Gas exchange occurs in the alveoli where oxygen is exchanged with carbon dioxide between the alveoli and the blood in the capillaries. This is driven by the change in partial pressure from the alveoli to the capillaries.
In the human body, oxygen is used by cells of the body’s tissues to produce ATP, while carbon dioxide is produced as a waste product. The ratio of carbon dioxide production to oxygen consumption is referred to as the respiratory quotient (RQ), which typically varies between 0.7 and 1.0. The RQ is a key factor because it is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung: the alveolar PO2 (PALVO2). The lungs never fully deflate with an exhalation; therefore, the inspired air mixes with this residual air, lowering the partial pressure of oxygen within the alveoli. This results in a lower concentration of oxygen in the lungs than is found in the air outside the body. When the RQ is known, the partial pressure of oxygen in the alveoli is calculated using the equation: alveolar PO2 = inspired PO2−((alveolar PO2)/RQ).
Henry’s law states that the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas. The higher the partial pressure of the gas, the higher the number of gas molecules that will dissolve in the liquid. The concentration of the gas in a fluid is also dependent on the solubility of the gas in the liquid.
In the lungs, oxygen diffuses out of the alveoli and into the capillaries surrounding the alveoli. Oxygen (about 98 per cent) binds reversibly to the respiratory pigment haemoglobin found in red blood cells. These red blood cells carry oxygen to the tissues where oxygen dissociates from the haemoglobin, diffusing into the cells of tissues. More specifically, alveolar PO2 is higher in the alveoli than blood PO2 in the capillaries. Since this pressure gradient exists, oxygen can diffuse down its pressure gradient, moving out of the alveoli and entering the blood of the capillaries where O2 binds to haemoglobin. At the same time, alveolar PCO2 is lower than blood PCO2. Due to this gradient, CO2 diffuses down its pressure gradient, moving out of the capillaries and entering the alveoli.
Oxygen and carbon dioxide move independently of each other; they diffuse down their pressure gradients as blood leaves the lungs through the pulmonary veins, the venous PO2=100mmHg, whereas the venous PCO2=40mmHg. As blood enters the systemic capillaries, the blood will lose oxygen and gain carbon dioxide because of the pressure difference between the tissues and blood. In systemic capillaries, PO2=100mmHg, but in the tissue cells, PO2=40mmHg. This pressure gradient drives the diffusion of oxygen out of the capillaries and into the tissue cells. At the same time, blood PCO2=40mmHg and systemic tissue PCO2=45mmHg. The pressure gradient drives CO2 out of tissue cells and into the capillaries. The blood returning to the lungs through the pulmonary arteries has a venous PO2=40mmHg and a PCO2=45mmHg. The blood enters the lung capillaries, where the process of exchanging gases between the capillaries and alveoli begins again.
Practice Questions
MCAT Official Prep (AAMC)
Official Guide B/B Section Question 29
Biology Question Pack, Vol. 2 Passage 18 Question 115
Practice Exam 4 B/B Section Question 12
Key Points
• The change in partial pressure from the alveoli (high concentration) to the capillaries (low concentration) drives the oxygen into the tissue and the carbon dioxide into the blood (high concentration) from the tissues (low concentration), which is then returned to the lungs and exhaled.
• Alveolar PO2 is higher in the alveoli than blood PO2 in the capillaries. Since this pressure gradient exists, oxygen can diffuse down its pressure gradient, moving out of the alveoli and entering the blood of the capillaries where O2 binds to haemoglobin.
• Once in the blood, the O2 binds to haemoglobin which carries it to the tissues where it dissociates to enter the cells of the tissues.
• The lungs never fully deflate, so the air that is inhaled mixes with the residual air left from the previous respiration, resulting in a lower partial pressure of oxygen within the alveoli.
• Henry’s law states that the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas.
Key Terms
Haemoglobin: iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)
Residual air: the volume of unexpended air that remains in the lungs following maximum expiration
Partial pressure: the pressure one component of a mixture of gases would contribute to the total pressure
Respiratory quotient: the ratio of carbon dioxide production to oxygen consumption
ATP: adenosine triphosphate commonly produced in respiration as a source of energy
Alveolar PO2: the partial pressure of oxygen in the alveoli
Inspired: air that is breathed in
Henry’s law: that the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas
Diffuses: moves from an area of high concentration to low concentration
Alveolar PCO2: the partial pressure of carbon dioxide in the alveoli
Pulmonary: the vascular system relating to the lungs
Systemic: the vascular system from the heart to the rest of the body
Capillaries: one cell thick small blood vessels that join arteries to veins
