Gas exchange in the muscles

Arterial-venous oxygen difference

At rest, the oxygen content of arterial blood is about 20ml of oxygen per 100ml of blood. As shown in the figure a below, this value decreases to 15 to 16ml of oxygen per 100ml after the blood has passed through the capillaries into the venous system. This difference in oxygen content between arterial and venous blood is referred to as the arterial-mixed venous oxygen difference, or (a-ṽ difference) O₂ difference. The term mixed venous(ṽ) refers to the oxygen content of blood in the right atrium, which comes from all parts of the body, both active and inactive. The difference between arterial and mixed venous oxygen content reflects to the 4 to 5ml of oxygen per 100ml of blood taken up by the tissues. The amount of oxygen taken up is proportional to its use for oxidative energy production. Thus, as the rate of oxygen use increases, the (a-ṽ) O₂ difference also increases. It can increase to 15 to 16ml per 100ml of blood during maximal levels of endurance exercise(figure b). However, at the level of the contracting muscle, the (a-ṽ) O₂ difference during intense exercise can increase to 17 to 18ml per 100ml of blood. Note that there is not a bar over the v in this instance because we are now looking at local muscle venous blood, not mixed venous blood in the right atrium. During such an effort, more oxygen is unloaded to the active muscles because the PO₂ in the muscles is substantially lower than in arterial blood.

Oxygen transport in the muscle

Oxygen is transported in the muscle to the mitochondria by a molecule called myoglobin where it is used in oxidative metabolism. Myoglobin is similar in structure to hemoglobin, but myoglobin has a much greater affinity for oxygen than hemoglobin. This concept is illustrated in the figure below. At PO₂ values less than 20, the myoglobin dissociation curve is much steeper than the dissociation curve for hemoglobin. Myoglobin releases its oxygen content only under conditions in which the PO₂ is very low. Note from the figure below that at a PO₂ at which venous blood is unloading oxygen, myoglobin is loading oxygen. It is estimated that the PO₂ in the mitochondria of an exercising muscle may be as low as 1 to 2 mmHg; thus myoglobin readily delivers oxygen to the mitochondria.

Factors influencing oxygen delivery and uptake

The rates of oxygen delivery and uptake depend on three major variables:

• Oxygen content of blood
• Blood flow
• Local conditions(e.g., pH, temperature).

With exercise, each of these variables is adjusted to ensure increased oxygen delivery to active muscle. Under normal circumstances, hemoglobin is about 98% saturated with oxygen. Any reduction in the blood’s normal oxygen-carrying capacity would hinder oxygen delivery and reduce cellular uptake of oxygen. Likewise, a reduction in the PO2 of the arterial blood would lower the partial pressure gradient, limiting the unloading of oxygen at the tissue level. Exercise increases blood flow through the muscles, less oxygen must be removed from each 100ml of blood(assuming the demand is unchanged). Thus, increased blood flow improves oxygen delivery.

Many local changes in the muscle during exercise affect oxygen delivery and uptake. For example, muscle activity increases muscle acidity because lactate production. Also, muscle temperature and carbon dioxide concentration both increase because of increased metabolism. All these changes increase oxygen unloading from the hemoglobin molecule, facilitating oxygen unloading from the hemoglobin molecule, facilitating oxygen delivery and uptake by the muscles.

Carbon dioxide removal

Carbon dioxide exits the cells by simple diffusion in response to the partial pressure gradient between the tissue and the capillary blood. For example, muscles generate carbon dioxide through oxidative metabolism, so the PCO₂ in muscles is relatively high compared with that in the capillary blood. Consequently, CO₂ diffuses out of the muscles and into the blood to be transported to the lungs.