen-carrying capacity of the blood. This could lead to widespread and serious disturbances in tissue functions all over the body. Low red blood cell levels is known as anemia. Most of the time it is due to iron deficiency, hence the term iron-deficiency anemia. High levels of red blood cells can also cause a problem. This is known as polycythemia.
Read more on iron deficiency anemia.
Functions of the Red Blood Cells
As described previously, the main function of red blood cells is to carry oxygen from the lungs and deliver it to tissues all over the body. Apart from this, red blood cells also transport carbon-dioxide and maintain pH of the blood by acting as acid-base buffers. However, other organs like the kidneys also play a role in maintaining blood pH while oxygen and carbon dioxide transport are exclusively the function of the red blood cells.
Cellular metabolism requires oxygen to work. One of the main byproducts of cellular metabolic processes is carbon-dioxide. This gas needs to be transported back to the lungs and eliminated. The carbonic anhydrase enzyme in red blood cells catalyzes the conversion of gaseous carbon-dioxide into water-soluble bicarbonate, which allows transport of carbon-dioxide in the blood.
Structure of Red Blood Cells
Shape and Size
A mature red blood cell has a biconcave disc-shaped structure. When the erythrocytes form, they are round and have a nucleus. However, by the time they mature, red blood cells have lost their nuclei and have adopted the biconcave disc-shape (almost like a doughnut).
The average diameter of a red blood cell is about 7.8 microns. The average thickness of a red blood cell is about 2 microns. Due to the biconcave shape, a red blood cell is about 2.5 micron at its thickest point on the periphery and less than 1 micron at this thinnest point in the center.
Red blood cells are bigger in diameter than the thinnest capillaries in the body. However, the peculiar shape of the red blood cells makes them flexible enough to squeeze through the thin capillaries without breaking or getting stuck. After exit from the capillaries, the red blood cells regain their original shapes.
Read more on large red blood cells.
Red blood cells have a peculiar composition and structure. Unlike other cells of the body, red blood cells are devoid of key cellular organelles such as nucleus, endoplasmic reticulum, and mitochondria. Even without these key organelles, red blood cells manage to produce energy in the form of ATP through the metabolic process of glycolysis.
A critical component of red blood cells is the oxygen-carrying protein, hemoglobin.
Without hemoglobin, the red blood cells would not be able to distribute oxygen to various tissues of the body. The red color of the blood in humans is due to hemoglobin. Hemoglobin is actually a metalloprotein made up of two parts: heme and globin.
The heme part is made up of a combination of iron and a heterocyclic compound known as porphyrin. Porphyrin is formed through a combination of four pyrrole molecules, which are formed when glycine combines with succinyl-CoA.
Globin is the polypeptide component of the hemoglobin molecule. Four globin chains need to combine to form the complete hemoglobin molecule. However, all four globin chains in a hemoglobin molecule are not identical. There are slight variations in the sequence of globin chains, due to which they are categorized as alpha, beta, gamma, and delta chains.
The hemoglobin molecule in adult humans is composed of two alpha and two beta globin chains. This adult hemoglobin type is also referred to as hemoglobin A. Fetal hemoglobin differs from adult hemoglobin in the composition of the globin chains.
Each hemoglobin molecule is capable of carrying eight oxygen atoms. Each iron atom can bind to two oxygen atoms (or one oxygen molecule). There is one iron atom in each globin chain. Therefore, there a total of four iron atoms in each hemoglobin molecule. The concentration of hemoglobin in blood is about 15 g per 100mL of blood.
Amount of Red Blood Cells in the Human Body
The exact quantity of red blood cells in the body varies from person to person according to a number of factors. An average male body contains around 5 million red blood cells in every cubic milliliter of blood. An average female body contains about 4.5 million red blood cells in very cubic milliliter of blood. The average number of red blood cells in the body can increase or decrease according to environmental and disease states. For example, living at high altitudes increases the red blood cell count. Diseases such as anemia decrease the red blood cell count.
Life Cycle of Red Blood Cells
The precursors of red blood cells are the hematopoietic stem cells that reside within the bone marrow. These hematopoietic stem cells differentiate to form all the cell types that are found within the blood. The formation of red blood cells from erythropoietic stem cells in the bone marrow takes about seven days. The last stages of differentiation into mature red blood cells require folic acid and vitamin B12. Deficiency of these vitamins can lead to anemia.
The production of red blood cells is controlled by the hormone, erythropoietin. The kidneys are the primary site of synthesis of erythropoietin. Low oxygen level in the blood (technically referred to as hypoxia) also stimulates the production of red blood cells (also known as erythropoiesis).
After synthesis and maturation, the red blood cells can live up to 120 days in the bloodstream. Non-functional and damaged red blood cells are removed from the circulation by the spleen. Destruction of red blood cells releases the hemoglobin into the blood.
Macrophages in the spleen and bone marrow, along with the Kupffer cells in the liver, take up the free hemoglobin and metabolize it. The iron is stripped off the hemoglobin molecule, and is reused for the production of new red blood cells in the bone marrow. The porphyrin ring is metabolized into bilirubin pigment, which is excreted into the duodenum via the bile.