HUMAN BLOOD

 

HUMAN BLOOD

PREPARED BY MR. ABHIJIT DAS

All living cells have to be provided with nutrients, oxygen and other essential substances. Also, the waste substances produced, have to be removed continuously for healthy functioning of the tissues.

So it is essential to have efficient mechanism for movement of these substances to the cell and from the cells. Blood is the most commonly used body fluid by humans for this purpose.

BLOOD: Blood is a special connective tissue consisting of plasma and cells.

COMPOSITION OF BLOOD

PLASMA

Plasma is a straw/yellowish coloured, viscous fluid consisting nearly 55% of the blood.

90-92% of plasma is water and proteins contribute 6-8% of plasma. Major proteins of plasma are albumins, globulins and fibrinogens.

Plasma also contains some amount of minerals like Na⁺, Ca⁺⁺, Mg⁺⁺, Cl^- etc.

Albumins help in water retention, fibrinogens are needed for clotting of blood. Globulins are mainly involved in defense mechanism of the body.

Factors for clotting of blood are also present in the plasma in inactive form. Plasma without the clotting factor is called the serum.

BLOOD CELLS

Erythrocytes (RBCs), leucocytes (WBCs) and platelets are collectively known as blood cells or formed elements.

They constitute nearly 45% of the blood.

ERYTHROCYTES

Erythrocytes or red blood cells are the most abundant of all the cells in the blood. A healthy human being has, on an average, 5 million to 5.5 million of RBC per mm³ of blood.

RBCs are formed in the red bone marrow in the adults and RBCs are devoid of nucleus.

RBCs are biconcave in shape.

They have red coloured iron containing complex protein called haemoglobin.

A healthy individual has 12-16gms of haemoglobin in every 100ml of blood. These haemoglobin molecules play a significant role in transportation of respiratory gasses (oxygen and carbon dioxide).

RBCs have an average life span of 120 days after which they are destroyed in the spleen (graveyard of RBCs).

LEUCOCYTES

Leucocytes are also known as white blood cells as they are colourless due to lack of haemoglobin.

They are relatively lesser in number which averages 6000-8000 per mm³ of blood.

There are two main categories of WBCs – granulocytes and agranulocytes.

Neutrophils, eosinophils and basophils are different types of granulocytes, while monocytes and lymphocytes are agranulocytes.

Neutrophils are the most abundant cells (60-65%) of the total leucocytes and basophils are the least (0.1- 1%) among them.

Neutrophils and monocytes are phagocytic in nature which destroy pathogens inside the body.

Basophils secret histamine and heparin and are involved in allergic reactions.

Eosinophils (2-3%) fight against parasitic infections.

Lymphocytes (20-30%) are of two different types ‘B’ and ‘T’ lymphocytes. Both B and T lymphocytes are responsible for immune responses of the body.

PLATELETS

Platelets are also known as thrombocytes. They are cell fragments produced from Megakaryocytes.

Blood normally contains 150000-350000 platelets per mm³ of blood.

Platelets can release a variety of substances which are involved in clotting of blood.

CIRCULATORY PATHWAYS

The circulatory patterns are of two types – open and closed.

In open circulatory system blood pumped by the heart passes through large vessels (arteries) into body cavities. Capillaries are absent in case of open circulatory system.

In closed circulatory system blood pumped by the heart is always circulated into organs through capillaries. Closed circulatory system is considered to be more advantageous as the flow of blood can be more precisely regulated.

BLOOD COAGULATION (BLOOD CLOTTING)

When a blood vessel is damaged, loss of blood is stopped and healing occurs in a series of processes.

Platelets play an important role in blood clotting.

The more badly damaged the vessel wall is, the faster coagulation begins, sometimes as quickly as 15 seconds after injury.

There are 3 steps:

VASOCONSTRICTION

When platelets come into contact with a damaged blood vessel, their surface becomes sticky and they get adhered to the damaged endothelium (wall of blood vessel).

Then they constrict the vessel, reducing the blood flow through it.

PLATELET PLUG FORMATION

The sticky platelets clump together and release other substances which attract more platelets to the site.

That’s why many platelets rapidly gather at the site of vascular damage and quickly form a temporary seal (the platelet plug).

Now the seal can plug small holes in walls of blood vessel.

COAGULATION

Damaged blood vessel releases thromboplastin which initiates blood coagulation.

Damaged platelets activates prothrobinase.

Then prothrobinase converts prothrombin into thrombin.

Thrombin converts inactive fibrinogens into threads of fibrin.

Then the platelet plug is stabilized by threads of fibrin.

CLOTTING FACTORS

                                  I.            FIBRINOGEN

                               II.            PROTHROMBIN

                            III.            THROMBOPLASTIN

                           IV.            CALCIUM

                              V.            LABILE FACTOR

                           VI.            There is no factor VI

                        VII.            PROCONVERTIN

                     VIII.            ANTIHAEMOPHILIC FACTOR A

                           IX.            ANTIHAEMOPHILIC FACTOR B

                              X.            STUART-PROWER FACTOR

                           XI.            ANTIHAEMOPHILIC FACTOR C

                        XII.            HAGEMAN FACTOR

 

ERYTHROPOIESIS

Matured red blood cells are responsible for transporting oxygen around the body.

They all arise from the bone marrow.

Red blood cells come from stem cells within the bone marrow.

Erythropoiesis is the term used to describe erythrocyte synthesis or the production of red blood cells.

 Figure Credit: Jyotirmayee Sahoo

Stem cells can be converted into proerythroblast and then early erythroblast.

In the early erythroblast stage, ribosome synthesis takes place. Because once the early erythroblast becomes a late erythroblast, the ribosomes are synthesizing a lot of haemoglobins and so the late erythroblast have haemoglobin accumulation.

Haemoglobin is the molecule within red blood cells that actually carry the oxygen.

This late erythroblast can then develop into a normoblast and normoblast already contains haemoglobins.

A normoblast will then become a reticulocyte. A reticulocyte loses its nucleus and it is not a matured erythrocyte yet.

A reticulocyte actually stays in the bone marrow for several days before entering circulation and once entered into circulation the reticulocyte after about 24 to 48 hours will be matured and become a erythrocyte.

Erythrocytes are the red blood cells that circulate and carry oxygen around the body. But erythrocytes do not circulate in our body forever.

It has a life span of about 120days. When it is damaged it has to be removed.

Red blood cell removal mainly occurs in spleen.

MATURATION FACTORS

ERYTHROPOIETIN

VITAMIN B₁₂

VITAMIN B₉

ERYTHROPOIETIN

Erythropoiesis is stimulated by decreased level of O₂ in blood circulation which is detected by the kidneys, which then secrete the hormone erythropoietin.

This hormone stimulates synthesis of red blood cells.

VITAMIN B₁₂

Vitamin B₁₂ (or cyanocobalamin), also known as extrinsic factor is essential for maturation of RBCs.

It is required for the synthesis of DNA.

VITAMIN B₉

Vitamin B₉ (or folic acid) is needed for the formation of heme (iron containing portion of the haemoglobin) in RBCs.

 

 

BLOOD GROUPS

Depending upon the type of antigen present or absent on the RBC membranes two types of blood grouping systems have been developed- ABO grouping and Rh grouping.

LANDSTEINER’S LAW

Karl Landsteiner framed a law in relation to antigens and antibodies.

The Landsteiner law is applicable to ABO blood group system only.

This law states that:

If an antigen (Ex- A antigen) is present on the RBC membrane of an individual, the antibody against that antigen (i.e Anti-A antibody) must be absent in the plasma.

Similarly, if an antigen (Ex- A antigen) is absent on the cell membrane of RBC of an individual, the antibody against that antigen (i.e Anti-A antibody) must be present in the plasma.

ABO GROUPING

A AND B ANTIGENS

The ABO blood grouping system is based on the presence or absence of A and B antigens on the cell membrane of RBCs.

ANTI-A AND ANTI-B ANTIBODIES

Anti-A antibody and Anti-B antibody are two types of antibodies in ABO blood grouping systems.

TYPES OF ABO BLOOD GROUPS

Depending upon the presence or absence of  A and B antigens and Anti-A and Anti-B antibodies there are four types of blood groups.

BLOOD GROUP A

Presence of A antigen on the cell membrane of RBC.

Presence of Anti-B antibody in the plasma.

BLOOD GROUP B

Presence of B antigen on the membrane of RBC.

Presence of Anti-A antibody in the plasma.

BLOOD GROUP AB

Presence of both A antigen and B antigen on the cell membrane of RBC.

Absence of both Anti-A antibody and Anti-B antibody in the plasma.

BLOOD GROUP O

Absence of both A antigen and B antigen on the cell membrane of RBC.

Presence of both Anti-A antibody and Anti-B antibody in the plasma.

 

So persons with ‘AB’ blood group can accept blood from persons with any blood group. That’s why such persons are called ‘universal recipients’.

Similarly, group ‘O’ blood can be donated to persons with any other blood group and hence ‘O’ blood group individuals are called ‘universal donors’.

RH GROUPING

Another antigen, the Rh antigen, is also observed on the surface of RBCs of nearly 80% of humans.

Such individuals are called Rh positive (Rh +ve) and those in whom this antigen is absent are called Rh negative (Rh –ve).

An Rh –ve person, if exposed to Rh +ve blood, will form specific antibodies against the Rh antigens. That’s why Rh group should also be matched before transfusion.

ERYTHROBLASTOSIS FOETALIS

A special case of Rh mismatching can be seen between the Rh –ve blood of a pregnant mother with Rh +ve blood of the foetus.

Rh antigens of the foetus do not get exposed to the Rh –ve blood of the mother in the first pregnancy as the two bloods are well separated by the placenta.

But, during the delivery of the first child, there is a possibility of exposure of the maternal blood to the Rh +ve blood of the foetus.

In that case, the mother starts preparing antibodies (Rh antibodies) against Rh antigen in her blood.

In case of her second pregnancy, the Rh antibodies from the mother can leak into the blood of the foetus (Rh +ve) and can destroy the foetal blood.

This condition is called erythroblastosis foetalis.

This can be prevented by administering Anti-Rh antibodies to the mother immediately after the delivery of the first child.

FUNCTIONS OF BLOOD

NUTRITIVE FUNCTION

Blood carries the nutritive substances such as glucose, amino acids, fatty acids, vitamins, minerals etc. from the GIT to the cells where they are utilized.

TRANSPORT FUNCTION

Various hormones produced by endocrine glands and certain biological antibodies are transported by the blood to the tissue.

PROTECTIVE FUNCTION

Blood plays an important role in the defence mechanism of the body.

Various types of WBCs initiate immune response

RESPIRATORY FUNCTION

Blood picks up oxygen from the lungs and delivers it to the cells of the body.

It also carries away carbon dioxide from the cells to the lungs.

EXCRETORY FUNCTION

Blood transports various metabolic waste products such as urea, uric acid etc. to excretory organs for their disposal.

MAINTENANCE OF BODY TEMPERATURE

Blood also plays an important role in regulation of the body temperature.

HOMEOSTATIC FUNCTION

Blood plays an important role in maintaining the internal environment of our body. Plasma proteins and haemoglobin act as buffers and help in maintaining the acid-base balance of the body fluids.

STORAGE FUNCTION

Blood stores some substances such as glucose, water, proteins and electrolytes for use in emergency conditions like starvation, fluid loss and electrolyte loss.

BLOOD’S FUNCTION AS CONNECTIVE TISSUE

Blood connects the body systems together bringing the needed oxygen, nutrients, hormones and other signaling molecules.