Hemoglobin

Chemical structure and metabolism

Red blood cells (erythrocytes) are almost one hundred percent filled with this specific respiratory pigment.

Structural features By its chemical nature, hemoglobin is a complex coordination compound. Two key structural components are distinguished in its architecture:

• The protein component, which is called globin.
• The non-protein component, represented by four heme molecules.

The unique feature of heme is that a ferrous iron atom ($Fe^{2+}$) is located at its center. It is thanks to this structure that the ability to reversibly bind respiratory gases is ensured.

Metabolic cycle in the body The process of hemoglobin biosynthesis actively proceeds in red bone marrow precursor cells — erythroblasts and normoblasts. For the full course of this process, an adequate dietary intake of iron is critically important.

The regular destruction of the molecule occurs at the moment of hemolysis, i.e., the natural death of old erythrocytes. The released pigment undergoes a series of transformations and ultimately turns into bilirubin. The excretion of the formed metabolites is carried out via two main pathways:

1. Predominantly through the gastrointestinal tract along with secreted bile.
2. Partially excreted by the kidneys in the urine as urobilin.

Gas transport and buffer functions

The key purpose of the molecule is to ensure tissue respiration through gas transport via the bloodstream.

Mechanism of oxygen transport ($O_2$) This is the leading and vital function. In the capillary network of the lungs, conditions with a high partial pressure of oxygen ($pO_2$) are created. Here, hemoglobin actively binds gas molecules, resulting in the synthesis of oxyhemoglobin ($HbO_2$). It should be noted that the overwhelming majority of oxygen — up to 90% of the total volume — is transported precisely in this chemical form. When blood reaches peripheral tissues, oxyhemoglobin undergoes dissociation. Oxygen is detached and diffuses into cells, while the pigment itself passes into a reduced state, turning into deoxygenated hemoglobin (or $HbH$).

Carbon dioxide transport ($CO_2$) Carbon dioxide transport is a secondary but important task. The binding of $CO_2$ occurs in the capillaries of the systemic circulation directly in the tissues. The compound formed in this process is called carbaminohemoglobin. Using this mechanism, about 20% of the generated carbon dioxide is effectively removed from the body.

Regulation of acid-base balance (ABB) In addition to direct gas transport, hemoglobin plays a crucial homeostatic role. It functions as the main blood buffer system, actively participating in maintaining the stable acid-base equilibrium of the body's internal environment.

Physiological and pathological forms

Upon interaction with various chemical agents, hemoglobin is capable of forming both normal and pathological complexes that disrupt tissue respiration.

Physiological compounds:

• Oxyhemoglobin ($HbO_2$) — normal transport form with oxygen.
• Deoxygenated hemoglobin ($HbH$) — reduced form that has released oxygen.
• Carbaminohemoglobin ($HbCO_2$) — physiological complex for carbon dioxide transport from tissues.

Pathological compounds:

1. Carboxyhemoglobin ($HbCO$)

A specific complex formed as a result of the protein's reaction with carbon monoxide ($CO$). It is characterized by an extremely high bond strength. Having captured carbon monoxide, hemoglobin completely loses its main ability — to transport oxygen to cells. In mild poisoning, the situation is reversible: it is enough to switch the victim to breathing pure air, and carbon monoxide will begin to gradually detach, restoring the normal gas transport function of the blood.

1. Methemoglobin ($MetHb$)

This pathological form is formed when the iron atom in the heme undergoes oxidation and passes from the normal ferrous to the ferric state ($Fe^{3+}$).

• Pathogenesis basics: The resulting $MetHb$ binds the oxygen molecule too tightly. This critically hinders its dissociation and release to cells in the periphery, which inevitably leads to severe gas exchange impairment.
• Etiological factors: The pathology can have a hereditary genesis or develop as an acquired condition. In the second case, the triggering factor is exposure to strong chemical oxidizing agents. Such substances include inorganic nitrites, nitrates, sulfonamide drugs, and certain local anesthetics (specifically, lidocaine).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of hemoglobin participation in the blood buffer system (role in maintaining ABB)
• Stages of globin and heme molecule biosynthesis in red bone marrow cells
• The process of oxyhemoglobin dissociation in tissue capillaries and factors influencing it
• Metabolic pathways of hemoglobin conversion into bilirubin during erythrocyte hemolysis
• Effect of local anesthetics and sulfonamides on the formation of methemoglobinemia

Other topics in the "Blood" section

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