Erythrocyte Structure and Hemoglobin Function

At the center of every heme group lies an iron atom. The oxidation state of this iron is the "switch" that determines how the protein interacts with gases. Specifically, the iron must be in the ferrous state ($Fe^{2+}$) to bind oxygen. If the iron is oxidized to the ferric state ($Fe^{3+}$), it loses its ability to bind oxygen effectively.

The States of Hemoglobin

Hemoglobin changes its name and appearance based on the gases it is currently carrying. Understanding these states is essential for grasping how blood moves between the lungs and the tissues:

1. Oxyhemoglobin ($HbO_{2}$): When oxygen concentrations are high (such as in the lungs), $Fe^{2+}$ binds to $O_{2}$. This state results in bright red blood.
2. Deoxyhemoglobin: When oxygen concentrations are low (such as in the tissues), hemoglobin releases its $O_{2}$. In this state, the iron remains as $Fe^{2+}$, but the blood appears dark red.
3. Carbaminohemoglobin: Hemoglobin also assists in waste removal. Approximately 23% of the body's $CO_{2}$ is transported by binding directly to the hemoglobin protein to form carbaminohemoglobin.

Study Tip: Summary of Hemoglobin States To keep these straight for exams, remember the "plus/minus" relationship:

• Oxyhemoglobin: $Hb + O_{2}$
• Deoxyhemoglobin: $Hb - O_{2}$
• Carbaminohemoglobin: $Hb + CO_{2}$
• Carboxyhemoglobin: $Hb + CO$

Clinical Significance: Carbon Monoxide Poisoning

The high affinity of hemoglobin for certain gases can lead to life-threatening situations. A primary example is carbon monoxide (CO) poisoning. When CO is inhaled, it binds to hemoglobin to form carboxyhemoglobin.

The danger lies in the strength of this bond: carbon monoxide has an affinity for hemoglobin that is 200 times greater than that of oxygen. This means that even in small amounts, CO will "outcompete" oxygen for the binding sites on the hemoglobin subunits. This prevents oxygen from binding in the first place, or if oxygen is already bound, it prevents the hemoglobin from releasing that oxygen into the body's tissues. This leads to cellular hypoxia (lack of oxygen) and can be fatal.

Summary of Gas Exchange Dynamics

The entire process of gas exchange relies on the concentration gradients of these gases.

• In the Lungs: High $O_{2}$ concentration $\rightarrow$ Hemoglobin binds $O_{2}$ $\rightarrow$ Oxyhemoglobin is formed.
• In the Tissues: Low $O_{2}$ concentration $\rightarrow$ Hemoglobin releases $O_{2}$ $\rightarrow$ Deoxyhemoglobin is formed.
• Waste Removal: High $CO_{2}$ concentration $\rightarrow$ Hemoglobin binds $CO_{2}$ $\rightarrow$ Carbaminohemoglobin is formed.

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