Blood and circulatory system


The human body contains 70% of fluid ( water) in the form of blood, lymph and other body secretions. The solid mass is accountable for only the remaining 30%, made up of body tissues, bones, and organs. The fluid compartment is further divided into the blood and blood and circulatory system, lymph and lymphatic fluid, humerus fluids (tears), sweat and intracellular fluid, and interstitial ( intercellular)  fluid. Blood and circulatory system is the most fundamental fluid compartment in our body. Blood is the only connective tissue that can circulate throughout the body to fulfil the metabolic, hormonal and excretory demands of each individual cell.


Composition and circulation of blood

The oxygenated blood is pushed from the left side( left ventricle) of the heart towards systemic circulation whereas the deoxygenated blood is received from the body( peripheral venous circulation) into the right side of the heart( right atrium). The blood and the blood vessels together form the hematologic system. Blood is the only fluid connective tissue in our body. Blood shares 55% of the liquid plasma and the remaining 45%  in the form of solids. Solid components are further divided into the cellular elements, blood proteins, and minerals. 3 principal blood proteins are albumin, globulin, and fibrinogen. The solid composition includes electrolytes, and micronutrients such as potassium, iron, calcium, etc. Furthermore, blood is the reservoir for many enzymes, clotting factors, and fibrinogens necessary for the tissue repair. Apart from these, blood is temporary space for the circulating body waters, nutrients, hormones, toxins and many unwanted substances. The cellular components of blood are split into 3 types; 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. 


Arterial and venous circulation 



Blood volume

Under normal conditions, a human being can produce 175 billion RBCs, 70 billion neutrophils (a mature form of a WBC), and 175 billion platelets (thrombocytes) each day. The total volume of blood in a normal healthy adult is approximately 7% to 10% of the total body weight which amounts to 5-6 litres. However, physiological variations are possible depending upon the age, body weight, diseases conditions etc.


Blood cells

Red blood cells

RBCs originate from the premature erythroblasts found in the red bone marrow.RBCs are enucleated ( don’t have a nucleus ) and disc-shaped( half-moon) with the approximate diameter,  8 μm. They are so flexible that they can pass easily through the tiniest capillaries that are as small as 2.8 μm in diameter. The major composition (95%)of RBCs is made up of up haemoglobin. Haemoglobin helps in binding the oxygen with it( oxyhaemoglobin) to transfer it to every single cell of our body. The  2 components of haemoglobin are so named, where heam refers to - iron and globulin is a protein that helps in oxygen binding. Haemoglobin percentage is the total weightage of haemoglobin per every 100 ML of blood. In a normal adult, the haemoglobin percentage is about 15.



White blood cells

WBCs are also known as leukocytes or leucocytes. They are mainly responsible for protecting our body by looking after our immune functions by resisting the entry of microorganisms and many allergies. White blood cells carry a nucleus at their centre, they are irregular in the shape, and they are usually larger than RBCs measuring about 15 microns. WBCs can be classified as granulocytes(look like granules)  and agranulocytes (no granules) based on their morphological features.   For more information on WBCs, click here.




Platelets are otherwise known as thrombocytes, help in the process of blood clotting. Platelets produce a fibre-like matter called fibrin, fibrin help in building a web of threads around the injured area, as a result, the bleeding gets arrested. An average adult has a bleeding time of 3-5 minutes and the clotting time of 5-8 minutes. Platelets may vary in their numbers depending upon the age group. A normal adult has 150,000 to 450,000 platelets per microlitres of blood. Some of the vitamin deficiencies such as vitamin K can substantially reduce the platelet counts.



Plasma and plasma proteins

More than 90% of plasma is water and the remaining part consists of plasma proteins, clotting factors (particularly fibrinogen), and traces of nutrients, enzymes, waste products, as well as gases. If blood plasma is allowed to clot, the leftover fluid is called serum. Serum has essentially the same composition as plasma, except that, there is no fibrinogen and clotting factors in it. The 2 fundamental plasma proteins are albumins and globulins. The globulins can be separated into three main subcategories —alpha, beta, and gamma—each of which consists of distinct proteins performing different functions. The proteins present in the alpha and beta fractions are made in the liver. The transport-globulins( one of the primary globulin type)  carry various substances. For example, thyroid-binding globulin carries thyroxine, and transferrin carries iron. The clotting factors including fibrinogen remain inactive until the clotting process begins. 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. On the other hand, albumin is essential for the maintenance of fluid balance within the vascular system. Since the capillary walls are impermeable to albumin, it can create an osmotic force that retains the required amount of fluid within the vascular space. Albumin is produced by the liver and it has the capacity to bind with several other substances such as medications, bilirubin and some hormones. People with poor hepatic function may have low concentrations of albumin resulting in decreased osmotic pressure which in turn causes generalized oedema(swelling).

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Blood groups


Blood vessels in human beings   




Anatomy of human heart


Check your understanding

  1. What is oxyhaemoglobin?

  2. Explain the composition of blood plasma

  3. What are plasma proteins, name them?

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

  5. Classify WBCs and explain the difference between them.

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