Studying Blood Composition

It's a well-known fact that a human being has an average 5 liters of blood in his body. Human blood is composed of two parts, viz, fluid and cellular. The fluid part is known as blood plasma that makes 55% of total blood volume and the rest comprises the cellular components or the formed elements.

When a blood sample is centrifuged, it gets separated into different layers, based on their density. Three complete layers are visible after centrifugation. The bottom layer is red in color due to presence of Red Blood Cells (RBCs). A white band is formed in the middle layer, which is also known as buffy coat and it's made up of White Blood Cells (WBCs) and platelets. The third or the topmost layer is a straw colored liquid containing the blood plasma. The cells present in the blood are also known as formed elements.

The following content explains you the composition and functions of each element.

Composition of Formed Elements
Red Blood Cells
RBCs are also known as red blood corpuscles or erythrocytes. They constitute 45% of blood by volume. They contain hemoglobin, that renders blood red in color. RBCs are produced in the bone marrow and they have a life cycle of 100-120 days. Mature RBCs are biconcave and flexible, lacking cell nucleus and organelles. The principle function is to deliver oxygen to different tissues of the body.

White Blood Cells
WBCs or white blood corpuscles are known as leukocytes. They make for 1% by volume of total blood. Leukocytes are cells of the immune system that provide protection to the body from foreign particles and infectious diseases. They are derived from hematopoietic stem cells. Leukocytes are classified as granulocytes and agranulocytes. Granulocytes cells include neutrophils, basophils and eosinophils, whereas agranulocytes cells are lymphocytes, monocytes and macrophages. The count of leukocytes in blood is an important factor for normal functioning of body.

Platelets are also known as thrombocytes. They are derived from precursor cells known as megakaryocytes and are devoid of nucleus. The lifespan of platelets is 5-9 days. The most important function of platelets is blood coagulation or blood clotting. If the count of platelets in blood is low, they cause excessive bleeding while the effect is just reverse if the count is more than normal. They are also a source of natural growth factors.

Composition of Blood Plasma
Blood Plasma is the straw colored liquid portion of the blood. 92% is composed of water and the rest 8% is made up of plasma proteins. It is mostly composed of dissolved proteins, mineral ions, glucose, clotting factors and carbon dioxide. It circulates dissolved nutrients (amino acids, fatty acids and glucose) and removes waste products (carbon dioxide, lactic acid and urea) from the body. Other components of blood plasma are serum albumin, lipoprotein particles, immunoglobulins, electrolytes, etc.

Functions of Blood

Blood performs a wide range of functions in human body. We list them down over here:
Supplies oxygen and nutrients to different tissues of our body.
Removes waste products like, urea, lactic acid and carbon dioxide from our body.
Provides immunity to body against foreign particles.
Helps in transportation of hormones throughout the body.
Aids in blood clotting which is a natural repair mechanism of cells.
Regulates and maintains normal temperature in our body.
Maintains pH balance inside the body.
The components of blood help in homeostasis.
The pH of blood lies in the range of 7.35 to 7.45, which is necessary for its normal functioning.

Hematopoiesis-process of making formed elements of blood

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates as well as the tissues of some invertebrates. Hemoglobin in the blood carries oxygen from the respiratory organs (lungs or gills) to the rest of the body (i.e. the tissues) where it releases the oxygen to burn nutrients to provide energy to power the functions of the organism, and collects the resultant carbon dioxide to bring it back to the respiratory organs to be dispensed from the organism.
In mammals, the protein makes up about 97% of the red blood cells' dry content, and around 35% of the total content (including water). Hemoglobin has an oxygen binding capacity of 1.34 ml O2 per gram of hemoglobin,which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules.
Hemoglobin is involved in the transport of other gases: it carries some of the body's respiratory carbon dioxide (about 10% of the total) as carbaminohemoglobin, in which CO2 is bound to the globin protein. The molecule also carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen.
Hemoglobin is also found outside red blood cells and their progenitor lines. Other cells that contain hemoglobin include the A9 dopaminergic neurons in the substantia nigra, macrophages, alveolar cells, and mesangial cells in the kidney. In these tissues, hemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism.
Hemoglobin and hemoglobin-like molecules are also found in many invertebrates, fungi, and plants. In these organisms, hemoglobins may carry oxygen, or they may act to transport and regulate other things such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant of the molecule, called leghemoglobin, is used to scavenge oxygen, to keep it from poisoning anaerobic systems, such as nitrogen-fixing nodules of leguminous plants.

hemoglobin breakdown

White Blood Cell

White blood cells, or leukocytes (also spelled "leucocytes"; from the Greek word leuko- meaning "white"), are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. They live for about 3 to 4 days in the average human body. Leukocytes are found throughout the body, including the blood and lymphatic system.
The number of leukocytes in the blood is often an indicator of disease. There are normally between 4×109 and 1.1×1010 white blood cells in a litre of blood, and ranging from 7 and 21 micrometres in diameter, they make up approximately 1% of blood in a healthy adult. An increase in the number of leukocytes over the upper limits is called leukocytosis, and a decrease below the lower limit is called leukopenia. The physical properties of leukocytes, such as volume, conductivity, and granularity, may change due to activation, the presence of immature cells, or the presence of malignant leukocytes in leukemia.


Neutrophils defend against bacterial or fungal infection and other very small inflammatory processes that are usually first responders to microbial infection; their activity and death in large numbers forms pus. They are commonly referred to as polymorphonuclear (PMN) leukocytes, although, in the technical sense, PMN refers to all granulocytes. They have a multi-lobed nucleus that may appear like multiple nuclei, hence the name polymorphonuclear leukocyte. The cytoplasm may look transparent because of fine granules that are pale lilac. Neutrophils are very active in phagocytosing bacteria and are present in large amount in the pus of wounds. These cells are not able to renew their lysosomes (used in digesting microbes) and die after having phagocytosed a few pathogens. Neutrophils are the most common cell type seen in the early stages of acute inflammation, and make up 60-70% of total leukocyte count in human blood.The life span of a circulating human neutrophil is about 5.4 days.


Eosinophils primarily deal with parasitic infections. Eosinophils are also the predominant inflammatory cells in allergic reactions. The most important causes of eosinophilia include allergies such as asthma, hay fever, and hives; and also parasitic infections. In general, their nucleus is bi-lobed. The cytoplasm is full of granules that assume a characteristic pink-orange color with eosin stain.


Basophils are chiefly responsible for allergic and antigen response by releasing the chemical histamine causing vasodilation. The nucleus is bi- or tri-lobed, but it is hard to see because of the number of coarse granules that hide it. They are characterized by their large blue granules.


Lymphocytes are much more common in the lymphatic system. Lymphocytes are distinguished by having a deeply staining nucleus that may be eccentric in location, and a relatively small amount of cytoplasm. The blood has three types of lymphocytes:
B cells: B cells make antibodies that bind to pathogens to enable their destruction. (B cells not only make antibodies that bind to pathogens, but after an attack, some B cells will retain the ability to produce an antibody to serve as a 'memory' system.)
T cells:
CD4+ (helper) T cells having co-receptor CD4 is known as CD4+ T cells.These cells binds with the antigen having MHC II receptor on its surface. Then it presents this antigen to B cells. B cells produce antibodies to destroy antigen. Thus CD4+ T cell is also known as antigen presenting cells.T cells co-ordinate the immune response and are important in the defense against intracellular bacteria. In acute HIV infection, these T cells are the main index to identify the individual's immune system activity. Research has shown that CD8+ cells are also another index to identify human's immune activity.
CD8+ cytotoxic T cells are able to kill virus-infected and tumor cells.T cells having co-receptor CD8 are known as CD8+ T cells. These cells kills damaged or cancerous cells. CD8 binds with MHC I receptor of damaged cells(carrying antigen). All nucleated cells possess MHC I on its surface.
γδ T cells possess an alternative T cell receptor as opposed to CD4+ and CD8+ αβ T cells and share characteristics of helper T cells, cytotoxic T cells and natural killer cells.
Natural killer cells: Natural killer cells are able to kill cells of the body that are displaying a signal to kill them, as they have been infected by a virus or have become cancerous.


Monocytes share the "vacuum cleaner" (phagocytosis) function of neutrophils, but are much longer lived as they have an additional role: they present pieces of pathogens to T cells so that the pathogens may be recognized again and killed, or so that an antibody response may be mounted. Monocytes eventually leave the bloodstream to become tissue macrophages, which remove dead cell debris as well as attacking microorganisms. Neither of these can be dealt with effectively by the neutrophils. Unlike neutrophils, monocytes are able to replace their lysosomal contents and are thought to have a much longer active life. They have the kidney shaped nucleus and are typically agranulated. They also possess abundant cytoplasm.
Once monocytes move from the bloodstream out into the body tissues, they undergo changes (differentiate) allowing phagocytosis and are then known as macrophages.


The heart is one of the most important organs in the entire human body. It is really nothing more than a pump, composed of muscle which pumps blood throughout the body, beating approximately 72 times per minute of our lives. The heart pumps the blood, which carries all the vital materials which help our bodies function and removes the waste products that we do not need. For example, the brain requires oxygen and glucose, which, if not received continuously, will cause it to loose consciousness. Muscles need oxygen, glucose and amino acids, as well as the proper ratio of sodium, calcium and potassium salts in order to contract normally. The glands need sufficient supplies of raw materials from which to manufacture the specific secretions. If the heart ever ceases to pump blood the body begins to shut down and after a very short period of time will die.

sistematic flow of blood through heart

blood vessel layer and comparison


Veins in the body