Blood and circulatory system.

Introduction to the blood and circulatory system

The human body has 70% of fluid ( water) in the form of blood, lymph and body secretions, and the solid mass is only  30%  of the total composition. The solid mass is 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 intercellular fluid. Blood and circulatory system is the most primary fluid compartment in our body. Unlike many other body systems, blood in the hematologic system (circulatory system) circulates throughout the body to fulfil the metabolic, hormonal and excretory demands. The oxygenated blood is pushed from the left side 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. The hematologic system consists of the connective tissue-blood and the blood vessels. This is the only fluid connective tissue in our body. Blood shares 55% of the liquid plasma and the remaining 45% is the solid part. Solid components are cellular elements, blood proteins, and minerals. Blood proteins are albumin, globulin, and fibrinogen. In addition to these, blood also clotting factors, and fibrinogens necessary for the tissue repair, electrolytes, waste products, and, hormones, excretory products and nutrients. Thee cellular components of blood is divided into 3 primary types; RBCs (red blood cells or erythrocytes), WBCs (white blood cells or leukocytes) and platelets or thrombocytes. The primary site for blood production (hematopoiesis) is the bone marrow, however, the liver also takes part in blood production in neonates. 


Table of content

1. Introduction

2. Blood volume

3. Blood cells in human beings

    3.1 Red blood cells

    3.2 White blood cells

    3.3 Platelets

4. Plasma Proteins



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 liters.


Blood cells in human beings

Blood cells are divided into red blood cells( RBCs), white blood cells( WBCs)  and platelets( Thrombocytes). 


Red blood cells

RBCs originate from the premature erythroblasts found in the bone marrow.RBCs are enucleated ( don’t have a nucleus ) and disc-shaped with approximately  8 μm in diameter. They are so flexible that they can pass easily through the tiniest capillaries of as small as 2.8 μm in diameter. 95% of the red cell mass is made up of iron. Haemoglobin present within the RBCs helps to trap the oxygen to transfer it to every single cell of our body. In a haemoglobin molecule, there are 2 components, namely hem- iron and globin- a protein that helps in oxygen binding. 100 ML of whole blood contains about 15 g of haemoglobin and this is known as haemoglobin %.


White blood cells

WBCs are also known as leukocytes or leucocytes. They are mainly responsible for immune function that helps to fight against infections and illnesses. White blood cells have a nucleus and they are usually larger than RBCs. WBCs can be classified as granulocytes( look like granules)  or agranulocytes ( no granules) based on their morphological features.  



Platelets are otherwise known as thrombocytes, which help in the process of blood clotting. Platelets produce a fibre-like matter that will form a web of threads around the injured area so that bleeding ceases within 5 minutes from the time of injury. Platelets contain a material known as fibrin that play an important role in the clotting process.


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 plasma is allowed to clot, the remaining fluid is called serum. Serum has essentially the same composition as plasma, except that, there is no fibrinogen and clotting factors in it. 2 types of plasma proteins are albumins and globulins. The globulins can be separated into three main fractions—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 thyroxin, 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 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.




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