E-cigarettes pose a dilemma for public health officials. Because e-cigarettes produce fewer toxic chemicals than regular cigarettes, they may be a safer alternative for people who want to quit smoking but still crave nicotine. On the other hand, e-cigarettes may encourage nicotine addiction among teenagers. Evaluate the scientific evidence. Are e-cigarettes an effective aid for quitting cigarettes? What evidence supports the assertion that e-cigarettes are especially harmful to adolescents? The Centers for Disease Control website is a good place to start. cdc.gov/tobacco/basic_information/e-cigarettes/

During exercise, the cardiovascular system must supply muscles with large amounts of oxygen and fuel and get rid of a lot of waste.

How do the cardiovascular systems of athletes respond to prolonged exercise?

When athletes exercise, what is the primary physiological variable responsible for their sustained increase in ventilation rate?

a. Decreased blood PO₂

b. Increased blood PCO₂

c. Increased blood pH

d. Increased body temperature

Partial pressure reflects the relative amount of gas in a mixture and is measured in millimeters of mercury (mm Hg). Llamas are native to the Andes Mountains in South America. The partial pressure of O₂ (abbreviated P_O₂) in the atmosphere where llamas live is about half of the P_O₂ at sea level. As a result, the PO₂ in the lungs of llamas is about 50 mm Hg, whereas that in human lungs at sea level is about 100 mm Hg. A dissociation curve for hemoglobin shows the percentage of saturation (the amount of O₂ bound to hemoglobin) at increasing values of P_O₂ As you see in the graph below, the dissociation curves for llama and human hemoglobin differ. Compare these two curves and explain how the hemoglobin of llamas is an adaptation to living where the air is 'thin.'

Mountain climbers often spend weeks adjusting to the lower partial pressure of oxygen at high altitudes before and during their ascent of high peaks. During that time, their bodies begin to produce more red blood cells. Some runners and cyclists prepare for competition by training at high altitudes or by sleeping in a tent in which P_O₂ is kept artificially low. Explain why this training strategy may improve an athlete's performance.