Gas Exchange and Control

Gas Exchange

1. Partial pressure is a measurement of the amount of force exerted by one particular substance in a mixture.

• Blood contains a mixture of gases, each of which exert pressure on the sides of the blood vessels
• Measured in mmHg
• The partial pressures of Oxygen and CO2 are important for understanding gas exchange in the circulatory system

• The higher the concentration of a gas, the higher the pressure it will exert.
• When there is a difference in partial pressure (or gas concentration) across a membrane, the gas will naturally diffuse from the area of high pressure (or high concentration) to an area of low pressure (or low concentration)

• Pulmonary circulation is the movement of blood between the heart and the lungs to become oxygenated
• Systemic circulation is the movement of blood between the heart and the rest of the body
• Gas exchange occurs in both of these pathways
• When blood reaches the tissues, it delivers oxygen and picks up CO2
• When blood reaches the lungs, it gets rid of the CO2 and picks up oxygen. In the lungs, the partial pressure of oxygen in the blood is very low (~40 mmHg). This allows oxygen in the alveoli to diffuse into the capillaries. Once the blood leaves the lungs, the partial pressure of oxygen is ~100 mmHg.

• The partial pressure of oxygen is a measure of the blood’s oxygen saturation.
• A constant level of saturation above 90% is ideal (arterial partial pressure of 100 mmHg)

Carbon Dioxide Gas Exchange

1. Dissolved in the plasma - small amount (7%)
2. Attached to a blood protein called globin (not full hemoglobin like O2)
3. Bicarbonate ions (about 70%)

• CO2 combines with water to make a carbonic acid. It dissociates into HCO3- and H+

• This is an important system in helping maintain the pH of our blood.
• higher levels of H+ can also help O2 dissociated from hemoglobin.