Hemoglobin and Oxygen Transport in the Human Body
Hemoglobin’s Capacity for Oxygen Transport
As illustrated, the blood vessel carrying plasma Hemoglobin (Hb) (right) demonstrates a higher capacity to capture oxygen molecules from the alveoli. This is because each Hb molecule can carry four O2 molecules, in addition to the oxygen particles that are dissolved in the plasma. In contrast, the blood vessel carrying only dissolved oxygen in plasma (left) carries fewer O2 particles. Consequently, the oxygen-carrying capacity is significantly greater for vessels containing Hb compared to those carrying oxygen solely dissolved in plasma. Vessels with oxygen dissolved in plasma carry approximately 0.3 mm O2 per 100 ml of blood, whereas vessels carrying Hb transport about 20 mm O2 per 100 ml of blood.
The enhanced oxygen uptake capacity of blood vessels carrying Hb molecules is attributed to the presence of iron (Fe) within Hb, which exhibits a high affinity for O2. Therefore, Hb effectively attracts available O2. As O2 from the alveoli enters the blood, it is absorbed by Hb, forming a compound that does not contribute to the O2 partial pressure. However, when Hb reaches its saturation point and can no longer capture more O2, the pressure increases until it reaches 100 ml. This is a continuous process. Hb’s ability to capture O2 increases the oxygen transport capacity by 70 times.
Typically, humans have an O2 saturation level of 95 to 96%. When it falls below 95%, tissues may be affected. Low saturation can lead to respiratory problems, among other issues.
In tissues, where O2 pressure is low, Hb releases O2 to equalize the pressure between the tissues and blood vessels. Thus, Hb saturation is dependent on pressure.
- 1 gram of Hb transports 1.34 ml of O2.
- 100 ml of blood contains 14.7 grams of Hb (14.7 x 1.34 = 19.7 ml O2).
If arterial Hb saturation is 95%, the O2 content will be:
95% of 19.7 = 18.715 ml O2 per 100 ml.
If venous Hb saturation is 70%, the O2 content will be:
70% of 19.7 = 13.79 ml O2 per 100 ml.
Partial Pressure Differences in Various Tissues
According to Dalton’s Law of Partial Pressures, in a gaseous mixture, each gas exerts a pressure as if it were alone, and this is termed the partial pressure of that gas. The significance of partial pressures lies in their role in determining the movement of each specific gas, which is crucial in the diffusion of gases.
Ideally, blood would reach all areas where oxygen is present. However, there are always areas where oxygen exchange cannot occur. This amount of unexchanged oxygen, in addition to the anatomic dead space (air in the respiratory airways), is termed physiologic dead space.
Respiration involves the exchange of gases between the atmosphere and cells. In unicellular organisms, this process is straightforward, occurring via diffusion according to partial pressure gradients. To oxygenate the rest of the tissues, several stages are necessary:
- Ventilation
- Alveolar-capillary diffusion of gases
- Ventilation/circulation
- Blood gas transport
- Regulation of breathing
Ventilation is the process of transporting air from the atmosphere to the lungs. Perfusion refers to the flow of venous blood through the pulmonary circulation to the capillaries and the return of oxygenated blood to the left ventricle.
The systemic circulation distributes blood from the left ventricle to all body tissues and returns it to the right atrium.
As depicted, air from the outside enters the lungs through the alveoli (alveolar air), where alveolar exchange occurs. Typically, the oxygen in the alveoli equilibrates with the blood. Blood entering the left ventricle via the pulmonary veins carries more oxygen than carbon dioxide. Subsequently, blood leaves the heart through the systemic arteries and reaches the tissues via the capillaries. In the tissues, an exchange takes place, altering the pressures of oxygen and carbon dioxide. The blood leaving the tissues is less oxygen-rich and has an increased amount of carbon dioxide. This deoxygenated blood then enters the right ventricle and subsequently the alveoli, where the exchange process is repeated, and atmospheric air is expelled.