Now let’s turn our focus to how we transport oxygen and carbon dioxide in the blood. Oxygen and carbon dioxide are carried through the bloodstream in different forms. For oxygen, 98.5% is bound to hemoglobin in RBCs and 1.5% is dissolved in the plasma. With carbon dioxide, 70% travels through the bloodstream as bicarbonate (HCO₃ ^–), 23% is bound to hemoglobin in RBCs, and 7% is dissolved in the plasma.

As was mentioned, 98.5% of oxygen is carried bound to hemoglobin in RBCs and 1.5% is dissolved in the plasma. The O₂-hemoglobin saturation curve shows the proportion of Hb bound to O₂ . The higher the PO₂ (X axis, left-right) the more oxygen is bound to Hb (Y axis, up-down).

Certain conditions influence the O₂ -Hb saturation curve. During conditions such as exercise, tissues need more oxygen.

Actively working tissues generate acids as waste, lowering the pH. A drop in pH shifts the curve to the right, causing more O₂ to be released at the tissues (see the teal line).

Actively working tissues also generate more CO₂ as waste. Higher PCO₂ levels shift the curve to the right and more O₂ is delivered to the tissues. A tissue that is resting generates less CO₂ . This shifts the curve to the left and less O₂ is delivered to the tissues.

Actively working tissues also generate heat. Higher temperatures shift the curve to the right, and more O₂ is delivered. Resting tissue generates less heat, and less O₂ is delivered.

When we’re sick and have a fever, more O₂ is delivered to the tissues helping them fight infection. Ingenious!

Fetal Hb has a higher affinity for O₂ than adult Hb. When PO₂ is low, fetal Hb can carry up to 30% more O₂ than maternal Hb. Because of this, oxygen diffuses easily from maternal RBCs to fetal RBCs across the placenta.

Carbon Dioxide

As discussed earlier, 70% of CO₂ travels through the bloodstream as bicarbonate (HCO₃ ^–), 23% is bound to hemoglobin in RBCs, and 7% is dissolved in the plasma.

The conversion of CO₂ to bicarbonate to be carried in the plasma is a somewhat complicated process.

First, CO₂ combines with H₂O in the red blood cell to form carbonic acid (HCO₃ ^–). Acids donate H⁺ ions, so carbonic acid dissociates to HCO₃ ^– and H⁺ . The bicarbonate ions leave the RBC in exchange for Cl– ions. At the alveoli, the reaction reverses and CO₂ is exhaled.

Buffers prevent rapid, drastic changes in pH of body fluids by converting strong acids and bases into weak acids and bases. This is a rapid conversion which takes place in fractions of a second. Most buffer systems in the body consist of a weak acid and a weak base.

Buffer reactions, like that in the equation above, can proceed from left to right or right to left. In the carbonic acid-bicarbonate buffer system, excess hydrogen ions can be instantly converted to water and carbon dioxide. Under alkaline conditions, water and carbon dioxide are converted to hydrogen and bicarbonate ions. We will revisit this concept of buffers and pH control in a later objective.