Which Blood Cells Contain Respiratory Pigment?

Blood is a mixture of cells floating in a fluid known as plasma. Plasma, which is 90-percent water, transports not only blood cells but many essential molecules needed by the body, such as hormones, immunoglobulins (antibodies), enzymes, clotting agents, hormones, vitamins, cholesterol, nutrients (such as glucose), electrolytes (such as sodium, potassium and calcium) and waste products. The cells found in blood are of three kinds: red blood cells, called erythrocytes, make up 40 to 45 percent of blood, white blood cells, called leucocytes, and platelets, called thrombocytes. Essentially, red blood cells carry oxygen from the lungs to the tissues throughout the body, white blood cells help fight infection, and platelets help in clotting.

Respiratory Pigment

• Oxygen molecules are carried from the lungs to the tissues of the body by a protein molecule in red blood cells, called hemoglobin. Hemoglobin contains atoms of iron, and it is the iron component of the hemoglobin that gives blood its red color. As such, hemoglobin is known as the respiratory pigment.

Hemoglobin Structure

• A hemoglobin molecule is a tetrameric heme protein, meaning that it consists of four polypeptide (amino acid) chains folded to form four globular protein structures, individually known as a monomeric heme protein. Each monomeric heme protein contains a heme prosthetic group, an essential component of which is the single iron atom at its center, which, in its oxidized Fe2+ state, is responsible for attracting and binding oxygen molecules in the oxygen-rich environment of the lungs. Each iron atom can bind one molecule of oxygen and, since each entire hemoglobin molecule contains four iron atoms, this allows each hemoglobin molecule to carry four oxygen molecules. Each red blood cell contains about 250 million hemoglobin molecules, thus respiratory pigments dramatically increase the oxygen carrying capacity of blood.

Hemoglobin Function

• The ability of hemoglobin to attract oxygen in the lungs and then repel it in the tissues is due to the differences in pH between the oxygen-rich environment of the lungs and the oxygen-poor environment of the tissues. The pH in tissues is lower (more acidic) than the pH in the lungs, and this acidic environment lowers the affinity of hemoglobin for oxygen and increases its affinity for hydrogen ions (H+ ions). Hydrogen ions are abundant in tissues, due to the reaction between carbon dioxide (CO2) that is produced in the tissues and water to form bicarbonate, a reaction that takes place when carbon dioxide diffuses into the red blood cells and reacts with water.

  As such, all four oxygen molecules are released from hemoglobin to the tissues and replaced with hydrogen ions in the hemoglobin molecule. Once hemoglobin returns to the lungs it is able to shed its hydrogen ions, because the higher pH of the lungs makes the H+ ions more attracted to the environment than to the heme, so hydrogen leaves and is replaced with oxygen and the cycle starts over.

  This system has the added benefit of allowing the transport of carbon dioxide to the lungs for removal from the body. CO2 is insoluble in water, but the bicarbonate ion is water soluble and therefore travels in the blood to the lungs, where it then reacts with the released H+ ions to revert back to CO2 gas which can be exhaled with the next breath.

Carbon Monoxide

• Carbon monoxide poisoning prevents oxygen transport.

  Carbon monoxide has a much higher affinity for hemoglobin than does oxygen. In carbon monoxide poisoning, this preferential binding means that oxygen cannot be transported and released to body tissues, thus resulting in asphyxiation and death.