Blood carries oxygen to each and every corner of the body. Substances of all kinds that the body requires are carried to the relevant organs by the blood. Nutrients such as glucose, amino acids and minerals—and most importantly, oxygen—are just a few of these.

Blood also has a major role in the body's defense against infection. Cells of the blood and constituents of the plasma (the colorless fluid part of blood, lymph, in which corpuscles or fat globules are suspended) interact in complex ways to confer immunity to infectious agents, to resist or destroy invading organisms, to produce the inflammatory response, and to destroy and remove foreign materials and dead cells.

The leukocytes (white blood cells) have a primary role in these reactions: granulocytes and monocytes phagocytize (ingest) bacteria and other organisms, migrate to sites of infection or inflammation and to areas containing dead tissue, and participate in the enzymatic breakdown and removal of cellular debris; lymphocytes are concerned with the development of immunity.

Thus, there is no substitute for blood. It cannot be made or manufactured. Donors are the only source of blood for patients who need it.

Blood is red in color due to the presence of heme protein. Blood is red from the protein, hemoglobin. Hemoglobin has a molecule called a "heme" which has the metal iron in it. When the iron is oxygenated, it becomes red. When the iron is deoxygenated, it becomes blue. This is why your veins are blue.

In living mammals, blood is red in color. It is very well known fact. But when we wonder why it is so.... science gives us the answer. Erythrocytes are red in color due to the presence of heme protein. The protein is made up of globin and heme. Heme gives blood cells red color.

Each molecule of heme protein has four heme groups. These heme groups have iron molecules attached to them. They alter their shape and color on interaction with various molecules. Thus heme protein gives bright red color to the blood when it combines with oxygen (oxygenated) and dark red color when oxygen is released.

In vertebrates, arterial blood and capillary blood is bright in color as it is oxygenated, whereas in the veins it is very dark red as they carry deoxygenated blood. The interesting part is, in few molluscs and arthropods, the blood is gray to pale yellow in color but it turns to dark blue when exposed to oxygen in air, as seen when they bleed. This happens due to the presence of copper containing protein in the blood known as hemocyanin. Similarly, the blood in some species of ascidians and tunicates such as sea squirts and sea cucumbers, contains a protein called vanadium. Though its not clear whether they carry oxygen, when their blood gets exposed to oxygen it turns mustard yellow. In order to know the reason for blood being red in color, we need to know more about RBC.

RBC transport oxygen from lungs to all the cells of the body through heart. Blood transports materials around the body and protects against disease. It consists of cells, solutes and liquid. Blood is pumped away from the heart at high pressure in arteries and returns to the heart at low pressure in veins.

Responsibility for transportation: Red blood cells are responsible for transport of oxygen from lungs to all the cells of the body through heart. This is because of the presence of hemoglobin in RBC. Hemoglobin has iron that carries oxygen and enables its transfer to all body cells. After releasing oxygen at the body cells, iron of the hemoglobin joins to carbon dioxide (waste product of the cell). The RBC then carry carbon dioxide to lungs from where it is exhaled out.

Oxygen transport through metal complexes: When at rest, an average adult consumes about 250ml of pure oxygen that is sufficient for all the tissues and organs of the body. But during exercises and rigorous work, our body requires oxygen more than 250ml. Large amounts of oxygen is required by the body for various activities. Transporting large amounts of oxygen by the blood to the tissues that demand continuous supply, requires a very sophisticated mechanism.

Thus, the human body is designed meticulously and intelligently. It has all the required tools and gadgets to perform its best. To meet this challenge it is well equipped with finely planned transport system that constitutes heme.

Hemoglobin present in blood is a globular shaped protein. When blood binds to oxygen it gets slightly darker red. It is always red because of the porphyrin in the heme group. The heme group in hemoglobin (the protein responsible for transporting gases in the red blood cells) consists of a porphyrin and an iron ion.

Hemoglobin present in blood is a globular shaped protein. Proteins are formed by linking amino acids through polypeptide chains. Hemoglobin consists of four protein subunits. Each subunit consists of alpha helically arranged amino acids that are binded to one another by hydrogen bonds. Each protein subunit contains heme group, which in turn contains an iron atom that binds to one oxygen molecule. Each hemoglobin protein can bind to four oxygen molecules.

The heme group alters its shape when it gets oxygenated. When a single iron atom of the heme group binds with oxygen, the whole protein changes its shape from domed to planar. When the heme group is in its original shape i.e.domed, only one oxygen molecule can bind to it. But when it attains planar shape, it can bind to three more oxygen molecules and gets more oxygenated. So, the binding of one molecule of oxygen to hemoglobin, enables it to bind to more oxygen molecules.

This property of hemoglobin is known as co–operative binding. From this, a microscopic observation noting the color of blood can tell if the observed blood is oxygenated or deoxygenated. Still, we may wonder what causes the color change in blood. The answer is here. The color change or difference of color between oxygenated and deoxygenated blood is due to the different light absorbing properties of the oxygenated and deoxygenated blood.