Circulatory system: Functions of blood-1

Formed Elements

There are three main categories of formed blood components:

• Red blood cells (Erythrocytes)
• White blood cells (Leukocytes)
• Platelets (Thrombocytes)

With the exception of the platelets, they are all cells. Certain bone marrow cells are broken down into platelets. Because they are encased in a plasma membrane, have a distinct shape, and are visible, they are referred to as formed elements.

• Red Blood Cells
• Monocyte
• White Blood Cells

Formation of Blood Cells

Before entering the blood, all blood cells undergo a number of developmental stages and derive from stem cells (haemocytoblasts). Hemopoiesis is the process by which the blood's formed components grow. Hemopoiesis takes place in the fetus's liver, spleen, thymus, and lymph nodes prior to birth. After birth and throughout life, red bone marrow produces all blood cells.

Red Blood Cells Production

Erythropoiesis is the process of producing red blood cells (RBCs). The proerythroblast is the first committed cell in this process. Large cells with a nucleus are called proerythroblasts. Proerythroblasts are stimulated to divide several times by the hormone erythropoietin (EPO), which is mostly released by the kidneys. This results in cells that start to make hemoglobin.

Basophil erythroblast (early normoblast), polychromatic erythroblast (intermediate normoblast), and orthochromatic erythroblast are the next stages of development after proerythroblast (late nomoblast). They all have a nucleus and are arranged in decreasing size. A cell finally releases its nucleus reticulocytes, also known as immature RBCS, when it is nearing the end of the development sequence. Within one to two days of being released from red bone marrow, reticulocytes mature into RBCs.

White Blood Cells Productions

White blood cell production is referred to as leukopoesis (leukos = white; poiesis = making). Hemocytoblasts divide to form three distinct types of committed cells:

• B progenitors, destined to become B lymphocytes
• T progenitors, which become T lymphocytes
• Granulocyte macrophage colony forming units (GM-CFU) which become granulocytes and monocytes.

The committed cells are equipped with colony-stimulating factor receptors (CSFs). Multiple CSF types are secreted by mature macrophages and lymphocytes in response to infections and other immunological stressors. In response to particular needs, each CSF promotes the differentiation of a different type of WBC. For instance, a bacterial infection may cause neutrophils to be produced, but allergies cause eosinophils to be produced. Each procedure utilizes its own CSF. Granulocytes and monocytes are kept in reserve in the red bone marrow. In the bone marrow, lymphocyte development starts. Some types reach maturity there, while others migrate to the thymus gland to finish growing. The spleen, lymph nodes, and other lymphoid organs and tissues are subsequently colonized by mature lymphocytes from both sites.

Platelets Production

The process of producing platelets is known as thrombopoiesis. In the bone marrow, platelets are created. When a hemocytoblast produces receptors for the hormone thrombopoietin, which, like erythropoietin, is made by the liver and kidneys, thrombopoiesis is said to have begun. The hemocytoblast eventually transforms into a huge multinucleated cell called a megakaryocyte under the influence of thrombopoietin. Within the cytoplasm, a megakaryocyte produces a demarcation membrane.

The cytoplasm then fragments into the minute pieces that make up platelets along these lines of demarcation. The spleen stores between 25 and 40 percent of the platelets, which are then released as needed. The remains do live for about 4 days and can freely circulate in the blood.

Red blood cells (Erythrocytes)

Erythrocyte is another name for a red blood cell (RBC) (erythros = red; cytes = cell). RBCs are minuscule, biconcave disc-shaped cells with a 7.2–7.5 micron diameter. It is thicker (2.2 u) near the edge and thinner (1.0 u) in the middle. The biconcave form is what causes this variation in thickness. The mature form of these cells, which can be found in the bloodstream, differs from other cells in that it lacks a nucleus. About 5 million RBCs are present in each cubic millimeter of blood in a typical healthy adult. A mature RBC's plasma membrane contains glycolipids that identify a person's blood type. All of the internal space in mature RBCs is available for the transportation of oxygen because they lack a nucleus and other internal structures.

RBCs can fit through tiny blood capillaries without rupturing because of their biconcave shape and robust plasma membrane. Hemoglobin, a red pigment that gives RBCs their red color and name, is a component of RBCs. 4.5 to 5.5 million/mm3 (45 to 55 lakh/mm3) is the average count.

• Male: 5-5.5 million/mm3
• Female: 4.5-5 million/mm3

Functions of RBC

• It takes oxygen from the lungs and transfers it to other tissues.
• Carbon dioxide is taken up from other tissues and released in the lung.
• The blood group of an individual is determined by the antigens on their RBCs' plasma membrane.
• The iron balance is influenced by RBC as well.

Hemoglobin

Hemoglobin makes up about 280 million molecules per RBC. Haemoglobin and oxygen are joined in the RBCs. It is made up of four globin-like protein chains (two alpha and two beta chains). Heme, a non-protein pigment found in each chain, is responsible for the center-located binding of oxygen and iron. One oxygen molecule can be transported by one heme, or up to four by one hemoglobin molecule.

The amount of oxygen that the blood can carry is determined by the hemoglobin concentration (Hb%), which is an essential piece of clinical information. Men typically have hemoglobin concentrations of 13 to 18 g/dl, women 12 to 16 g/dl, and infants 10 to 20 g/dl.

 Functions of Hemoglobin:

• Transport of oxygen.
• Transport of carbon dioxide.
• Buffer action

Gas Transport by Hemoglobin:

• Oxygen diffuses from the alveoli into the circulation as oxygen-poor blood flows through the lungs, where it subsequently enters the RBCs where it binds to hemoglobin. Oxyhemoglobin, sometimes known as bright red hemoglobin, is produced when oxygen binds to iron.
• The iron-oxygen reaction is reversed as the blood reaches the tissue. Iron and oxygen separate, and the oxygen diffuses into the tissue fluid before entering the tissue cells. Deoxyhemoglobin, which is now a dark red substance, is produced as hemoglobin releases oxygen.
• Hemoglobin also carries about 20% of the blood's carbon dioxide, but it is connected to the globin rather than the heme. When hemoglobin is in the reduced state, carbamino-hemoglobin rapidly forms.

Factors affecting Erythropoiesis:

The production of RBCs or erythropoiesis mainly depends on three factors:

• Erythropoietin: It is a hormone that stimulates the production of RRC and is primarily secreted by the kidneys.
• Hypoxia: Hypoxia, or a lack of cellular oxygen, can happen if there is insufficient oxygen entering the blood. The kidney cells are stimulated by hypoxia to increase the release of erythropoietin which stimulates proerythroblasts to differentiate into reticulocytes in the red bone marrow. Three or four days later, the RBC count begins to rise and reverses hypoxia that started the process.
• Dietary Factors: Erythropoiesis depends on a few dietary components. For instance, amino acids, iron, and vitamin B12 and folic acid activity are necessary for the synthesis of globin, haeme, and red blood cell maturation, respectively. Therefore, a lack of any of these elements lowers erythropoietic activity.

Destruction of RBCs

RBCs that are in circulation have a lifespan of 120 days and are primarily eliminated in the spleen. The membranes of the mature RBCS become more and more brittle. As the old, frail RBCs try to fit through the spleen's tiny channels, they become caught, split apart, and killed. Macrophages engulf and consume dying erythrocytes.

The first step in the elimination process is the separation of globin from heme by macrophages. The globin (protein) component is disassembled into free amino acids, which are then used to synthesize other proteins. Iron is liberated from the heme component and repurposed in the bone marrow for the production of new cells. The remaining portion of heme is transformed into bilirubin, a bile pigment required for the breakdown of fat.

The Fate of Expired RBCs and Hemoglobin

• RBCs lose elasticity with age.
• RBCs disintegrate when they pass through blood capillaries and sinusoids.
• In the spleen and liver, macrophages phagocytize cell fragments.
• Hemoglobin decomposes into:
  • Globin is hydrolyzed into amino acids that can be used again.
• Heme portion- further decomposed into:
  • Iron:
    • Transported to the bone marrow by albumin.
    • Some are used to produce new hemoglobin in the bone marrow.
    • Excess stored as ferritin in the liver.
  • Bilverdin:
    • Albumin-bound and converted to bilirubin by the liver is expelled and bile is produced
    • Concentrated and kept in the gall bladder
    • Small intestine discharged
    • Converted to urobilinogen by gut bacteria
    • feces excreted
    • A portion of urobilinogen is reabsorbed into the circulation and transformed into urobilin.
    • Urinated out.