Physiology of Blood

 

Physiology of Blood

Introduction

Blood is a body fluid and is necessary for the transportation of essential nutrients and oxygen to the cells and to carry out waste products from the cells detaching it from the human body. The basic components of the blood are Red Blood Cells, White Blood Cells, Platelets and Plasma and these are the four components of the blood that are necessary for the smooth functioning of the human body. Red Blood Cells are also known as erythrocytes and are responsible for the transport of hemoglobin and in turn are responsible for the transport of oxygen from the lungs to the tissues. It is seen that in some lower animals the hemoglobin is present in the plasma as free protein and in the human being it is important for the hemoglobin to remain inside the Red Blood Corpuscles because if it will be present as free protein in the plasma than 3 % of it will leak through the capillary membrane into the tissue space or through the glomerular membrane of the kidney into the glomerular filtrate each time the blood passes through the capillaries.

            Red Blood Corpuscles contains carbonic anhydrase a catalytic enzyme that is responsible for the reversible reaction between carbon dioxide and water to form carbonic acid and the rapidity of the reaction makes possible for the water of the blood to transport carbon dioxide in the form of bicarbonate ion from the tissues to the longs, where it is converted to carbon dioxide and is expelled to the atmosphere as a metabolic waste product. The hemoglobin in Red blood Corpuscles is an excellent acid base buffer.

Features of Red Blood Corpuscles

·         Shape of Red Blood Corpuscles: - Biconcave

·         Size of Red Blood Corpuscles: - 7.8 micrometers in diameter

·         Thickness: - 2.5 micrometers at the thickest point or less in the centre.

·         Volume of Red Blood Corpuscles: - 90 to 95 cubic micrometers.

Concentration of RBC in blood

·         Men: - 5,200,000 per cubic millimeter.

·         Women: - 4,700,000 per cubic millimeter.

·         Persons living in high altitude usually have higher concentration of RBC.

Quantity of hemoglobin in the cells

·         Men:- 15 gram of hemoglobin per 100 milliliters of cells.

·         Women:- 14 grams per 100 milliliter of cells.

Transportation of oxygen by the Blood

·         Each gram of pure hemoglobin is capable of combining with 1.34 ml of oxygen.

·         Men:- 20 milliliters of oxygen can be carried in combination with hemoglobin in each 100 milliliters of blood.

·         Women:- 19 milliliters of oxygen can be carried in combination with hemoglobin in each 100 milliliters of blood.

Production of Red Blood Cells

·         Early week of embryonic life – Yolk sac.

·         Middle trimester of gestation – liver but reasonably numbers are also produced in the spleen and lymph nodes.

·         Last month or so of gestation and after birth- RBC are produced exclusively in the bone marrow.

P   Production of RBC occurs in the marrow of all bones till the age of 5 years. Proximal portion of the humerous and tibia becomes quite fatty and produce no RBC after the age of 20 years. Beyond this age RBC continue to be produced in the marrow of the membranous bone such as vertebrae, Sternum, Ribs. Even in these bones, the marrow becomes less productive as age increases.

     Beginning of life of Blood cells

     The blood cells begins their life in the marrow from a single type of cells called pluripotential hematopoietic stem cells and from it only other types of circulating blood cells are eventually derived. Circulating blood cells- monocytes, lymphocytes, neutrophils, eosinophils, basophils and macrophages, RBC and platelets. As these cells reproduces a small portion of them remain exactly like pleuripotent cells in the marrow to maintain constant supply whenever needed. But their number gradually diminishes with age. However most of the reproduced cells however differentiate to form other cell types like macrocytes, megakaryocytes, platelets, T-lymphocytes and B-lymphocytes.

Committed Stem Cells

The progenitor cells are very much like pluripotential stem cell, even though they have already become committed to a particular line of cells and are called committed stem cells. The different committed stem cells when grown in cultural media they give rise to specific blood cells.

Committed stem cells that produce erythrocytes are known as colony forming unit erythrocytes. Likewise granulocytes and monocytes were also been designated the same. Growth and reproduction of stem cells are mainly done by proteins and are known as growth inducers. 4 such growth inducers are known hereby and one among them is interleulin-3 which promotes growth and development of almost all kinds of committed stem cells while other induces growth of specific cell types.

The growth inducers are mainly responsible for the growth but have no role in differentiation which is done another set of protein known as differentiation inducers and they differentiate committed stem cells to developed adult blood cells. Formation of growth inducers and differentiation inducers are itself controlled by factors outside the bone marrow.

Stages of differentiation of RBC

The first cell belonging to the blood series is pro-erythroblasts and under appropriate stimulation large number of these cells is produced from CFU-E stem cells. Once the pro-erythroblasts are formed they eventually divide multiple times and produce many mature Red Blood Cells. The first generation cells are called basophil erythroblasts and at this stage they accumulate very little hemoglobin. This basophil erythroblast differentiates to polychromatophil erythroblasts, orthochromatic erythroblast. In the succeeding generation, the cells become filled with hemoglobin to a concentration of about 34%, the nucleus become condensed to a smaller size and the final remnants is absorbed or extruded from the cells. The cell at this stage is called reticulocytes. The cell at this stage contains small amount of remnants of basophilic material, remnants of Golgi apparatus, mitochondria and a few other cytoplasmic organelles. During this reticulocyte stage, the cell passes from the bone marrow to the capillaries by diapedesis (squeezing through the pores of the capillary membrane). The remaining basophilic material of the reticulocyte normally disappears within 1 to 2 days and the cell is then a mature erythrocyte.

Regulation of Red Blood Cell production and what is the role of erythropoietin

            The total mass of RBC in the circulatory system is within normal limit and sufficient amount of RBC is always present to transport sufficient amount of oxygen from lungs to the tissues and yet the quantity always remains in normal limit.

Oxygenation of tissues is the most essential factor of RBC production

In any condition when the oxygen supply to the tissues decreases in condition such as anaemia or destruction of the bone marrow due to X-ray therapy, increases the RBC production by bone marrow. At high altitude when the quantity of oxygen decreases and insufficient amount of oxygen is transported to the tissues and RBC production increases. Increasement of RBC production increases because the demand of oxygen by the tissues increases. Various diseases of the circulatory system that leads of lack of blood supply to the tissues and decreases of oxygen absorption by the blood in conditions such as prolong cardiac failure and in several lung diseases increases the hypoxic condition of the tissues and increase RBC production. The resultant factor also increases the percentage of volume of RBC and also increases total blood volume.

Erythropoietin stimulates Red Cell Production and its formation increases in response to Hypoxia

 The principle protein that is responsible for the production of RBC is a glycoprotein named erythropoietin. it is a glycoprotein with a molecular weight of 34,000. It is a circulating hormone. when the hypoxic condition persists enhances erythropoietin production and this condition enhances increase RBC production until hypoxia is relieved. 

Role of Kidney in the formation of erythropoietin  

90% of the erythropoietin occurs in the kidney and the rest 10% occurs in the liver. some studies suggest that the fibroblasts like interstitial cells surrounding the tubules of the cortex and outer medulla secrete where much of the kidney's oxygen consumption occurs. renal epithelial cells also secretes erythropoietin in response to hypoxia. renal tissue hypoxia leads to increased tissue level of hypoxia inducible factor -1 which serves as a transcription factor for a large number of hypoxia inducible gene, including the erythropoietin gene. Hypoxia inducible factor-1 binds with hypoxia response element residing in the erythropoietin gene, inducing transcription of mRNA and ultimately erythropoietin synthesis occurs. when there is development of hypoxia in other parts of the body, non renal sensors sends signal to the kidney and secretion of erythropoietin occurs. Nor-epinephrine and epinephrine and several prostaglandins also stimulate erythropoietin secretion.

Importance of erythropoietin in erythrogenesis

In the low atmospheric condition when the oxygen concentration is low, secretion of erythropoietin occurs within minutes to hours and reaches maximum within 24 hours. But till now production of new RBC do not occur till about 5 days. Within this period the erythropoietin stimulates the production of proerythroblasts from pluripotential hematopoietic stem cells in the bone marrow thus speeding up the process of producing RBC. The rapid production continues as long as a person remains in low oxygen level or enough RBC have been produced to carry enough oxygen to the tissues. The rate of erythropoietin production decreases to a level that maintain required RBC number but not to an access. In the presence of erythropoietin and in the availability of plenty of iron and other nutrients the rate of RBC production can rise to 10 or more time than normal.

Maturation of Red Blood Cells- requirement for Vitamin B12 and folic acid

As the RBC need to be replenished continually, the erythropoietic cells of the bone marrow are among the moist growing and reproducing cells of the human body. two kinds of vitamin are essential for the maturation of RBC. they are Vitamin B12 and folic acid. they are also required for the synthesis of DNA because production of thymidine triphosphate occurs which is the building block of DNA. lack of vitamin B12 and folic acid causes abnormal and diminished DNA synthesis and consequently failure of nuclear maturation and cell division occurs. the erythroblasts cells of the bone marrow fail to proliferate rapidly and as a result formation of macrocytes occurs which are larger than normal RBC. they are oval and irregular in shape and are capable of carrying oxygen in the circulating system. They have short life span.  the deficiency of vitamin B12 and folic acid causes maturation failure in the process of erythropoiesis.

Maturation failure cause by poor absorption of vitamin B12 from the GI tract

A common cause for failure of maturation of RBC is failure of absorption of vitamin B12 from the GI tract. This is mainly seen in pernicious anemia disease in which basic abnormality is an atrophic gastric mucosa that fails to produce normal gastric secretions. The parietal cells of the gastric gland secretes a glycoprotein called intrinsic factor, which combine with vitamin B12 in food and makes B12 available for absorption by the gut. It follows the following step:- 

1. Intrinsic factor binds with vitamin B12 and vitamin B12 is protected from digestion by gastro intestinal secretions.

2. Intrinsic factor binds to specific receptor sites on the brush border membrane of the mucosal cells in the limen.

3. Vitamin B12 is transported in the blood during the next few hours by the process of pinocytosis, carrying Intrinsic Factor and vitamin together through the membrane. Lack of Intrinsic factor, therefore decreases availability of vitamin B12 because of faulty absorption of the vitamin.

    Vitamin B12 after absorption is stored in the liver in large quantity and then is released slowly as needed by bone marrow. For normal maturation of RBC - 1 to 3 micrograms of vitamin B12 is required. Normal storage of vitamin B12 in the liver is 1000 times this amount.

Failure of RBC maturation by deficiency of folic acid

Folic acid is normally present in vegetables, fruits, meat etc. and is destroyed easily during cooking. Small intestinal disease called sprue leads to failure in absorption of vitamin B12 and folic acid.\

Synthesis of hemoglobin

Starts in the proerythroblasts and continues in the reticulocyte stage of the RBC and when reticulocyte leaves the bone marrow and passes to the blood stream, they continue to form minute hemoglobin for another day or so until they become mature erythrocytes.

Steps of hemoglobin formation

Acetyl-CoA synthesized from Krebs's metabolic cycle combines with glycine to form pyrrole molecules. In turn 4 pyrrole molecules combine to protoporphyrin IX, which then combine with iron to form heme molecule. Finally each heme molecules combine with a long polypeptide chain, a globin synthesized from Ribosomes, forming a subunit of hemoglobin called a hemoglobin chain. each chain has a molecular weight of 16,000; 4 of these in turn bind together loosely to form the whole hemoglobin molecule. Depending upon the amino -acid composition of the polypeptide, there are slight variations in the different hemoglobin chains. the different types of chains are alpha, beta, gama and delta chains. The most common form of hemoglobin in the adult human is hemoglobin A and it is a combination of 2 alpha chains and 2 beta chains. the hemoglobin A molecule has a molecular weight of 64,458. Each hemoglobin chain contains heme prosthetic group having one iron atom and 4 hemoglobin chains are present in one hemoglobin molecule, thus one molecule of hemoglobin finds 4 iron atoms. each hemoglobin chain combines with one molecule of oxygen there 4 molecules of oxygen can be transported by one hemoglobin molecules. The type of hemoglobin chain in the hemoglobin molecules determines the affinity of hemoglobin for oxygen. In sickle cell anemia, the amino acid valine is substituted for glutamic acid at one point of the two beta chains. when this kind of hemoglobin are exposed to low oxygen concentration, it forms crystals inside the RBC and some are 15 micrometers in length making it impossible for the cells to pass through the capillaries and as a result rupture of cell membrane occurs leading to the development of sickle cell anemia. 

Iron Metabolism

Iron is essential for the formation of hemoglobin and other essential components such as - myoglobin, cytochromes, cytochrome oxidase, peroxidase and catalase. 

1. Total quantity - (4-56) gms.

2. 65% is in the form of hemoglobin.

3. 4% is in the form of myoglobin.

4. 1% is in the form of heme compounds that promotes intracellular oxidation.

5. 0.1% combines with protein transferrin in the blood plasma.

6. (15-30)% is stored in reticuloendothelial system and l9iver parenchyma cells in the form of ferritin.

Transport and storage of Iron

When iron is absorbed from the small intestine, it immediately combines with beta globulin, apo transferrin to form transferrin. The iron is loosely bound in the transferrin and can be released into the tissues at any point of time. excess of iron is stored in the liver hepatocytes and less in the reticuloendothelial cells of the bone marrow. In the cell cytoplasm the iron combines with a protein called apoferritin to form ferritin and apoferritin has a molecular weight of 4,60,000. Varying quantities of iron in the form of iron radicles can combine as iron radicles with apoferritin. hence ferritin may have large or small amount of iron. the iron stored in ferritin is called storage iron. Small quantity of iron in the storage pool is in an extremely insoluble form and is called hemosiderin. Hemosiderin collects in the cells in the form of large clusters and can be seen in microscope. Ferritin particles are small and dispersed and are usually seen in cell cytoplasm with electron microscope. When the quantity of iron in the plasma drops down then some of the iron is removed from the ferritin storage pool and is delivered in the form of transferrin to the area where the iron is in need. bonding of transferrin with the receptor of the cell membrane in the erythroblast of the bone marrow is very strong and it is ingested into the erythroblasts by endocytosis. Transferrin directly delivers the iron into the mitochondria where heme is synthesized. After the destruction of RBC, the hemoglobin is released and is ingested by monocyte-macrophage cells. The iron liberated is stored in the ferritin iron pool and is needed for the future generation of hemoglobin.

Daily loss of Iron

0.6 mg of iron is lost every day mainly through faeces.

Absorption of Iron from the intestinal tract

The liver secretes apo transferrin in the bile and passes in the bile duct to the duodenum. The transferrin binds with free iron along with certain iron compounds in the form of hemoglobin and myoglobin. The transferrin binds with receptors in the membranes of the intestinal epithelial cells. the transferrin along with its iron store is absorbed in the intestinal epithelial cells and is released into the capillaries in the form of plasma transferrin. Iron absorption from the intestine is extremely slow and only few miligram is absorbed per day.

Life span of RBC is about 120 days

RBC are delivered from the bone marrow into the circulatory system and they circulate for the period of 120 days before being destroyed. they do not have mitochondria, endoplasmic reticulum and nucleus but they do have cytoplasmic enzymes that are capable of metabolizing glucose and produce small amount of ATP. 

    The cytoplasmic proteins maintains

1. Pliability of the cell membrane.

2. Membrane transport of ions.

3. Keeps the iron of cell hemoglobin in ferrous form rather than ferric form.

4. Prevent oxidation of protein in the red cells.

When the cell membrane of the RBC become fragile the cell ruptures while passing through tight spots of the circulatory system. Some of the cell self destructs itself in the spleen when they squeeze through the red-pulp of the spleen. The space between the structural trabeculae of the red pulp, through which most of cells pass is only 3 micrometer while RBC is 8 micrometer in diameter. when the spleen is removed the number of old RBC in the circulatory system increases considerably.

Destruction of hemoglobin

When Red Blood Cells bursts the hemoglobin is phagocytozed by the macrophages especially by the macrophages of Kupfer cells, spleen and bone marrow. The macrophages release iron from hemoglobin into the blood to be carried by transferrin either to the bone marrow for the production of new RBC or to the liver and other tissues from storage in the form of ferritin. the porphyrin portion of the hemoglobin is converted into bilirubin by macrophages and is released into the blood. 

Source:-

Guyton and Hall, Textbook of Medical Physiology, 12th edition, Chapter Number 32, Page Number- 413-421.