Gas transport in blood

Once the oxygen has diffused from the alveoli into pulmonary circulation, it must be carried, or transported, in the blood to cells and tissues that need it. Furthermore, once the carbon dioxide has diffused from the tissues into the systemic circulation, it must be transported to the lungs, where it can be eliminated. This section considers mechanisms by which these gases are transported.

Transport of oxygen. Oxygen is carried in the blood in two forms:

• Physically dissolved

• Chemically combined with hemoglobin

Oxygen is poorly soluble in plasma. At a PO2 of 100 mmHg, only 3 ml of oxygen is physically dissolved in 1 l of blood. Assuming a blood volume of 5 l, a total of 15 ml of oxygen is in the dissolved form. A normal rate of oxygen consumption at rest is about 250 ml/min. During exercise, oxygen consumption may increase to 3.5 to 5.5 l/min. Therefore, the amount of dissolved oxygen is clearly insufficient to meet the needs of the tissues. Most of the oxygen in the blood (98.5%) is transported chemically combined with hemoglobin. A large complex molecule, hemoglobin consists of four polypeptide chains (globin portion), each of which contains a ferrous iron atom (heme portion). Each iron atom can bind reversibly with an oxygen molecule:

The binding of oxygen to hemoglobin follows the law of mass action so that, as the PO2 increases (as it does in the lungs), more will combine with hemoglobin. When the PO2 decreases, as it does in the tissues consuming it, the reaction moves to the left and the hemoglobin releases the oxygen. Each gram of hemoglobin can combine with up to 1.34 ml of oxygen. In a healthy individual, there are 15 g of hemoglobin per 100 ml of blood. Therefore, the oxygen content of the blood is 20.1 ml O2/100 ml blood:

100 ml blood g Hb 100 ml blood

It is important to note that oxygen bound to hemoglobin has no effect on the PO2 of the blood. The amount of oxygen bound to hemoglobin determines oxygen content of the blood. The PO2 of the blood is determined by the amount of oxygen dissolved in the plasma.

The PO2 of blood is the major factor determining the amount of oxygen chemically combined with hemoglobin, or the percent of hemoglobin saturation. The relationship between these two variables is illustrated graphically by the oxyhemoglobin dissociation curve (see Figure 17.6). This relationship is not

Blood PO2 (mmHg)

Figure 17.6 Oxyhemoglobin dissociation curve. The percent of hemoglobin saturation depends upon the PO2 of the blood, which in pulmonary capillaries is 100 mmHg. Consequently, in the lungs the hemoglobin loads up with oxygen and becomes 97.5% saturated. The average PO2 of the blood in systemic capillaries is 40 mmHg. Therefore, in the tissues, hemoglobin releases oxygen and saturation falls to 75%. Increased PCO2, H+ ion concentration, temperature and 2,3-bispho-sphoglycerate shifts the oxyhemoglobin dissociation curve to the right. As a result, at any given PO2, the hemoglobin releases more oxygen to the tissue.

Blood PO2 (mmHg)

Figure 17.6 Oxyhemoglobin dissociation curve. The percent of hemoglobin saturation depends upon the PO2 of the blood, which in pulmonary capillaries is 100 mmHg. Consequently, in the lungs the hemoglobin loads up with oxygen and becomes 97.5% saturated. The average PO2 of the blood in systemic capillaries is 40 mmHg. Therefore, in the tissues, hemoglobin releases oxygen and saturation falls to 75%. Increased PCO2, H+ ion concentration, temperature and 2,3-bispho-sphoglycerate shifts the oxyhemoglobin dissociation curve to the right. As a result, at any given PO2, the hemoglobin releases more oxygen to the tissue.

linear. The amount of oxygen carried by hemoglobin increases steeply up to a PO2 of about 60 mmHg. Beyond this point, the curve becomes much flatter, so little change occurs in the percent of hemoglobin saturation as PO2 continues to increase. At a PO2 of 100 mmHg, which is the normal PO2 of the alveoli, and therefore the arterial blood, the hemoglobin is 97.5% saturated with oxygen.

Each region of the curve (the steep and flat plateau portions) has important physiological significance. The steep portion of the curve, between 0 and 60 mmHg, is the PO2 range found in the cells and tissues. On average, the PO2 of the tissues and therefore the mixed venous blood is about 40 mmHg at rest. At a PO2 of 40 mmHg, the hemoglobin is 75% saturated with oxygen. In other words, as the blood flows through the systemic capillaries, the hemoglobin releases 22.5% of its oxygen to the tissues. An increase in the metabolic activity of a tissue, and thus an increase in oxygen consumption, will decrease the PO2 in that tissue. The fall in PO2 in this region of the oxyhemoglobin dissociation curve has a profound effect on the percent of hemoglobin saturation. At a PO2 of 15 mmHg, the hemoglobin is only 25% saturated with oxygen; in this case, the hemoglobin has released 72.5% of its oxygen to the tissue. Therefore, a small drop in PO2 (from 40 to 15 mmHg) results in a marked increase in the unloading of oxygen (more than three times as much oxygen has been released to the tissue that needs it).

The plateau portion of the curve, between 60 and 100 mmHg, is the PO2 range found in the alveoli. As the mixed venous blood flows through the pulmonary capillaries in the walls of the alveoli, the hemoglobin loads up with oxygen. As mentioned earlier, at a normal alveolar PO2 of 100 mmHg, the hemoglobin becomes almost fully saturated with oxygen (97.5%). Interestingly, at a PO2 of 60 mmHg, the hemoglobin still becomes 90% saturated with oxygen. In other words, the hemoglobin remains quite saturated with oxygen even with a marked fall in PO2 (40 mmHg). This provides a good margin of safety for the oxygen-carrying capacity of blood. Therefore, if an individual ascends to some altitude above sea level or has pulmonary disease such that the alveolar PO2 falls, the oxygen content of the blood remains high.

Factors affecting transport of oxygen. Several factors affect the transport of oxygen, including:

• 2,3-Bisphosphoglycerate

Essentials of Human Physiology

Essentials of Human Physiology

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