Gas Exchange at the Muscles • Now we have considered how our respiratory and cardio-vascular system brings air into our lungs, exchange oxygen and carbon dioxide in the alveoli, and transport oxygen to the muscles (and CO2 away from them). • All that remains is for us to consider the delivery of oxygen to the muscles from the capillary blood. • This gas exchange between the tissue and the blood in the capillaries is known as – internal respiration.
The Arterial-venous Oxygen Difference • At rest, the oxygen content of arterial blood is about 20ml of oxygen per 100 ml of blood. • This value drops to 15 or 16ml of O2 per 100ml as the blood passes through the capillaries into the venous system. • This difference in oxygen content between arterial and venous blood is referred to as the arterial-venous oxygen difference (a-VO2diff).
a-vO2 diff this expresses the difference between the oxygen carried by blood in arteries and veins and represents the amount of oxygen delivered to working tissue in the capillary system ARTERIOVENOUS OXYGEN DIFFERENCE a-vO2 diff - AT REST venule capillary arteriole blood flow 15ml O2 per 100ml blood 20ml O2 per 100ml blood a-vO2 diff = 5ml per 100ml blood a-vO2 diff - DURING INTENSE EXERCISE venule capillary arteriole blood flow 5ml O2 per 100ml blood 20ml O2 per 100ml blood a-vO2 diff = 15ml per 100ml blood
The Arterial-venous Oxygen Difference • It reflects the 4-5 ml of oxygen per 100 ml 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-vO2 diff also increases.
The Arterial-venous Oxygen Difference • E.g. during intense exercise the a-vO2 diff in contracting muscles can increase to 15 to 16 ml per 100ml of blood. During such an effort, the blood unloads more oxygen to the active muscles because the PO2 in the muscles is drastically lower than in arterial blood.
Key Point • The a-vO2 diff increases from a resting value of about 4 to 5 ml per 100 ml of blood up to values of 15 to 16 ml per 100 ml of blood during exercise. • This increase reflects an increase extraction of oxygen from arterial blood by active muscle, thus decreasing the oxygen content of the venous blood.
Factors Influencing Oxygen Delivery and Uptake. • The rates of oxygen delivery and uptake depend on the three major variables. • The oxygen content of blood. • The amount of blood flow. • The local conditions. • As we begin to exercise, each of these variables must be adjusted to ensure increased oxygen delivery to our active muscles.
Factors Influencing Oxygen Delivery and Uptake. • We have discussed in class that under normal circumstances haemoglobin is 98% saturated with oxygen. • Any reduction in the blood’s normal oxygen carrying capacity would hinder oxygen delivery and reduce cellular uptake of oxygen.
Factors Influencing Oxygen Delivery and Uptake. • Exercise causes increased blood flow through the muscles. As more blood carries oxygen through the muscles, less oxygen must be removed from each 100 ml of blood (assuming the demand remains unchanged). • Thus increasing blood flow improves oxygen delivery and uptake.
Factors Influencing Oxygen Delivery and Uptake. • Many local changes in the muscle during exercise affect oxygen delivery and uptake. • Muscle activity increases muscle acidity because of lactate production. • Muscle temperature and carbon dioxide concentration both increase because of increased metabolism. • All of these increase oxygen uploading from haemoglobin molecule, facilitating oxygen delivery and uptake by the muscles.
Factors Influencing Oxygen Delivery and Uptake. • During maximal exercise,however, when we push our bodies to the limit, changes in any of these areas can impair oxygen delivery and restrict out abilities to meet oxidative demands.