Red blood cells | definition, structure and function

Red blood cells

 The red blood cells are most numerous formed elements of the blood . They are the most abundant cells in the human body . A unique features of the red blood cells is the presence of a red, oxygen – carrying pigment , the hemoglobin ,in their cytoplasm.

Red blood cells

  • What are red blood cells ? 
Red blood cells also known as RBCs ( red blood corpuscles)  or  erythrocytes . RBCs are formed in the myeloid tissue or most commonly known as red bone marrow , although when the body is under severe conditions the yellow bone marrow , which is also in the fatty places of the marrow in the body will also make RBCs . The formation of RBCs is called erythropoiesis ( erythro means red ; poiesis means formation ) .Red blood cells lose nuclei upon maturation and take on a biconcave ,dimple – shape . They are about 7-8 micrometres in diameter . There are about 1000 × more red blood cells than white blood cells .
  • What is the lifespan and disposal of red blood cells ? 
       Human RBCs have short  lifespan of about 120 days and do not self repair . However , as produce about 2-3 million RBCs per second . That is about 200 billion a day . This allows us to have more to replace the ones we lose . RBCs  are formed mainly in bone marrow .
                   The worn out RBCs are destroyed by phagocytosis in the blood itself and in the spleen and liver in particular . Their iron is returned to the red bone marrow for reuse in the synthesis of fresh hemoglobin .
  • What are the components of red blood cells ?

RBCs contains hemoglobin and are covered with membrane composed of proteins and lipids . Hemoglobin is an iron- rich proteins that gives its red color from their respiratory pigments . RBCs contain hemoglobin which transports oxygen from the lungs to the rest of the body , such as to the muscles,where it releases the oxygen lead .

  • What are the shape of red blood cells ?

RBCs have a shape of a disk that appears to be ‘ caved in ‘ or almost flattened in the middle ; this is called biconcave . This biconcave allows the RBC to carry oxygen and pass through even the smallest capillaries in the lungs . This shape also allows RBCs to stack like dinner plates and bend as they flow smoothly through the narrow blood vessels in the body . RBCs lack a nucleus ( no DNA ) and no organelles , meaning that these cells cannot divide or replicate themselves like the cells in our skin and muscles .

Shape of red blood cell

The shape of  red blood cells varies in different vertebrate classes In fishes , amphibians , reptiles and birds , they are oval , biconvex and nucleated . In mammals (e.g., man ), they are circular , biconcave , denucleated discs . Their central part is thinner than the margin . This shape provides flexibility and results in a 20 to 30 % increase in surface area as compared to a sphere. This favours quick diffusion of gases . Camel and llama are exceptional among mammals in having oval RBCs.  

  • What size  of red blood cells ?

Human RBCs are smaller than the white blood cells . They are 7-8 micron metres in diameter and 2 micron meter thick near the rim . Small size of RBC provides it greater surface area for quick diffusion of O2 into it .

  • What is the normal number of RBC in human body ?

The normal RBC range of healthy adult man and woman have 5 and 4.5 million RBCs per cubic milimeter of blood respectively . This is called the total RBC count . 

  • What does a low red blood count mean ?

The RBC count decrease or low in anaemia . Anaemia  may be caused by loss of blood (haemorrhage) , destruction of RBCs (haemolysis) , or faulty formation of blood . Symptoms of anaemia is feeling fatigue and weakness .

  • What does a high red blood count mean ? 

The RBC count increase or high it means that there is a shortage of oxygen in the system and the body creating more red blood cells to carry more oxygen to make up for the shortage . It is a risk marker for some  very serious illness : heart disease , kidney disease and stroke . Doctor  may recommend a procedure or medication to lower . In a procedure called a phlebotomy , a healthy professional inserts a needle into your vein and drains blood through a tube into a bag or container .

  • What does an abnormal red blood count mean ?

An abnormal rise in red blood cells count is called  polycythemia .

Structure of  red blood cells 

A red blood cells is bounded by an elastic and semipermeable plasma membrane . This enables it to squeeze through capillaries having  a diameter less than its own.  It loses plasticity in sickle – cell anaemia.  In this disorder, the RBCs block the capillaries, leading to grave consequences.   An erythrocyte contains homogeneous cytoplasm which loses the nucleus , endoplasmic reticulum , mitochondria , ribosomes and centrioles during the development of the corpuscle or cells .  This gives a double advantage . The corpuscle or cells has more space to hold hemoglobin . It’s oxygen consumption is very low due to lack of organelles so that it can supply more oxygen carried by hemoglobin to the tissue cells . Red blood cells cannot reproduce or carry out cellular metabolism due to lack of organelles . Besides hemoglobin , a red blood cells also contains several inorganic ions , including those of sodium , potassium , calcium , magnesium , chloride and phosphate . The adult red blood cells of mammals are described as enucleated ( denucleated ) as , when young , they have a nucleus that later disappears. 

Formation of red blood cells

Formation of red blood cells is called erythropoiesis . It occurs in the liver and spleen in the foetus and in red bone marrow after birth . Proteins and iron are components of hemoglobin , and vitamin B12 and folic acid stimulate erythropoiesis . Deficiency of any of these materials may cause anaemia . Excess RBCs are stored in the spleen . 

Colour of Red blood cells

  • What is colur of red blood cells ?

The colour of red blood cells look yellowish when seen singly and red viewed in bulk . They impart red colour to the blood . The colour is due to the presence of a solution of iron – containing pigment, hemoglobin in them .


Special property of red blood cells

In resting ( drawn ) and slow flowing blood , the RBCs form piles called rouleaux by adhering together due to surface tension . 

Function of RBCs 

The main function of RBCs is to transport oxygen from the lungs to the tissues and to  carry  carbon dioxide a waste product of metabolism  to the lungs , where it is excreted .

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