Question

Explain how acute exercise would challenge the delivery of oxygen and release of carbon dioxide as well as how acute exercise may actually enhance the delivery and release of oxygen at the tissues via an explanation of shifting of the oxyhemoglobin dissociation curve, etc. Make sure to include information about hemoglobin and myoglobin structure and a full explanation of the sigmoidal shape of the dissociation curve (i.e. loading and unloading portions).

Answer 1

FACTORS AFFECTING THE DISSOCIATION CURVE

The strength with which oxygen binds to hemoglobin is affected by several factors. These factors shift or reshape the oxyhemoglobin dissociation curve. A rightward shift indicates that the hemoglobin under study has a decreased affinity for oxygen. This makes it more difficult for hemoglobin to bind to oxygen (requiring a higher partial pressure of oxygen to achieve the same oxygen saturation), but it makes it easier for the hemoglobin to release oxygen bound to it. The effect of this rightward shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, such as during exercise, or hemorrhagic shock.

In contrast, the curve is shifted to the left by the opposite of these conditions. This leftward shift indicates that the hemoglobin under study has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly. Left shift of the curve is a sign of hemoglobin's increased affinity for oxygen (e.g. at the lungs).

Exercise causes an IMCREASE IN ACIDITY, TEMPERATURE AND INCREASE IN METABOLISM INTERMEDIATES (2,3DPG) and a decrease in oxygen in your muscle tissues. This causes an increased dissociation of oxygen from your blood flowing through your muscles, supplying them with much needed oxygen.

THUS, EXERCISE CAUSES OXYGEN-HAEMOGLOBIN DISSOCIATION CURVE TO SHIFT RIGHT DUE TO DECREASED AFFINITY OF HAEMOGLOBIN TO OXYGEN. THIS DECREASED AFFINITY TO OXYGEN PROMPTS RELEASE OF OXYGEN FROM HAEMOGLOBIN TO TISSUES MORE EFFICIENTLY.

(The Bohr effect describes how the affinity of hemoglobin for oxygen changes depending on the local biochemical conditions. An increase in acidity, temperature and the concentration of intermediate chemicals in the conversion of sugar to energy—specifically 2,3-diphosphoglycerate—decreases hemoglobin's affinity for oxygen, causing oxygen to diffuse into the tissues.)

REASONS FOR THE SIGMOIDAL SHAPE OF DISSOCIATION CURVE

A hemoglobin molecule can bind up to four oxygen molecules in a reversible method. Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape. At pressures above about 60 mmHg, the standard dissociation curve is relatively flat, which means that the oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure.

The 'plateau' portion of the oxyhaemoglobin dissociation curve is the range that exists at the pulmonary capillaries (minimal reduction of oxygen transported until the p(O2) falls 50 mmHg).

The 'steep' portion of the oxyhaemoglobin dissociation curve is the range that exists at the systemic capillaries (a small drop in systemic capillary p(O2) can result in the release of large amounts of oxygen for the metabolically active cells).