Wize High School Grade 11 Biology Textbook > Human Physiology: The Respiratory System
Gas Exchange and Control
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Gas Exchange
- 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
2. Diffusion and partial pressure
- 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)
3. Pulmonary and systemic circulation
- 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.
4. Oxygen saturation
- 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)
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Carbon Dioxide Gas Exchange
From the tissues to the lungs, there is a large amount of CO2 waste being carried. It is transported in 3 ways
- Dissolved in the plasma - small amount (7%)
- Attached to a blood protein called globin (not full hemoglobin like O2)
- 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.
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Example: Oxygen Saturation Curves
Given the three oxygen saturation curves (labelled normal, A and B) describe some of the potential causes of the left and right shifts in the curves.
Leftward shift (A, blue): hemoglobin has a HIGH affinity for oxygen and is less likely to give it up. Shift could be describing fetal hemoglobin (has a higher affinity for O2 due to the relatively low pressure of oxygen in the placenta circulation) OR low CO2 levels (causing a higher blood pH)
Rightward shift (B, red): hemoglobin has a LOW affinity for oxygen and is more likely to give it up. Shift could be describing people with blood disorders like anemia (production of low-affinity version of Hb) or sickle cell anemia (Hb is mutated so cannot hold on to oxygen very well) OR high CO2 (low blood pH) or high temperatures - both happen during exercise
Which of the following statements is true regarding gas exchange?
Practice: Gas Exchange
Answer the following questions regarding gas exchange:
- What is the name of the iron-carrying protein that binds oxygen?
- Having high bicarbonate causes blood pH to be?
Practice: Carbon Monoxide
Carbon monoxide is an odourless gas that can cause death by binding to hemoglobin and blocking oxygen transport to organs. During low levels of carbon monoxide exposure, in which direction would the oxygen saturation curve be shifted?