Blood and circulatory system

The circulatory system provides a route map in which blood has to flow. Systematic blood flow leads to the formation of the well-organised vascular network in our body. The circulatory system is otherwise known as the cardiovascular system or the vascular system. Although the 2 terms cardio and vascular are observed together, there is a huge difference between the cardiac system and the vascular systems but the circulatory system is complete only when the cardiac and vascular components join together

Structure of the circulatory system


Arterial and venous circulation 

Many people tend to understand that the circulatory system is a component that carries blood alone, but it is an inclusive term used to represent the channel that carries both blood and lymph. The entire circulatory system begins at heart, slowly progresses to each corner of the body through a network of blood vessels. The 2 circuits or systems that divide blood into 2  different channels include pulmonary and systemic circuits. Pulmonary circulation refers to the movement of blood between the heart and the lungs while the systemic circulation moves blood between the heart and the rest of the body. Pulmonary circulation transports deoxygenated blood from the body into the lungs to absorb oxygen and release carbon dioxide back to the lungs. Following this, the oxygenated blood then flows back to the heart to enter into the systemic circulation. Conversely, the oxygenated blood that enters from the lungs into the heart moves to the systemic circulation. The systemic circulation pushes the pure blood into the entire body while receiving the deoxygenated blood from all the cells. The blood collected from each cell of the body moves back to the heart from where it enters into the pulmonary circulation for oxygenation.

Fluid composition

The human body comprises 70% of fluid ( water) which is mostly blood, while the rest is filled by lymph and other body secretions while the solid components like body tissues, bones, and organs make up the remaining  30%. The entire fluid compartment is distributed as the blood and circulatory system, lymph and lymphatic fluid, humerus fluids (tears), sweat, intracellular fluid, and interstitial ( intercellular)  fluid. However, among all these, blood and the circulatory system forms the most fundamental component of the fluid compartment in our body. The fluid compartment is generally divided into ECF( Extracellular fluid) and ICF( Intracellular fluid), however, understanding the detailed description of the fluid compartment is beyond the scope of this post.

Composition of blood

Blood is the only connective tissue that circulates throughout the body to fulfil the metabolic, hormonal and excretory demands of the cell.  About 7% of our body weight is contributed by the blood and it has an average density of 1060 kg/m3 which almost equal to pure water's density of 1000 kg/m3. An adult has approximately 5 litres or 1.3 gallons. The total blood composition is divided into 55%  plasma and 45% is solid or which, the solid component has 4% WBCs and 41% RBCs.  Haematology is the most frequently used term to indicate the study of blood, blood-forming organs and blood diseases. It broadly includes the treatment of blood disorders and some malignancies, including types of haemophilia, leukaemia, lymphomas and sickle-cell anaemias. The vascular system is the study of Blood vessels, which is one of the components used along with the hematologic system. Our forms the very strong foundation of fluidity in the body. It is made up of 55%  plasma while the leftover component of  45% is a solid portion.  Solid elements are again divided into cellular elements, blood proteins, and minerals. Three important blood proteins are albumin, globulin, and fibrinogen. Other solids include electrolytes, and micronutrients such as potassium, iron, calcium, etc. Moreover, our blood is a reservoir of many enzymes necessary for hematologic processes, clotting factors that help in the coagulation of blood, fibrinogens necessary for the tissue repair. Though blood does not store circulating nutrients, hormones, toxins and many unwanted substances, it affords some temporary space necessary for them. The most vital component of the blood is the cellular component has 3 types of cells: RBCs (red blood cells or erythrocytes), WBCs (white blood cells or leukocytes) and platelets or thrombocytes. The primary site of the blood production (hematopoiesis) is the bone marrow, however, the liver also takes part in the blood production in young babies. Depending upon whether our blood is oxygenated or deoxygenated, its colour varies. Venous blood is a bit darker while arterial blood is brighter. Haemoglobin plays a vital role in giving our blood a normal colour.

Different blood cells 



Red blood cells/ red blood corpuscles

Blood cell count is a highly varied component that differs from an individual to another. Young children are mostly distinguished by having more number of cells, haemoglobin and mineral composition of blood when compared to adults. An average adult can produce 175 billion RBCs, 70 billion neutrophils (a mature form of a WBC), and 175 billion platelets (thrombocytes) each day. On examining the cell count, 4.7 to 6.1 million RBCs are found in a male while it is  4.2 to 5.4 million RBCs in a typical female.  RBCs originate from the premature erythroblasts found in the red bone marrow as our marrow is responsible for production of new cells. Normal RBCs are enucleated ( don’t have a nucleus ) and disc-shaped( half-moon) and measure approximately 8 μm in their diameter. They can pass easily through the tiniest capillaries of as small as 2.8 μm in diameter.

Haemoglobin and its function

The major composition- about 95% of RBCs is made up of up haemoglobin which helps in binding oxygen to transfer it to every single cell of our body. The  2 components of haemoglobin: heam refers to - iron and globin is a protein that helps in oxygen binding. Haemoglobin percentage is the total weightage of haemoglobin per 100 ML of blood. A normal adult`s blood has 15% haemoglobin which is slightly lesser in women.

White blood cells

WBCs are also known as leukocytes or leucocytes and are mainly responsible for protecting our body by giving us immunity from the adverse actions of microorganisms and allergies. White blood cells are larger than RBCs and have a nucleus at their centre. WBCs are highly irregular in size and shape because of many varieties. They are classified as granulocytes(look like granules)  and agranulocytes (no granules) based on their morphological features. Eosinophils, neutrophils and basophils are the granulocytes while monocytes and lymphocytes are the agranulocytes. Different cells types and their percentages of WBCs are: neutrophils of 40% to 60%, lymphocytes make up 20% to 40%, monocytes are of  2% to 8%, eosinophils are 1% to 4%, basophils make up 0.5% to 1% and and band (young neutrophil) are  0% to 3%. For more information on WBCs, click here

Platelets and their functions

Platelets are otherwise known as thrombocytes, help in blood clotting. Fibrin, a fibre-like material produced by platelets help in building a web of threads that lock the bleeding area during injuries. Fibrin quantity determines the bleeding and clotting times of blood. Bleeding time is the time within which blood stops bleeding from the time of incision while clotting time is the total duration necessary for complete coagulation. An average adult has a bleeding time of 3-5 minutes while clotting time is 5-8 minutes. The number of platelets is determined by various factors, while a normal adult has 150,000 to 450,000 platelets per microlitres of blood. Vitamin deficiencies like vitamin K can substantially reduce the platelet counts.

Plasma and plasma proteins

More than 90% of plasma is water while the remaining part consists of plasma proteins, clotting factors (particularly fibrinogen), and traces of nutrients, enzymes, waste products, and some gases. When the blood plasma is allowed to clot, the leftover fluid is called serum which has essentially the same composition as plasma except that it has no fibrinogen and clotting factors.


The 2 principal plasma proteins are globulins and albumins.  Globulins are classified into  3 types: alpha, beta, and gamma. Alpha and beta globulins are transport proteins, serve as substrates through which other substances are formed, and perform other diverse functions while gamma globulins play a vital role in natural and acquired immunity to infection. Additionally, the transport-globulins are one of the subtypes of globulin help to carry various substances. For example, thyroid-binding globulin carries thyroxine, and transferrin carries iron. The gamma globulin fraction refers to the percentage of immunoglobulins or antibodies found in the plasma. Gamma globulins are produced by well-differentiated lymphocytes and plasma cells. The actual fraction of the globulins can be measured through a specific laboratory test known as serum protein electrophoresis.


Albumin maintains fluid balance within the vascular system. As the capillary walls are impermeable to albumin, it remains inside the vessels. Albumin can create a negative osmotic force that helps to retain great amounts of fluid within the vascular space. It is synthesized by the liver and it has the capacity to bind with several substances including medications, bilirubin and some hormones. So, it indirectly helps to transport these substances across various areas of our body.  People with poor hepatic function may have low concentrations of albumin resulting in decreased osmotic pressure which may be a cause of generalized oedema(swelling).

Read more 

  1. Blood groups

  2. Blood vessels in human beings 

  3. WBCs

  4. Anatomy of the human heart

Check your understanding

  1. What is oxyhaemoglobin?

  2. What are the 2 components of the cardiovascular system?

  3. Explain the structure of the circulatory system

  4. Explain the composition of blood plasma.

  5. What are plasma proteins? Explain their functions?

  6. What is the platelet count in a healthy adult?

  7. Classify WBCs and explain the difference between them.

  8. Describe the composition of blood

  9. Explain about red blood cells.

  10. Describe the composition nf functions of WBCs.

  11. What is bleeding time and clotting time?

  12. What are platelets? Explain their functions.


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